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rfc:rfc4880

Network Working Group J. Callas Request for Comments: 4880 PGP Corporation Obsoletes: 1991, 2440 L. Donnerhacke Category: Standards Track IKS GmbH

                                                             H. Finney
                                                       PGP Corporation
                                                               D. Shaw
                                                             R. Thayer
                                                         November 2007
                       OpenPGP Message Format

Status of This Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Abstract

 This document is maintained in order to publish all necessary
 information needed to develop interoperable applications based on the
 OpenPGP format.  It is not a step-by-step cookbook for writing an
 application.  It describes only the format and methods needed to
 read, check, generate, and write conforming packets crossing any
 network.  It does not deal with storage and implementation questions.
 It does, however, discuss implementation issues necessary to avoid
 security flaws.
 OpenPGP software uses a combination of strong public-key and
 symmetric cryptography to provide security services for electronic
 communications and data storage.  These services include
 confidentiality, key management, authentication, and digital
 signatures.  This document specifies the message formats used in
 OpenPGP.

Callas, et al Standards Track [Page 1] RFC 4880 OpenPGP Message Format November 2007

Table of Contents

 1. Introduction ....................................................5
    1.1. Terms ......................................................5
 2. General functions ...............................................6
    2.1. Confidentiality via Encryption .............................6
    2.2. Authentication via Digital Signature .......................7
    2.3. Compression ................................................7
    2.4. Conversion to Radix-64 .....................................8
    2.5. Signature-Only Applications ................................8
 3. Data Element Formats ............................................8
    3.1. Scalar Numbers .............................................8
    3.2. Multiprecision Integers ....................................9
    3.3. Key IDs ....................................................9
    3.4. Text .......................................................9
    3.5. Time Fields ...............................................10
    3.6. Keyrings ..................................................10
    3.7. String-to-Key (S2K) Specifiers ............................10
         3.7.1. String-to-Key (S2K) Specifier Types ................10
                3.7.1.1. Simple S2K ................................10
                3.7.1.2. Salted S2K ................................11
                3.7.1.3. Iterated and Salted S2K ...................11
         3.7.2. String-to-Key Usage ................................12
                3.7.2.1. Secret-Key Encryption .....................12
                3.7.2.2. Symmetric-Key Message Encryption ..........13
 4. Packet Syntax ..................................................13
    4.1. Overview ..................................................13
    4.2. Packet Headers ............................................13
         4.2.1. Old Format Packet Lengths ..........................14
         4.2.2. New Format Packet Lengths ..........................15
                4.2.2.1. One-Octet Lengths .........................15
                4.2.2.2. Two-Octet Lengths .........................15
                4.2.2.3. Five-Octet Lengths ........................15
                4.2.2.4. Partial Body Lengths ......................16
         4.2.3. Packet Length Examples .............................16
    4.3. Packet Tags ...............................................17
 5. Packet Types ...................................................17
    5.1. Public-Key Encrypted Session Key Packets (Tag 1) ..........17
    5.2. Signature Packet (Tag 2) ..................................19
         5.2.1. Signature Types ....................................19
         5.2.2. Version 3 Signature Packet Format ..................21
         5.2.3. Version 4 Signature Packet Format ..................24
                5.2.3.1. Signature Subpacket Specification .........25
                5.2.3.2. Signature Subpacket Types .................27
                5.2.3.3. Notes on Self-Signatures ..................27
                5.2.3.4. Signature Creation Time ...................28
                5.2.3.5. Issuer ....................................28
                5.2.3.6. Key Expiration Time .......................28

Callas, et al Standards Track [Page 2] RFC 4880 OpenPGP Message Format November 2007

                5.2.3.7. Preferred Symmetric Algorithms ............28
                5.2.3.8. Preferred Hash Algorithms .................29
                5.2.3.9. Preferred Compression Algorithms ..........29
                5.2.3.10. Signature Expiration Time ................29
                5.2.3.11. Exportable Certification .................29
                5.2.3.12. Revocable ................................30
                5.2.3.13. Trust Signature ..........................30
                5.2.3.14. Regular Expression .......................31
                5.2.3.15. Revocation Key ...........................31
                5.2.3.16. Notation Data ............................31
                5.2.3.17. Key Server Preferences ...................32
                5.2.3.18. Preferred Key Server .....................33
                5.2.3.19. Primary User ID ..........................33
                5.2.3.20. Policy URI ...............................33
                5.2.3.21. Key Flags ................................33
                5.2.3.22. Signer's User ID .........................34
                5.2.3.23. Reason for Revocation ....................35
                5.2.3.24. Features .................................36
                5.2.3.25. Signature Target .........................36
                5.2.3.26. Embedded Signature .......................37
         5.2.4. Computing Signatures ...............................37
                5.2.4.1. Subpacket Hints ...........................38
    5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3) .......38
    5.4. One-Pass Signature Packets (Tag 4) ........................39
    5.5. Key Material Packet .......................................40
         5.5.1. Key Packet Variants ................................40
                5.5.1.1. Public-Key Packet (Tag 6) .................40
                5.5.1.2. Public-Subkey Packet (Tag 14) .............40
                5.5.1.3. Secret-Key Packet (Tag 5) .................41
                5.5.1.4. Secret-Subkey Packet (Tag 7) ..............41
         5.5.2. Public-Key Packet Formats ..........................41
         5.5.3. Secret-Key Packet Formats ..........................43
    5.6. Compressed Data Packet (Tag 8) ............................45
    5.7. Symmetrically Encrypted Data Packet (Tag 9) ...............45
    5.8. Marker Packet (Obsolete Literal Packet) (Tag 10) ..........46
    5.9. Literal Data Packet (Tag 11) ..............................46
    5.10. Trust Packet (Tag 12) ....................................47
    5.11. User ID Packet (Tag 13) ..................................48
    5.12. User Attribute Packet (Tag 17) ...........................48
         5.12.1. The Image Attribute Subpacket .....................48
    5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18) ..49
    5.14. Modification Detection Code Packet (Tag 19) ..............52
 6. Radix-64 Conversions ...........................................53
    6.1. An Implementation of the CRC-24 in "C" ....................54
    6.2. Forming ASCII Armor .......................................54
    6.3. Encoding Binary in Radix-64 ...............................57
    6.4. Decoding Radix-64 .........................................58
    6.5. Examples of Radix-64 ......................................59

Callas, et al Standards Track [Page 3] RFC 4880 OpenPGP Message Format November 2007

    6.6. Example of an ASCII Armored Message .......................59
 7. Cleartext Signature Framework ..................................59
    7.1. Dash-Escaped Text .........................................60
 8. Regular Expressions ............................................61
 9. Constants ......................................................61
    9.1. Public-Key Algorithms .....................................62
    9.2. Symmetric-Key Algorithms ..................................62
    9.3. Compression Algorithms ....................................63
    9.4. Hash Algorithms ...........................................63
 10. IANA Considerations ...........................................63
    10.1. New String-to-Key Specifier Types ........................64
    10.2. New Packets ..............................................64
         10.2.1. User Attribute Types ..............................64
                10.2.1.1. Image Format Subpacket Types .............64
         10.2.2. New Signature Subpackets ..........................64
                10.2.2.1. Signature Notation Data Subpackets .......65
                10.2.2.2. Key Server Preference Extensions .........65
                10.2.2.3. Key Flags Extensions .....................65
                10.2.2.4. Reason For Revocation Extensions .........65
                10.2.2.5. Implementation Features ..................66
         10.2.3. New Packet Versions ...............................66
    10.3. New Algorithms ...........................................66
         10.3.1. Public-Key Algorithms .............................66
         10.3.2. Symmetric-Key Algorithms ..........................67
         10.3.3. Hash Algorithms ...................................67
         10.3.4. Compression Algorithms ............................67
 11. Packet Composition ............................................67
    11.1. Transferable Public Keys .................................67
    11.2. Transferable Secret Keys .................................69
    11.3. OpenPGP Messages .........................................69
    11.4. Detached Signatures ......................................70
 12. Enhanced Key Formats ..........................................70
    12.1. Key Structures ...........................................70
    12.2. Key IDs and Fingerprints .................................71
 13. Notes on Algorithms ...........................................72
    13.1. PKCS#1 Encoding in OpenPGP ...............................72
         13.1.1. EME-PKCS1-v1_5-ENCODE .............................73
         13.1.2. EME-PKCS1-v1_5-DECODE .............................73
         13.1.3. EMSA-PKCS1-v1_5 ...................................74
    13.2. Symmetric Algorithm Preferences ..........................75
    13.3. Other Algorithm Preferences ..............................76
         13.3.1. Compression Preferences ...........................76
         13.3.2. Hash Algorithm Preferences ........................76
    13.4. Plaintext ................................................77
    13.5. RSA ......................................................77
    13.6. DSA ......................................................77
    13.7. Elgamal ..................................................78
    13.8. Reserved Algorithm Numbers ...............................78

Callas, et al Standards Track [Page 4] RFC 4880 OpenPGP Message Format November 2007

    13.9. OpenPGP CFB Mode .........................................78
    13.10. Private or Experimental Parameters ......................79
    13.11. Extension of the MDC System .............................80
    13.12. Meta-Considerations for Expansion .......................80
 14. Security Considerations .......................................81
 15. Implementation Nits ...........................................84
 16. References ....................................................86
    16.1. Normative References .....................................86
    16.2. Informative References ...................................88

1. Introduction

 This document provides information on the message-exchange packet
 formats used by OpenPGP to provide encryption, decryption, signing,
 and key management functions.  It is a revision of RFC 2440, "OpenPGP
 Message Format", which itself replaces RFC 1991, "PGP Message
 Exchange Formats" [RFC1991] [RFC2440].

1.1. Terms

  • OpenPGP - This is a term for security software that uses PGP 5.x

as a basis, formalized in RFC 2440 and this document.

  • PGP - Pretty Good Privacy. PGP is a family of software systems

developed by Philip R. Zimmermann from which OpenPGP is based.

  • PGP 2.6.x - This version of PGP has many variants, hence the term

PGP 2.6.x. It used only RSA, MD5, and IDEA for its cryptographic

     transforms.  An informational RFC, RFC 1991, was written
     describing this version of PGP.
  • PGP 5.x - This version of PGP is formerly known as "PGP 3" in the

community and also in the predecessor of this document, RFC 1991.

     It has new formats and corrects a number of problems in the PGP
     2.6.x design.  It is referred to here as PGP 5.x because that
     software was the first release of the "PGP 3" code base.
  • GnuPG - GNU Privacy Guard, also called GPG. GnuPG is an OpenPGP

implementation that avoids all encumbered algorithms.

     Consequently, early versions of GnuPG did not include RSA public
     keys.  GnuPG may or may not have (depending on version) support
     for IDEA or other encumbered algorithms.
 "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of PGP
 Corporation and are used with permission.  The term "OpenPGP" refers
 to the protocol described in this and related documents.

Callas, et al Standards Track [Page 5] RFC 4880 OpenPGP Message Format November 2007

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in [RFC2119].
 The key words "PRIVATE USE", "HIERARCHICAL ALLOCATION", "FIRST COME
 FIRST SERVED", "EXPERT REVIEW", "SPECIFICATION REQUIRED", "IESG
 APPROVAL", "IETF CONSENSUS", and "STANDARDS ACTION" that appear in
 this document when used to describe namespace allocation are to be
 interpreted as described in [RFC2434].

2. General functions

 OpenPGP provides data integrity services for messages and data files
 by using these core technologies:
  1. digital signatures
  1. encryption
  1. compression
  1. Radix-64 conversion
 In addition, OpenPGP provides key management and certificate
 services, but many of these are beyond the scope of this document.

2.1. Confidentiality via Encryption

 OpenPGP combines symmetric-key encryption and public-key encryption
 to provide confidentiality.  When made confidential, first the object
 is encrypted using a symmetric encryption algorithm.  Each symmetric
 key is used only once, for a single object.  A new "session key" is
 generated as a random number for each object (sometimes referred to
 as a session).  Since it is used only once, the session key is bound
 to the message and transmitted with it.  To protect the key, it is
 encrypted with the receiver's public key.  The sequence is as
 follows:
 1.  The sender creates a message.
 2.  The sending OpenPGP generates a random number to be used as a
     session key for this message only.
 3.  The session key is encrypted using each recipient's public key.
     These "encrypted session keys" start the message.

Callas, et al Standards Track [Page 6] RFC 4880 OpenPGP Message Format November 2007

 4.  The sending OpenPGP encrypts the message using the session key,
     which forms the remainder of the message.  Note that the message
     is also usually compressed.
 5.  The receiving OpenPGP decrypts the session key using the
     recipient's private key.
 6.  The receiving OpenPGP decrypts the message using the session key.
     If the message was compressed, it will be decompressed.
 With symmetric-key encryption, an object may be encrypted with a
 symmetric key derived from a passphrase (or other shared secret), or
 a two-stage mechanism similar to the public-key method described
 above in which a session key is itself encrypted with a symmetric
 algorithm keyed from a shared secret.
 Both digital signature and confidentiality services may be applied to
 the same message.  First, a signature is generated for the message
 and attached to the message.  Then the message plus signature is
 encrypted using a symmetric session key.  Finally, the session key is
 encrypted using public-key encryption and prefixed to the encrypted
 block.

2.2. Authentication via Digital Signature

 The digital signature uses a hash code or message digest algorithm,
 and a public-key signature algorithm.  The sequence is as follows:
 1.  The sender creates a message.
 2.  The sending software generates a hash code of the message.
 3.  The sending software generates a signature from the hash code
     using the sender's private key.
 4.  The binary signature is attached to the message.
 5.  The receiving software keeps a copy of the message signature.
 6.  The receiving software generates a new hash code for the received
     message and verifies it using the message's signature.  If the
     verification is successful, the message is accepted as authentic.

2.3. Compression

 OpenPGP implementations SHOULD compress the message after applying
 the signature but before encryption.

Callas, et al Standards Track [Page 7] RFC 4880 OpenPGP Message Format November 2007

 If an implementation does not implement compression, its authors
 should be aware that most OpenPGP messages in the world are
 compressed.  Thus, it may even be wise for a space-constrained
 implementation to implement decompression, but not compression.
 Furthermore, compression has the added side effect that some types of
 attacks can be thwarted by the fact that slightly altered, compressed
 data rarely uncompresses without severe errors.  This is hardly
 rigorous, but it is operationally useful.  These attacks can be
 rigorously prevented by implementing and using Modification Detection
 Codes as described in sections following.

2.4. Conversion to Radix-64

 OpenPGP's underlying native representation for encrypted messages,
 signature certificates, and keys is a stream of arbitrary octets.
 Some systems only permit the use of blocks consisting of seven-bit,
 printable text.  For transporting OpenPGP's native raw binary octets
 through channels that are not safe to raw binary data, a printable
 encoding of these binary octets is needed.  OpenPGP provides the
 service of converting the raw 8-bit binary octet stream to a stream
 of printable ASCII characters, called Radix-64 encoding or ASCII
 Armor.
 Implementations SHOULD provide Radix-64 conversions.

2.5. Signature-Only Applications

 OpenPGP is designed for applications that use both encryption and
 signatures, but there are a number of problems that are solved by a
 signature-only implementation.  Although this specification requires
 both encryption and signatures, it is reasonable for there to be
 subset implementations that are non-conformant only in that they omit
 encryption.

3. Data Element Formats

 This section describes the data elements used by OpenPGP.

3.1. Scalar Numbers

 Scalar numbers are unsigned and are always stored in big-endian
 format.  Using n[k] to refer to the kth octet being interpreted, the
 value of a two-octet scalar is ((n[0] << 8) + n[1]).  The value of a
 four-octet scalar is ((n[0] << 24) + (n[1] << 16) + (n[2] << 8) +
 n[3]).

Callas, et al Standards Track [Page 8] RFC 4880 OpenPGP Message Format November 2007

3.2. Multiprecision Integers

 Multiprecision integers (also called MPIs) are unsigned integers used
 to hold large integers such as the ones used in cryptographic
 calculations.
 An MPI consists of two pieces: a two-octet scalar that is the length
 of the MPI in bits followed by a string of octets that contain the
 actual integer.
 These octets form a big-endian number; a big-endian number can be
 made into an MPI by prefixing it with the appropriate length.
 Examples:
 (all numbers are in hexadecimal)
 The string of octets [00 01 01] forms an MPI with the value 1.  The
 string [00 09 01 FF] forms an MPI with the value of 511.
 Additional rules:
 The size of an MPI is ((MPI.length + 7) / 8) + 2 octets.
 The length field of an MPI describes the length starting from its
 most significant non-zero bit.  Thus, the MPI [00 02 01] is not
 formed correctly.  It should be [00 01 01].
 Unused bits of an MPI MUST be zero.
 Also note that when an MPI is encrypted, the length refers to the
 plaintext MPI.  It may be ill-formed in its ciphertext.

3.3. Key IDs

 A Key ID is an eight-octet scalar that identifies a key.
 Implementations SHOULD NOT assume that Key IDs are unique.  The
 section "Enhanced Key Formats" below describes how Key IDs are
 formed.

3.4. Text

 Unless otherwise specified, the character set for text is the UTF-8
 [RFC3629] encoding of Unicode [ISO10646].

Callas, et al Standards Track [Page 9] RFC 4880 OpenPGP Message Format November 2007

3.5. Time Fields

 A time field is an unsigned four-octet number containing the number
 of seconds elapsed since midnight, 1 January 1970 UTC.

3.6. Keyrings

 A keyring is a collection of one or more keys in a file or database.
 Traditionally, a keyring is simply a sequential list of keys, but may
 be any suitable database.  It is beyond the scope of this standard to
 discuss the details of keyrings or other databases.

3.7. String-to-Key (S2K) Specifiers

 String-to-key (S2K) specifiers are used to convert passphrase strings
 into symmetric-key encryption/decryption keys.  They are used in two
 places, currently: to encrypt the secret part of private keys in the
 private keyring, and to convert passphrases to encryption keys for
 symmetrically encrypted messages.

3.7.1. String-to-Key (S2K) Specifier Types

 There are three types of S2K specifiers currently supported, and
 some reserved values:
     ID          S2K Type
     --          --------
     0           Simple S2K
     1           Salted S2K
     2           Reserved value
     3           Iterated and Salted S2K
     100 to 110  Private/Experimental S2K
 These are described in Sections 3.7.1.1 - 3.7.1.3.

3.7.1.1. Simple S2K

 This directly hashes the string to produce the key data.  See below
 for how this hashing is done.
     Octet 0:        0x00
     Octet 1:        hash algorithm
 Simple S2K hashes the passphrase to produce the session key.  The
 manner in which this is done depends on the size of the session key
 (which will depend on the cipher used) and the size of the hash

Callas, et al Standards Track [Page 10] RFC 4880 OpenPGP Message Format November 2007

 algorithm's output.  If the hash size is greater than the session key
 size, the high-order (leftmost) octets of the hash are used as the
 key.
 If the hash size is less than the key size, multiple instances of the
 hash context are created -- enough to produce the required key data.
 These instances are preloaded with 0, 1, 2, ... octets of zeros (that
 is to say, the first instance has no preloading, the second gets
 preloaded with 1 octet of zero, the third is preloaded with two
 octets of zeros, and so forth).
 As the data is hashed, it is given independently to each hash
 context.  Since the contexts have been initialized differently, they
 will each produce different hash output.  Once the passphrase is
 hashed, the output data from the multiple hashes is concatenated,
 first hash leftmost, to produce the key data, with any excess octets
 on the right discarded.

3.7.1.2. Salted S2K

 This includes a "salt" value in the S2K specifier -- some arbitrary
 data -- that gets hashed along with the passphrase string, to help
 prevent dictionary attacks.
     Octet 0:        0x01
     Octet 1:        hash algorithm
     Octets 2-9:     8-octet salt value
 Salted S2K is exactly like Simple S2K, except that the input to the
 hash function(s) consists of the 8 octets of salt from the S2K
 specifier, followed by the passphrase.

3.7.1.3. Iterated and Salted S2K

 This includes both a salt and an octet count.  The salt is combined
 with the passphrase and the resulting value is hashed repeatedly.
 This further increases the amount of work an attacker must do to try
 dictionary attacks.
     Octet  0:        0x03
     Octet  1:        hash algorithm
     Octets 2-9:      8-octet salt value
     Octet  10:       count, a one-octet, coded value

Callas, et al Standards Track [Page 11] RFC 4880 OpenPGP Message Format November 2007

 The count is coded into a one-octet number using the following
 formula:
     #define EXPBIAS 6
         count = ((Int32)16 + (c & 15)) << ((c >> 4) + EXPBIAS);
 The above formula is in C, where "Int32" is a type for a 32-bit
 integer, and the variable "c" is the coded count, Octet 10.
 Iterated-Salted S2K hashes the passphrase and salt data multiple
 times.  The total number of octets to be hashed is specified in the
 encoded count in the S2K specifier.  Note that the resulting count
 value is an octet count of how many octets will be hashed, not an
 iteration count.
 Initially, one or more hash contexts are set up as with the other S2K
 algorithms, depending on how many octets of key data are needed.
 Then the salt, followed by the passphrase data, is repeatedly hashed
 until the number of octets specified by the octet count has been
 hashed.  The one exception is that if the octet count is less than
 the size of the salt plus passphrase, the full salt plus passphrase
 will be hashed even though that is greater than the octet count.
 After the hashing is done, the data is unloaded from the hash
 context(s) as with the other S2K algorithms.

3.7.2. String-to-Key Usage

 Implementations SHOULD use salted or iterated-and-salted S2K
 specifiers, as simple S2K specifiers are more vulnerable to
 dictionary attacks.

3.7.2.1. Secret-Key Encryption

 An S2K specifier can be stored in the secret keyring to specify how
 to convert the passphrase to a key that unlocks the secret data.
 Older versions of PGP just stored a cipher algorithm octet preceding
 the secret data or a zero to indicate that the secret data was
 unencrypted.  The MD5 hash function was always used to convert the
 passphrase to a key for the specified cipher algorithm.
 For compatibility, when an S2K specifier is used, the special value
 254 or 255 is stored in the position where the hash algorithm octet
 would have been in the old data structure.  This is then followed
 immediately by a one-octet algorithm identifier, and then by the S2K
 specifier as encoded above.

Callas, et al Standards Track [Page 12] RFC 4880 OpenPGP Message Format November 2007

 Therefore, preceding the secret data there will be one of these
 possibilities:
     0:           secret data is unencrypted (no passphrase)
     255 or 254:  followed by algorithm octet and S2K specifier
     Cipher alg:  use Simple S2K algorithm using MD5 hash
 This last possibility, the cipher algorithm number with an implicit
 use of MD5 and IDEA, is provided for backward compatibility; it MAY
 be understood, but SHOULD NOT be generated, and is deprecated.
 These are followed by an Initial Vector of the same length as the
 block size of the cipher for the decryption of the secret values, if
 they are encrypted, and then the secret-key values themselves.

3.7.2.2. Symmetric-Key Message Encryption

 OpenPGP can create a Symmetric-key Encrypted Session Key (ESK) packet
 at the front of a message.  This is used to allow S2K specifiers to
 be used for the passphrase conversion or to create messages with a
 mix of symmetric-key ESKs and public-key ESKs.  This allows a message
 to be decrypted either with a passphrase or a public-key pair.
 PGP 2.X always used IDEA with Simple string-to-key conversion when
 encrypting a message with a symmetric algorithm.  This is deprecated,
 but MAY be used for backward-compatibility.

4. Packet Syntax

 This section describes the packets used by OpenPGP.

4.1. Overview

 An OpenPGP message is constructed from a number of records that are
 traditionally called packets.  A packet is a chunk of data that has a
 tag specifying its meaning.  An OpenPGP message, keyring,
 certificate, and so forth consists of a number of packets.  Some of
 those packets may contain other OpenPGP packets (for example, a
 compressed data packet, when uncompressed, contains OpenPGP packets).
 Each packet consists of a packet header, followed by the packet body.
 The packet header is of variable length.

4.2. Packet Headers

 The first octet of the packet header is called the "Packet Tag".  It
 determines the format of the header and denotes the packet contents.
 The remainder of the packet header is the length of the packet.

Callas, et al Standards Track [Page 13] RFC 4880 OpenPGP Message Format November 2007

 Note that the most significant bit is the leftmost bit, called bit 7.
 A mask for this bit is 0x80 in hexadecimal.
            +---------------+
       PTag |7 6 5 4 3 2 1 0|
            +---------------+
       Bit 7 -- Always one
       Bit 6 -- New packet format if set
 PGP 2.6.x only uses old format packets.  Thus, software that
 interoperates with those versions of PGP must only use old format
 packets.  If interoperability is not an issue, the new packet format
 is RECOMMENDED.  Note that old format packets have four bits of
 packet tags, and new format packets have six; some features cannot be
 used and still be backward-compatible.
 Also note that packets with a tag greater than or equal to 16 MUST
 use new format packets.  The old format packets can only express tags
 less than or equal to 15.
 Old format packets contain:
       Bits 5-2 -- packet tag
       Bits 1-0 -- length-type
 New format packets contain:
       Bits 5-0 -- packet tag

4.2.1. Old Format Packet Lengths

 The meaning of the length-type in old format packets is:
 0 - The packet has a one-octet length.  The header is 2 octets long.
 1 - The packet has a two-octet length.  The header is 3 octets long.
 2 - The packet has a four-octet length.  The header is 5 octets long.
 3 - The packet is of indeterminate length.  The header is 1 octet
     long, and the implementation must determine how long the packet
     is.  If the packet is in a file, this means that the packet
     extends until the end of the file.  In general, an implementation
     SHOULD NOT use indeterminate-length packets except where the end
     of the data will be clear from the context, and even then it is
     better to use a definite length, or a new format header.  The new
     format headers described below have a mechanism for precisely
     encoding data of indeterminate length.

Callas, et al Standards Track [Page 14] RFC 4880 OpenPGP Message Format November 2007

4.2.2. New Format Packet Lengths

 New format packets have four possible ways of encoding length:
 1. A one-octet Body Length header encodes packet lengths of up to 191
    octets.
 2. A two-octet Body Length header encodes packet lengths of 192 to
    8383 octets.
 3. A five-octet Body Length header encodes packet lengths of up to
    4,294,967,295 (0xFFFFFFFF) octets in length.  (This actually
    encodes a four-octet scalar number.)
 4. When the length of the packet body is not known in advance by the
    issuer, Partial Body Length headers encode a packet of
    indeterminate length, effectively making it a stream.

4.2.2.1. One-Octet Lengths

 A one-octet Body Length header encodes a length of 0 to 191 octets.
 This type of length header is recognized because the one octet value
 is less than 192.  The body length is equal to:
     bodyLen = 1st_octet;

4.2.2.2. Two-Octet Lengths

 A two-octet Body Length header encodes a length of 192 to 8383
 octets.  It is recognized because its first octet is in the range 192
 to 223.  The body length is equal to:
     bodyLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192

4.2.2.3. Five-Octet Lengths

 A five-octet Body Length header consists of a single octet holding
 the value 255, followed by a four-octet scalar.  The body length is
 equal to:
     bodyLen = (2nd_octet << 24) | (3rd_octet << 16) |
               (4th_octet << 8)  | 5th_octet
 This basic set of one, two, and five-octet lengths is also used
 internally to some packets.

Callas, et al Standards Track [Page 15] RFC 4880 OpenPGP Message Format November 2007

4.2.2.4. Partial Body Lengths

 A Partial Body Length header is one octet long and encodes the length
 of only part of the data packet.  This length is a power of 2, from 1
 to 1,073,741,824 (2 to the 30th power).  It is recognized by its one
 octet value that is greater than or equal to 224, and less than 255.
 The Partial Body Length is equal to:
     partialBodyLen = 1 << (1st_octet & 0x1F);
 Each Partial Body Length header is followed by a portion of the
 packet body data.  The Partial Body Length header specifies this
 portion's length.  Another length header (one octet, two-octet,
 five-octet, or partial) follows that portion.  The last length header
 in the packet MUST NOT be a Partial Body Length header.  Partial Body
 Length headers may only be used for the non-final parts of the
 packet.
 Note also that the last Body Length header can be a zero-length
 header.
 An implementation MAY use Partial Body Lengths for data packets, be
 they literal, compressed, or encrypted.  The first partial length
 MUST be at least 512 octets long.  Partial Body Lengths MUST NOT be
 used for any other packet types.

4.2.3. Packet Length Examples

 These examples show ways that new format packets might encode the
 packet lengths.
 A packet with length 100 may have its length encoded in one octet:
 0x64.  This is followed by 100 octets of data.
 A packet with length 1723 may have its length encoded in two octets:
 0xC5, 0xFB.  This header is followed by the 1723 octets of data.
 A packet with length 100000 may have its length encoded in five
 octets: 0xFF, 0x00, 0x01, 0x86, 0xA0.
 It might also be encoded in the following octet stream: 0xEF, first
 32768 octets of data; 0xE1, next two octets of data; 0xE0, next one
 octet of data; 0xF0, next 65536 octets of data; 0xC5, 0xDD, last 1693
 octets of data.  This is just one possible encoding, and many
 variations are possible on the size of the Partial Body Length
 headers, as long as a regular Body Length header encodes the last
 portion of the data.

Callas, et al Standards Track [Page 16] RFC 4880 OpenPGP Message Format November 2007

 Please note that in all of these explanations, the total length of
 the packet is the length of the header(s) plus the length of the
 body.

4.3. Packet Tags

 The packet tag denotes what type of packet the body holds.  Note that
 old format headers can only have tags less than 16, whereas new
 format headers can have tags as great as 63.  The defined tags (in
 decimal) are as follows:
     0        -- Reserved - a packet tag MUST NOT have this value
     1        -- Public-Key Encrypted Session Key Packet
     2        -- Signature Packet
     3        -- Symmetric-Key Encrypted Session Key Packet
     4        -- One-Pass Signature Packet
     5        -- Secret-Key Packet
     6        -- Public-Key Packet
     7        -- Secret-Subkey Packet
     8        -- Compressed Data Packet
     9        -- Symmetrically Encrypted Data Packet
     10       -- Marker Packet
     11       -- Literal Data Packet
     12       -- Trust Packet
     13       -- User ID Packet
     14       -- Public-Subkey Packet
     17       -- User Attribute Packet
     18       -- Sym. Encrypted and Integrity Protected Data Packet
     19       -- Modification Detection Code Packet
     60 to 63 -- Private or Experimental Values

5. Packet Types

5.1. Public-Key Encrypted Session Key Packets (Tag 1)

 A Public-Key Encrypted Session Key packet holds the session key used
 to encrypt a message.  Zero or more Public-Key Encrypted Session Key
 packets and/or Symmetric-Key Encrypted Session Key packets may
 precede a Symmetrically Encrypted Data Packet, which holds an
 encrypted message.  The message is encrypted with the session key,
 and the session key is itself encrypted and stored in the Encrypted
 Session Key packet(s).  The Symmetrically Encrypted Data Packet is
 preceded by one Public-Key Encrypted Session Key packet for each
 OpenPGP key to which the message is encrypted.  The recipient of the
 message finds a session key that is encrypted to their public key,
 decrypts the session key, and then uses the session key to decrypt
 the message.

Callas, et al Standards Track [Page 17] RFC 4880 OpenPGP Message Format November 2007

 The body of this packet consists of:
  1. A one-octet number giving the version number of the packet type.

The currently defined value for packet version is 3.

  1. An eight-octet number that gives the Key ID of the public key to

which the session key is encrypted. If the session key is

     encrypted to a subkey, then the Key ID of this subkey is used
     here instead of the Key ID of the primary key.
  1. A one-octet number giving the public-key algorithm used.
  1. A string of octets that is the encrypted session key. This

string takes up the remainder of the packet, and its contents are

     dependent on the public-key algorithm used.
 Algorithm Specific Fields for RSA encryption
  1. multiprecision integer (MPI) of RSA encrypted value me mod n. Algorithm Specific Fields for Elgamal encryption: - MPI of Elgamal (Diffie-Hellman) value gk mod p.
  1. MPI of Elgamal (Diffie-Hellman) value m * yk mod p. The value "m" in the above formulas is derived from the session key as follows. First, the session key is prefixed with a one-octet algorithm identifier that specifies the symmetric encryption algorithm used to encrypt the following Symmetrically Encrypted Data Packet. Then a two-octet checksum is appended, which is equal to the sum of the preceding session key octets, not including the algorithm identifier, modulo 65536. This value is then encoded as described in PKCS#1 block encoding EME-PKCS1-v1_5 in Section 7.2.1 of [RFC3447] to form the "m" value used in the formulas above. See Section 13.1 of this document for notes on OpenPGP's use of PKCS#1. Note that when an implementation forms several PKESKs with one session key, forming a message that can be decrypted by several keys, the implementation MUST make a new PKCS#1 encoding for each key. An implementation MAY accept or use a Key ID of zero as a "wild card" or "speculative" Key ID. In this case, the receiving implementation would try all available private keys, checking for a valid decrypted session key. This format helps reduce traffic analysis of messages. Callas, et al Standards Track [Page 18] RFC 4880 OpenPGP Message Format November 2007 5.2. Signature Packet (Tag 2) A Signature packet describes a binding between some public key and some data. The most common signatures are a signature of a file or a block of text, and a signature that is a certification of a User ID. Two versions of Signature packets are defined. Version 3 provides basic signature information, while version 4 provides an expandable format with subpackets that can specify more information about the signature. PGP 2.6.x only accepts version 3 signatures. Implementations SHOULD accept V3 signatures. Implementations SHOULD generate V4 signatures. Note that if an implementation is creating an encrypted and signed message that is encrypted to a V3 key, it is reasonable to create a V3 signature. 5.2.1. Signature Types There are a number of possible meanings for a signature, which are indicated in a signature type octet in any given signature. Please note that the vagueness of these meanings is not a flaw, but a feature of the system. Because OpenPGP places final authority for validity upon the receiver of a signature, it may be that one signer's casual act might be more rigorous than some other authority's positive act. See Section 5.2.4, "Computing Signatures", for detailed information on how to compute and verify signatures of each type. These meanings are as follows: 0x00: Signature of a binary document. This means the signer owns it, created it, or certifies that it has not been modified. 0x01: Signature of a canonical text document. This means the signer owns it, created it, or certifies that it has not been modified. The signature is calculated over the text data with its line endings converted to <CR><LF>. 0x02: Standalone signature. This signature is a signature of only its own subpacket contents. It is calculated identically to a signature over a zero-length binary document. Note that it doesn't make sense to have a V3 standalone signature. Callas, et al Standards Track [Page 19] RFC 4880 OpenPGP Message Format November 2007 0x10: Generic certification of a User ID and Public-Key packet. The issuer of this certification does not make any particular assertion as to how well the certifier has checked that the owner of the key is in fact the person described by the User ID. 0x11: Persona certification of a User ID and Public-Key packet. The issuer of this certification has not done any verification of the claim that the owner of this key is the User ID specified. 0x12: Casual certification of a User ID and Public-Key packet. The issuer of this certification has done some casual verification of the claim of identity. 0x13: Positive certification of a User ID and Public-Key packet. The issuer of this certification has done substantial verification of the claim of identity. Most OpenPGP implementations make their "key signatures" as 0x10 certifications. Some implementations can issue 0x11-0x13 certifications, but few differentiate between the types. 0x18: Subkey Binding Signature This signature is a statement by the top-level signing key that indicates that it owns the subkey. This signature is calculated directly on the primary key and subkey, and not on any User ID or other packets. A signature that binds a signing subkey MUST have an Embedded Signature subpacket in this binding signature that contains a 0x19 signature made by the signing subkey on the primary key and subkey. 0x19: Primary Key Binding Signature This signature is a statement by a signing subkey, indicating that it is owned by the primary key and subkey. This signature is calculated the same way as a 0x18 signature: directly on the primary key and subkey, and not on any User ID or other packets. 0x1F: Signature directly on a key This signature is calculated directly on a key. It binds the information in the Signature subpackets to the key, and is appropriate to be used for subpackets that provide information about the key, such as the Revocation Key subpacket. It is also appropriate for statements that non-self certifiers want to make about the key itself, rather than the binding between a key and a name. Callas, et al Standards Track [Page 20] RFC 4880 OpenPGP Message Format November 2007 0x20: Key revocation signature The signature is calculated directly on the key being revoked. A revoked key is not to be used. Only revocation signatures by the key being revoked, or by an authorized revocation key, should be considered valid revocation signatures. 0x28: Subkey revocation signature The signature is calculated directly on the subkey being revoked. A revoked subkey is not to be used. Only revocation signatures by the top-level signature key that is bound to this subkey, or by an authorized revocation key, should be considered valid revocation signatures. 0x30: Certification revocation signature This signature revokes an earlier User ID certification signature (signature class 0x10 through 0x13) or direct-key signature (0x1F). It should be issued by the same key that issued the revoked signature or an authorized revocation key. The signature is computed over the same data as the certificate that it revokes, and should have a later creation date than that certificate. 0x40: Timestamp signature. This signature is only meaningful for the timestamp contained in it. 0x50: Third-Party Confirmation signature. This signature is a signature over some other OpenPGP Signature packet(s). It is analogous to a notary seal on the signed data. A third-party signature SHOULD include Signature Target subpacket(s) to give easy identification. Note that we really do mean SHOULD. There are plausible uses for this (such as a blind party that only sees the signature, not the key or source document) that cannot include a target subpacket. 5.2.2. Version 3 Signature Packet Format The body of a version 3 Signature Packet contains: - One-octet version number (3). - One-octet length of following hashed material. MUST be 5. - One-octet signature type. - Four-octet creation time. - Eight-octet Key ID of signer. Callas, et al Standards Track [Page 21] RFC 4880 OpenPGP Message Format November 2007 - One-octet public-key algorithm. - One-octet hash algorithm. - Two-octet field holding left 16 bits of signed hash value. - One or more multiprecision integers comprising the signature. This portion is algorithm specific, as described below. The concatenation of the data to be signed, the signature type, and creation time from the Signature packet (5 additional octets) is hashed. The resulting hash value is used in the signature algorithm. The high 16 bits (first two octets) of the hash are included in the Signature packet to provide a quick test to reject some invalid signatures. Algorithm-Specific Fields for RSA signatures: - multiprecision integer (MPI) of RSA signature value md mod n.
 Algorithm-Specific Fields for DSA signatures:
  1. MPI of DSA value r.
  1. MPI of DSA value s.
 The signature calculation is based on a hash of the signed data, as
 described above.  The details of the calculation are different for
 DSA signatures than for RSA signatures.
 With RSA signatures, the hash value is encoded using PKCS#1 encoding
 type EMSA-PKCS1-v1_5 as described in Section 9.2 of RFC 3447.  This
 requires inserting the hash value as an octet string into an ASN.1
 structure.  The object identifier for the type of hash being used is
 included in the structure.  The hexadecimal representations for the
 currently defined hash algorithms are as follows:
  1. MD5: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05
  1. RIPEMD-160: 0x2B, 0x24, 0x03, 0x02, 0x01
  1. SHA-1: 0x2B, 0x0E, 0x03, 0x02, 0x1A
  1. SHA224: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04
  1. SHA256: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01
  1. SHA384: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02

Callas, et al Standards Track [Page 22] RFC 4880 OpenPGP Message Format November 2007

  1. SHA512: 0x60, 0x86, 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03
 The ASN.1 Object Identifiers (OIDs) are as follows:
  1. MD5: 1.2.840.113549.2.5
  1. RIPEMD-160: 1.3.36.3.2.1
  1. SHA-1: 1.3.14.3.2.26
  1. SHA224: 2.16.840.1.101.3.4.2.4
  1. SHA256: 2.16.840.1.101.3.4.2.1
  1. SHA384: 2.16.840.1.101.3.4.2.2
  1. SHA512: 2.16.840.1.101.3.4.2.3
 The full hash prefixes for these are as follows:
     MD5:        0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
                 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x05, 0x05, 0x00,
                 0x04, 0x10
     RIPEMD-160: 0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2B, 0x24,
                 0x03, 0x02, 0x01, 0x05, 0x00, 0x04, 0x14
     SHA-1:      0x30, 0x21, 0x30, 0x09, 0x06, 0x05, 0x2b, 0x0E,
                 0x03, 0x02, 0x1A, 0x05, 0x00, 0x04, 0x14
     SHA224:     0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x04, 0x05,
                 0x00, 0x04, 0x1C
     SHA256:     0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
                 0x00, 0x04, 0x20
     SHA384:     0x30, 0x41, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x02, 0x05,
                 0x00, 0x04, 0x30
     SHA512:     0x30, 0x51, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
                 0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x03, 0x05,
                 0x00, 0x04, 0x40
 DSA signatures MUST use hashes that are equal in size to the number
 of bits of q, the group generated by the DSA key's generator value.

Callas, et al Standards Track [Page 23] RFC 4880 OpenPGP Message Format November 2007

 If the output size of the chosen hash is larger than the number of
 bits of q, the hash result is truncated to fit by taking the number
 of leftmost bits equal to the number of bits of q.  This (possibly
 truncated) hash function result is treated as a number and used
 directly in the DSA signature algorithm.

5.2.3. Version 4 Signature Packet Format

 The body of a version 4 Signature packet contains:
  1. One-octet version number (4).
  1. One-octet signature type.
  1. One-octet public-key algorithm.
  1. One-octet hash algorithm.
  1. Two-octet scalar octet count for following hashed subpacket data.

Note that this is the length in octets of all of the hashed

     subpackets; a pointer incremented by this number will skip over
     the hashed subpackets.
  1. Hashed subpacket data set (zero or more subpackets).
  1. Two-octet scalar octet count for the following unhashed subpacket

data. Note that this is the length in octets of all of the

     unhashed subpackets; a pointer incremented by this number will
     skip over the unhashed subpackets.
  1. Unhashed subpacket data set (zero or more subpackets).
  1. Two-octet field holding the left 16 bits of the signed hash

value.

  1. One or more multiprecision integers comprising the signature.

This portion is algorithm specific, as described above.

 The concatenation of the data being signed and the signature data
 from the version number through the hashed subpacket data (inclusive)
 is hashed.  The resulting hash value is what is signed.  The left 16
 bits of the hash are included in the Signature packet to provide a
 quick test to reject some invalid signatures.
 There are two fields consisting of Signature subpackets.  The first
 field is hashed with the rest of the signature data, while the second
 is unhashed.  The second set of subpackets is not cryptographically

Callas, et al Standards Track [Page 24] RFC 4880 OpenPGP Message Format November 2007

 protected by the signature and should include only advisory
 information.
 The algorithms for converting the hash function result to a signature
 are described in a section below.

5.2.3.1. Signature Subpacket Specification

 A subpacket data set consists of zero or more Signature subpackets.
 In Signature packets, the subpacket data set is preceded by a two-
 octet scalar count of the length in octets of all the subpackets.  A
 pointer incremented by this number will skip over the subpacket data
 set.
 Each subpacket consists of a subpacket header and a body.  The header
 consists of:
  1. the subpacket length (1, 2, or 5 octets),
  1. the subpacket type (1 octet),
 and is followed by the subpacket-specific data.
 The length includes the type octet but not this length.  Its format
 is similar to the "new" format packet header lengths, but cannot have
 Partial Body Lengths.  That is:
     if the 1st octet <  192, then
         lengthOfLength = 1
         subpacketLen = 1st_octet
     if the 1st octet >= 192 and < 255, then
         lengthOfLength = 2
         subpacketLen = ((1st_octet - 192) << 8) + (2nd_octet) + 192
     if the 1st octet = 255, then
         lengthOfLength = 5
         subpacket length = [four-octet scalar starting at 2nd_octet]
 The value of the subpacket type octet may be:
          0 = Reserved
          1 = Reserved
          2 = Signature Creation Time
          3 = Signature Expiration Time
          4 = Exportable Certification
          5 = Trust Signature
          6 = Regular Expression

Callas, et al Standards Track [Page 25] RFC 4880 OpenPGP Message Format November 2007

          7 = Revocable
          8 = Reserved
          9 = Key Expiration Time
         10 = Placeholder for backward compatibility
         11 = Preferred Symmetric Algorithms
         12 = Revocation Key
         13 = Reserved
         14 = Reserved
         15 = Reserved
         16 = Issuer
         17 = Reserved
         18 = Reserved
         19 = Reserved
         20 = Notation Data
         21 = Preferred Hash Algorithms
         22 = Preferred Compression Algorithms
         23 = Key Server Preferences
         24 = Preferred Key Server
         25 = Primary User ID
         26 = Policy URI
         27 = Key Flags
         28 = Signer's User ID
         29 = Reason for Revocation
         30 = Features
         31 = Signature Target
         32 = Embedded Signature
 100 To 110 = Private or experimental
 An implementation SHOULD ignore any subpacket of a type that it does
 not recognize.
 Bit 7 of the subpacket type is the "critical" bit.  If set, it
 denotes that the subpacket is one that is critical for the evaluator
 of the signature to recognize.  If a subpacket is encountered that is
 marked critical but is unknown to the evaluating software, the
 evaluator SHOULD consider the signature to be in error.
 An evaluator may "recognize" a subpacket, but not implement it.  The
 purpose of the critical bit is to allow the signer to tell an
 evaluator that it would prefer a new, unknown feature to generate an
 error than be ignored.
 Implementations SHOULD implement the three preferred algorithm
 subpackets (11, 21, and 22), as well as the "Reason for Revocation"
 subpacket.  Note, however, that if an implementation chooses not to
 implement some of the preferences, it is required to behave in a
 polite manner to respect the wishes of those users who do implement
 these preferences.

Callas, et al Standards Track [Page 26] RFC 4880 OpenPGP Message Format November 2007

5.2.3.2. Signature Subpacket Types

 A number of subpackets are currently defined.  Some subpackets apply
 to the signature itself and some are attributes of the key.
 Subpackets that are found on a self-signature are placed on a
 certification made by the key itself.  Note that a key may have more
 than one User ID, and thus may have more than one self-signature, and
 differing subpackets.
 A subpacket may be found either in the hashed or unhashed subpacket
 sections of a signature.  If a subpacket is not hashed, then the
 information in it cannot be considered definitive because it is not
 part of the signature proper.

5.2.3.3. Notes on Self-Signatures

 A self-signature is a binding signature made by the key to which the
 signature refers.  There are three types of self-signatures, the
 certification signatures (types 0x10-0x13), the direct-key signature
 (type 0x1F), and the subkey binding signature (type 0x18).  For
 certification self-signatures, each User ID may have a self-
 signature, and thus different subpackets in those self-signatures.
 For subkey binding signatures, each subkey in fact has a self-
 signature.  Subpackets that appear in a certification self-signature
 apply to the user name, and subpackets that appear in the subkey
 self-signature apply to the subkey.  Lastly, subpackets on the
 direct-key signature apply to the entire key.
 Implementing software should interpret a self-signature's preference
 subpackets as narrowly as possible.  For example, suppose a key has
 two user names, Alice and Bob.  Suppose that Alice prefers the
 symmetric algorithm CAST5, and Bob prefers IDEA or TripleDES.  If the
 software locates this key via Alice's name, then the preferred
 algorithm is CAST5; if software locates the key via Bob's name, then
 the preferred algorithm is IDEA.  If the key is located by Key ID,
 the algorithm of the primary User ID of the key provides the
 preferred symmetric algorithm.
 Revoking a self-signature or allowing it to expire has a semantic
 meaning that varies with the signature type.  Revoking the self-
 signature on a User ID effectively retires that user name.  The
 self-signature is a statement, "My name X is tied to my signing key
 K" and is corroborated by other users' certifications.  If another
 user revokes their certification, they are effectively saying that
 they no longer believe that name and that key are tied together.
 Similarly, if the users themselves revoke their self-signature, then
 the users no longer go by that name, no longer have that email
 address, etc.  Revoking a binding signature effectively retires that

Callas, et al Standards Track [Page 27] RFC 4880 OpenPGP Message Format November 2007

 subkey.  Revoking a direct-key signature cancels that signature.
 Please see the "Reason for Revocation" subpacket (Section 5.2.3.23)
 for more relevant detail.
 Since a self-signature contains important information about the key's
 use, an implementation SHOULD allow the user to rewrite the self-
 signature, and important information in it, such as preferences and
 key expiration.
 It is good practice to verify that a self-signature imported into an
 implementation doesn't advertise features that the implementation
 doesn't support, rewriting the signature as appropriate.
 An implementation that encounters multiple self-signatures on the
 same object may resolve the ambiguity in any way it sees fit, but it
 is RECOMMENDED that priority be given to the most recent self-
 signature.

5.2.3.4. Signature Creation Time

 (4-octet time field)
 The time the signature was made.
 MUST be present in the hashed area.

5.2.3.5. Issuer

 (8-octet Key ID)
 The OpenPGP Key ID of the key issuing the signature.

5.2.3.6. Key Expiration Time

 (4-octet time field)
 The validity period of the key.  This is the number of seconds after
 the key creation time that the key expires.  If this is not present
 or has a value of zero, the key never expires.  This is found only on
 a self-signature.

5.2.3.7. Preferred Symmetric Algorithms

 (array of one-octet values)
 Symmetric algorithm numbers that indicate which algorithms the key
 holder prefers to use.  The subpacket body is an ordered list of
 octets with the most preferred listed first.  It is assumed that only

Callas, et al Standards Track [Page 28] RFC 4880 OpenPGP Message Format November 2007

 algorithms listed are supported by the recipient's software.
 Algorithm numbers are in Section 9.  This is only found on a self-
 signature.

5.2.3.8. Preferred Hash Algorithms

 (array of one-octet values)
 Message digest algorithm numbers that indicate which algorithms the
 key holder prefers to receive.  Like the preferred symmetric
 algorithms, the list is ordered.  Algorithm numbers are in Section 9.
 This is only found on a self-signature.

5.2.3.9. Preferred Compression Algorithms

 (array of one-octet values)
 Compression algorithm numbers that indicate which algorithms the key
 holder prefers to use.  Like the preferred symmetric algorithms, the
 list is ordered.  Algorithm numbers are in Section 9.  If this
 subpacket is not included, ZIP is preferred.  A zero denotes that
 uncompressed data is preferred; the key holder's software might have
 no compression software in that implementation.  This is only found
 on a self-signature.

5.2.3.10. Signature Expiration Time

 (4-octet time field)
 The validity period of the signature.  This is the number of seconds
 after the signature creation time that the signature expires.  If
 this is not present or has a value of zero, it never expires.

5.2.3.11. Exportable Certification

 (1 octet of exportability, 0 for not, 1 for exportable)
 This subpacket denotes whether a certification signature is
 "exportable", to be used by other users than the signature's issuer.
 The packet body contains a Boolean flag indicating whether the
 signature is exportable.  If this packet is not present, the
 certification is exportable; it is equivalent to a flag containing a
 1.
 Non-exportable, or "local", certifications are signatures made by a
 user to mark a key as valid within that user's implementation only.

Callas, et al Standards Track [Page 29] RFC 4880 OpenPGP Message Format November 2007

 Thus, when an implementation prepares a user's copy of a key for
 transport to another user (this is the process of "exporting" the
 key), any local certification signatures are deleted from the key.
 The receiver of a transported key "imports" it, and likewise trims
 any local certifications.  In normal operation, there won't be any,
 assuming the import is performed on an exported key.  However, there
 are instances where this can reasonably happen.  For example, if an
 implementation allows keys to be imported from a key database in
 addition to an exported key, then this situation can arise.
 Some implementations do not represent the interest of a single user
 (for example, a key server).  Such implementations always trim local
 certifications from any key they handle.

5.2.3.12. Revocable

 (1 octet of revocability, 0 for not, 1 for revocable)
 Signature's revocability status.  The packet body contains a Boolean
 flag indicating whether the signature is revocable.  Signatures that
 are not revocable have any later revocation signatures ignored.  They
 represent a commitment by the signer that he cannot revoke his
 signature for the life of his key.  If this packet is not present,
 the signature is revocable.

5.2.3.13. Trust Signature

 (1 octet "level" (depth), 1 octet of trust amount)
 Signer asserts that the key is not only valid but also trustworthy at
 the specified level.  Level 0 has the same meaning as an ordinary
 validity signature.  Level 1 means that the signed key is asserted to
 be a valid trusted introducer, with the 2nd octet of the body
 specifying the degree of trust.  Level 2 means that the signed key is
 asserted to be trusted to issue level 1 trust signatures, i.e., that
 it is a "meta introducer".  Generally, a level n trust signature
 asserts that a key is trusted to issue level n-1 trust signatures.
 The trust amount is in a range from 0-255, interpreted such that
 values less than 120 indicate partial trust and values of 120 or
 greater indicate complete trust.  Implementations SHOULD emit values
 of 60 for partial trust and 120 for complete trust.

Callas, et al Standards Track [Page 30] RFC 4880 OpenPGP Message Format November 2007

5.2.3.14. Regular Expression

 (null-terminated regular expression)
 Used in conjunction with trust Signature packets (of level > 0) to
 limit the scope of trust that is extended.  Only signatures by the
 target key on User IDs that match the regular expression in the body
 of this packet have trust extended by the trust Signature subpacket.
 The regular expression uses the same syntax as the Henry Spencer's
 "almost public domain" regular expression [REGEX] package.  A
 description of the syntax is found in Section 8 below.

5.2.3.15. Revocation Key

 (1 octet of class, 1 octet of public-key algorithm ID, 20 octets of
 fingerprint)
 Authorizes the specified key to issue revocation signatures for this
 key.  Class octet must have bit 0x80 set.  If the bit 0x40 is set,
 then this means that the revocation information is sensitive.  Other
 bits are for future expansion to other kinds of authorizations.  This
 is found on a self-signature.
 If the "sensitive" flag is set, the keyholder feels this subpacket
 contains private trust information that describes a real-world
 sensitive relationship.  If this flag is set, implementations SHOULD
 NOT export this signature to other users except in cases where the
 data needs to be available: when the signature is being sent to the
 designated revoker, or when it is accompanied by a revocation
 signature from that revoker.  Note that it may be appropriate to
 isolate this subpacket within a separate signature so that it is not
 combined with other subpackets that need to be exported.

5.2.3.16. Notation Data

     (4 octets of flags, 2 octets of name length (M),
                         2 octets of value length (N),
                         M octets of name data,
                         N octets of value data)
 This subpacket describes a "notation" on the signature that the
 issuer wishes to make.  The notation has a name and a value, each of
 which are strings of octets.  There may be more than one notation in
 a signature.  Notations can be used for any extension the issuer of
 the signature cares to make.  The "flags" field holds four octets of
 flags.

Callas, et al Standards Track [Page 31] RFC 4880 OpenPGP Message Format November 2007

 All undefined flags MUST be zero.  Defined flags are as follows:
     First octet: 0x80 = human-readable.  This note value is text.
     Other octets: none.
 Notation names are arbitrary strings encoded in UTF-8.  They reside
 in two namespaces: The IETF namespace and the user namespace.
 The IETF namespace is registered with IANA.  These names MUST NOT
 contain the "@" character (0x40).  This is a tag for the user
 namespace.
 Names in the user namespace consist of a UTF-8 string tag followed by
 "@" followed by a DNS domain name.  Note that the tag MUST NOT
 contain an "@" character.  For example, the "sample" tag used by
 Example Corporation could be "sample@example.com".
 Names in a user space are owned and controlled by the owners of that
 domain.  Obviously, it's bad form to create a new name in a DNS space
 that you don't own.
 Since the user namespace is in the form of an email address,
 implementers MAY wish to arrange for that address to reach a person
 who can be consulted about the use of the named tag.  Note that due
 to UTF-8 encoding, not all valid user space name tags are valid email
 addresses.
 If there is a critical notation, the criticality applies to that
 specific notation and not to notations in general.

5.2.3.17. Key Server Preferences

 (N octets of flags)
 This is a list of one-bit flags that indicate preferences that the
 key holder has about how the key is handled on a key server.  All
 undefined flags MUST be zero.
 First octet: 0x80 = No-modify
     the key holder requests that this key only be modified or updated
     by the key holder or an administrator of the key server.
 This is found only on a self-signature.

Callas, et al Standards Track [Page 32] RFC 4880 OpenPGP Message Format November 2007

5.2.3.18. Preferred Key Server

 (String)
 This is a URI of a key server that the key holder prefers be used for
 updates.  Note that keys with multiple User IDs can have a preferred
 key server for each User ID.  Note also that since this is a URI, the
 key server can actually be a copy of the key retrieved by ftp, http,
 finger, etc.

5.2.3.19. Primary User ID

 (1 octet, Boolean)
 This is a flag in a User ID's self-signature that states whether this
 User ID is the main User ID for this key.  It is reasonable for an
 implementation to resolve ambiguities in preferences, etc. by
 referring to the primary User ID.  If this flag is absent, its value
 is zero.  If more than one User ID in a key is marked as primary, the
 implementation may resolve the ambiguity in any way it sees fit, but
 it is RECOMMENDED that priority be given to the User ID with the most
 recent self-signature.
 When appearing on a self-signature on a User ID packet, this
 subpacket applies only to User ID packets.  When appearing on a
 self-signature on a User Attribute packet, this subpacket applies
 only to User Attribute packets.  That is to say, there are two
 different and independent "primaries" -- one for User IDs, and one
 for User Attributes.

5.2.3.20. Policy URI

 (String)
 This subpacket contains a URI of a document that describes the policy
 under which the signature was issued.

5.2.3.21. Key Flags

 (N octets of flags)
 This subpacket contains a list of binary flags that hold information
 about a key.  It is a string of octets, and an implementation MUST
 NOT assume a fixed size.  This is so it can grow over time.  If a
 list is shorter than an implementation expects, the unstated flags
 are considered to be zero.  The defined flags are as follows:

Callas, et al Standards Track [Page 33] RFC 4880 OpenPGP Message Format November 2007

     First octet:
     0x01 - This key may be used to certify other keys.
     0x02 - This key may be used to sign data.
     0x04 - This key may be used to encrypt communications.
     0x08 - This key may be used to encrypt storage.
     0x10 - The private component of this key may have been split
            by a secret-sharing mechanism.
     0x20 - This key may be used for authentication.
     0x80 - The private component of this key may be in the
            possession of more than one person.
 Usage notes:
 The flags in this packet may appear in self-signatures or in
 certification signatures.  They mean different things depending on
 who is making the statement -- for example, a certification signature
 that has the "sign data" flag is stating that the certification is
 for that use.  On the other hand, the "communications encryption"
 flag in a self-signature is stating a preference that a given key be
 used for communications.  Note however, that it is a thorny issue to
 determine what is "communications" and what is "storage".  This
 decision is left wholly up to the implementation; the authors of this
 document do not claim any special wisdom on the issue and realize
 that accepted opinion may change.
 The "split key" (0x10) and "group key" (0x80) flags are placed on a
 self-signature only; they are meaningless on a certification
 signature.  They SHOULD be placed only on a direct-key signature
 (type 0x1F) or a subkey signature (type 0x18), one that refers to the
 key the flag applies to.

5.2.3.22. Signer's User ID

 (String)
 This subpacket allows a keyholder to state which User ID is
 responsible for the signing.  Many keyholders use a single key for
 different purposes, such as business communications as well as
 personal communications.  This subpacket allows such a keyholder to
 state which of their roles is making a signature.

Callas, et al Standards Track [Page 34] RFC 4880 OpenPGP Message Format November 2007

 This subpacket is not appropriate to use to refer to a User Attribute
 packet.

5.2.3.23. Reason for Revocation

 (1 octet of revocation code, N octets of reason string)
 This subpacket is used only in key revocation and certification
 revocation signatures.  It describes the reason why the key or
 certificate was revoked.
 The first octet contains a machine-readable code that denotes the
 reason for the revocation:
      0  - No reason specified (key revocations or cert revocations)
      1  - Key is superseded (key revocations)
      2  - Key material has been compromised (key revocations)
      3  - Key is retired and no longer used (key revocations)
      32 - User ID information is no longer valid (cert revocations)
 100-110 - Private Use
 Following the revocation code is a string of octets that gives
 information about the Reason for Revocation in human-readable form
 (UTF-8).  The string may be null, that is, of zero length.  The
 length of the subpacket is the length of the reason string plus one.
 An implementation SHOULD implement this subpacket, include it in all
 revocation signatures, and interpret revocations appropriately.
 There are important semantic differences between the reasons, and
 there are thus important reasons for revoking signatures.
 If a key has been revoked because of a compromise, all signatures
 created by that key are suspect.  However, if it was merely
 superseded or retired, old signatures are still valid.  If the
 revoked signature is the self-signature for certifying a User ID, a
 revocation denotes that that user name is no longer in use.  Such a
 revocation SHOULD include a 0x20 code.
 Note that any signature may be revoked, including a certification on
 some other person's key.  There are many good reasons for revoking a
 certification signature, such as the case where the keyholder leaves
 the employ of a business with an email address.  A revoked
 certification is no longer a part of validity calculations.

Callas, et al Standards Track [Page 35] RFC 4880 OpenPGP Message Format November 2007

5.2.3.24. Features

 (N octets of flags)
 The Features subpacket denotes which advanced OpenPGP features a
 user's implementation supports.  This is so that as features are
 added to OpenPGP that cannot be backwards-compatible, a user can
 state that they can use that feature.  The flags are single bits that
 indicate that a given feature is supported.
 This subpacket is similar to a preferences subpacket, and only
 appears in a self-signature.
 An implementation SHOULD NOT use a feature listed when sending to a
 user who does not state that they can use it.
 Defined features are as follows:
     First octet:
     0x01 - Modification Detection (packets 18 and 19)
 If an implementation implements any of the defined features, it
 SHOULD implement the Features subpacket, too.
 An implementation may freely infer features from other suitable
 implementation-dependent mechanisms.

5.2.3.25. Signature Target

 (1 octet public-key algorithm, 1 octet hash algorithm, N octets hash)
 This subpacket identifies a specific target signature to which a
 signature refers.  For revocation signatures, this subpacket
 provides explicit designation of which signature is being revoked.
 For a third-party or timestamp signature, this designates what
 signature is signed.  All arguments are an identifier of that target
 signature.
 The N octets of hash data MUST be the size of the hash of the
 signature.  For example, a target signature with a SHA-1 hash MUST
 have 20 octets of hash data.

Callas, et al Standards Track [Page 36] RFC 4880 OpenPGP Message Format November 2007

5.2.3.26. Embedded Signature

 (1 signature packet body)
 This subpacket contains a complete Signature packet body as
 specified in Section 5.2 above.  It is useful when one signature
 needs to refer to, or be incorporated in, another signature.

5.2.4. Computing Signatures

 All signatures are formed by producing a hash over the signature
 data, and then using the resulting hash in the signature algorithm.
 For binary document signatures (type 0x00), the document data is
 hashed directly.  For text document signatures (type 0x01), the
 document is canonicalized by converting line endings to <CR><LF>,
 and the resulting data is hashed.
 When a signature is made over a key, the hash data starts with the
 octet 0x99, followed by a two-octet length of the key, and then body
 of the key packet.  (Note that this is an old-style packet header for
 a key packet with two-octet length.)  A subkey binding signature
 (type 0x18) or primary key binding signature (type 0x19) then hashes
 the subkey using the same format as the main key (also using 0x99 as
 the first octet).  Key revocation signatures (types 0x20 and 0x28)
 hash only the key being revoked.
 A certification signature (type 0x10 through 0x13) hashes the User
 ID being bound to the key into the hash context after the above
 data.  A V3 certification hashes the contents of the User ID or
 attribute packet packet, without any header.  A V4 certification
 hashes the constant 0xB4 for User ID certifications or the constant
 0xD1 for User Attribute certifications, followed by a four-octet
 number giving the length of the User ID or User Attribute data, and
 then the User ID or User Attribute data.
 When a signature is made over a Signature packet (type 0x50), the
 hash data starts with the octet 0x88, followed by the four-octet
 length of the signature, and then the body of the Signature packet.
 (Note that this is an old-style packet header for a Signature packet
 with the length-of-length set to zero.)  The unhashed subpacket data
 of the Signature packet being hashed is not included in the hash, and
 the unhashed subpacket data length value is set to zero.
 Once the data body is hashed, then a trailer is hashed.  A V3
 signature hashes five octets of the packet body, starting from the
 signature type field.  This data is the signature type, followed by
 the four-octet signature time.  A V4 signature hashes the packet body

Callas, et al Standards Track [Page 37] RFC 4880 OpenPGP Message Format November 2007

 starting from its first field, the version number, through the end
 of the hashed subpacket data.  Thus, the fields hashed are the
 signature version, the signature type, the public-key algorithm, the
 hash algorithm, the hashed subpacket length, and the hashed
 subpacket body.
 V4 signatures also hash in a final trailer of six octets: the
 version of the Signature packet, i.e., 0x04; 0xFF; and a four-octet,
 big-endian number that is the length of the hashed data from the
 Signature packet (note that this number does not include these final
 six octets).
 After all this has been hashed in a single hash context, the
 resulting hash field is used in the signature algorithm and placed
 at the end of the Signature packet.

5.2.4.1. Subpacket Hints

 It is certainly possible for a signature to contain conflicting
 information in subpackets.  For example, a signature may contain
 multiple copies of a preference or multiple expiration times.  In
 most cases, an implementation SHOULD use the last subpacket in the
 signature, but MAY use any conflict resolution scheme that makes
 more sense.  Please note that we are intentionally leaving conflict
 resolution to the implementer; most conflicts are simply syntax
 errors, and the wishy-washy language here allows a receiver to be
 generous in what they accept, while putting pressure on a creator to
 be stingy in what they generate.
 Some apparent conflicts may actually make sense -- for example,
 suppose a keyholder has a V3 key and a V4 key that share the same
 RSA key material.  Either of these keys can verify a signature
 created by the other, and it may be reasonable for a signature to
 contain an issuer subpacket for each key, as a way of explicitly
 tying those keys to the signature.

5.3. Symmetric-Key Encrypted Session Key Packets (Tag 3)

 The Symmetric-Key Encrypted Session Key packet holds the
 symmetric-key encryption of a session key used to encrypt a message.
 Zero or more Public-Key Encrypted Session Key packets and/or
 Symmetric-Key Encrypted Session Key packets may precede a
 Symmetrically Encrypted Data packet that holds an encrypted message.
 The message is encrypted with a session key, and the session key is
 itself encrypted and stored in the Encrypted Session Key packet or
 the Symmetric-Key Encrypted Session Key packet.

Callas, et al Standards Track [Page 38] RFC 4880 OpenPGP Message Format November 2007

 If the Symmetrically Encrypted Data packet is preceded by one or
 more Symmetric-Key Encrypted Session Key packets, each specifies a
 passphrase that may be used to decrypt the message.  This allows a
 message to be encrypted to a number of public keys, and also to one
 or more passphrases.  This packet type is new and is not generated
 by PGP 2.x or PGP 5.0.
 The body of this packet consists of:
  1. A one-octet version number. The only currently defined version

is 4.

  1. A one-octet number describing the symmetric algorithm used.
  1. A string-to-key (S2K) specifier, length as defined above.
  1. Optionally, the encrypted session key itself, which is decrypted

with the string-to-key object.

 If the encrypted session key is not present (which can be detected
 on the basis of packet length and S2K specifier size), then the S2K
 algorithm applied to the passphrase produces the session key for
 decrypting the file, using the symmetric cipher algorithm from the
 Symmetric-Key Encrypted Session Key packet.
 If the encrypted session key is present, the result of applying the
 S2K algorithm to the passphrase is used to decrypt just that
 encrypted session key field, using CFB mode with an IV of all zeros.
 The decryption result consists of a one-octet algorithm identifier
 that specifies the symmetric-key encryption algorithm used to
 encrypt the following Symmetrically Encrypted Data packet, followed
 by the session key octets themselves.
 Note: because an all-zero IV is used for this decryption, the S2K
 specifier MUST use a salt value, either a Salted S2K or an
 Iterated-Salted S2K.  The salt value will ensure that the decryption
 key is not repeated even if the passphrase is reused.

5.4. One-Pass Signature Packets (Tag 4)

 The One-Pass Signature packet precedes the signed data and contains
 enough information to allow the receiver to begin calculating any
 hashes needed to verify the signature.  It allows the Signature
 packet to be placed at the end of the message, so that the signer
 can compute the entire signed message in one pass.
 A One-Pass Signature does not interoperate with PGP 2.6.x or
 earlier.

Callas, et al Standards Track [Page 39] RFC 4880 OpenPGP Message Format November 2007

 The body of this packet consists of:
  1. A one-octet version number. The current version is 3.
  1. A one-octet signature type. Signature types are described in

Section 5.2.1.

  1. A one-octet number describing the hash algorithm used.
  1. A one-octet number describing the public-key algorithm used.
  1. An eight-octet number holding the Key ID of the signing key.
  1. A one-octet number holding a flag showing whether the signature

is nested. A zero value indicates that the next packet is

     another One-Pass Signature packet that describes another
     signature to be applied to the same message data.
 Note that if a message contains more than one one-pass signature,
 then the Signature packets bracket the message; that is, the first
 Signature packet after the message corresponds to the last one-pass
 packet and the final Signature packet corresponds to the first
 one-pass packet.

5.5. Key Material Packet

 A key material packet contains all the information about a public or
 private key.  There are four variants of this packet type, and two
 major versions.  Consequently, this section is complex.

5.5.1. Key Packet Variants

5.5.1.1. Public-Key Packet (Tag 6)

 A Public-Key packet starts a series of packets that forms an OpenPGP
 key (sometimes called an OpenPGP certificate).

5.5.1.2. Public-Subkey Packet (Tag 14)

 A Public-Subkey packet (tag 14) has exactly the same format as a
 Public-Key packet, but denotes a subkey.  One or more subkeys may be
 associated with a top-level key.  By convention, the top-level key
 provides signature services, and the subkeys provide encryption
 services.
 Note: in PGP 2.6.x, tag 14 was intended to indicate a comment
 packet.  This tag was selected for reuse because no previous version
 of PGP ever emitted comment packets but they did properly ignore

Callas, et al Standards Track [Page 40] RFC 4880 OpenPGP Message Format November 2007

 them.  Public-Subkey packets are ignored by PGP 2.6.x and do not
 cause it to fail, providing a limited degree of backward
 compatibility.

5.5.1.3. Secret-Key Packet (Tag 5)

 A Secret-Key packet contains all the information that is found in a
 Public-Key packet, including the public-key material, but also
 includes the secret-key material after all the public-key fields.

5.5.1.4. Secret-Subkey Packet (Tag 7)

 A Secret-Subkey packet (tag 7) is the subkey analog of the Secret
 Key packet and has exactly the same format.

5.5.2. Public-Key Packet Formats

 There are two versions of key-material packets.  Version 3 packets
 were first generated by PGP 2.6.  Version 4 keys first appeared in
 PGP 5.0 and are the preferred key version for OpenPGP.
 OpenPGP implementations MUST create keys with version 4 format.  V3
 keys are deprecated; an implementation MUST NOT generate a V3 key,
 but MAY accept it.
 A version 3 public key or public-subkey packet contains:
  1. A one-octet version number (3).
  1. A four-octet number denoting the time that the key was created.
  1. A two-octet number denoting the time in days that this key is

valid. If this number is zero, then it does not expire.

  1. A one-octet number denoting the public-key algorithm of this key.
  1. A series of multiprecision integers comprising the key material:
  1. a multiprecision integer (MPI) of RSA public modulus n;
  1. an MPI of RSA public encryption exponent e.
 V3 keys are deprecated.  They contain three weaknesses.  First, it is
 relatively easy to construct a V3 key that has the same Key ID as any
 other key because the Key ID is simply the low 64 bits of the public
 modulus.  Secondly, because the fingerprint of a V3 key hashes the
 key material, but not its length, there is an increased opportunity
 for fingerprint collisions.  Third, there are weaknesses in the MD5

Callas, et al Standards Track [Page 41] RFC 4880 OpenPGP Message Format November 2007

 hash algorithm that make developers prefer other algorithms.  See
 below for a fuller discussion of Key IDs and fingerprints.
 V2 keys are identical to the deprecated V3 keys except for the
 version number.  An implementation MUST NOT generate them and MAY
 accept or reject them as it sees fit.
 The version 4 format is similar to the version 3 format except for
 the absence of a validity period.  This has been moved to the
 Signature packet.  In addition, fingerprints of version 4 keys are
 calculated differently from version 3 keys, as described in the
 section "Enhanced Key Formats".
 A version 4 packet contains:
  1. A one-octet version number (4).
  1. A four-octet number denoting the time that the key was created.
  1. A one-octet number denoting the public-key algorithm of this key.
  1. A series of multiprecision integers comprising the key material.

This algorithm-specific portion is:

     Algorithm-Specific Fields for RSA public keys:
  1. multiprecision integer (MPI) of RSA public modulus n;
  1. MPI of RSA public encryption exponent e.
     Algorithm-Specific Fields for DSA public keys:
  1. MPI of DSA prime p;
  1. MPI of DSA group order q (q is a prime divisor of p-1);
  1. MPI of DSA group generator g;
  1. MPI of DSA public-key value y (= gx mod p where x is secret). Algorithm-Specific Fields for Elgamal public keys: - MPI of Elgamal prime p; - MPI of Elgamal group generator g; Callas, et al Standards Track [Page 42] RFC 4880 OpenPGP Message Format November 2007 - MPI of Elgamal public key value y (= gx mod p where x

is secret).

5.5.3. Secret-Key Packet Formats

 The Secret-Key and Secret-Subkey packets contain all the data of the
 Public-Key and Public-Subkey packets, with additional algorithm-
 specific secret-key data appended, usually in encrypted form.
 The packet contains:
  1. A Public-Key or Public-Subkey packet, as described above.
  1. One octet indicating string-to-key usage conventions. Zero

indicates that the secret-key data is not encrypted. 255 or 254

     indicates that a string-to-key specifier is being given.  Any
     other value is a symmetric-key encryption algorithm identifier.
  1. [Optional] If string-to-key usage octet was 255 or 254, a one-

octet symmetric encryption algorithm.

  1. [Optional] If string-to-key usage octet was 255 or 254, a

string-to-key specifier. The length of the string-to-key

     specifier is implied by its type, as described above.
  1. [Optional] If secret data is encrypted (string-to-key usage octet

not zero), an Initial Vector (IV) of the same length as the

     cipher's block size.
  1. Plain or encrypted multiprecision integers comprising the secret

key data. These algorithm-specific fields are as described

     below.
  1. If the string-to-key usage octet is zero or 255, then a two-octet

checksum of the plaintext of the algorithm-specific portion (sum

     of all octets, mod 65536).  If the string-to-key usage octet was
     254, then a 20-octet SHA-1 hash of the plaintext of the
     algorithm-specific portion.  This checksum or hash is encrypted
     together with the algorithm-specific fields (if string-to-key
     usage octet is not zero).  Note that for all other values, a
     two-octet checksum is required.
     Algorithm-Specific Fields for RSA secret keys:
  1. multiprecision integer (MPI) of RSA secret exponent d.
  1. MPI of RSA secret prime value p.

Callas, et al Standards Track [Page 43] RFC 4880 OpenPGP Message Format November 2007

  1. MPI of RSA secret prime value q (p < q).
  1. MPI of u, the multiplicative inverse of p, mod q.
     Algorithm-Specific Fields for DSA secret keys:
  1. MPI of DSA secret exponent x.
     Algorithm-Specific Fields for Elgamal secret keys:
  1. MPI of Elgamal secret exponent x.
 Secret MPI values can be encrypted using a passphrase.  If a string-
 to-key specifier is given, that describes the algorithm for
 converting the passphrase to a key, else a simple MD5 hash of the
 passphrase is used.  Implementations MUST use a string-to-key
 specifier; the simple hash is for backward compatibility and is
 deprecated, though implementations MAY continue to use existing
 private keys in the old format.  The cipher for encrypting the MPIs
 is specified in the Secret-Key packet.
 Encryption/decryption of the secret data is done in CFB mode using
 the key created from the passphrase and the Initial Vector from the
 packet.  A different mode is used with V3 keys (which are only RSA)
 than with other key formats.  With V3 keys, the MPI bit count prefix
 (i.e., the first two octets) is not encrypted.  Only the MPI non-
 prefix data is encrypted.  Furthermore, the CFB state is
 resynchronized at the beginning of each new MPI value, so that the
 CFB block boundary is aligned with the start of the MPI data.
 With V4 keys, a simpler method is used.  All secret MPI values are
 encrypted in CFB mode, including the MPI bitcount prefix.
 The two-octet checksum that follows the algorithm-specific portion is
 the algebraic sum, mod 65536, of the plaintext of all the algorithm-
 specific octets (including MPI prefix and data).  With V3 keys, the
 checksum is stored in the clear.  With V4 keys, the checksum is
 encrypted like the algorithm-specific data.  This value is used to
 check that the passphrase was correct.  However, this checksum is
 deprecated; an implementation SHOULD NOT use it, but should rather
 use the SHA-1 hash denoted with a usage octet of 254.  The reason for
 this is that there are some attacks that involve undetectably
 modifying the secret key.

Callas, et al Standards Track [Page 44] RFC 4880 OpenPGP Message Format November 2007

5.6. Compressed Data Packet (Tag 8)

 The Compressed Data packet contains compressed data.  Typically, this
 packet is found as the contents of an encrypted packet, or following
 a Signature or One-Pass Signature packet, and contains a literal data
 packet.
 The body of this packet consists of:
  1. One octet that gives the algorithm used to compress the packet.
  1. Compressed data, which makes up the remainder of the packet.
 A Compressed Data Packet's body contains an block that compresses
 some set of packets.  See section "Packet Composition" for details on
 how messages are formed.
 ZIP-compressed packets are compressed with raw RFC 1951 [RFC1951]
 DEFLATE blocks.  Note that PGP V2.6 uses 13 bits of compression.  If
 an implementation uses more bits of compression, PGP V2.6 cannot
 decompress it.
 ZLIB-compressed packets are compressed with RFC 1950 [RFC1950] ZLIB-
 style blocks.
 BZip2-compressed packets are compressed using the BZip2 [BZ2]
 algorithm.

5.7. Symmetrically Encrypted Data Packet (Tag 9)

 The Symmetrically Encrypted Data packet contains data encrypted with
 a symmetric-key algorithm.  When it has been decrypted, it contains
 other packets (usually a literal data packet or compressed data
 packet, but in theory other Symmetrically Encrypted Data packets or
 sequences of packets that form whole OpenPGP messages).
 The body of this packet consists of:
  1. Encrypted data, the output of the selected symmetric-key cipher

operating in OpenPGP's variant of Cipher Feedback (CFB) mode.

 The symmetric cipher used may be specified in a Public-Key or
 Symmetric-Key Encrypted Session Key packet that precedes the
 Symmetrically Encrypted Data packet.  In that case, the cipher
 algorithm octet is prefixed to the session key before it is
 encrypted.  If no packets of these types precede the encrypted data,
 the IDEA algorithm is used with the session key calculated as the MD5
 hash of the passphrase, though this use is deprecated.

Callas, et al Standards Track [Page 45] RFC 4880 OpenPGP Message Format November 2007

 The data is encrypted in CFB mode, with a CFB shift size equal to the
 cipher's block size.  The Initial Vector (IV) is specified as all
 zeros.  Instead of using an IV, OpenPGP prefixes a string of length
 equal to the block size of the cipher plus two to the data before it
 is encrypted.  The first block-size octets (for example, 8 octets for
 a 64-bit block length) are random, and the following two octets are
 copies of the last two octets of the IV.  For example, in an 8-octet
 block, octet 9 is a repeat of octet 7, and octet 10 is a repeat of
 octet 8.  In a cipher of length 16, octet 17 is a repeat of octet 15
 and octet 18 is a repeat of octet 16.  As a pedantic clarification,
 in both these examples, we consider the first octet to be numbered 1.
 After encrypting the first block-size-plus-two octets, the CFB state
 is resynchronized.  The last block-size octets of ciphertext are
 passed through the cipher and the block boundary is reset.
 The repetition of 16 bits in the random data prefixed to the message
 allows the receiver to immediately check whether the session key is
 incorrect.  See the "Security Considerations" section for hints on
 the proper use of this "quick check".

5.8. Marker Packet (Obsolete Literal Packet) (Tag 10)

 An experimental version of PGP used this packet as the Literal
 packet, but no released version of PGP generated Literal packets with
 this tag.  With PGP 5.x, this packet has been reassigned and is
 reserved for use as the Marker packet.
 The body of this packet consists of:
  1. The three octets 0x50, 0x47, 0x50 (which spell "PGP" in UTF-8).
 Such a packet MUST be ignored when received.  It may be placed at the
 beginning of a message that uses features not available in PGP 2.6.x
 in order to cause that version to report that newer software is
 necessary to process the message.

5.9. Literal Data Packet (Tag 11)

 A Literal Data packet contains the body of a message; data that is
 not to be further interpreted.
 The body of this packet consists of:
  1. A one-octet field that describes how the data is formatted.

Callas, et al Standards Track [Page 46] RFC 4880 OpenPGP Message Format November 2007

 If it is a 'b' (0x62), then the Literal packet contains binary data.
 If it is a 't' (0x74), then it contains text data, and thus may need
 line ends converted to local form, or other text-mode changes.  The
 tag 'u' (0x75) means the same as 't', but also indicates that
 implementation believes that the literal data contains UTF-8 text.
 Early versions of PGP also defined a value of 'l' as a 'local' mode
 for machine-local conversions.  RFC 1991 [RFC1991] incorrectly stated
 this local mode flag as '1' (ASCII numeral one).  Both of these local
 modes are deprecated.
  1. File name as a string (one-octet length, followed by a file

name). This may be a zero-length string. Commonly, if the

     source of the encrypted data is a file, this will be the name of
     the encrypted file.  An implementation MAY consider the file name
     in the Literal packet to be a more authoritative name than the
     actual file name.
 If the special name "_CONSOLE" is used, the message is considered to
 be "for your eyes only".  This advises that the message data is
 unusually sensitive, and the receiving program should process it more
 carefully, perhaps avoiding storing the received data to disk, for
 example.
  1. A four-octet number that indicates a date associated with the

literal data. Commonly, the date might be the modification date

     of a file, or the time the packet was created, or a zero that
     indicates no specific time.
  1. The remainder of the packet is literal data.
     Text data is stored with <CR><LF> text endings (i.e., network-
     normal line endings).  These should be converted to native line
     endings by the receiving software.

5.10. Trust Packet (Tag 12)

 The Trust packet is used only within keyrings and is not normally
 exported.  Trust packets contain data that record the user's
 specifications of which key holders are trustworthy introducers,
 along with other information that implementing software uses for
 trust information.  The format of Trust packets is defined by a given
 implementation.
 Trust packets SHOULD NOT be emitted to output streams that are
 transferred to other users, and they SHOULD be ignored on any input
 other than local keyring files.

Callas, et al Standards Track [Page 47] RFC 4880 OpenPGP Message Format November 2007

5.11. User ID Packet (Tag 13)

 A User ID packet consists of UTF-8 text that is intended to represent
 the name and email address of the key holder.  By convention, it
 includes an RFC 2822 [RFC2822] mail name-addr, but there are no
 restrictions on its content.  The packet length in the header
 specifies the length of the User ID.

5.12. User Attribute Packet (Tag 17)

 The User Attribute packet is a variation of the User ID packet.  It
 is capable of storing more types of data than the User ID packet,
 which is limited to text.  Like the User ID packet, a User Attribute
 packet may be certified by the key owner ("self-signed") or any other
 key owner who cares to certify it.  Except as noted, a User Attribute
 packet may be used anywhere that a User ID packet may be used.
 While User Attribute packets are not a required part of the OpenPGP
 standard, implementations SHOULD provide at least enough
 compatibility to properly handle a certification signature on the
 User Attribute packet.  A simple way to do this is by treating the
 User Attribute packet as a User ID packet with opaque contents, but
 an implementation may use any method desired.
 The User Attribute packet is made up of one or more attribute
 subpackets.  Each subpacket consists of a subpacket header and a
 body.  The header consists of:
  1. the subpacket length (1, 2, or 5 octets)
  1. the subpacket type (1 octet)
 and is followed by the subpacket specific data.
 The only currently defined subpacket type is 1, signifying an image.
 An implementation SHOULD ignore any subpacket of a type that it does
 not recognize.  Subpacket types 100 through 110 are reserved for
 private or experimental use.

5.12.1. The Image Attribute Subpacket

 The Image Attribute subpacket is used to encode an image, presumably
 (but not required to be) that of the key owner.
 The Image Attribute subpacket begins with an image header.  The first
 two octets of the image header contain the length of the image
 header.  Note that unlike other multi-octet numerical values in this
 document, due to a historical accident this value is encoded as a

Callas, et al Standards Track [Page 48] RFC 4880 OpenPGP Message Format November 2007

 little-endian number.  The image header length is followed by a
 single octet for the image header version.  The only currently
 defined version of the image header is 1, which is a 16-octet image
 header.  The first three octets of a version 1 image header are thus
 0x10, 0x00, 0x01.
 The fourth octet of a version 1 image header designates the encoding
 format of the image.  The only currently defined encoding format is
 the value 1 to indicate JPEG.  Image format types 100 through 110 are
 reserved for private or experimental use.  The rest of the version 1
 image header is made up of 12 reserved octets, all of which MUST be
 set to 0.
 The rest of the image subpacket contains the image itself.  As the
 only currently defined image type is JPEG, the image is encoded in
 the JPEG File Interchange Format (JFIF), a standard file format for
 JPEG images [JFIF].
 An implementation MAY try to determine the type of an image by
 examination of the image data if it is unable to handle a particular
 version of the image header or if a specified encoding format value
 is not recognized.

5.13. Sym. Encrypted Integrity Protected Data Packet (Tag 18)

 The Symmetrically Encrypted Integrity Protected Data packet is a
 variant of the Symmetrically Encrypted Data packet.  It is a new
 feature created for OpenPGP that addresses the problem of detecting a
 modification to encrypted data.  It is used in combination with a
 Modification Detection Code packet.
 There is a corresponding feature in the features Signature subpacket
 that denotes that an implementation can properly use this packet
 type.  An implementation MUST support decrypting these packets and
 SHOULD prefer generating them to the older Symmetrically Encrypted
 Data packet when possible.  Since this data packet protects against
 modification attacks, this standard encourages its proliferation.
 While blanket adoption of this data packet would create
 interoperability problems, rapid adoption is nevertheless important.
 An implementation SHOULD specifically denote support for this packet,
 but it MAY infer it from other mechanisms.
 For example, an implementation might infer from the use of a cipher
 such as Advanced Encryption Standard (AES) or Twofish that a user
 supports this feature.  It might place in the unhashed portion of
 another user's key signature a Features subpacket.  It might also
 present a user with an opportunity to regenerate their own self-
 signature with a Features subpacket.

Callas, et al Standards Track [Page 49] RFC 4880 OpenPGP Message Format November 2007

 This packet contains data encrypted with a symmetric-key algorithm
 and protected against modification by the SHA-1 hash algorithm.  When
 it has been decrypted, it will typically contain other packets (often
 a Literal Data packet or Compressed Data packet).  The last decrypted
 packet in this packet's payload MUST be a Modification Detection Code
 packet.
 The body of this packet consists of:
  1. A one-octet version number. The only currently defined value is

1.

  1. Encrypted data, the output of the selected symmetric-key cipher

operating in Cipher Feedback mode with shift amount equal to the

     block size of the cipher (CFB-n where n is the block size).
 The symmetric cipher used MUST be specified in a Public-Key or
 Symmetric-Key Encrypted Session Key packet that precedes the
 Symmetrically Encrypted Data packet.  In either case, the cipher
 algorithm octet is prefixed to the session key before it is
 encrypted.
 The data is encrypted in CFB mode, with a CFB shift size equal to the
 cipher's block size.  The Initial Vector (IV) is specified as all
 zeros.  Instead of using an IV, OpenPGP prefixes an octet string to
 the data before it is encrypted.  The length of the octet string
 equals the block size of the cipher in octets, plus two.  The first
 octets in the group, of length equal to the block size of the cipher,
 are random; the last two octets are each copies of their 2nd
 preceding octet.  For example, with a cipher whose block size is 128
 bits or 16 octets, the prefix data will contain 16 random octets,
 then two more octets, which are copies of the 15th and 16th octets,
 respectively.  Unlike the Symmetrically Encrypted Data Packet, no
 special CFB resynchronization is done after encrypting this prefix
 data.  See "OpenPGP CFB Mode" below for more details.
 The repetition of 16 bits in the random data prefixed to the message
 allows the receiver to immediately check whether the session key is
 incorrect.
 The plaintext of the data to be encrypted is passed through the SHA-1
 hash function, and the result of the hash is appended to the
 plaintext in a Modification Detection Code packet.  The input to the
 hash function includes the prefix data described above; it includes
 all of the plaintext, and then also includes two octets of values
 0xD3, 0x14.  These represent the encoding of a Modification Detection
 Code packet tag and length field of 20 octets.

Callas, et al Standards Track [Page 50] RFC 4880 OpenPGP Message Format November 2007

 The resulting hash value is stored in a Modification Detection Code
 (MDC) packet, which MUST use the two octet encoding just given to
 represent its tag and length field.  The body of the MDC packet is
 the 20-octet output of the SHA-1 hash.
 The Modification Detection Code packet is appended to the plaintext
 and encrypted along with the plaintext using the same CFB context.
 During decryption, the plaintext data should be hashed with SHA-1,
 including the prefix data as well as the packet tag and length field
 of the Modification Detection Code packet.  The body of the MDC
 packet, upon decryption, is compared with the result of the SHA-1
 hash.
 Any failure of the MDC indicates that the message has been modified
 and MUST be treated as a security problem.  Failures include a
 difference in the hash values, but also the absence of an MDC packet,
 or an MDC packet in any position other than the end of the plaintext.
 Any failure SHOULD be reported to the user.
 Note: future designs of new versions of this packet should consider
 rollback attacks since it will be possible for an attacker to change
 the version back to 1.
    NON-NORMATIVE EXPLANATION
    The MDC system, as packets 18 and 19 are called, were created to
    provide an integrity mechanism that is less strong than a
    signature, yet stronger than bare CFB encryption.
    It is a limitation of CFB encryption that damage to the ciphertext
    will corrupt the affected cipher blocks and the block following.
    Additionally, if data is removed from the end of a CFB-encrypted
    block, that removal is undetectable.  (Note also that CBC mode has
    a similar limitation, but data removed from the front of the block
    is undetectable.)
    The obvious way to protect or authenticate an encrypted block is
    to digitally sign it.  However, many people do not wish to
    habitually sign data, for a large number of reasons beyond the
    scope of this document.  Suffice it to say that many people
    consider properties such as deniability to be as valuable as
    integrity.
    OpenPGP addresses this desire to have more security than raw
    encryption and yet preserve deniability with the MDC system.  An
    MDC is intentionally not a MAC.  Its name was not selected by
    accident.  It is analogous to a checksum.

Callas, et al Standards Track [Page 51] RFC 4880 OpenPGP Message Format November 2007

    Despite the fact that it is a relatively modest system, it has
    proved itself in the real world.  It is an effective defense to
    several attacks that have surfaced since it has been created.  It
    has met its modest goals admirably.
    Consequently, because it is a modest security system, it has
    modest requirements on the hash function(s) it employs.  It does
    not rely on a hash function being collision-free, it relies on a
    hash function being one-way.  If a forger, Frank, wishes to send
    Alice a (digitally) unsigned message that says, "I've always
    secretly loved you, signed Bob", it is far easier for him to
    construct a new message than it is to modify anything intercepted
    from Bob.  (Note also that if Bob wishes to communicate secretly
    with Alice, but without authentication or identification and with
    a threat model that includes forgers, he has a problem that
    transcends mere cryptography.)
    Note also that unlike nearly every other OpenPGP subsystem, there
    are no parameters in the MDC system.  It hard-defines SHA-1 as its
    hash function.  This is not an accident.  It is an intentional
    choice to avoid downgrade and cross-grade attacks while making a
    simple, fast system.  (A downgrade attack would be an attack that
    replaced SHA-256 with SHA-1, for example.  A cross-grade attack
    would replace SHA-1 with another 160-bit hash, such as RIPE-
    MD/160, for example.)
    However, given the present state of hash function cryptanalysis
    and cryptography, it may be desirable to upgrade the MDC system to
    a new hash function.  See Section 13.11 in the "IANA
    Considerations" for guidance.

5.14. Modification Detection Code Packet (Tag 19)

 The Modification Detection Code packet contains a SHA-1 hash of
 plaintext data, which is used to detect message modification.  It is
 only used with a Symmetrically Encrypted Integrity Protected Data
 packet.  The Modification Detection Code packet MUST be the last
 packet in the plaintext data that is encrypted in the Symmetrically
 Encrypted Integrity Protected Data packet, and MUST appear in no
 other place.
 A Modification Detection Code packet MUST have a length of 20 octets.

Callas, et al Standards Track [Page 52] RFC 4880 OpenPGP Message Format November 2007

 The body of this packet consists of:
  1. A 20-octet SHA-1 hash of the preceding plaintext data of the

Symmetrically Encrypted Integrity Protected Data packet,

     including prefix data, the tag octet, and length octet of the
     Modification Detection Code packet.
 Note that the Modification Detection Code packet MUST always use a
 new format encoding of the packet tag, and a one-octet encoding of
 the packet length.  The reason for this is that the hashing rules for
 modification detection include a one-octet tag and one-octet length
 in the data hash.  While this is a bit restrictive, it reduces
 complexity.

6. Radix-64 Conversions

 As stated in the introduction, OpenPGP's underlying native
 representation for objects is a stream of arbitrary octets, and some
 systems desire these objects to be immune to damage caused by
 character set translation, data conversions, etc.
 In principle, any printable encoding scheme that met the requirements
 of the unsafe channel would suffice, since it would not change the
 underlying binary bit streams of the native OpenPGP data structures.
 The OpenPGP standard specifies one such printable encoding scheme to
 ensure interoperability.
 OpenPGP's Radix-64 encoding is composed of two parts: a base64
 encoding of the binary data and a checksum.  The base64 encoding is
 identical to the MIME base64 content-transfer-encoding [RFC2045].
 The checksum is a 24-bit Cyclic Redundancy Check (CRC) converted to
 four characters of radix-64 encoding by the same MIME base64
 transformation, preceded by an equal sign (=).  The CRC is computed
 by using the generator 0x864CFB and an initialization of 0xB704CE.
 The accumulation is done on the data before it is converted to
 radix-64, rather than on the converted data.  A sample implementation
 of this algorithm is in the next section.
 The checksum with its leading equal sign MAY appear on the first line
 after the base64 encoded data.
 Rationale for CRC-24: The size of 24 bits fits evenly into printable
 base64.  The nonzero initialization can detect more errors than a
 zero initialization.

Callas, et al Standards Track [Page 53] RFC 4880 OpenPGP Message Format November 2007

6.1. An Implementation of the CRC-24 in "C"

    #define CRC24_INIT 0xB704CEL
    #define CRC24_POLY 0x1864CFBL
    typedef long crc24;
    crc24 crc_octets(unsigned char *octets, size_t len)
    {
        crc24 crc = CRC24_INIT;
        int i;
        while (len--) {
            crc ^= (*octets++) << 16;
            for (i = 0; i < 8; i++) {
                crc <<= 1;
                if (crc & 0x1000000)
                    crc ^= CRC24_POLY;
            }
        }
        return crc & 0xFFFFFFL;
    }

6.2. Forming ASCII Armor

 When OpenPGP encodes data into ASCII Armor, it puts specific headers
 around the Radix-64 encoded data, so OpenPGP can reconstruct the data
 later.  An OpenPGP implementation MAY use ASCII armor to protect raw
 binary data.  OpenPGP informs the user what kind of data is encoded
 in the ASCII armor through the use of the headers.
 Concatenating the following data creates ASCII Armor:
  1. An Armor Header Line, appropriate for the type of data
  1. Armor Headers
  1. A blank (zero-length, or containing only whitespace) line
  1. The ASCII-Armored data
  1. An Armor Checksum
  1. The Armor Tail, which depends on the Armor Header Line
 An Armor Header Line consists of the appropriate header line text
 surrounded by five (5) dashes ('-', 0x2D) on either side of the
 header line text.  The header line text is chosen based upon the type
 of data that is being encoded in Armor, and how it is being encoded.
 Header line texts include the following strings:

Callas, et al Standards Track [Page 54] RFC 4880 OpenPGP Message Format November 2007

 BEGIN PGP MESSAGE
     Used for signed, encrypted, or compressed files.
 BEGIN PGP PUBLIC KEY BLOCK
     Used for armoring public keys.
 BEGIN PGP PRIVATE KEY BLOCK
     Used for armoring private keys.
 BEGIN PGP MESSAGE, PART X/Y
     Used for multi-part messages, where the armor is split amongst Y
     parts, and this is the Xth part out of Y.
 BEGIN PGP MESSAGE, PART X
     Used for multi-part messages, where this is the Xth part of an
     unspecified number of parts.  Requires the MESSAGE-ID Armor
     Header to be used.
 BEGIN PGP SIGNATURE
     Used for detached signatures, OpenPGP/MIME signatures, and
     cleartext signatures.  Note that PGP 2.x uses BEGIN PGP MESSAGE
     for detached signatures.
 Note that all these Armor Header Lines are to consist of a complete
 line.  That is to say, there is always a line ending preceding the
 starting five dashes, and following the ending five dashes.  The
 header lines, therefore, MUST start at the beginning of a line, and
 MUST NOT have text other than whitespace following them on the same
 line.  These line endings are considered a part of the Armor Header
 Line for the purposes of determining the content they delimit.  This
 is particularly important when computing a cleartext signature (see
 below).
 The Armor Headers are pairs of strings that can give the user or the
 receiving OpenPGP implementation some information about how to decode
 or use the message.  The Armor Headers are a part of the armor, not a
 part of the message, and hence are not protected by any signatures
 applied to the message.
 The format of an Armor Header is that of a key-value pair.  A colon
 (':' 0x38) and a single space (0x20) separate the key and value.
 OpenPGP should consider improperly formatted Armor Headers to be
 corruption of the ASCII Armor.  Unknown keys should be reported to
 the user, but OpenPGP should continue to process the message.
 Note that some transport methods are sensitive to line length.  While
 there is a limit of 76 characters for the Radix-64 data (Section
 6.3), there is no limit to the length of Armor Headers.  Care should

Callas, et al Standards Track [Page 55] RFC 4880 OpenPGP Message Format November 2007

 be taken that the Armor Headers are short enough to survive
 transport.  One way to do this is to repeat an Armor Header key
 multiple times with different values for each so that no one line is
 overly long.
 Currently defined Armor Header Keys are as follows:
  1. "Version", which states the OpenPGP implementation and version

used to encode the message.

  1. "Comment", a user-defined comment. OpenPGP defines all text to

be in UTF-8. A comment may be any UTF-8 string. However, the

     whole point of armoring is to provide seven-bit-clean data.
     Consequently, if a comment has characters that are outside the
     US-ASCII range of UTF, they may very well not survive transport.
  1. "MessageID", a 32-character string of printable characters. The

string must be the same for all parts of a multi-part message

     that uses the "PART X" Armor Header.  MessageID strings should be
     unique enough that the recipient of the mail can associate all
     the parts of a message with each other.  A good checksum or
     cryptographic hash function is sufficient.
     The MessageID SHOULD NOT appear unless it is in a multi-part
     message.  If it appears at all, it MUST be computed from the
     finished (encrypted, signed, etc.) message in a deterministic
     fashion, rather than contain a purely random value.  This is to
     allow the legitimate recipient to determine that the MessageID
     cannot serve as a covert means of leaking cryptographic key
     information.
  1. "Hash", a comma-separated list of hash algorithms used in this

message. This is used only in cleartext signed messages.

  1. "Charset", a description of the character set that the plaintext

is in. Please note that OpenPGP defines text to be in UTF-8. An

     implementation will get best results by translating into and out
     of UTF-8.  However, there are many instances where this is easier
     said than done.  Also, there are communities of users who have no
     need for UTF-8 because they are all happy with a character set
     like ISO Latin-5 or a Japanese character set.  In such instances,
     an implementation MAY override the UTF-8 default by using this
     header key.  An implementation MAY implement this key and any
     translations it cares to; an implementation MAY ignore it and
     assume all text is UTF-8.

Callas, et al Standards Track [Page 56] RFC 4880 OpenPGP Message Format November 2007

     The Armor Tail Line is composed in the same manner as the Armor
     Header Line, except the string "BEGIN" is replaced by the string
     "END".

6.3. Encoding Binary in Radix-64

 The encoding process represents 24-bit groups of input bits as output
 strings of 4 encoded characters.  Proceeding from left to right, a
 24-bit input group is formed by concatenating three 8-bit input
 groups.  These 24 bits are then treated as four concatenated 6-bit
 groups, each of which is translated into a single digit in the
 Radix-64 alphabet.  When encoding a bit stream with the Radix-64
 encoding, the bit stream must be presumed to be ordered with the most
 significant bit first.  That is, the first bit in the stream will be
 the high-order bit in the first 8-bit octet, and the eighth bit will
 be the low-order bit in the first 8-bit octet, and so on.
       +--first octet--+-second octet--+--third octet--+
       |7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|7 6 5 4 3 2 1 0|
       +-----------+---+-------+-------+---+-----------+
       |5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|5 4 3 2 1 0|
       +--1.index--+--2.index--+--3.index--+--4.index--+
 Each 6-bit group is used as an index into an array of 64 printable
 characters from the table below.  The character referenced by the
 index is placed in the output string.
   Value Encoding  Value Encoding  Value Encoding  Value Encoding
       0 A            17 R            34 i            51 z
       1 B            18 S            35 j            52 0
       2 C            19 T            36 k            53 1
       3 D            20 U            37 l            54 2
       4 E            21 V            38 m            55 3
       5 F            22 W            39 n            56 4
       6 G            23 X            40 o            57 5
       7 H            24 Y            41 p            58 6
       8 I            25 Z            42 q            59 7
       9 J            26 a            43 r            60 8
      10 K            27 b            44 s            61 9
      11 L            28 c            45 t            62 +
      12 M            29 d            46 u            63 /
      13 N            30 e            47 v
      14 O            31 f            48 w         (pad) =
      15 P            32 g            49 x
      16 Q            33 h            50 y
 The encoded output stream must be represented in lines of no more
 than 76 characters each.

Callas, et al Standards Track [Page 57] RFC 4880 OpenPGP Message Format November 2007

 Special processing is performed if fewer than 24 bits are available
 at the end of the data being encoded.  There are three possibilities:
 1. The last data group has 24 bits (3 octets).  No special processing
    is needed.
 2. The last data group has 16 bits (2 octets).  The first two 6-bit
    groups are processed as above.  The third (incomplete) data group
    has two zero-value bits added to it, and is processed as above.  A
    pad character (=) is added to the output.
 3. The last data group has 8 bits (1 octet).  The first 6-bit group
    is processed as above.  The second (incomplete) data group has
    four zero-value bits added to it, and is processed as above.  Two
    pad characters (=) are added to the output.

6.4. Decoding Radix-64

 In Radix-64 data, characters other than those in the table, line
 breaks, and other white space probably indicate a transmission error,
 about which a warning message or even a message rejection might be
 appropriate under some circumstances.  Decoding software must ignore
 all white space.
 Because it is used only for padding at the end of the data, the
 occurrence of any "=" characters may be taken as evidence that the
 end of the data has been reached (without truncation in transit).  No
 such assurance is possible, however, when the number of octets
 transmitted was a multiple of three and no "=" characters are
 present.

Callas, et al Standards Track [Page 58] RFC 4880 OpenPGP Message Format November 2007

6.5. Examples of Radix-64

 Input data:  0x14FB9C03D97E
 Hex:     1   4    F   B    9   C     | 0   3    D   9    7   E
 8-bit:   00010100 11111011 10011100  | 00000011 11011001 11111110
 6-bit:   000101 001111 101110 011100 | 000000 111101 100111 111110
 Decimal: 5      15     46     28       0      61     37     62
 Output:  F      P      u      c        A      9      l      +
 Input data:  0x14FB9C03D9
 Hex:     1   4    F   B    9   C     | 0   3    D   9
 8-bit:   00010100 11111011 10011100  | 00000011 11011001
                                                 pad with 00
 6-bit:   000101 001111 101110 011100 | 000000 111101 100100
 Decimal: 5      15     46     28       0      61     36
                                                    pad with =
 Output:  F      P      u      c        A      9      k      =
 Input data:  0x14FB9C03
 Hex:     1   4    F   B    9   C     | 0   3
 8-bit:   00010100 11111011 10011100  | 00000011
                                        pad with 0000
 6-bit:   000101 001111 101110 011100 | 000000 110000
 Decimal: 5      15     46     28       0      48
                                             pad with =      =
 Output:  F      P      u      c        A      w      =      =

6.6. Example of an ASCII Armored Message

  1. —-BEGIN PGP MESSAGE—–

Version: OpenPrivacy 0.99

 yDgBO22WxBHv7O8X7O/jygAEzol56iUKiXmV+XmpCtmpqQUKiQrFqclFqUDBovzS
 vBSFjNSiVHsuAA==
 =njUN
 -----END PGP MESSAGE-----
 Note that this example has extra indenting; an actual armored message
 would have no leading whitespace.

7. Cleartext Signature Framework

 It is desirable to be able to sign a textual octet stream without
 ASCII armoring the stream itself, so the signed text is still
 readable without special software.  In order to bind a signature to
 such a cleartext, this framework is used.  (Note that this framework
 is not intended to be reversible.  RFC 3156 [RFC3156] defines another
 way to sign cleartext messages for environments that support MIME.)

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 The cleartext signed message consists of:
  1. The cleartext header '—–BEGIN PGP SIGNED MESSAGE—–' on a

single line,

  1. One or more "Hash" Armor Headers,
  1. Exactly one empty line not included into the message digest,
  1. The dash-escaped cleartext that is included into the message

digest,

  1. The ASCII armored signature(s) including the '—–BEGIN PGP

SIGNATURE—–' Armor Header and Armor Tail Lines.

 If the "Hash" Armor Header is given, the specified message digest
 algorithm(s) are used for the signature.  If there are no such
 headers, MD5 is used.  If MD5 is the only hash used, then an
 implementation MAY omit this header for improved V2.x compatibility.
 If more than one message digest is used in the signature, the "Hash"
 armor header contains a comma-delimited list of used message digests.
 Current message digest names are described below with the algorithm
 IDs.
 An implementation SHOULD add a line break after the cleartext, but
 MAY omit it if the cleartext ends with a line break.  This is for
 visual clarity.

7.1. Dash-Escaped Text

 The cleartext content of the message must also be dash-escaped.
 Dash-escaped cleartext is the ordinary cleartext where every line
 starting with a dash '-' (0x2D) is prefixed by the sequence dash '-'
 (0x2D) and space ' ' (0x20).  This prevents the parser from
 recognizing armor headers of the cleartext itself.  An implementation
 MAY dash-escape any line, SHOULD dash-escape lines commencing "From"
 followed by a space, and MUST dash-escape any line commencing in a
 dash.  The message digest is computed using the cleartext itself, not
 the dash-escaped form.
 As with binary signatures on text documents, a cleartext signature is
 calculated on the text using canonical <CR><LF> line endings.  The
 line ending (i.e., the <CR><LF>) before the '-----BEGIN PGP
 SIGNATURE-----' line that terminates the signed text is not
 considered part of the signed text.

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 When reversing dash-escaping, an implementation MUST strip the string
 "- " if it occurs at the beginning of a line, and SHOULD warn on "-"
 and any character other than a space at the beginning of a line.
 Also, any trailing whitespace -- spaces (0x20) and tabs (0x09) -- at
 the end of any line is removed when the cleartext signature is
 generated.

8. Regular Expressions

 A regular expression is zero or more branches, separated by '|'.  It
 matches anything that matches one of the branches.
 A branch is zero or more pieces, concatenated.  It matches a match
 for the first, followed by a match for the second, etc.
 A piece is an atom possibly followed by '*', '+', or '?'.  An atom
 followed by '*' matches a sequence of 0 or more matches of the atom.
 An atom followed by '+' matches a sequence of 1 or more matches of
 the atom.  An atom followed by '?' matches a match of the atom, or
 the null string.
 An atom is a regular expression in parentheses (matching a match for
 the regular expression), a range (see below), '.' (matching any
 single character), '^' (matching the null string at the beginning of
 the input string), '$' (matching the null string at the end of the
 input string), a '\' followed by a single character (matching that
 character), or a single character with no other significance
 (matching that character).
 A range is a sequence of characters enclosed in '[]'.  It normally
 matches any single character from the sequence.  If the sequence
 begins with '^', it matches any single character not from the rest of
 the sequence.  If two characters in the sequence are separated
 by '-', this is shorthand for the full list of ASCII characters
 between them (e.g., '[0-9]' matches any decimal digit).  To include a
 literal ']' in the sequence, make it the first character (following a
 possible '^').  To include a literal '-', make it the first or last
 character.

9. Constants

 This section describes the constants used in OpenPGP.
 Note that these tables are not exhaustive lists; an implementation
 MAY implement an algorithm not on these lists, so long as the
 algorithm numbers are chosen from the private or experimental
 algorithm range.

Callas, et al Standards Track [Page 61] RFC 4880 OpenPGP Message Format November 2007

 See the section "Notes on Algorithms" below for more discussion of
 the algorithms.

9.1. Public-Key Algorithms

    ID           Algorithm
    --           ---------
    1          - RSA (Encrypt or Sign) [HAC]
    2          - RSA Encrypt-Only [HAC]
    3          - RSA Sign-Only [HAC]
    16         - Elgamal (Encrypt-Only) [ELGAMAL] [HAC]
    17         - DSA (Digital Signature Algorithm) [FIPS186] [HAC]
    18         - Reserved for Elliptic Curve
    19         - Reserved for ECDSA
    20         - Reserved (formerly Elgamal Encrypt or Sign)
    21         - Reserved for Diffie-Hellman (X9.42,
                 as defined for IETF-S/MIME)
    100 to 110 - Private/Experimental algorithm
 Implementations MUST implement DSA for signatures, and Elgamal for
 encryption.  Implementations SHOULD implement RSA keys (1).  RSA
 Encrypt-Only (2) and RSA Sign-Only are deprecated and SHOULD NOT be
 generated, but may be interpreted.  See Section 13.5.  See Section
 13.8 for notes on Elliptic Curve (18), ECDSA (19), Elgamal Encrypt or
 Sign (20), and X9.42 (21).  Implementations MAY implement any other
 algorithm.

9.2. Symmetric-Key Algorithms

     ID           Algorithm
     --           ---------
     0          - Plaintext or unencrypted data
     1          - IDEA [IDEA]
     2          - TripleDES (DES-EDE, [SCHNEIER] [HAC] -
                  168 bit key derived from 192)
     3          - CAST5 (128 bit key, as per [RFC2144])
     4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
     5          - Reserved
     6          - Reserved
     7          - AES with 128-bit key [AES]
     8          - AES with 192-bit key
     9          - AES with 256-bit key
     10         - Twofish with 256-bit key [TWOFISH]
     100 to 110 - Private/Experimental algorithm
 Implementations MUST implement TripleDES.  Implementations SHOULD
 implement AES-128 and CAST5.  Implementations that interoperate with

Callas, et al Standards Track [Page 62] RFC 4880 OpenPGP Message Format November 2007

 PGP 2.6 or earlier need to support IDEA, as that is the only
 symmetric cipher those versions use.  Implementations MAY implement
 any other algorithm.

9.3. Compression Algorithms

     ID           Algorithm
     --           ---------
     0          - Uncompressed
     1          - ZIP [RFC1951]
     2          - ZLIB [RFC1950]
     3          - BZip2 [BZ2]
     100 to 110 - Private/Experimental algorithm
 Implementations MUST implement uncompressed data.  Implementations
 SHOULD implement ZIP.  Implementations MAY implement any other
 algorithm.

9.4. Hash Algorithms

    ID           Algorithm                             Text Name
    --           ---------                             ---------
    1          - MD5 [HAC]                             "MD5"
    2          - SHA-1 [FIPS180]                       "SHA1"
    3          - RIPE-MD/160 [HAC]                     "RIPEMD160"
    4          - Reserved
    5          - Reserved
    6          - Reserved
    7          - Reserved
    8          - SHA256 [FIPS180]                      "SHA256"
    9          - SHA384 [FIPS180]                      "SHA384"
    10         - SHA512 [FIPS180]                      "SHA512"
    11         - SHA224 [FIPS180]                      "SHA224"
    100 to 110 - Private/Experimental algorithm
 Implementations MUST implement SHA-1.  Implementations MAY implement
 other algorithms.  MD5 is deprecated.

10. IANA Considerations

 OpenPGP is highly parameterized, and consequently there are a number
 of considerations for allocating parameters for extensions.  This
 section describes how IANA should look at extensions to the protocol
 as described in this document.

Callas, et al Standards Track [Page 63] RFC 4880 OpenPGP Message Format November 2007

10.1. New String-to-Key Specifier Types

 OpenPGP S2K specifiers contain a mechanism for new algorithms to turn
 a string into a key.  This specification creates a registry of S2K
 specifier types.  The registry includes the S2K type, the name of the
 S2K, and a reference to the defining specification.  The initial
 values for this registry can be found in Section 3.7.1.  Adding a new
 S2K specifier MUST be done through the IETF CONSENSUS method, as
 described in [RFC2434].

10.2. New Packets

 Major new features of OpenPGP are defined through new packet types.
 This specification creates a registry of packet types.  The registry
 includes the packet type, the name of the packet, and a reference to
 the defining specification.  The initial values for this registry can
 be found in Section 4.3.  Adding a new packet type MUST be done
 through the IETF CONSENSUS method, as described in [RFC2434].

10.2.1. User Attribute Types

 The User Attribute packet permits an extensible mechanism for other
 types of certificate identification.  This specification creates a
 registry of User Attribute types.  The registry includes the User
 Attribute type, the name of the User Attribute, and a reference to
 the defining specification.  The initial values for this registry can
 be found in Section 5.12.  Adding a new User Attribute type MUST be
 done through the IETF CONSENSUS method, as described in [RFC2434].

10.2.1.1. Image Format Subpacket Types

 Within User Attribute packets, there is an extensible mechanism for
 other types of image-based user attributes.  This specification
 creates a registry of Image Attribute subpacket types.  The registry
 includes the Image Attribute subpacket type, the name of the Image
 Attribute subpacket, and a reference to the defining specification.
 The initial values for this registry can be found in Section 5.12.1.
 Adding a new Image Attribute subpacket type MUST be done through the
 IETF CONSENSUS method, as described in [RFC2434].

10.2.2. New Signature Subpackets

 OpenPGP signatures contain a mechanism for signed (or unsigned) data
 to be added to them for a variety of purposes in the Signature
 subpackets as discussed in Section 5.2.3.1.  This specification
 creates a registry of Signature subpacket types.  The registry
 includes the Signature subpacket type, the name of the subpacket, and
 a reference to the defining specification.  The initial values for

Callas, et al Standards Track [Page 64] RFC 4880 OpenPGP Message Format November 2007

 this registry can be found in Section 5.2.3.1.  Adding a new
 Signature subpacket MUST be done through the IETF CONSENSUS method,
 as described in [RFC2434].

10.2.2.1. Signature Notation Data Subpackets

 OpenPGP signatures further contain a mechanism for extensions in
 signatures.  These are the Notation Data subpackets, which contain a
 key/value pair.  Notations contain a user space that is completely
 unmanaged and an IETF space.
 This specification creates a registry of Signature Notation Data
 types.  The registry includes the Signature Notation Data type, the
 name of the Signature Notation Data, its allowed values, and a
 reference to the defining specification.  The initial values for this
 registry can be found in Section 5.2.3.16.  Adding a new Signature
 Notation Data subpacket MUST be done through the EXPERT REVIEW
 method, as described in [RFC2434].

10.2.2.2. Key Server Preference Extensions

 OpenPGP signatures contain a mechanism for preferences to be
 specified about key servers.  This specification creates a registry
 of key server preferences.  The registry includes the key server
 preference, the name of the preference, and a reference to the
 defining specification.  The initial values for this registry can be
 found in Section 5.2.3.17.  Adding a new key server preference MUST
 be done through the IETF CONSENSUS method, as described in [RFC2434].

10.2.2.3. Key Flags Extensions

 OpenPGP signatures contain a mechanism for flags to be specified
 about key usage.  This specification creates a registry of key usage
 flags.  The registry includes the key flags value, the name of the
 flag, and a reference to the defining specification.  The initial
 values for this registry can be found in Section 5.2.3.21.  Adding a
 new key usage flag MUST be done through the IETF CONSENSUS method, as
 described in [RFC2434].

10.2.2.4. Reason for Revocation Extensions

 OpenPGP signatures contain a mechanism for flags to be specified
 about why a key was revoked.  This specification creates a registry
 of "Reason for Revocation" flags.  The registry includes the "Reason
 for Revocation" flags value, the name of the flag, and a reference to
 the defining specification.  The initial values for this registry can
 be found in Section 5.2.3.23.  Adding a new feature flag MUST be done
 through the IETF CONSENSUS method, as described in [RFC2434].

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10.2.2.5. Implementation Features

 OpenPGP signatures contain a mechanism for flags to be specified
 stating which optional features an implementation supports.  This
 specification creates a registry of feature-implementation flags.
 The registry includes the feature-implementation flags value, the
 name of the flag, and a reference to the defining specification.  The
 initial values for this registry can be found in Section 5.2.3.24.
 Adding a new feature-implementation flag MUST be done through the
 IETF CONSENSUS method, as described in [RFC2434].
 Also see Section 13.12 for more information about when feature flags
 are needed.

10.2.3. New Packet Versions

 The core OpenPGP packets all have version numbers, and can be revised
 by introducing a new version of an existing packet.  This
 specification creates a registry of packet types.  The registry
 includes the packet type, the number of the version, and a reference
 to the defining specification.  The initial values for this registry
 can be found in Section 5.  Adding a new packet version MUST be done
 through the IETF CONSENSUS method, as described in [RFC2434].

10.3. New Algorithms

 Section 9 lists the core algorithms that OpenPGP uses.  Adding in a
 new algorithm is usually simple.  For example, adding in a new
 symmetric cipher usually would not need anything more than allocating
 a constant for that cipher.  If that cipher had other than a 64-bit
 or 128-bit block size, there might need to be additional
 documentation describing how OpenPGP-CFB mode would be adjusted.
 Similarly, when DSA was expanded from a maximum of 1024-bit public
 keys to 3072-bit public keys, the revision of FIPS 186 contained
 enough information itself to allow implementation.  Changes to this
 document were made mainly for emphasis.

10.3.1. Public-Key Algorithms

 OpenPGP specifies a number of public-key algorithms.  This
 specification creates a registry of public-key algorithm identifiers.
 The registry includes the algorithm name, its key sizes and
 parameters, and a reference to the defining specification.  The
 initial values for this registry can be found in Section 9.  Adding a
 new public-key algorithm MUST be done through the IETF CONSENSUS
 method, as described in [RFC2434].

Callas, et al Standards Track [Page 66] RFC 4880 OpenPGP Message Format November 2007

10.3.2. Symmetric-Key Algorithms

 OpenPGP specifies a number of symmetric-key algorithms.  This
 specification creates a registry of symmetric-key algorithm
 identifiers.  The registry includes the algorithm name, its key sizes
 and block size, and a reference to the defining specification.  The
 initial values for this registry can be found in Section 9.  Adding a
 new symmetric-key algorithm MUST be done through the IETF CONSENSUS
 method, as described in [RFC2434].

10.3.3. Hash Algorithms

 OpenPGP specifies a number of hash algorithms.  This specification
 creates a registry of hash algorithm identifiers.  The registry
 includes the algorithm name, a text representation of that name, its
 block size, an OID hash prefix, and a reference to the defining
 specification.  The initial values for this registry can be found in
 Section 9 for the algorithm identifiers and text names, and Section
 5.2.2 for the OIDs and expanded signature prefixes.  Adding a new
 hash algorithm MUST be done through the IETF CONSENSUS method, as
 described in [RFC2434].

10.3.4. Compression Algorithms

 OpenPGP specifies a number of compression algorithms.  This
 specification creates a registry of compression algorithm
 identifiers.  The registry includes the algorithm name and a
 reference to the defining specification.  The initial values for this
 registry can be found in Section 9.3.  Adding a new compression key
 algorithm MUST be done through the IETF CONSENSUS method, as
 described in [RFC2434].

11. Packet Composition

 OpenPGP packets are assembled into sequences in order to create
 messages and to transfer keys.  Not all possible packet sequences are
 meaningful and correct.  This section describes the rules for how
 packets should be placed into sequences.

11.1. Transferable Public Keys

 OpenPGP users may transfer public keys.  The essential elements of a
 transferable public key are as follows:
  1. One Public-Key packet
  1. Zero or more revocation signatures

Callas, et al Standards Track [Page 67] RFC 4880 OpenPGP Message Format November 2007

  1. One or more User ID packets
  1. After each User ID packet, zero or more Signature packets

(certifications)

  1. Zero or more User Attribute packets
  1. After each User Attribute packet, zero or more Signature packets

(certifications)

  1. Zero or more Subkey packets
  1. After each Subkey packet, one Signature packet, plus optionally a

revocation

 The Public-Key packet occurs first.  Each of the following User ID
 packets provides the identity of the owner of this public key.  If
 there are multiple User ID packets, this corresponds to multiple
 means of identifying the same unique individual user; for example, a
 user may have more than one email address, and construct a User ID
 for each one.
 Immediately following each User ID packet, there are zero or more
 Signature packets.  Each Signature packet is calculated on the
 immediately preceding User ID packet and the initial Public-Key
 packet.  The signature serves to certify the corresponding public key
 and User ID.  In effect, the signer is testifying to his or her
 belief that this public key belongs to the user identified by this
 User ID.
 Within the same section as the User ID packets, there are zero or
 more User Attribute packets.  Like the User ID packets, a User
 Attribute packet is followed by zero or more Signature packets
 calculated on the immediately preceding User Attribute packet and the
 initial Public-Key packet.
 User Attribute packets and User ID packets may be freely intermixed
 in this section, so long as the signatures that follow them are
 maintained on the proper User Attribute or User ID packet.
 After the User ID packet or Attribute packet, there may be zero or
 more Subkey packets.  In general, subkeys are provided in cases where
 the top-level public key is a signature-only key.  However, any V4
 key may have subkeys, and the subkeys may be encryption-only keys,
 signature-only keys, or general-purpose keys.  V3 keys MUST NOT have
 subkeys.

Callas, et al Standards Track [Page 68] RFC 4880 OpenPGP Message Format November 2007

 Each Subkey packet MUST be followed by one Signature packet, which
 should be a subkey binding signature issued by the top-level key.
 For subkeys that can issue signatures, the subkey binding signature
 MUST contain an Embedded Signature subpacket with a primary key
 binding signature (0x19) issued by the subkey on the top-level key.
 Subkey and Key packets may each be followed by a revocation Signature
 packet to indicate that the key is revoked.  Revocation signatures
 are only accepted if they are issued by the key itself, or by a key
 that is authorized to issue revocations via a Revocation Key
 subpacket in a self-signature by the top-level key.
 Transferable public-key packet sequences may be concatenated to allow
 transferring multiple public keys in one operation.

11.2. Transferable Secret Keys

 OpenPGP users may transfer secret keys.  The format of a transferable
 secret key is the same as a transferable public key except that
 secret-key and secret-subkey packets are used instead of the public
 key and public-subkey packets.  Implementations SHOULD include self-
 signatures on any user IDs and subkeys, as this allows for a complete
 public key to be automatically extracted from the transferable secret
 key.  Implementations MAY choose to omit the self-signatures,
 especially if a transferable public key accompanies the transferable
 secret key.

11.3. OpenPGP Messages

 An OpenPGP message is a packet or sequence of packets that
 corresponds to the following grammatical rules (comma represents
 sequential composition, and vertical bar separates alternatives):
 OpenPGP Message :- Encrypted Message | Signed Message |
                    Compressed Message | Literal Message.
 Compressed Message :- Compressed Data Packet.
 Literal Message :- Literal Data Packet.
 ESK :- Public-Key Encrypted Session Key Packet |
        Symmetric-Key Encrypted Session Key Packet.
 ESK Sequence :- ESK | ESK Sequence, ESK.
 Encrypted Data :- Symmetrically Encrypted Data Packet |
       Symmetrically Encrypted Integrity Protected Data Packet

Callas, et al Standards Track [Page 69] RFC 4880 OpenPGP Message Format November 2007

 Encrypted Message :- Encrypted Data | ESK Sequence, Encrypted Data.
 One-Pass Signed Message :- One-Pass Signature Packet,
             OpenPGP Message, Corresponding Signature Packet.
 Signed Message :- Signature Packet, OpenPGP Message |
             One-Pass Signed Message.
 In addition, decrypting a Symmetrically Encrypted Data packet or a
 Symmetrically Encrypted Integrity Protected Data packet as well as
 decompressing a Compressed Data packet must yield a valid OpenPGP
 Message.

11.4. Detached Signatures

 Some OpenPGP applications use so-called "detached signatures".  For
 example, a program bundle may contain a file, and with it a second
 file that is a detached signature of the first file.  These detached
 signatures are simply a Signature packet stored separately from the
 data for which they are a signature.

12. Enhanced Key Formats

12.1. Key Structures

 The format of an OpenPGP V3 key is as follows.  Entries in square
 brackets are optional and ellipses indicate repetition.
         RSA Public Key
            [Revocation Self Signature]
             User ID [Signature ...]
            [User ID [Signature ...] ...]
 Each signature certifies the RSA public key and the preceding User
 ID.  The RSA public key can have many User IDs and each User ID can
 have many signatures.  V3 keys are deprecated.  Implementations MUST
 NOT generate new V3 keys, but MAY continue to use existing ones.
 The format of an OpenPGP V4 key that uses multiple public keys is
 similar except that the other keys are added to the end as "subkeys"
 of the primary key.

Callas, et al Standards Track [Page 70] RFC 4880 OpenPGP Message Format November 2007

         Primary-Key
            [Revocation Self Signature]
            [Direct Key Signature...]
             User ID [Signature ...]
            [User ID [Signature ...] ...]
            [User Attribute [Signature ...] ...]
            [[Subkey [Binding-Signature-Revocation]
                    Primary-Key-Binding-Signature] ...]
 A subkey always has a single signature after it that is issued using
 the primary key to tie the two keys together.  This binding signature
 may be in either V3 or V4 format, but SHOULD be V4.  Subkeys that can
 issue signatures MUST have a V4 binding signature due to the REQUIRED
 embedded primary key binding signature.
 In the above diagram, if the binding signature of a subkey has been
 revoked, the revoked key may be removed, leaving only one key.
 In a V4 key, the primary key MUST be a key capable of certification.
 The subkeys may be keys of any other type.  There may be other
 constructions of V4 keys, too.  For example, there may be a single-
 key RSA key in V4 format, a DSA primary key with an RSA encryption
 key, or RSA primary key with an Elgamal subkey, etc.
 It is also possible to have a signature-only subkey.  This permits a
 primary key that collects certifications (key signatures), but is
 used only for certifying subkeys that are used for encryption and
 signatures.

12.2. Key IDs and Fingerprints

 For a V3 key, the eight-octet Key ID consists of the low 64 bits of
 the public modulus of the RSA key.
 The fingerprint of a V3 key is formed by hashing the body (but not
 the two-octet length) of the MPIs that form the key material (public
 modulus n, followed by exponent e) with MD5.  Note that both V3 keys
 and MD5 are deprecated.
 A V4 fingerprint is the 160-bit SHA-1 hash of the octet 0x99,
 followed by the two-octet packet length, followed by the entire
 Public-Key packet starting with the version field.  The Key ID is the
 low-order 64 bits of the fingerprint.  Here are the fields of the
 hash material, with the example of a DSA key:
 a.1) 0x99 (1 octet)
 a.2) high-order length octet of (b)-(e) (1 octet)

Callas, et al Standards Track [Page 71] RFC 4880 OpenPGP Message Format November 2007

 a.3) low-order length octet of (b)-(e) (1 octet)
   b) version number = 4 (1 octet);
   c) timestamp of key creation (4 octets);
   d) algorithm (1 octet): 17 = DSA (example);
   e) Algorithm-specific fields.
 Algorithm-Specific Fields for DSA keys (example):
 e.1) MPI of DSA prime p;
 e.2) MPI of DSA group order q (q is a prime divisor of p-1);
 e.3) MPI of DSA group generator g;
 e.4) MPI of DSA public-key value y (= g**x mod p where x is secret).
 Note that it is possible for there to be collisions of Key IDs -- two
 different keys with the same Key ID.  Note that there is a much
 smaller, but still non-zero, probability that two different keys have
 the same fingerprint.
 Also note that if V3 and V4 format keys share the same RSA key
 material, they will have different Key IDs as well as different
 fingerprints.
 Finally, the Key ID and fingerprint of a subkey are calculated in the
 same way as for a primary key, including the 0x99 as the first octet
 (even though this is not a valid packet ID for a public subkey).

13. Notes on Algorithms

13.1. PKCS#1 Encoding in OpenPGP

 This standard makes use of the PKCS#1 functions EME-PKCS1-v1_5 and
 EMSA-PKCS1-v1_5.  However, the calling conventions of these functions
 has changed in the past.  To avoid potential confusion and
 interoperability problems, we are including local copies in this
 document, adapted from those in PKCS#1 v2.1 [RFC3447].  RFC 3447
 should be treated as the ultimate authority on PKCS#1 for OpenPGP.
 Nonetheless, we believe that there is value in having a self-
 contained document that avoids problems in the future with needed
 changes in the conventions.

Callas, et al Standards Track [Page 72] RFC 4880 OpenPGP Message Format November 2007

13.1.1. EME-PKCS1-v1_5-ENCODE

 Input:
 k  = the length in octets of the key modulus
 M  = message to be encoded, an octet string of length mLen, where
      mLen <= k - 11
 Output:
 EM = encoded message, an octet string of length k
 Error:   "message too long"
   1. Length checking: If mLen > k - 11, output "message too long" and
      stop.
   2. Generate an octet string PS of length k - mLen - 3 consisting of
      pseudo-randomly generated nonzero octets.  The length of PS will
      be at least eight octets.
   3. Concatenate PS, the message M, and other padding to form an
      encoded message EM of length k octets as
      EM = 0x00 || 0x02 || PS || 0x00 || M.
   4. Output EM.

13.1.2. EME-PKCS1-v1_5-DECODE

 Input:
 EM = encoded message, an octet string
 Output:
 M  = message, an octet string
 Error:   "decryption error"
 To decode an EME-PKCS1_v1_5 message, separate the encoded message EM
 into an octet string PS consisting of nonzero octets and a message M
 as follows
   EM = 0x00 || 0x02 || PS || 0x00 || M.

Callas, et al Standards Track [Page 73] RFC 4880 OpenPGP Message Format November 2007

 If the first octet of EM does not have hexadecimal value 0x00, if the
 second octet of EM does not have hexadecimal value 0x02, if there is
 no octet with hexadecimal value 0x00 to separate PS from M, or if the
 length of PS is less than 8 octets, output "decryption error" and
 stop.  See also the security note in Section 14 regarding differences
 in reporting between a decryption error and a padding error.

13.1.3. EMSA-PKCS1-v1_5

 This encoding method is deterministic and only has an encoding
 operation.
 Option:
 Hash - a hash function in which hLen denotes the length in octets of
       the hash function output
 Input:
 M  = message to be encoded
 mL = intended length in octets of the encoded message, at least tLen
      + 11, where tLen is the octet length of the DER encoding T of a
      certain value computed during the encoding operation
 Output:
 EM = encoded message, an octet string of length emLen
 Errors: "message too long"; "intended encoded message length too
 short"
 Steps:
   1. Apply the hash function to the message M to produce a hash value
      H:
      H = Hash(M).
      If the hash function outputs "message too long," output "message
      too long" and stop.
   2. Using the list in Section 5.2.2, produce an ASN.1 DER value for
      the hash function used.  Let T be the full hash prefix from
      Section 5.2.2, and let tLen be the length in octets of T.
   3. If emLen < tLen + 11, output "intended encoded message length
      too short" and stop.

Callas, et al Standards Track [Page 74] RFC 4880 OpenPGP Message Format November 2007

   4. Generate an octet string PS consisting of emLen - tLen - 3
      octets with hexadecimal value 0xFF.  The length of PS will be at
      least 8 octets.
   5. Concatenate PS, the hash prefix T, and other padding to form the
      encoded message EM as
      EM = 0x00 || 0x01 || PS || 0x00 || T.
   6. Output EM.

13.2. Symmetric Algorithm Preferences

 The symmetric algorithm preference is an ordered list of algorithms
 that the keyholder accepts.  Since it is found on a self-signature,
 it is possible that a keyholder may have multiple, different
 preferences.  For example, Alice may have TripleDES only specified
 for "alice@work.com" but CAST5, Blowfish, and TripleDES specified for
 "alice@home.org".  Note that it is also possible for preferences to
 be in a subkey's binding signature.
 Since TripleDES is the MUST-implement algorithm, if it is not
 explicitly in the list, it is tacitly at the end.  However, it is
 good form to place it there explicitly.  Note also that if an
 implementation does not implement the preference, then it is
 implicitly a TripleDES-only implementation.
 An implementation MUST NOT use a symmetric algorithm that is not in
 the recipient's preference list.  When encrypting to more than one
 recipient, the implementation finds a suitable algorithm by taking
 the intersection of the preferences of the recipients.  Note that the
 MUST-implement algorithm, TripleDES, ensures that the intersection is
 not null.  The implementation may use any mechanism to pick an
 algorithm in the intersection.
 If an implementation can decrypt a message that a keyholder doesn't
 have in their preferences, the implementation SHOULD decrypt the
 message anyway, but MUST warn the keyholder that the protocol has
 been violated.  For example, suppose that Alice, above, has software
 that implements all algorithms in this specification.  Nonetheless,
 she prefers subsets for work or home.  If she is sent a message
 encrypted with IDEA, which is not in her preferences, the software
 warns her that someone sent her an IDEA-encrypted message, but it
 would ideally decrypt it anyway.

Callas, et al Standards Track [Page 75] RFC 4880 OpenPGP Message Format November 2007

13.3. Other Algorithm Preferences

 Other algorithm preferences work similarly to the symmetric algorithm
 preference, in that they specify which algorithms the keyholder
 accepts.  There are two interesting cases that other comments need to
 be made about, though, the compression preferences and the hash
 preferences.

13.3.1. Compression Preferences

 Compression has been an integral part of PGP since its first days.
 OpenPGP and all previous versions of PGP have offered compression.
 In this specification, the default is for messages to be compressed,
 although an implementation is not required to do so.  Consequently,
 the compression preference gives a way for a keyholder to request
 that messages not be compressed, presumably because they are using a
 minimal implementation that does not include compression.
 Additionally, this gives a keyholder a way to state that it can
 support alternate algorithms.
 Like the algorithm preferences, an implementation MUST NOT use an
 algorithm that is not in the preference vector.  If the preferences
 are not present, then they are assumed to be [ZIP(1),
 Uncompressed(0)].
 Additionally, an implementation MUST implement this preference to the
 degree of recognizing when to send an uncompressed message.  A robust
 implementation would satisfy this requirement by looking at the
 recipient's preference and acting accordingly.  A minimal
 implementation can satisfy this requirement by never generating a
 compressed message, since all implementations can handle messages
 that have not been compressed.

13.3.2. Hash Algorithm Preferences

 Typically, the choice of a hash algorithm is something the signer
 does, rather than the verifier, because a signer rarely knows who is
 going to be verifying the signature.  This preference, though, allows
 a protocol based upon digital signatures ease in negotiation.
 Thus, if Alice is authenticating herself to Bob with a signature, it
 makes sense for her to use a hash algorithm that Bob's software uses.
 This preference allows Bob to state in his key which algorithms Alice
 may use.
 Since SHA1 is the MUST-implement hash algorithm, if it is not
 explicitly in the list, it is tacitly at the end.  However, it is
 good form to place it there explicitly.

Callas, et al Standards Track [Page 76] RFC 4880 OpenPGP Message Format November 2007

13.4. Plaintext

 Algorithm 0, "plaintext", may only be used to denote secret keys that
 are stored in the clear.  Implementations MUST NOT use plaintext in
 Symmetrically Encrypted Data packets; they must use Literal Data
 packets to encode unencrypted or literal data.

13.5. RSA

 There are algorithm types for RSA Sign-Only, and RSA Encrypt-Only
 keys.  These types are deprecated.  The "key flags" subpacket in a
 signature is a much better way to express the same idea, and
 generalizes it to all algorithms.  An implementation SHOULD NOT
 create such a key, but MAY interpret it.
 An implementation SHOULD NOT implement RSA keys of size less than
 1024 bits.

13.6. DSA

 An implementation SHOULD NOT implement DSA keys of size less than
 1024 bits.  It MUST NOT implement a DSA key with a q size of less
 than 160 bits.  DSA keys MUST also be a multiple of 64 bits, and the
 q size MUST be a multiple of 8 bits.  The Digital Signature Standard
 (DSS) [FIPS186] specifies that DSA be used in one of the following
 ways:
  • 1024-bit key, 160-bit q, SHA-1, SHA-224, SHA-256, SHA-384, or

SHA-512 hash

  • 2048-bit key, 224-bit q, SHA-224, SHA-256, SHA-384, or SHA-512

hash

  • 2048-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash
  • 3072-bit key, 256-bit q, SHA-256, SHA-384, or SHA-512 hash
 The above key and q size pairs were chosen to best balance the
 strength of the key with the strength of the hash.  Implementations
 SHOULD use one of the above key and q size pairs when generating DSA
 keys.  If DSS compliance is desired, one of the specified SHA hashes
 must be used as well.  [FIPS186] is the ultimate authority on DSS,
 and should be consulted for all questions of DSS compliance.
 Note that earlier versions of this standard only allowed a 160-bit q
 with no truncation allowed, so earlier implementations may not be
 able to handle signatures with a different q size or a truncated
 hash.

Callas, et al Standards Track [Page 77] RFC 4880 OpenPGP Message Format November 2007

13.7. Elgamal

 An implementation SHOULD NOT implement Elgamal keys of size less than
 1024 bits.

13.8. Reserved Algorithm Numbers

 A number of algorithm IDs have been reserved for algorithms that
 would be useful to use in an OpenPGP implementation, yet there are
 issues that prevent an implementer from actually implementing the
 algorithm.  These are marked in Section 9.1, "Public-Key Algorithms",
 as "reserved for".
 The reserved public-key algorithms, Elliptic Curve (18), ECDSA (19),
 and X9.42 (21), do not have the necessary parameters, parameter
 order, or semantics defined.
 Previous versions of OpenPGP permitted Elgamal [ELGAMAL] signatures
 with a public-key identifier of 20.  These are no longer permitted.
 An implementation MUST NOT generate such keys.  An implementation
 MUST NOT generate Elgamal signatures.  See [BLEICHENBACHER].

13.9. OpenPGP CFB Mode

 OpenPGP does symmetric encryption using a variant of Cipher Feedback
 mode (CFB mode).  This section describes the procedure it uses in
 detail.  This mode is what is used for Symmetrically Encrypted Data
 Packets; the mechanism used for encrypting secret-key material is
 similar, and is described in the sections above.
 In the description below, the value BS is the block size in octets of
 the cipher.  Most ciphers have a block size of 8 octets.  The AES and
 Twofish have a block size of 16 octets.  Also note that the
 description below assumes that the IV and CFB arrays start with an
 index of 1 (unlike the C language, which assumes arrays start with a
 zero index).
 OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and
 prefixes the plaintext with BS+2 octets of random data, such that
 octets BS+1 and BS+2 match octets BS-1 and BS.  It does a CFB
 resynchronization after encrypting those BS+2 octets.
 Thus, for an algorithm that has a block size of 8 octets (64 bits),
 the IV is 10 octets long and octets 7 and 8 of the IV are the same as
 octets 9 and 10.  For an algorithm with a block size of 16 octets
 (128 bits), the IV is 18 octets long, and octets 17 and 18 replicate
 octets 15 and 16.  Those extra two octets are an easy check for a
 correct key.

Callas, et al Standards Track [Page 78] RFC 4880 OpenPGP Message Format November 2007

 Step by step, here is the procedure:
 1.  The feedback register (FR) is set to the IV, which is all zeros.
 2.  FR is encrypted to produce FRE (FR Encrypted).  This is the
     encryption of an all-zero value.
 3.  FRE is xored with the first BS octets of random data prefixed to
     the plaintext to produce C[1] through C[BS], the first BS octets
     of ciphertext.
 4.  FR is loaded with C[1] through C[BS].
 5.  FR is encrypted to produce FRE, the encryption of the first BS
     octets of ciphertext.
 6.  The left two octets of FRE get xored with the next two octets of
     data that were prefixed to the plaintext.  This produces C[BS+1]
     and C[BS+2], the next two octets of ciphertext.
 7.  (The resynchronization step) FR is loaded with C[3] through
     C[BS+2].
 8.  FR is encrypted to produce FRE.
 9.  FRE is xored with the first BS octets of the given plaintext, now
     that we have finished encrypting the BS+2 octets of prefixed
     data.  This produces C[BS+3] through C[BS+(BS+2)], the next BS
     octets of ciphertext.
 10. FR is loaded with C[BS+3] to C[BS + (BS+2)] (which is C11-C18 for
     an 8-octet block).
     11. FR is encrypted to produce FRE.
     12. FRE is xored with the next BS octets of plaintext, to produce
     the next BS octets of ciphertext.  These are loaded into FR, and
     the process is repeated until the plaintext is used up.

13.10. Private or Experimental Parameters

 S2K specifiers, Signature subpacket types, user attribute types,
 image format types, and algorithms described in Section 9 all reserve
 the range 100 to 110 for private and experimental use.  Packet types
 reserve the range 60 to 63 for private and experimental use.  These
 are intentionally managed with the PRIVATE USE method, as described
 in [RFC2434].

Callas, et al Standards Track [Page 79] RFC 4880 OpenPGP Message Format November 2007

 However, implementations need to be careful with these and promote
 them to full IANA-managed parameters when they grow beyond the
 original, limited system.

13.11. Extension of the MDC System

 As described in the non-normative explanation in Section 5.13, the
 MDC system is uniquely unparameterized in OpenPGP.  This was an
 intentional decision to avoid cross-grade attacks.  If the MDC system
 is extended to a stronger hash function, care must be taken to avoid
 downgrade and cross-grade attacks.
 One simple way to do this is to create new packets for a new MDC.
 For example, instead of the MDC system using packets 18 and 19, a new
 MDC could use 20 and 21.  This has obvious drawbacks (it uses two
 packet numbers for each new hash function in a space that is limited
 to a maximum of 60).
 Another simple way to extend the MDC system is to create new versions
 of packet 18, and reflect this in packet 19.  For example, suppose
 that V2 of packet 18 implicitly used SHA-256.  This would require
 packet 19 to have a length of 32 octets.  The change in the version
 in packet 18 and the size of packet 19 prevent a downgrade attack.
 There are two drawbacks to this latter approach.  The first is that
 using the version number of a packet to carry algorithm information
 is not tidy from a protocol-design standpoint.  It is possible that
 there might be several versions of the MDC system in common use, but
 this untidiness would reflect untidiness in cryptographic consensus
 about hash function security.  The second is that different versions
 of packet 19 would have to have unique sizes.  If there were two
 versions each with 256-bit hashes, they could not both have 32-octet
 packet 19s without admitting the chance of a cross-grade attack.
 Yet another, complex approach to extend the MDC system would be a
 hybrid of the two above -- create a new pair of MDC packets that are
 fully parameterized, and yet protected from downgrade and cross-
 grade.
 Any change to the MDC system MUST be done through the IETF CONSENSUS
 method, as described in [RFC2434].

13.12. Meta-Considerations for Expansion

 If OpenPGP is extended in a way that is not backwards-compatible,
 meaning that old implementations will not gracefully handle their

Callas, et al Standards Track [Page 80] RFC 4880 OpenPGP Message Format November 2007

 absence of a new feature, the extension proposal can be declared in
 the key holder's self-signature as part of the Features signature
 subpacket.
 We cannot state definitively what extensions will not be upwards-
 compatible, but typically new algorithms are upwards-compatible,
 whereas new packets are not.
 If an extension proposal does not update the Features system, it
 SHOULD include an explanation of why this is unnecessary.  If the
 proposal contains neither an extension to the Features system nor an
 explanation of why such an extension is unnecessary, the proposal
 SHOULD be rejected.

14. Security Considerations

  • As with any technology involving cryptography, you should check the

current literature to determine if any algorithms used here have

   been found to be vulnerable to attack.
  • This specification uses Public-Key Cryptography technologies. It

is assumed that the private key portion of a public-private key

   pair is controlled and secured by the proper party or parties.
  • Certain operations in this specification involve the use of random

numbers. An appropriate entropy source should be used to generate

   these numbers (see [RFC4086]).
  • The MD5 hash algorithm has been found to have weaknesses, with

collisions found in a number of cases. MD5 is deprecated for use

   in OpenPGP.  Implementations MUST NOT generate new signatures using
   MD5 as a hash function.  They MAY continue to consider old
   signatures that used MD5 as valid.
  • SHA-224 and SHA-384 require the same work as SHA-256 and SHA-512,

respectively. In general, there are few reasons to use them

   outside of DSS compatibility.  You need a situation where one needs
   more security than smaller hashes, but does not want to have the
   full 256-bit or 512-bit data length.
  • Many security protocol designers think that it is a bad idea to use

a single key for both privacy (encryption) and integrity

   (signatures).  In fact, this was one of the motivating forces
   behind the V4 key format with separate signature and encryption
   keys.  If you as an implementer promote dual-use keys, you should
   at least be aware of this controversy.

Callas, et al Standards Track [Page 81] RFC 4880 OpenPGP Message Format November 2007

  • The DSA algorithm will work with any hash, but is sensitive to the

quality of the hash algorithm. Verifiers should be aware that even

   if the signer used a strong hash, an attacker could have modified
   the signature to use a weak one.  Only signatures using acceptably
   strong hash algorithms should be accepted as valid.
  • As OpenPGP combines many different asymmetric, symmetric, and hash

algorithms, each with different measures of strength, care should

   be taken that the weakest element of an OpenPGP message is still
   sufficiently strong for the purpose at hand.  While consensus about
   the strength of a given algorithm may evolve, NIST Special
   Publication 800-57 [SP800-57] recommends the following list of
   equivalent strengths:
         Asymmetric  |  Hash  |  Symmetric
          key size   |  size  |   key size
         ------------+--------+-----------
            1024        160         80
            2048        224        112
            3072        256        128
            7680        384        192
           15360        512        256
  • There is a somewhat-related potential security problem in

signatures. If an attacker can find a message that hashes to the

   same hash with a different algorithm, a bogus signature structure
   can be constructed that evaluates correctly.
   For example, suppose Alice DSA signs message M using hash algorithm
   H.  Suppose that Mallet finds a message M' that has the same hash
   value as M with H'.  Mallet can then construct a signature block
   that verifies as Alice's signature of M' with H'.  However, this
   would also constitute a weakness in either H or H' or both.  Should
   this ever occur, a revision will have to be made to this document
   to revise the allowed hash algorithms.
  • If you are building an authentication system, the recipient may

specify a preferred signing algorithm. However, the signer would

   be foolish to use a weak algorithm simply because the recipient
   requests it.
  • Some of the encryption algorithms mentioned in this document have

been analyzed less than others. For example, although CAST5 is

   presently considered strong, it has been analyzed less than
   TripleDES.  Other algorithms may have other controversies
   surrounding them.

Callas, et al Standards Track [Page 82] RFC 4880 OpenPGP Message Format November 2007

  • In late summer 2002, Jallad, Katz, and Schneier published an

interesting attack on the OpenPGP protocol and some of its

   implementations [JKS02].  In this attack, the attacker modifies a
   message and sends it to a user who then returns the erroneously
   decrypted message to the attacker.  The attacker is thus using the
   user as a random oracle, and can often decrypt the message.
   Compressing data can ameliorate this attack.  The incorrectly
   decrypted data nearly always decompresses in ways that defeat the
   attack.  However, this is not a rigorous fix, and leaves open some
   small vulnerabilities.  For example, if an implementation does not
   compress a message before encryption (perhaps because it knows it
   was already compressed), then that message is vulnerable.  Because
   of this happenstance -- that modification attacks can be thwarted
   by decompression errors -- an implementation SHOULD treat a
   decompression error as a security problem, not merely a data
   problem.
   This attack can be defeated by the use of Modification Detection,
   provided that the implementation does not let the user naively
   return the data to the attacker.  An implementation MUST treat an
   MDC failure as a security problem, not merely a data problem.
   In either case, the implementation MAY allow the user access to the
   erroneous data, but MUST warn the user as to potential security
   problems should that data be returned to the sender.
   While this attack is somewhat obscure, requiring a special set of
   circumstances to create it, it is nonetheless quite serious as it
   permits someone to trick a user to decrypt a message.
   Consequently, it is important that:
    1. Implementers treat MDC errors and decompression failures as
       security problems.
    2. Implementers implement Modification Detection with all due
       speed and encourage its spread.
    3. Users migrate to implementations that support Modification
       Detection with all due speed.
  • PKCS#1 has been found to be vulnerable to attacks in which a system

that reports errors in padding differently from errors in

   decryption becomes a random oracle that can leak the private key in
   mere millions of queries.  Implementations must be aware of this
   attack and prevent it from happening.  The simplest solution is to
   report a single error code for all variants of decryption errors so
   as not to leak information to an attacker.

Callas, et al Standards Track [Page 83] RFC 4880 OpenPGP Message Format November 2007

  • Some technologies mentioned here may be subject to government

control in some countries.

  • In winter 2005, Serge Mister and Robert Zuccherato from Entrust

released a paper describing a way that the "quick check" in OpenPGP

   CFB mode can be used with a random oracle to decrypt two octets of
   every cipher block [MZ05].  They recommend as prevention not using
   the quick check at all.
   Many implementers have taken this advice to heart for any data that
   is symmetrically encrypted and for which the session key is
   public-key encrypted.  In this case, the quick check is not needed
   as the public-key encryption of the session key should guarantee
   that it is the right session key.  In other cases, the
   implementation should use the quick check with care.
   On the one hand, there is a danger to using it if there is a random
   oracle that can leak information to an attacker.  In plainer
   language, there is a danger to using the quick check if timing
   information about the check can be exposed to an attacker,
   particularly via an automated service that allows rapidly repeated
   queries.
   On the other hand, it is inconvenient to the user to be informed
   that they typed in the wrong passphrase only after a petabyte of
   data is decrypted.  There are many cases in cryptographic
   engineering where the implementer must use care and wisdom, and
   this is one.

15. Implementation Nits

 This section is a collection of comments to help an implementer,
 particularly with an eye to backward compatibility.  Previous
 implementations of PGP are not OpenPGP compliant.  Often the
 differences are small, but small differences are frequently more
 vexing than large differences.  Thus, this is a non-comprehensive
 list of potential problems and gotchas for a developer who is trying
 to be backward-compatible.
  • The IDEA algorithm is patented, and yet it is required for PGP

2.x interoperability. It is also the de-facto preferred

     algorithm for a V3 key with a V3 self-signature (or no self-
     signature).
  • When exporting a private key, PGP 2.x generates the header "BEGIN

PGP SECRET KEY BLOCK" instead of "BEGIN PGP PRIVATE KEY BLOCK".

     All previous versions ignore the implied data type, and look
     directly at the packet data type.

Callas, et al Standards Track [Page 84] RFC 4880 OpenPGP Message Format November 2007

  • PGP 2.0 through 2.5 generated V2 Public-Key packets. These are

identical to the deprecated V3 keys except for the version

     number.  An implementation MUST NOT generate them and may accept
     or reject them as it sees fit.  Some older PGP versions generated
     V2 PKESK packets (Tag 1) as well.  An implementation may accept
     or reject V2 PKESK packets as it sees fit, and MUST NOT generate
     them.
  • PGP 2.6.x will not accept key-material packets with versions

greater than 3.

  • There are many ways possible for two keys to have the same key

material, but different fingerprints (and thus Key IDs). Perhaps

     the most interesting is an RSA key that has been "upgraded" to V4
     format, but since a V4 fingerprint is constructed by hashing the
     key creation time along with other things, two V4 keys created at
     different times, yet with the same key material will have
     different fingerprints.
  • If an implementation is using zlib to interoperate with PGP 2.x,

then the "windowBits" parameter should be set to -13.

  • The 0x19 back signatures were not required for signing subkeys

until relatively recently. Consequently, there may be keys in

     the wild that do not have these back signatures.  Implementing
     software may handle these keys as it sees fit.
  • OpenPGP does not put limits on the size of public keys. However,

larger keys are not necessarily better keys. Larger keys take

     more computation time to use, and this can quickly become
     impractical.  Different OpenPGP implementations may also use
     different upper bounds for public key sizes, and so care should
     be taken when choosing sizes to maintain interoperability.  As of
     2007 most implementations have an upper bound of 4096 bits.
  • ASCII armor is an optional feature of OpenPGP. The OpenPGP

working group strives for a minimal set of mandatory-to-implement

     features, and since there could be useful implementations that
     only use binary object formats, this is not a "MUST" feature for
     an implementation.  For example, an implementation that is using
     OpenPGP as a mechanism for file signatures may find ASCII armor
     unnecessary. OpenPGP permits an implementation to declare what
     features it does and does not support, but ASCII armor is not one
     of these.  Since most implementations allow binary and armored
     objects to be used indiscriminately, an implementation that does
     not implement ASCII armor may find itself with compatibility
     issues with general-purpose implementations.  Moreover,
     implementations of OpenPGP-MIME [RFC3156] already have a

Callas, et al Standards Track [Page 85] RFC 4880 OpenPGP Message Format November 2007

     requirement for ASCII armor so those implementations will
     necessarily have support.

16. References

16.1. Normative References

 [AES]            NIST, FIPS PUB 197, "Advanced Encryption Standard
                  (AES)," November 2001.
                  http://csrc.nist.gov/publications/fips/fips197/fips-
                  197.{ps,pdf}
 [BLOWFISH]       Schneier, B. "Description of a New Variable-Length
                  Key, 64-Bit Block Cipher (Blowfish)" Fast Software
                  Encryption, Cambridge Security Workshop Proceedings
                  (December 1993), Springer-Verlag, 1994, pp191-204
                  <http://www.counterpane.com/bfsverlag.html>
 [BZ2]            J. Seward, jseward@acm.org, "The Bzip2 and libbzip2
                  home page" <http://www.bzip.org/>
 [ELGAMAL]        T. Elgamal, "A Public-Key Cryptosystem and a
                  Signature Scheme Based on Discrete Logarithms," IEEE
                  Transactions on Information Theory, v. IT-31, n. 4,
                  1985, pp. 469-472.
 [FIPS180]        Secure Hash Signature Standard (SHS) (FIPS PUB 180-
                  2).
                  <http://csrc.nist.gov/publications/fips/fips180-
                  2/fips180-2withchangenotice.pdf>
 [FIPS186]        Digital Signature Standard (DSS) (FIPS PUB 186-2).
                  <http://csrc.nist.gov/publications/fips/fips186-2/
                   fips186-2-change1.pdf> FIPS 186-3 describes keys
                  greater than 1024 bits.  The latest draft is at:
                  <http://csrc.nist.gov/publications/drafts/
                  fips_186-3/Draft-FIPS-186-3%20_March2006.pdf>
 [HAC]            Alfred Menezes, Paul van Oorschot, and Scott
                  Vanstone, "Handbook of Applied Cryptography," CRC
                  Press, 1996.
                  <http://www.cacr.math.uwaterloo.ca/hac/>
 [IDEA]           Lai, X, "On the design and security of block
                  ciphers", ETH Series in Information Processing, J.L.
                  Massey (editor), Vol. 1, Hartung-Gorre Verlag
                  Knostanz, Technische Hochschule (Zurich), 1992

Callas, et al Standards Track [Page 86] RFC 4880 OpenPGP Message Format November 2007

 [ISO10646]       ISO/IEC 10646-1:1993. International Standard --
                  Information technology -- Universal Multiple-Octet
                  Coded Character Set (UCS) -- Part 1: Architecture
                  and Basic Multilingual Plane.
 [JFIF]           JPEG File Interchange Format (Version 1.02).  Eric
                  Hamilton, C-Cube Microsystems, Milpitas, CA,
                  September 1, 1992.
 [RFC1950]        Deutsch, P. and J-L. Gailly, "ZLIB Compressed Data
                  Format Specification version 3.3", RFC 1950, May
                  1996.
 [RFC1951]        Deutsch, P., "DEFLATE Compressed Data Format
                  Specification version 1.3", RFC 1951, May 1996.
 [RFC2045]        Freed, N. and N. Borenstein, "Multipurpose Internet
                  Mail Extensions (MIME) Part One: Format of Internet
                  Message Bodies", RFC 2045, November 1996
 [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2144]        Adams, C., "The CAST-128 Encryption Algorithm", RFC
                  2144, May 1997.
 [RFC2434]        Narten, T. and H. Alvestrand, "Guidelines for
                  Writing an IANA Considerations Section in RFCs", BCP
                  26, RFC 2434, October 1998.
 [RFC2822]        Resnick, P., "Internet Message Format", RFC 2822,
                  April 2001.
 [RFC3156]        Elkins, M., Del Torto, D., Levien, R., and T.
                  Roessler, "MIME Security with OpenPGP", RFC 3156,
                  August 2001.
 [RFC3447]        Jonsson, J. and B. Kaliski, "Public-Key Cryptography
                  Standards (PKCS) #1: RSA Cryptography Specifications
                  Version 2.1", RFC 3447, February 2003.
 [RFC3629]        Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, November 2003.
 [RFC4086]        Eastlake, D., 3rd, Schiller, J., and S. Crocker,
                  "Randomness Requirements for Security", BCP 106, RFC
                  4086, June 2005.

Callas, et al Standards Track [Page 87] RFC 4880 OpenPGP Message Format November 2007

 [SCHNEIER]      Schneier, B., "Applied Cryptography Second Edition:
                  protocols, algorithms, and source code in C", 1996.
 [TWOFISH]        B. Schneier, J. Kelsey, D. Whiting, D. Wagner, C.
                  Hall, and N. Ferguson, "The Twofish Encryption
                  Algorithm", John Wiley & Sons, 1999.

16.2. Informative References

 [BLEICHENBACHER] Bleichenbacher, Daniel, "Generating Elgamal
                  signatures without knowing the secret key,"
                  Eurocrypt 96. Note that the version in the
                  proceedings has an error. A revised version is
                  available at the time of writing from
                  <ftp://ftp.inf.ethz.ch/pub/publications/papers/ti
                  /isc/ElGamal.ps>
 [JKS02]          Kahil Jallad, Jonathan Katz, Bruce Schneier
                  "Implementation of Chosen-Ciphertext Attacks against
                  PGP and GnuPG" http://www.counterpane.com/pgp-
                  attack.html
 [MAURER]         Ueli Maurer, "Modelling a Public-Key
                  Infrastructure", Proc. 1996 European Symposium on
                  Research in Computer Security (ESORICS' 96), Lecture
                  Notes in Computer Science, Springer-Verlag, vol.
                  1146, pp. 325-350, Sep 1996.
 [MZ05]           Serge Mister, Robert Zuccherato, "An Attack on CFB
                  Mode Encryption As Used By OpenPGP," IACR ePrint
                  Archive: Report 2005/033, 8 Feb 2005
                  http://eprint.iacr.org/2005/033
 [REGEX]          Jeffrey Friedl, "Mastering Regular Expressions,"
                  O'Reilly, ISBN 0-596-00289-0.
 [RFC1423]        Balenson, D., "Privacy Enhancement for Internet
                  Electronic Mail: Part III: Algorithms, Modes, and
                  Identifiers", RFC 1423, February 1993.
 [RFC1991]        Atkins, D., Stallings, W., and P. Zimmermann, "PGP
                  Message Exchange Formats", RFC 1991, August 1996.
 [RFC2440]        Callas, J., Donnerhacke, L., Finney, H., and R.
                  Thayer, "OpenPGP Message Format", RFC 2440, November
                  1998.

Callas, et al Standards Track [Page 88] RFC 4880 OpenPGP Message Format November 2007

 [SP800-57]       NIST Special Publication 800-57, Recommendation on
                  Key Management
                  <http://csrc.nist.gov/publications/nistpubs/ 800-
                  57/SP800-57-Part1.pdf>
                  <http://csrc.nist.gov/publications/nistpubs/ 800-
                  57/SP800-57-Part2.pdf>

Acknowledgements

 This memo also draws on much previous work from a number of other
 authors, including: Derek Atkins, Charles Breed, Dave Del Torto, Marc
 Dyksterhouse, Gail Haspert, Gene Hoffman, Paul Hoffman, Ben Laurie,
 Raph Levien, Colin Plumb, Will Price, David Shaw, William Stallings,
 Mark Weaver, and Philip R. Zimmermann.

Authors' Addresses

 The working group can be contacted via the current chair:
    Derek Atkins
    IHTFP Consulting, Inc.
    4 Farragut Ave
    Somerville, MA  02144  USA
    EMail: derek@ihtfp.com
    Tel: +1 617 623 3745
 The principal authors of this document are as follows:
    Jon Callas
    EMail: jon@callas.org
    Lutz Donnerhacke
    IKS GmbH
    Wildenbruchstr. 15
    07745 Jena, Germany
    EMail: lutz@iks-jena.de
    Hal Finney
    EMail: hal@finney.org
    David Shaw
    EMail: dshaw@jabberwocky.com
    Rodney Thayer
    EMail: rodney@canola-jones.com

Callas, et al Standards Track [Page 89] RFC 4880 OpenPGP Message Format November 2007

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Callas, et al Standards Track [Page 90]

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