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rfc:rfc2440

Network Working Group J. Callas Request for Comments: 2440 Network Associates Category: Standards Track L. Donnerhacke

                                   IN-Root-CA Individual Network e.V.
                                                            H. Finney
                                                   Network Associates
                                                            R. Thayer
                                                      EIS Corporation
                                                        November 1998
                       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.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

IESG Note

 This document defines many tag values, yet it doesn't describe a
 mechanism for adding new tags (for new features).  Traditionally the
 Internet Assigned Numbers Authority (IANA) handles the allocation of
 new values for future expansion and RFCs usually define the procedure
 to be used by the IANA.  However, there are subtle (and not so
 subtle) interactions that may occur in this protocol between new
 features and existing features which result in a significant
 reduction in over all security.  Therefore, this document does not
 define an extension procedure.  Instead requests to define new tag
 values (say for new encryption algorithms for example) should be
 forwarded to the IESG Security Area Directors for consideration or
 forwarding to the appropriate IETF Working Group for consideration.

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,

Callas, et. al. Standards Track [Page 1] RFC 2440 OpenPGP Message Format November 1998

 however, discuss implementation issues necessary to avoid security
 flaws.
 Open-PGP 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.

Table of Contents

          Status of this Memo                                       1
          IESG Note                                                 1
          Abstract                                                  1
          Table of Contents                                         2
 1.       Introduction                                              4
 1.1.     Terms                                                     5
 2.       General functions                                         5
 2.1.     Confidentiality via Encryption                            5
 2.2.     Authentication via Digital signature                      6
 2.3.     Compression                                               7
 2.4.     Conversion to Radix-64                                    7
 2.5.     Signature-Only Applications                               7
 3.       Data Element Formats                                      7
 3.1.     Scalar numbers                                            8
 3.2.     Multi-Precision Integers                                  8
 3.3.     Key IDs                                                   8
 3.4.     Text                                                      8
 3.5.     Time fields                                               9
 3.6.     String-to-key (S2K) specifiers                            9
 3.6.1.   String-to-key (S2k) specifier types                       9
 3.6.1.1. Simple S2K                                                9
 3.6.1.2. Salted S2K                                               10
 3.6.1.3. Iterated and Salted S2K                                  10
 3.6.2.   String-to-key usage                                      11
 3.6.2.1. Secret key encryption                                    11
 3.6.2.2. Symmetric-key message encryption                         11
 4.       Packet Syntax                                            12
 4.1.     Overview                                                 12
 4.2.     Packet Headers                                           12
 4.2.1.   Old-Format Packet Lengths                                13
 4.2.2.   New-Format Packet Lengths                                13
 4.2.2.1. One-Octet Lengths                                        14
 4.2.2.2. Two-Octet Lengths                                        14
 4.2.2.3. Five-Octet Lengths                                       14
 4.2.2.4. Partial Body Lengths                                     14
 4.2.3.   Packet Length Examples                                   14

Callas, et. al. Standards Track [Page 2] RFC 2440 OpenPGP Message Format November 1998

 4.3.     Packet Tags                                              15
 5.       Packet Types                                             16
 5.1.     Public-Key Encrypted Session Key Packets (Tag 1)         16
 5.2.     Signature Packet (Tag 2)                                 17
 5.2.1.   Signature Types                                          17
 5.2.2.   Version 3 Signature Packet Format                        19
 5.2.3.   Version 4 Signature Packet Format                        21
 5.2.3.1. Signature Subpacket Specification                        22
 5.2.3.2. Signature Subpacket Types                                24
 5.2.3.3. Signature creation time                                  25
 5.2.3.4. Issuer                                                   25
 5.2.3.5. Key expiration time                                      25
 5.2.3.6. Preferred symmetric algorithms                           25
 5.2.3.7. Preferred hash algorithms                                25
 5.2.3.8. Preferred compression algorithms                         26
 5.2.3.9. Signature expiration time                                26
 5.2.3.10.Exportable Certification                                 26
 5.2.3.11.Revocable                                                27
 5.2.3.12.Trust signature                                          27
 5.2.3.13.Regular expression                                       27
 5.2.3.14.Revocation key                                           27
 5.2.3.15.Notation Data                                            28
 5.2.3.16.Key server preferences                                   28
 5.2.3.17.Preferred key server                                     29
 5.2.3.18.Primary user id                                          29
 5.2.3.19.Policy URL                                               29
 5.2.3.20.Key Flags                                                29
 5.2.3.21.Signer's User ID                                         30
 5.2.3.22.Reason for Revocation                                    30
 5.2.4.   Computing Signatures                                     31
 5.2.4.1. Subpacket Hints                                          32
 5.3.     Symmetric-Key Encrypted Session-Key Packets (Tag 3)      32
 5.4.     One-Pass Signature Packets (Tag 4)                       33
 5.5.     Key Material Packet                                      34
 5.5.1.   Key Packet Variants                                      34
 5.5.1.1. Public Key Packet (Tag 6)                                34
 5.5.1.2. Public Subkey Packet (Tag 14)                            34
 5.5.1.3. Secret Key Packet (Tag 5)                                35
 5.5.1.4. Secret Subkey Packet (Tag 7)                             35
 5.5.2.   Public Key Packet Formats                                35
 5.5.3.   Secret Key Packet Formats                                37
 5.6.     Compressed Data Packet (Tag 8)                           38
 5.7.     Symmetrically Encrypted Data Packet (Tag 9)              39
 5.8.     Marker Packet (Obsolete Literal Packet) (Tag 10)         39
 5.9.     Literal Data Packet (Tag 11)                             40
 5.10.    Trust Packet (Tag 12)                                    40
 5.11.    User ID Packet (Tag 13)                                  41
 6.       Radix-64 Conversions                                     41

Callas, et. al. Standards Track [Page 3] RFC 2440 OpenPGP Message Format November 1998

 6.1.     An Implementation of the CRC-24 in "C"                   42
 6.2.     Forming ASCII Armor                                      42
 6.3.     Encoding Binary in Radix-64                              44
 6.4.     Decoding Radix-64                                        46
 6.5.     Examples of Radix-64                                     46
 6.6.     Example of an ASCII Armored Message                      47
 7.       Cleartext signature framework                            47
 7.1.     Dash-Escaped Text                                        47
 8.       Regular Expressions                                      48
 9.       Constants                                                49
 9.1.     Public Key Algorithms                                    49
 9.2.     Symmetric Key Algorithms                                 49
 9.3.     Compression Algorithms                                   50
 9.4.     Hash Algorithms                                          50
 10.      Packet Composition                                       50
 10.1.    Transferable Public Keys                                 50
 10.2.    OpenPGP Messages                                         52
 10.3.    Detached Signatures                                      52
 11.      Enhanced Key Formats                                     52
 11.1.    Key Structures                                           52
 11.2.    Key IDs and Fingerprints                                 53
 12.      Notes on Algorithms                                      54
 12.1.    Symmetric Algorithm Preferences                          54
 12.2.    Other Algorithm Preferences                              55
 12.2.1.  Compression Preferences                                  56
 12.2.2.  Hash Algorithm Preferences                               56
 12.3.    Plaintext                                                56
 12.4.    RSA                                                      56
 12.5.    Elgamal                                                  57
 12.6.    DSA                                                      58
 12.7.    Reserved Algorithm Numbers                               58
 12.8.    OpenPGP CFB mode                                         58
 13.      Security Considerations                                  59
 14.      Implementation Nits                                      60
 15.      Authors and Working Group Chair                          62
 16.      References                                               63
 17.      Full Copyright Statement                                 65

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 builds on the foundation provided in
 RFC 1991 "PGP Message Exchange Formats."

Callas, et. al. Standards Track [Page 4] RFC 2440 OpenPGP Message Format November 1998

1.1. Terms

  • OpenPGP - This is a definition for security software that uses

PGP 5.x as a basis.

  • 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.
 "PGP", "Pretty Good", and "Pretty Good Privacy" are trademarks of
 Network Associates, Inc. and are used with permission.
 This document uses the terms "MUST", "SHOULD", and "MAY" as defined
 in RFC 2119, along with the negated forms of those terms.

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 uses two encryption methods to provide confidentiality:
 symmetric-key encryption and public key encryption. With public-key
 encryption, the object is encrypted using a symmetric encryption
 algorithm.  Each symmetric key is used only once. A new "session key"
 is generated as a random number for each message. Since it is used

Callas, et. al. Standards Track [Page 5] RFC 2440 OpenPGP Message Format November 1998

 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.
 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.

Callas, et. al. Standards Track [Page 6] RFC 2440 OpenPGP Message Format November 1998

 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 MAY compress the message after applying the
 signature but before encryption.

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.
 Note that many applications, particularly messaging applications,
 will want more advanced features as described in the OpenPGP-MIME
 document, RFC 2015. An application that implements OpenPGP for
 messaging SHOULD implement OpenPGP-MIME.

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-comformant only in that they omit
 encryption.

3. Data Element Formats

 This section describes the data elements used by OpenPGP.

Callas, et. al. Standards Track [Page 7] RFC 2440 OpenPGP Message Format November 1998

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]).

3.2. Multi-Precision Integers

 Multi-Precision 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].

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

 The default character set for text is the UTF-8 [RFC2279] encoding of
 Unicode [ISO10646].

Callas, et. al. Standards Track [Page 8] RFC 2440 OpenPGP Message Format November 1998

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. 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.

3.6.1. String-to-key (S2k) specifier types

 There are three types of S2K specifiers currently supported, as
 follows:

3.6.1.1. 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
 algorithm's output. If the hash size is greater than or equal to 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.

Callas, et. al. Standards Track [Page 9] RFC 2440 OpenPGP Message Format November 1998

3.6.1.2. 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.

3.6.1.3. 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
 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.

Callas, et. al. Standards Track [Page 10] RFC 2440 OpenPGP Message Format November 1998

 After the hashing is done the data is unloaded from the hash
 context(s) as with the other S2K algorithms.

3.6.2. String-to-key usage

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

3.6.2.1. 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
 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.
 Therefore, preceding the secret data there will be one of these
 possibilities:
     0:           secret data is unencrypted (no pass phrase)
     255:         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 8-octet Initial Vector for the decryption of
 the secret values, if they are encrypted, and then the secret key
 values themselves.

3.6.2.2. 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.

Callas, et. al. Standards Track [Page 11] RFC 2440 OpenPGP Message Format November 1998

 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.
 Note that the most significant bit is the left-most 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, either format may be
 used. Note that old format packets have four bits of content tags,
 and new format packets have six; some features cannot be used and
 still be backward-compatible.
 Old format packets contain:
       Bits 5-2 -- content tag
       Bits 1-0 - length-type

Callas, et. al. Standards Track [Page 12] RFC 2440 OpenPGP Message Format November 1998

 New format packets contain:
       Bits 5-0 -- content 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.

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.

Callas, et. al. Standards Track [Page 13] RFC 2440 OpenPGP Message Format November 1998

4.2.2.1. One-Octet Lengths

 A one-octet Body Length header encodes a length of from 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 from 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

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 (of one of the three types --
 one octet, two-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.

4.2.3. Packet Length Examples

 These examples show ways that new-format packets might encode the
 packet lengths.

Callas, et. al. Standards Track [Page 14] RFC 2440 OpenPGP Message Format November 1998

 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 coded 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. 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.
 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:
     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

Callas, et. al. Standards Track [Page 15] RFC 2440 OpenPGP Message Format November 1998

     13       -- User ID Packet
     14       -- Public Subkey 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 Encrypted Session Key packets
 (either Public-Key or Symmetric-Key) 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.
 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. An

     implementation should accept, but not generate a version of 2,
     which is equivalent to V3 in all other respects.
  1. An eight-octet number that gives the key ID of the public key

that the session key is encrypted to.

  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. Callas, et. al. Standards Track [Page 16] RFC 2440 OpenPGP Message Format November 1998 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 padded as described in PKCS-1 block type 02 [RFC2313] to form the "m" value used in the formulas above. 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 new PKCS-1 padding 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. 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 MUST accept V3 signatures. Implementations SHOULD generate V4 signatures. Implementations MAY generate a V3 signature that can be verified by PGP 2.6.x. 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 specified in a signature type octet in any given signature. These meanings are: 0x00: Signature of a binary document. Typically, this means the signer owns it, created it, or certifies that it has not been modified. Callas, et. al. Standards Track [Page 17] RFC 2440 OpenPGP Message Format November 1998 0x01: Signature of a canonical text document. Typically, 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> and trailing blanks removed. 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. 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. Note that all PGP "key signatures" are this type of certification. 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. Please note that the vagueness of these certification claims is not a flaw, but a feature of the system. Because PGP places final authority for validity upon the receiver of a certification, it may be that one authority's casual certification might be more rigorous than some other authority's positive certification. These classifications allow a certification authority to issue fine-grained claims. 0x18: Subkey Binding Signature This signature is a statement by the top-level signing key indicates that it owns the subkey. This signature is calculated directly on the subkey itself, not on any User ID or other packets. Callas, et. al. Standards Track [Page 18] RFC 2440 OpenPGP Message Format November 1998 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. 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). It should be issued by the same key that issued the revoked signature or an authorized revocation key The signature should have a later creation date than the signature it revokes. 0x40: Timestamp signature. This signature is only meaningful for the timestamp contained in it. 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. - One-octet public key algorithm. Callas, et. al. Standards Track [Page 19] RFC 2440 OpenPGP Message Format November 1998 - One-octet hash algorithm. - Two-octet field holding left 16 bits of signed hash value. - One or more multi-precision integers comprising the signature. This portion is algorithm specific, as described below. The data being signed is hashed, and then the signature type and creation time from the signature packet are hashed (5 additional octets). 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.
 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 signature than for RSA signatures.
 With RSA signatures, the hash value is encoded as described in PKCS-1
 section 10.1.2, "Data encoding", producing an ASN.1 value of type
 DigestInfo, and then padded using PKCS-1 block type 01 [RFC2313].
 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:
  1. MD2: 0x2A, 0x86, 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02
  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

Callas, et. al. Standards Track [Page 20] RFC 2440 OpenPGP Message Format November 1998

 The ASN.1 OIDs are:
  1. MD2: 1.2.840.113549.2.2
  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
 The full hash prefixes for these are:
     MD2:        0x30, 0x20, 0x30, 0x0C, 0x06, 0x08, 0x2A, 0x86,
                 0x48, 0x86, 0xF7, 0x0D, 0x02, 0x02, 0x05, 0x00,
                 0x04, 0x10
     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
 DSA signatures MUST use hashes with a size of 160 bits, to match q,
 the size of the group generated by the DSA key's generator value.
 The hash function result is treated as a 160 bit 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.

Callas, et. al. Standards Track [Page 21] RFC 2440 OpenPGP Message Format November 1998

  1. Hashed subpacket data. (zero or more subpackets)
  1. Two-octet scalar octet count for 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. (zero or more subpackets)
  1. Two-octet field holding left 16 bits of signed hash value.
  1. One or more multi-precision integers comprising the signature.

This portion is algorithm specific, as described above.

 The data being signed is hashed, and then 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
 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

 The subpacket fields consist of zero or more signature subpackets.
 Each set of subpackets is preceded by a two-octet scalar count of the
 length of the set of 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 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:

Callas, et. al. Standards Track [Page 22] RFC 2440 OpenPGP Message Format November 1998

     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:
     2 = signature creation time
     3 = signature expiration time
     4 = exportable certification
     5 = trust signature
     6 = regular expression
     7 = revocable
     9 = key expiration time
     10 = placeholder for backward compatibility
     11 = preferred symmetric algorithms
     12 = revocation key
     16 = issuer key ID
     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 URL
     27 = key flags
     28 = signer's user id
     29 = reason for revocation
     100 to 110 = internal or user-defined
 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.

Callas, et. al. Standards Track [Page 23] RFC 2440 OpenPGP Message Format November 1998

 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 "preferences".

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 user id
 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 self-signature is a binding signature made by the key the signature
 refers to. 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 username, 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 usernames, Alice and Bob. Suppose that Alice prefers the
 symmetric algorithm CAST5, and Bob prefers IDEA or Triple-DES. 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,
 then algorithm of the default user id of the key provides the default
 symmetric algorithm.
 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.

Callas, et. al. Standards Track [Page 24] RFC 2440 OpenPGP Message Format November 1998

5.2.3.3. Signature creation time

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

5.2.3.4. Issuer

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

5.2.3.5. 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.

5.2.3.6. Preferred symmetric algorithms

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

5.2.3.7. 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 6.
 This is only found on a self-signature.

Callas, et. al. Standards Track [Page 25] RFC 2440 OpenPGP Message Format November 1998

5.2.3.8. 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 6. 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.

5.2.3.9. 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.

5.2.3.10. 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.
 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.

Callas, et. al. Standards Track [Page 26] RFC 2440 OpenPGP Message Format November 1998

5.2.3.11. Revocable

 (1 octet of revocability, 0 for not, 1 for revocable)
 Signature's revocability status.  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.

5.2.3.12. 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.

5.2.3.13. 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 package. A description of
 the syntax is found in a section below.

5.2.3.14. Revocation key

 (1 octet of class, 1 octet of algid, 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

Callas, et. al. Standards Track [Page 27] RFC 2440 OpenPGP Message Format November 1998

 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.

5.2.3.15. 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.
 All undefined flags MUST be zero. Defined flags are:
     First octet: 0x80 = human-readable. This note is text, a note
                         from one person to another, and has no
                         meaning to software.
     Other octets: none.

5.2.3.16. Key server preferences

 (N octets of flags)
 This is a list of 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 28] RFC 2440 OpenPGP Message Format November 1998

5.2.3.17. Preferred key server

 (String)
 This is a URL 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 URL, the
 key server can actually be a copy of the key retrieved by ftp, http,
 finger, etc.

5.2.3.18. 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.

5.2.3.19. Policy URL

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

5.2.3.20. Key Flags

 (Octet string)
 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:
     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.

Callas, et. al. Standards Track [Page 29] RFC 2440 OpenPGP Message Format November 1998

     0x10 - The private component of this key may have been split by a
     secret-sharing mechanism.
     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.

5.2.3.21. Signer's User ID

 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.

5.2.3.22. 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:

Callas, et. al. Standards Track [Page 30] RFC 2440 OpenPGP Message Format November 1998

     0x00 - No reason specified (key revocations or cert revocations)
     0x01 - Key is superceded (key revocations)
     0x02 - Key material has been compromised (key revocations)
     0x03 - Key is no longer used (key revocations)
     0x20 - User id information is no longer valid (cert revocations)
 Following the revocation code is a string of octets which 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.

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.
 The signature data is simple to compute for document signatures
 (types 0x00 and 0x01), for which the document itself is the data.
 For standalone signatures, this is a null string.
 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 signature (type 0x18)
 then hashes the subkey, using the same format as the main key. 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 name packet, without any
 header. A V4 certification hashes the constant 0xb4 (which is an
 old-style packet header with the length-of-length set to zero), a
 four-octet number giving the length of the username, and then the
 username data.
 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
 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.

Callas, et. al. Standards Track [Page 31] RFC 2440 OpenPGP Message Format November 1998

 V4 signatures also hash in a final trailer of six octets: the version
 of the signature packet, i.e. 0x04; 0xFF; 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, 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

 An implementation SHOULD put the two mandatory subpackets, creation
 time and issuer, as the first subpackets in the subpacket list,
 simply to make it easier for the implementer to find them.
 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 an 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 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.
 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

Callas, et. al. Standards Track [Page 32] RFC 2440 OpenPGP Message Format November 1998

 message to be encrypted to a number of public keys, and also to one
 or more pass phrases. 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 insure 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.
 The body of this packet consists of:

Callas, et. al. Standards Track [Page 33] RFC 2440 OpenPGP Message Format November 1998

  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 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.

Callas, et. al. Standards Track [Page 34] RFC 2440 OpenPGP Message Format November 1998

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 2 packets are identical in
 format to Version 3 packets, but are generated by PGP 2.5 or before.
 V2 packets are deprecated and they MUST NOT be generated.  PGP 5.0
 introduced version 4 packets, with new fields and semantics.  PGP
 2.6.x will not accept key-material packets with versions greater than
 3.
 OpenPGP implementations SHOULD create keys with version 4 format. An
 implementation MAY generate a V3 key to ensure interoperability with
 old software; note, however, that V4 keys correct some security
 deficiencies in V3 keys. These deficiencies are described below. An
 implementation MUST NOT create a V3 key with a public key algorithm
 other than RSA.
 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 multi-precision integers comprising the key

material:

  1. a multiprecision integer (MPI) of RSA public modulus n;
  1. an MPI of RSA public encryption exponent e.

Callas, et. al. Standards Track [Page 35] RFC 2440 OpenPGP Message Format November 1998

 V3 keys SHOULD only be used for backward compatibility because of
 three weaknesses in them. 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, which increases the opportunity for fingerprint collisions.
 Third, there are minor weaknesses in the MD5 hash algorithm that make
 developers prefer other algorithms. See below for a fuller discussion
 of key IDs and fingerprints.
 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 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 multi-precision 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 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 36] RFC 2440 OpenPGP Message Format November 1998 - MPI of Elgamal public key value y (= gx 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, 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. 0

indicates that the secret key data is not encrypted. 255

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

symmetric encryption algorithm.

  1. [Optional] If string-to-key usage octet was 255, 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, eight-octet Initial

Vector (IV).

  1. Encrypted multi-precision integers comprising the secret key

data. These algorithm-specific fields are as described below.

  1. Two-octet checksum of the plaintext of the algorithm-specific

portion (sum of all octets, mod 65536).

     Algorithm Specific Fields for RSA secret keys:
  1. multiprecision integer (MPI) of RSA secret exponent d.
  1. MPI of RSA secret prime value p.
  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.

Callas, et. al. Standards Track [Page 37] RFC 2440 OpenPGP Message Format November 1998

     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 SHOULD use a string-to-key
 specifier; the simple hash is for backward compatibility. 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 16-bit 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.

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 literal data
 packets.
 The body of this packet consists of:
  1. One octet that gives the algorithm used to compress the packet.
  1. The remainder of the packet is compressed data.
 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.

Callas, et. al. Standards Track [Page 38] RFC 2440 OpenPGP Message Format November 1998

 ZIP-compressed packets are compressed with raw RFC 1951 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 ZLIB-style
 blocks.

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 will
 typically contain other packets (often literal data packets or
 compressed data packets).
 The body of this packet consists of:
  1. Encrypted data, the output of the selected symmetric-key cipher

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

 The symmetric cipher used may be specified in an 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.
 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 10-octet string to
 the data before it is encrypted.  The first eight octets are random,
 and the 9th and 10th octets are copies of the 7th and 8th octets,
 respectively. After encrypting the first 10 octets, the CFB state is
 resynchronized if the cipher block size is 8 octets or less.  The
 last 8 octets of ciphertext are passed through the cipher and the
 block boundary is reset.
 The repetition of 16 bits in the 80 bits of random data prefixed to
 the message allows the receiver to immediately check whether the
 session key is incorrect.

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 re-assigned and is
 reserved for use as the Marker packet.

Callas, et. al. Standards Track [Page 39] RFC 2440 OpenPGP Message Format November 1998

 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.
 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.  RFC
 1991 also defined a value of 'l' as a 'local' mode for machine-local
 conversions.  This use is now deprecated.
  1. File name as a string (one-octet length, followed by file name),

if the encrypted data should be saved as a file.

 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 the modification date of the

file, or the creation time of the packet, or a zero that

     indicates the present 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,

Callas, et. al. Standards Track [Page 40] RFC 2440 OpenPGP Message Format November 1998

 along with other information that implementing software uses for
 trust information.
 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.

5.11. User ID Packet (Tag 13)

 A User ID packet consists of data that is intended to represent the
 name and email address of the key holder.  By convention, it includes
 an RFC 822 mail name, but there are no restrictions on its content.
 The packet length in the header specifies the length of the user id.
 If it is text, it is encoded in UTF-8.

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 [RFC2231,
 Section 6.8]. An OpenPGP implementation MAY use ASCII Armor to
 protect the raw binary data.
 The checksum is a 24-bit CRC converted to four characters of radix-64
 encoding by the same MIME base64 transformation, preceded by an
 equals 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 41] RFC 2440 OpenPGP Message Format November 1998

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 data, so OpenPGP can reconstruct the data later. 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 42] RFC 2440 OpenPGP Message Format November 1998

 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
     natures following clearsigned messages. Note that PGP 2.x s BEGIN
     PGP MESSAGE for detached signatures.
 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.
 Currently defined Armor Header Keys are:
  1. "Version", that states the OpenPGP Version used to encode the

message.

  1. "Comment", a user-defined comment.
  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

Callas, et. al. Standards Track [Page 43] RFC 2440 OpenPGP Message Format November 1998

     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.
  1. "Hash", a comma-separated list of hash algorithms used in this

message. This is used only in clear-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 by

     default. 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.
     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.
 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.

Callas, et. al. Standards Track [Page 44] RFC 2440 OpenPGP Message Format November 1998

       +--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.
 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.

Callas, et. al. Standards Track [Page 45] RFC 2440 OpenPGP Message Format November 1998

6.4. Decoding Radix-64

 Any characters outside of the base64 alphabet are ignored in Radix-64
 data. Decoding software must ignore all line breaks or other
 characters not found in the table above.
 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.
 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.

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      =      =

Callas, et. al. Standards Track [Page 46] RFC 2440 OpenPGP Message Format November 1998

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 is indented by two spaces.

7. Cleartext signature framework

 It is desirable 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 RFC 2015 defines another way to
 clear sign messages for environments that support MIME.)
 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 is used for the signature. If there are no such headers,
 MD5 is used, an implementation MAY omit them for 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.

7.1. Dash-Escaped Text

 The cleartext content of the message must also be dash-escaped.

Callas, et. al. Standards Track [Page 47] RFC 2440 OpenPGP Message Format November 1998

 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. 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.
 Also, any trailing whitespace (spaces, and tabs, 0x09) at the end of
 any line is ignored when the cleartext signature is calculated.

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.

Callas, et. al. Standards Track [Page 48] RFC 2440 OpenPGP Message Format November 1998

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.
 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)
     2          - RSA Encrypt-Only
     3          - RSA Sign-Only
     16         - Elgamal (Encrypt-Only), see [ELGAMAL]
     17         - DSA (Digital Signature Standard)
     18         - Reserved for Elliptic Curve
     19         - Reserved for ECDSA
     20         - 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.
 Implementations MAY implement any other algorithm.

9.2. Symmetric Key Algorithms

     ID           Algorithm
     --           ---------
     0          - Plaintext or unencrypted data
     1          - IDEA [IDEA]
     2          - Triple-DES (DES-EDE, as per spec -
                  168 bit key derived from 192)
     3          - CAST5 (128 bit key, as per RFC 2144)
     4          - Blowfish (128 bit key, 16 rounds) [BLOWFISH]
     5          - SAFER-SK128 (13 rounds) [SAFER]
     6          - Reserved for DES/SK
     7          - Reserved for AES with 128-bit key

Callas, et. al. Standards Track [Page 49] RFC 2440 OpenPGP Message Format November 1998

     8          - Reserved for AES with 192-bit key
     9          - Reserved for AES with 256-bit key
     100 to 110 - Private/Experimental algorithm.
 Implementations MUST implement Triple-DES. Implementations SHOULD
 implement IDEA and CAST5.Implementations MAY implement any other
 algorithm.

9.3. Compression Algorithms

     ID           Algorithm
     --           ---------
     0          - Uncompressed
     1          - ZIP (RFC 1951)
     2          - ZLIB (RFC 1950)
     100 to 110 - Private/Experimental algorithm.
 Implementations MUST implement uncompressed data. Implementations
 SHOULD implement ZIP. Implementations MAY implement ZLIB.

9.4. Hash Algorithms

     ID           Algorithm                              Text Name
     --           ---------                              ---- ----
     1          - MD5                                    "MD5"
     2          - SHA-1                                  "SHA1"
     3          - RIPE-MD/160                            "RIPEMD160"
     4          - Reserved for double-width SHA (experimental)
     5          - MD2                                    "MD2"
     6          - Reserved for TIGER/192                 "TIGER192"
     7          - Reserved for HAVAL (5 pass, 160-bit)
     "HAVAL-5-160"
     100 to 110 - Private/Experimental algorithm.
 Implementations MUST implement SHA-1. Implementations SHOULD
 implement MD5.

10. 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 describes the rules for how packets
 should be placed into sequences.

10.1. Transferable Public Keys

 OpenPGP users may transfer public keys. The essential elements of a
 transferable public key are:

Callas, et. al. Standards Track [Page 50] RFC 2440 OpenPGP Message Format November 1998

  1. One Public Key packet
  1. Zero or more revocation signatures
  1. One or more User ID packets
  1. After each User ID packet, zero or more signature packets

(certifications)

  1. Zero or more Subkey packets
  1. After each Subkey packet, one signature packet, 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.
 After the User ID packets there may be one 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.
 Each Subkey packet must be followed by one Signature packet, which
 should be a subkey binding signature issued by 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.

Callas, et. al. Standards Track [Page 51] RFC 2440 OpenPGP Message Format November 1998

10.2. 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 Message :- Symmetrically Encrypted Data Packet |
             ESK Sequence, Symmetrically Encrypted Data Packet.
 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 and
 decompressing a Compressed Data packet must yield a valid OpenPGP
 Message.

10.3. 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 that they are a signature of.

11. Enhanced Key Formats

11.1. Key Structures

 The format of an OpenPGP V3 key is as follows.  Entries in square
 brackets are optional and ellipses indicate repetition.

Callas, et. al. Standards Track [Page 52] RFC 2440 OpenPGP Message Format November 1998

         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.
 The format of an OpenPGP V4 key that uses two public keys is similar
 except that the other keys are added to the end as 'subkeys' of the
 primary key.
         Primary-Key
            [Revocation Self Signature]
            [Direct Key Self Signature...]
             User ID [Signature ...]
            [User ID [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 V4 is preferred, of course.
 In the above diagram, if the binding signature of a subkey has been
 revoked, the revoked binding signature may be removed, leaving only
 one signature.
 In a key that has a main key and subkeys, the primary key MUST be a
 key capable of signing. 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 used for certifying subkeys that are used for encryption and
 signatures.

11.2. 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.

Callas, et. al. Standards Track [Page 53] RFC 2440 OpenPGP Message Format November 1998

 A V4 fingerprint is the 160-bit SHA-1 hash of the one-octet Packet
 Tag, 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)-(f) (1 octet)
a.3) low order length octet of (b)-(f) (1 octet)
  b) version number = 4 (1 octet);
  c) time stamp 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 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.

12. Notes on Algorithms

12.1. 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 different preferences. For
 example, Alice may have TripleDES only specified for "alice@work.com"
 but CAST5, Blowfish, and TripleDES specified for "alice@home.org".

Callas, et. al. Standards Track [Page 54] RFC 2440 OpenPGP Message Format November 1998

 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 than 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.)
 An implementation that is striving for backward compatibility MAY
 consider a V3 key with a V3 self-signature to be an implicit
 preference for IDEA, and no ability to do TripleDES. This is
 technically non-compliant, but an implementation MAY violate the
 above rule in this case only and use IDEA to encrypt the message,
 provided that the message creator is warned. Ideally, though, the
 implementation would follow the rule by actually generating two
 messages, because it is possible that the OpenPGP user's
 implementation does not have IDEA, and thus could not read the
 message. Consequently, an implementation MAY, but SHOULD NOT use IDEA
 in an algorithm conflict with a V3 key.

12.2. 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.

Callas, et. al. Standards Track [Page 55] RFC 2440 OpenPGP Message Format November 1998

12.2.1. Compression Preferences

 Compression has been an integral part of PGP since its first days.
 OpenPGP and all previous versions of PGP have offered compression.
 And 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)].

12.2.2. Hash Algorithm Preferences

 Typically, the choice of a hash algorithm is something the signer
 does, rather than the verifier, because a signer does not typically
 know 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.

12.3. 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.

12.4. RSA

 There are algorithm types for RSA-signature-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 768
 bits.

Callas, et. al. Standards Track [Page 56] RFC 2440 OpenPGP Message Format November 1998

 It is permissible for an implementation to support RSA merely for
 backward compatibility; for example, such an implementation would
 support V3 keys with IDEA symmetric cryptography. Note that this is
 an exception to the other MUST-implement rules. An implementation
 that supports RSA in V4 keys MUST implement the MUST-implement
 features.

12.5. Elgamal

 If an Elgamal key is to be used for both signing and encryption,
 extra care must be taken in creating the key.
 An ElGamal key consists of a generator g, a prime modulus p, a secret
 exponent x, and a public value y = g^x mod p.
 The generator and prime must be chosen so that solving the discrete
 log problem is intractable.  The group g should generate the
 multiplicative group mod p-1 or a large subgroup of it, and the order
 of g should have at least one large prime factor.  A good choice is
 to use a "strong" Sophie-Germain prime in choosing p, so that both p
 and (p-1)/2 are primes. In fact, this choice is so good that
 implementors SHOULD do it, as it avoids a small subgroup attack.
 In addition, a result of Bleichenbacher [BLEICHENBACHER] shows that
 if the generator g has only small prime factors, and if g divides the
 order of the group it generates, then signatures can be forged.  In
 particular, choosing g=2 is a bad choice if the group order may be
 even. On the other hand, a generator of 2 is a fine choice for an
 encryption-only key, as this will make the encryption faster.
 While verifying Elgamal signatures, note that it is important to test
 that r and s are less than p.  If this test is not done then
 signatures can be trivially forged by using large r values of
 approximately twice the length of p.  This attack is also discussed
 in the Bleichenbacher paper.
 Details on safe use of Elgamal signatures may be found in [MENEZES],
 which discusses all the weaknesses described above.
 If an implementation allows Elgamal signatures, then it MUST use the
 algorithm identifier 20 for an Elgamal public key that can sign.
 An implementation SHOULD NOT implement Elgamal keys of size less than
 768 bits. For long-term security, Elgamal keys should be 1024 bits or
 longer.

Callas, et. al. Standards Track [Page 57] RFC 2440 OpenPGP Message Format November 1998

12.6. DSA

 An implementation SHOULD NOT implement DSA keys of size less than 768
 bits. Note that present DSA is limited to a maximum of 1024 bit keys,
 which are recommended for long-term use.

12.7. 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 implementor from actually implementing the
 algorithm. These are marked in the Public Algorithms section 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.
 The reserved symmetric key algorithm, DES/SK (6), does not have
 semantics defined.
 The reserved hash algorithms, TIGER192 (6), and HAVAL-5-160 (7), do
 not have OIDs. The reserved algorithm number 4, reserved for a
 double-width variant of SHA1, is not presently defined.
 We have reserver three algorithm IDs for the US NIST's Advanced
 Encryption Standard. This algorithm will work with (at least) 128,
 192, and 256-bit keys. We expect that this algorithm will be selected
 from the candidate algorithms in the year 2000.

12.8. 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, but described in those sections above.
 OpenPGP CFB mode uses an initialization vector (IV) of all zeros, and
 prefixes the plaintext with ten octets of random data, such that
 octets 9 and 10 match octets 7 and 8.  It does a CFB "resync" after
 encrypting those ten octets.
 Note that for an algorithm that has a larger block size than 64 bits,
 the equivalent function will be done with that entire block.  For
 example, a 16-octet block algorithm would operate on 16 octets, and
 then produce two octets of check, and then work on 16-octet blocks.

Callas, et. al. Standards Track [Page 58] RFC 2440 OpenPGP Message Format November 1998

 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 8 octets of random data prefixed to
     the plaintext to produce C1-C8, the first 8 octets of ciphertext.
 4.  FR is loaded with C1-C8.
 5.  FR is encrypted to produce FRE, the encryption of the first 8
     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 C9-C10,
     the next two octets of ciphertext.
 7.  (The resync step) FR is loaded with C3-C10.
 8.  FR is encrypted to produce FRE.
 9.  FRE is xored with the first 8 octets of the given plaintext, now
     that we have finished encrypting the 10 octets of prefixed data.
     This produces C11-C18, the next 8 octets of ciphertext.
 10.  FR is loaded with C11-C18
 11.  FR is encrypted to produce FRE.
 12.  FRE is xored with the next 8 octets of plaintext, to produce the
     next 8 octets of ciphertext.  These are loaded into FR and the
     process is repeated until the plaintext is used up.

13. 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.
 Possession of the private key portion of a public-private key pair is
 assumed to be controlled 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 RFC 1750.

Callas, et. al. Standards Track [Page 59] RFC 2440 OpenPGP Message Format November 1998

 The MD5 hash algorithm has been found to have weaknesses (pseudo-
 collisions in the compress function) that make some people deprecate
 its use.  They consider the SHA-1 algorithm better.
 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
 implementor promote dual-use keys, you should at least be aware of
 this controversy.
 The DSA algorithm will work with any 160-bit hash, but it is
 sensitive to the quality of the hash algorithm, if the hash algorithm
 is broken, it can leak the secret key. The Digital Signature Standard
 (DSS) specifies that DSA be used with SHA-1.  RIPEMD-160 is
 considered by many cryptographers to be as strong. An implementation
 should take care which hash algorithms are used with DSA, as a weak
 hash can not only allow a signature to be forged, but could leak the
 secret key. These same considerations about the quality of the hash
 algorithm apply to Elgamal signatures.
 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 Triple-
 DES. Other algorithms may have other controversies surrounding them.
 Some technologies mentioned here may be subject to government control
 in some countries.

14. 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 list of potential problems
 and gotchas for a developer who is trying to be backward-compatible.
  • PGP 5.x does not accept V4 signatures for anything other than

key material.

  • PGP 5.x does not recognize the "five-octet" lengths in new-format

headers or in signature subpacket lengths.

Callas, et. al. Standards Track [Page 60] RFC 2440 OpenPGP Message Format November 1998

  • PGP 5.0 rejects an encrypted session key if the keylength differs

from the S2K symmetric algorithm. This is a bug in its validation

     function.
  • PGP 5.0 does not handle multiple one-pass signature headers and

trailers. Signing one will compress the one-pass signed literal

     and prefix a V3 signature instead of doing a nested one-pass
     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.
  • In a clear-signed signature, PGP 5.0 will figure out the correct

hash algorithm if there is no "Hash:" header, but it will reject

     a mismatch between the header and the actual algorithm used. The
     "standard" (i.e. Zimmermann/Finney/et al.) version of PGP 2.x
     rejects the "Hash:" header and assumes MD5. There are a number of
     enhanced variants of PGP 2.6.x that have been modified for SHA-1
     signatures.
  • PGP 5.0 can read an RSA key in V4 format, but can only recognize

it with a V3 keyid, and can properly use only a V3 format RSA

     key.
  • Neither PGP 5.x nor PGP 6.0 recognize Elgamal Encrypt and Sign

keys. They only handle Elgamal Encrypt-only keys.

  • 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.

Callas, et. al. Standards Track [Page 61] RFC 2440 OpenPGP Message Format November 1998

15. Authors and Working Group Chair

 The working group can be contacted via the current chair:
 John W. Noerenberg, II
 Qualcomm, Inc
 6455 Lusk Blvd
 San Diego, CA 92131 USA
 Phone: +1 619-658-3510
 EMail: jwn2@qualcomm.com
 The principal authors of this memo are:
 Jon Callas
 Network Associates, Inc.
 3965 Freedom Circle
 Santa Clara, CA 95054, USA
 Phone: +1 408-346-5860
 EMail: jon@pgp.com, jcallas@nai.com
 Lutz Donnerhacke
 IKS GmbH
 Wildenbruchstr. 15
 07745 Jena, Germany
 Phone: +49-3641-675642
 EMail: lutz@iks-jena.de
 Hal Finney
 Network Associates, Inc.
 3965 Freedom Circle
 Santa Clara, CA 95054, USA
 EMail: hal@pgp.com
 Rodney Thayer
 EIS Corporation
 Clearwater, FL 33767, USA
 EMail: rodney@unitran.com

Callas, et. al. Standards Track [Page 62] RFC 2440 OpenPGP Message Format November 1998

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

16. 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>
 [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>
 [DONNERHACKE]    Donnerhacke, L., et. al, "PGP263in - an improved
                  international version of PGP", ftp://ftp.iks-
                  jena.de/mitarb/lutz/crypt/software/pgp/
 [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.
 [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
 [ISO-10646]      ISO/IEC 10646-1:1993. International Standard --
                  Information technology -- Universal Multiple-Octet
                  Coded Character Set (UCS) -- Part 1: Architecture
                  and Basic Multilingual Plane.  UTF-8 is described in
                  Annex R, adopted but not yet published.  UTF-16 is
                  described in Annex Q, adopted but not yet published.
 [MENEZES]        Alfred Menezes, Paul van Oorschot, and Scott
                  Vanstone, "Handbook of Applied Cryptography," CRC
                  Press, 1996.

Callas, et. al. Standards Track [Page 63] RFC 2440 OpenPGP Message Format November 1998

 [RFC822]         Crocker, D., "Standard for the format of ARPA
                  Internet text messages", STD 11, RFC 822, August
                  1982.
 [RFC1423]        Balenson, D., "Privacy Enhancement for Internet
                  Electronic Mail: Part III: Algorithms, Modes, and
                  Identifiers", RFC 1423, October 1993.
 [RFC1641]        Goldsmith, D. and M. Davis, "Using Unicode with
                  MIME", RFC 1641, July 1994.
 [RFC1750]        Eastlake, D., Crocker, S. and J. Schiller,
                  "Randomness Recommendations for Security", RFC 1750,
                  December 1994.
 [RFC1951]        Deutsch, P., "DEFLATE Compressed Data Format
                  Specification version 1.3.", RFC 1951, May 1996.
 [RFC1983]        Malkin, G., "Internet Users' Glossary", FYI 18, RFC
                  1983, August 1996.
 [RFC1991]        Atkins, D., Stallings, W. and P. Zimmermann, "PGP
                  Message Exchange Formats", RFC 1991, August 1996.
 [RFC2015]        Elkins, M., "MIME Security with Pretty Good Privacy
                  (PGP)", RFC 2015, October 1996.
 [RFC2231]        Borenstein, N. and N. Freed, "Multipurpose Internet
                  Mail Extensions (MIME) Part One: Format of Internet
                  Message Bodies.", RFC 2231, November 1996.
 [RFC2119]        Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Level", BCP 14, RFC 2119, March 1997.
 [RFC2144]        Adams, C., "The CAST-128 Encryption Algorithm", RFC
                  2144, May 1997.
 [RFC2279]        Yergeau., F., "UTF-8, a transformation format of
                  Unicode and ISO 10646", RFC 2279, January 1998.
 [RFC2313]        Kaliski, B., "PKCS #1: RSA Encryption Standard
                  version 1.5", RFC 2313, March 1998.
 [SAFER]          Massey, J.L. "SAFER K-64: One Year Later", B.
                  Preneel, editor, Fast Software Encryption, Second
                  International Workshop (LNCS 1008) pp212-241,
                  Springer-Verlag 1995

Callas, et. al. Standards Track [Page 64] RFC 2440 OpenPGP Message Format November 1998

17. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Callas, et. al. Standards Track [Page 65]

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