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rfc:rfc1423

Network Working Group D. Balenson Request for Comments: 1423 TIS Obsoletes: 1115 IAB IRTF PSRG, IETF PEM WG

                                                         February 1993
         Privacy Enhancement for Internet Electronic Mail:
            Part III: Algorithms, Modes, and Identifiers

Status of This Memo

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

Abstract

 This document provides definitions, formats, references, and
 citations for cryptographic algorithms, usage modes, and associated
 identifiers and parameters used in support of Privacy Enhanced Mail
 (PEM) in the Internet community.  It is intended to become one member
 of the set of related PEM RFCs.  This document is organized into four
 primary sections, dealing with message encryption algorithms, message
 integrity check algorithms, symmetric key management algorithms, and
 asymmetric key management algorithms (including both asymmetric
 encryption and asymmetric signature algorithms).
 Some parts of this material are cited by other documents and it is
 anticipated that some of the material herein may be changed, added,
 or replaced without affecting the citing documents.  Therefore,
 algorithm-specific material has been placed into this separate
 document.
 Use of other algorithms and/or modes will require case-by-case study
 to determine applicability and constraints.  The use of additional
 algorithms may be documented first in Prototype or Experimental RFCs.
 As experience is gained, these protocols may be considered for
 incorporation into the standard.  Additional algorithms and modes
 approved for use in PEM in this context will be specified in
 successors to this document.

Acknowledgments

 This specification was initially developed by the Internet Research
 Task Force's Privacy and Security Research Group (IRTF PSRG) and
 subsequently refined based on discussion in the Internet Engineering

Balenson [Page 1] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 Task Force's Privacy Enhanced Mail Working Group (IETF PEM WG).  John
 Linn contributed significantly to the predecessor of this document
 (RFC 1115).  I would like to thank the members of the PSRG and PEM
 WG, as well as all participants in discussions on the "pem-
 dev@tis.com" mailing list, for their contributions to this document.

Table of Contents

    1.  Message Encryption Algorithms ....................... 2
    1.1  DES in CBC Mode (DES-CBC) .......................... 2
    2.  Message Integrity Check Algorithms .................. 4
    2.1  RSA-MD2 Message Digest Algorithm ................... 4
    2.2  RSA-MD5 Message Digest Algorithm ................... 5
    3.  Symmetric Key Management Algorithms ................. 6
    3.1  DES in ECB mode (DES-ECB) .......................... 6
    3.2  DES in EDE mode (DES-EDE) .......................... 7
    4.  Asymmetric Key Management Algorithms ................ 7
    4.1  Asymmetric Keys .................................... 7
    4.1.1  RSA Keys ......................................... 7
    4.2  Asymmetric Encryption Algorithms ..................  9
    4.2.1  RSAEncryption ...................................  9
    4.3  Asymmetric Signature Algorithms ................... 10
    4.3.1  md2WithRSAEncryption ............................ 11
    5.  Descriptive Grammar ................................ 11
    References ............................................. 12
    Patent Statement ....................................... 13
    Security Considerations ................................ 14
    Author's Address ....................................... 14

1. Message Encryption Algorithms

 This section identifies the alternative message encryption algorithms
 and modes that shall be used to encrypt message text and, when
 asymmetric key management is employed in an ENCRYPTED PEM message, for
 encryption of message signatures.  Character string identifiers are
 assigned and any parameters required by the message encryption
 algorithm are defined for incorporation in an encapsulated "DEK-
 Info:" header field.
 Only one alternative is currently defined in this category.

1.1 DES in CBC Mode (DES-CBC)

 Message text and, if required, message signatures are encrypted using
 the Data Encryption Standard (DES) algorithm in the Cipher Block
 Chaining (CBC) mode of operation.  The DES algorithm is defined in
 FIPS PUB 46-1 [1], and is equivalent to the Data Encryption Algorithm
 (DEA) provided in ANSI X3.92-1981 [2].  The CBC mode of operation of

Balenson [Page 2] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 DES is defined in FIPS PUB 81 [3], and is equivalent to those
 provided in ANSI X3.106 [4] and in ISO IS 8372 [5].  The character
 string "DES-CBC" within an encapsulated PEM header field indicates
 the use of this algorithm/mode combination.
 The input to the DES CBC encryption process shall be padded to a
 multiple of 8 octets, in the following manner.  Let n be the length
 in octets of the input.  Pad the input by appending 8-(n mod 8)
 octets to the end of the message, each having the value 8-(n mod 8),
 the number of octets being added.  In hexadecimal, the possible
 paddings are:  01, 0202, 030303, 04040404, 0505050505, 060606060606,
 07070707070707, and 0808080808080808.  All input is padded with 1 to
 8 octets to produce a multiple of 8 octets in length.  The padding
 can be removed unambiguously after decryption.
 The DES CBC encryption process requires a 64-bit cryptographic key.
 A new, pseudorandom key shall be generated for each ENCRYPTED PEM
 message.  Of the 64 bits, 56 are used directly by the DES CBC
 process, and 8 are odd parity bits, with one parity bit occupying the
 right-most bit of each octet.  When symmetric key management is
 employed, the setting and checking of odd parity bits is encouraged,
 since these bits could detect an error in the decryption of a DES key
 encrypted under a symmetric key management algorithm (e.g., DES ECB).
 When asymmetric key management is employed, the setting of odd parity
 bits is encouraged, but the checking of odd parity bits is
 discouraged, in order to facilitate interoperability, and since an
 error in the decryption of a DES key can be detected by other means
 (e.g., an incorrect PKCS #1 encryption-block format).  In all cases,
 the encrypted form of a DES key shall carry all 64 bits of the key,
 including the 8 parity bits, though those bits may have no meaning.
 The DES CBC encryption process also requires a 64-bit Initialization
 Vector (IV).  A new, pseudorandom IV shall be generated for each
 ENCRYPTED PEM message.  Section 4.3.1 of [7] provides rationale for
 this requirement, even given the fact that individual DES keys are
 generated for individual messages.  The IV is transmitted with the
 message within an encapsulated PEM header field.
 When this algorithm/mode combination is used for message text
 encryption, the "DEK-Info:" header field carries exactly two
 arguments.  The first argument identifies the DES CBC algorithm/mode
 using the character string defined above.  The second argument
 contains the IV, represented as a contiguous string of 16 ASCII
 hexadecimal digits.
 When symmetric key management is employed with this algorithm/mode
 combination, a symmetrically encrypted DES key will be represented in
 the third argument of a "Key-Info:" header field as a contiguous

Balenson [Page 3] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 string of 16 ASCII hexadecimal digits (corresponding to a 64-bit
 key).
 To avoid any potential ambiguity regarding the ordering of the octets
 of a DES key that is input as a data value to another encryption
 process (e.g., RSAEncryption), the following holds true.  The first
 (or left-most displayed, if one thinks in terms of a key's "print"
 representation) (For purposes of discussion in this document, data
 values are normalized in terms of their "print" representation.  For a
 octet stream, the "first" octet would appear as the one on the "left",
 and the "last" octet would appear on the "right".) octet of the key
 (i.e., bits 1-8 per FIPS PUB 46-1), when considered as a data value,
 has numerical weight 2**56.  The last (or right-most displayed) octet
 (i.e., bits 57-64 per FIPS PUB 46-1) has numerical weight 2**0.

2. Message Integrity Check Algorithms

 This section identifies the alternative algorithms that shall be used
 to compute Message Integrity Check (MIC) values for PEM messages.
 Character string identifiers and ASN.1 object identifiers are
 assigned for incorporation in encapsulated "MIC-Info:" and "Key-
 Info:" header fields to indicate the choice of MIC algorithm
 employed.
 A compliant PEM implementation shall be able to process all of the
 alternative MIC algorithms defined here on incoming messages.  It is
 a sender option as to which alternative is employed on an outbound
 message.

2.1 RSA-MD2 Message Digest Algorithm

 The RSA-MD2 message digest is computed using the algorithm defined in
 RFC 1319 [9].  ( An error has been identified in RFC 1319.  The
 statement in the text of Section 3.2 which reads "Set C[j] to S[c xor
 L]" should read "Set C[j] to S[c xor L] xor C[j]".  Note that the C
 source code in the appendix of RFC 1319 is correct.)  The character
 string "RSA-MD2" within an encapsulated PEM header field indicates the
 use of this algorithm.  Also, as defined in RFC 1319, the ASN.1 object
 identifier
   md2 OBJECT IDENTIFIER ::= {
       iso(1) member-body(2) US(840) rsadsi(113549)
       digestAlgorithm(2) 2
   }
 identifies this algorithm.  When this object identifier is used with
 the ASN.1 type AlgorithmIdentifier, the parameters component of that
 type is the ASN.1 type NULL.

Balenson [Page 4] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 The RSA-MD2 message digest algorithm accepts as input a message of
 any length and produces as output a 16-octet quantity.  When
 symmetric key management is employed, an RSA-MD2 MIC is encrypted by
 splitting the MIC into two 8-octet halves, independently encrypting
 each half, and concatenating the results.
 When symmetric key management is employed with this MIC algorithm,
 the symmetrically encrypted MD2 message digest is represented in a
 the fourth argument of a "Key-Info:" header field as a contiguous
 string of 32 ASCII hexadecimal digits (corresponding to a 128-bit MD2
 message digest).
 To avoid any potential ambiguity regarding the ordering of the octets
 of an MD2 message digest that is input as a data value to another
 encryption process (e.g., RSAEncryption), the following holds true.
 The first (or left-most displayed, if one thinks in terms of a
 digest's "print" representation) octet of the digest (i.e., digest[0]
 as specified in RFC 1319), when considered as an RSA data value, has
 numerical weight 2**120.  The last (or right-most displayed) octet
 (i.e., digest[15] as specified in RFC 1319) has numerical weight
 2**0.

2.2 RSA-MD5 Message Digest Algorithm

 The RSA-MD5 message digest is computed using the algorithm defined in
 RFC 1321 [10].  The character string "RSA-MD5" within an encapsulated
 PEM header field indicates the use of this algorithm.  Also, as
 defined in RFC 1321, the object identifier
   md5 OBJECT IDENTIFIER ::= {
       iso(1) member-body(2) US(840) rsadsi(113549)
       digestAlgorithm(2) 5
   }
 identifies this algorithm.  When this object identifier is used with
 the ASN.1 type AlgorithmIdentifier, the parameters component of that
 type is the ASN.1 type NULL.
 The RSA-MD5 message digest algorithm accepts as input a message of
 any length and produces as output a 16-octet quantity.  When
 symmetric key management is employed, an RSA-MD5 MIC is encrypted by
 splitting the MIC into two 8-octet halves, independently encrypting
 each half, and concatenating the results.
 When symmetric key management is employed with this MIC algorithm,
 the symmetrically encrypted MD5 message digest is represented in the
 fourth argument of a "Key-Info:" header field as a contiguous string
 of 32 ASCII hexadecimal digits (corresponding to a 128-bit MD5

Balenson [Page 5] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 message digest).
 To avoid any potential ambiguity regarding the ordering of the octets
 of a MD5 message digest that is input as an RSA data value to the RSA
 encryption process, the following holds true.  The first (or left-
 most displayed, if one thinks in terms of a digest's "print"
 representation) octet of the digest (i.e., the low-order octet of A
 as specified in RFC 1321), when considered as an RSA data value, has
 numerical weight 2**120.  The last (or right-most displayed) octet
 (i.e., the high-order octet of D as specified in RFC 1321) has
 numerical weight 2**0.

3. Symmetric Key Management Algorithms

 This section identifies the alternative algorithms and modes that
 shall be used when symmetric key management is employed, to encrypt
 data encryption keys (DEKs) and message integrity check (MIC) values.
 Character string identifiers are assigned for incorporation in
 encapsulated "Key-Info:" header fields to indicate the choice of
 algorithm employed.
 All alternatives presently defined in this category correspond to
 different usage modes of the DES algorithm, rather than to other
 algorithms.
 When symmetric key management is employed, the symmetrically
 encrypted DEK and MIC, carried in the third and fourth arguments of a
 "Key-Info:" header field, respectively, are each represented as a
 string of contiguous ASCII hexadecimal digits.  The manner in which
 to use the following symmetric encryption algorithms and the length
 of the symmetrically encrypted DEK and MIC may vary depending on the
 length of the underlying DEK and MIC.  Section 1, Message Encryption
 Algorithms, and Section 2, Message Integrity Check Algorithms,
 provide information on the proper manner in which a DEK and MIC,
 respectively, are symmetrically encrypted when the size of the DEK or
 MIC is not equal to the symmetric encryption algorithm's input block
 size.  These sections also provide information on the proper format
 and length of the symmetrically encrypted DEK and MIC, respectively.

3.1 DES in ECB Mode (DES-ECB)

 The DES algorithm in Electronic Codebook (ECB) mode [1][3] is used
 for DEK and MIC encryption when symmetric key management is employed.
 The character string "DES-ECB" within an encapsulated PEM header
 field indicates use of this algorithm/mode combination.
 A compliant PEM implementation supporting symmetric key management
 shall support this algorithm/mode combination.

Balenson [Page 6] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

3.2 DES in EDE Mode (DES-EDE)

 The DES algorithm in Encrypt-Decrypt-Encrypt (EDE) multiple
 encryption mode, as defined by ANSI X9.17 [6] for encryption and
 decryption with pairs of 64-bit keys, may be used for DEK and MIC
 encryption when symmetric key management is employed.  The character
 string "DES-EDE" within an encapsulated a PEM header field indicates
 use of this algorithm/mode combination.
 A compliant PEM implementation supporting symmetric key management
 may optionally support this algorithm/mode combination.

4. Asymmetric Key Management Algorithms

 This section identifies the alternative asymmetric keys and the
 alternative asymmetric key management algorithms with which those
 keys shall be used, namely the asymmetric encryption algorithms with
 which DEKs and MICs are encrypted, and the asymmetric signature
 algorithms with which certificates and certificate revocation lists
 (CRLs) are signed.

4.1 Asymmetric Keys

 This section describes the asymmetric keys that shall be used with
 the asymmetric encryption algorithms and the signature algorithms
 described later.  ASN.1 object identifiers are identified for
 incorporation in a public-key certificate to identify the
 algorithm(s) with which the accompanying public key is to be
 employed.

4.1.1 RSA Keys

 An RSA asymmetric key pair is comprised of matching public and
 private keys.
 An RSA public key consists of an encryption exponent e and an
 arithmetic modulus n, which are both public quantities typically
 carried in a public-key certificate.  For the value of e, Annex C to
 X.509 suggests the use of Fermat's Number F4 (65537 decimal, or
 1+2**16) as a value "common to the whole environment in order to
 reduce transmission capacity and complexity of transformation", i.e.,
 the value can be transmitted as 3 octets and at most seventeen (17)
 multiplications are required to effect exponentiation.  As an
 alternative, the number three (3) can be employed as the value for e,
 requiring even less octets for transmission and yielding even faster
 exponentiation.  For purposes of PEM, the value of e shall be either
 F4 or the number three (3).  The use of the number three (3) for the
 value of e is encouraged, to permit rapid certificate validation.

Balenson [Page 7] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 An RSA private key consists of a decryption exponent d, which should
 be kept secret, and the arithmetic modulus n.  Other values may be
 stored with a private key to facilitate efficient private key
 operations (see PKCS #1 [11]).
 For purposes of PEM, the modulus n may vary in size from 508 to 1024
 bits.
 Two ASN.1 object identifiers have been defined to identify RSA public
 keys.  In Annex H of X.509 [8], the object identifier
   rsa OBJECT IDENTIFIER ::= {
       joint-iso-ccitt(2) ds(5) algorithm(8)
       encryptionAlgorithm(1) 1
   }
 is defined to identify an RSA public key.  A single parameter,
 KeySize, the length of the public key modulus in bits, is defined for
 use in conjunction with this object identifier.  When this object
 identifier is used with the ASN.1 type AlgorithmIdentifier, the
 parameters component of that type is the number of bits in the
 modulus, ASN.1 encoded as an INTEGER.
 Alternatively, in PKCS #1 [11], the ASN.1 object identifier
   rsaEncryption OBJECT IDENTIFIER ::= {
       iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
       pkcs-1(1) 1
   }
 is defined to identify both an RSA public key and the RSAEncryption
 process.  There are no parameters defined in conjunction with this
 object identifier, hence, when it is used with the ASN.1 type
 AlgorithmIdentifier, the parameters component of that type is the
 ASN.1 type NULL.
 A compliant PEM implementation may optionally generate an RSA
 public-key certificate that identifies the enclosed RSA public key
 (within the SubjectPublicKeyInformation component) with either the
 "rsa" or the "rsaEncryption" object identifier.  Use of the "rsa"
 object identifier is encouraged, since it is, in some sense, more
 generic in its identification of a key, without indicating how the
 key will be used.  However, to facilitate interoperability, a
 compliant PEM implementation shall accept RSA public-key certificates
 that identify the enclosed RSA public key with either the "rsa" or
 the "rsaEncryption" object identifier.  In all cases, an RSA public
 key identified in an RSA public-key certificate with either the "rsa"
 or "rsaEncryption" object identifier, shall be used according to the

Balenson [Page 8] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 procedures defined below for asymmetric encryption algorithms and
 asymmetric signature algorithms.

4.2 Asymmetric Encryption Algorithms

 This section identifies the alternative algorithms that shall be used
 when asymmetric key management is employed, to encrypt DEKs and MICs.
 Character string identifiers are assigned for incorporation in "MIC-
 Info:" and "Key-Info:" header fields to indicate the choice of
 algorithm employed.
 Only one alternative is presently defined in this category.

4.2.1 RSAEncryption

 The RSAEncryption public-key encryption algorithm, defined in PKCS #1
 [11], is used for DEK and MIC encryption when asymmetric key
 management is employed.  The character string "RSA" within a "MIC-
 Info:" or "Key-Info:" header field indicates the use of this
 algorithm.
 All PEM implementations supporting asymmetric key management shall
 support this algorithm.
 As described in PKCS #1, all quantities input as data values to the
 RSAEncryption process shall be properly justified and padded to the
 length of the modulus prior to the encryption process.  In general,
 an RSAEncryption input value is formed by concatenating a leading
 NULL octet, a block type BT, a padding string PS, a NULL octet, and
 the data quantity D, that is,
   RSA input value = 0x00 || BT || PS || 0x00 || D.
 To prepare a DEK for RSAEncryption, the PKCS #1 "block type 02"
 encryption-block formatting scheme is employed.  The block type BT is
 a single octet containing the value 0x02 and the padding string PS is
 one or more octets (enough octets to make the length of the complete
 RSA input value equal to the length of the modulus) each containing a
 pseudorandomly generated, non-zero value.  For multiple recipient
 messages, a different, pseudorandom padding string should be used for
 each recipient.  The data quantity D is the DEK itself, which is
 right-justified within the RSA input such that the last (or rightmost
 displayed, if one thinks in terms of the "print" representation)
 octet of the DEK is aligned with the right-most, or least-
 significant, octet of the RSA input.  Proceeding to the left, each of
 the remaining octets of the DEK, up through the first (or left-most
 displayed) octet, are each aligned in the next more significant octet
 of the RSA input.

Balenson [Page 9] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 To prepare a MIC for RSAEncryption, the PKCS #1 "block type 01"
 encryption-block formatting scheme is employed.  The block type BT is
 a single octet containing the value 0x01 and the padding string PS is
 one or more octets (enough octets to make the length of the complete
 RSA input value equal to the length of the modulus) each containing
 the value 0xFF.  The data quantity D is comprised of the MIC and the
 MIC algorithm identifier which are ASN.1 encoded as the following
 sequence.
   SEQUENCE {
       digestAlgorithm   AlgorithmIdentifier,
       digest            OCTET STRING
   }
 The ASN.1 type AlgorithmIdentifier is defined in X.509 as follows.
   AlgorithmIdentifier ::= SEQUENCE {
       algorithm         OBJECT IDENTIFIER,
       parameters        ANY DEFINED BY algorithm OPTIONAL
   }
 An RSA input block is encrypted using the RSA algorithm with the
 first (or left-most) octet taken as the most significant octet, and
 the last (or right-most) octet taken as the least significant octet.
 The resulting RSA output block is interpreted in a similar manner.
 When RSAEncryption is used to encrypt a DEK, the second argument in a
 "MIC-Info:" header field, an asymmetrically encrypted DEK, is
 represented using the printable encoding technique defined in Section
 4.3.2.4 of RFC 1421 [12].
 When RSAEncryption is used to sign a MIC, the third argument in a
 "MIC-Info:" header field, an asymmetrically signed MIC, is
 represented using the printable encoding technique defined in Section
 4.3.2.4 of RFC 1421.

4.3 Asymmetric Signature Algorithms

 This section identifies the alternative algorithms which shall be
 used to asymmetrically sign certificates and certificate revocation
 lists (CRLs) in accordance with the SIGNED macro defined in Annex G
 of X.509.  ASN.1 object identifiers are identified for incorporation
 in certificates and CRLs to indicate the choice of algorithm
 employed.
 Only one alternative is presently defined in this category.

Balenson [Page 10] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

4.3.1 md2WithRSAEncryption

 The md2WithRSAEncryption signature algorithm is used to sign
 certificates and CRLs.  The algorithm is defined in PKCS #1 [11].  It
 combines the RSA-MD2 message digest algorithm described here in
 Section 2.2 with the RSAEncryption asymmetric encryption algorithm
 described here in Section 4.2.1.  As defined in PKCS #1, the ASN.1
 object identifier
   md2WithRSAEncryption OBJECT IDENTIFIER ::= {
       iso(1) member-body(2) US(840) rsadsi(113549) pkcs(1)
       pkcs-1(1) 2
   }
 identifies this algorithm.  When this object identifier is used with
 the ASN.1 type AlgorithmIdentifier, the parameters component of that
 type is the ASN.1 type NULL.
 There is some ambiguity in X.509 regarding the definition of the
 SIGNED macro and, in particular, the representation of a signature in
 a certificate or a CRL.  The interpretation selected for PEM requires
 that the data to be signed (in our case, an MD2 message digest) is
 first ASN.1 encoded as an OCTET STRING and the result is encrypted
 (in our case, using RSAEncryption) to form the signed quantity, which
 is then ASN.1 encoded as a BIT STRING.

5. Descriptive Grammar

 ; Addendum to PEM BNF representation, using RFC 822 notation
 ; Provides specification for official PEM cryptographic algorithms,
 ; modes, identifiers and formats.
 ; Imports <hexchar> and <encbin> from RFC [1421]
     <dekalgid> ::= "DES-CBC"
     <ikalgid>  ::= "DES-EDE" / "DES-ECB" / "RSA"
     <sigalgid> ::= "RSA"
     <micalgid> ::= "RSA-MD2" / "RSA-MD5"
     <dekparameters> ::= <DESCBCparameters>
     <DESCBCparameters> ::= <IV>
     <IV> ::= <hexchar16>
     <symencdek> ::= <DESECBencDESCBC> / <DESEDEencDESCBC>
     <DESECBencDESCBC> ::= <hexchar16>
     <DESEDEencDESCBC> ::= <hexchar16>
     <symencmic> ::= <DESECBencRSAMD2> / <DESECBencRSAMD5>

Balenson [Page 11] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

     <DESECBencRSAMD2> ::= 2*2<hexchar16>
     <DESECBencRSAMD5> ::= 2*2<hexchar16>
     <asymsignmic> ::= <RSAsignmic>
     <RSAsignmic> ::= <encbin>
     <asymencdek> ::= <RSAencdek>
     <RSAencdek> ::= <encbin>
     <hexchar16> ::= 16*16<hexchar>

References

 [1] Federal Information Processing Standards Publication (FIPS PUB)
     46-1, Data Encryption Standard, Reaffirmed 1988 January 22
     (supersedes FIPS PUB 46, 1977 January 15).
 [2] ANSI X3.92-1981, American National Standard Data Encryption
     Algorithm, American National Standards Institute, Approved 30
     December 1980.
 [3] Federal Information Processing Standards Publication (FIPS PUB)
     81, DES Modes of Operation, 1980 December 2.
 [4] ANSI X3.106-1983, American National Standard for Information
     Systems - Data Encryption Algorithm - Modes of Operation,
     American National Standards Institute, Approved 16 May 1983.
 [5] ISO 8372, Information Processing Systems: Data Encipherment:
     Modes of Operation of a 64-bit Block Cipher.
 [6] ANSI X9.17-1985, American National Standard, Financial
     Institution Key Management (Wholesale), American Bankers
     Association, April 4, 1985, Section 7.2.
 [7] Voydock, V. L. and Kent, S. T., "Security Mechanisms in High-
     Level Network Protocols", ACM Computing Surveys, Vol. 15, No. 2,
     June 1983, pp. 135-171.
 [8] CCITT Recommendation X.509, "The Directory - Authentication
     Framework", November 1988, (Developed in collaboration, and
     technically aligned, with ISO 9594-8).
 [9] Kaliski, B., "The MD2 Message-Digest Algorithm", RFC 1319, RSA
     Laboratories, April 1992.
[10] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, MIT
     Laboratory for Computer Science and RSA Data Security, Inc.,

Balenson [Page 12] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

     April 1992.
[11] PKCS #1: RSA Encryption Standard, Version 1.4, RSA Data Security,
     Inc., June 3, 1991.
[12] Linn, J., "Privacy Enhancement for Internet Electronic Mail: Part
     I: Message Encryption and Authentication Procedures", RFC 1421,
     DEC, February 1993.
[13] Kent, S., "Privacy Enhancement for Internet Electronic Mail: Part
     II: Certificate-Based Key Management", RFC 1422, BBN, February
     1993.
[14] Kaliski, B., "Privacy Enhancement for Internet Electronic Mail:
     Part IV: Key Certification and Related Services", RFC 1424, RSA
     Laboratories, February 1993.

Patent Statement

 This version of Privacy Enhanced Mail (PEM) relies on the use of
 patented public key encryption technology for authentication and
 encryption.  The Internet Standards Process as defined in RFC 1310
 requires a written statement from the Patent holder that a license
 will be made available to applicants under reasonable terms and
 conditions prior to approving a specification as a Proposed, Draft or
 Internet Standard.
 The Massachusetts Institute of Technology and the Board of Trustees
 of the Leland Stanford Junior University have granted Public Key
 Partners (PKP) exclusive sub-licensing rights to the following
 patents issued in the United States, and all of their corresponding
 foreign patents:
    Cryptographic Apparatus and Method
    ("Diffie-Hellman")............................... No. 4,200,770
    Public Key Cryptographic Apparatus
    and Method ("Hellman-Merkle").................... No. 4,218,582
    Cryptographic Communications System and
    Method ("RSA")................................... No. 4,405,829
    Exponential Cryptographic Apparatus
    and Method ("Hellman-Pohlig").................... No. 4,424,414
 These patents are stated by PKP to cover all known methods of
 practicing the art of Public Key encryption, including the variations
 collectively known as El Gamal.

Balenson [Page 13] RFC 1423 PEM: Algorithms, Modes and Identifiers February 1993

 Public Key Partners has provided written assurance to the Internet
 Society that parties will be able to obtain, under reasonable,
 nondiscriminatory terms, the right to use the technology covered by
 these patents.  This assurance is documented in RFC 1170 titled
 "Public Key Standards and Licenses".  A copy of the written assurance
 dated April 20, 1990, may be obtained from the Internet Assigned
 Number Authority (IANA).
 The Internet Society, Internet Architecture Board, Internet
 Engineering Steering Group and the Corporation for National Research
 Initiatives take no position on the validity or scope of the patents
 and patent applications, nor on the appropriateness of the terms of
 the assurance.  The Internet Society and other groups mentioned above
 have not made any determination as to any other intellectual property
 rights which may apply to the practice of this standard. Any further
 consideration of these matters is the user's own responsibility.

Security Considerations

 This entire document is about security.

Author's Address

 David Balenson
 Trusted Information Systems
 3060 Washington Road
 Glenwood, Maryland 21738
 Phone: 301-854-6889
 EMail: balenson@tis.com

Balenson [Page 14]

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