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rfc:rfc2523

Network Working Group P. Karn Request for Comments: 2523 Qualcomm Category: Experimental W. Simpson

                                                            DayDreamer
                                                            March 1999
             Photuris: Extended Schemes and Attributes

Status of this Memo

 This document defines an Experimental Protocol for the Internet
 community.  It does not specify an Internet standard of any kind.
 Discussion and suggestions for improvement are requested.
 Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1999).  Copyright (C) Philip Karn
 and William Allen Simpson (1994-1999).  All Rights Reserved.

Abstract

 Photuris is a session-key management protocol.  Extensible Exchange-
 Schemes are provided to enable future implementation changes without
 affecting the basic protocol.
 Additional authentication attributes are included for use with the IP
 Authentication Header (AH) or the IP Encapsulating Security Protocol
 (ESP).
 Additional confidentiality attributes are included for use with ESP.

Karn & Simpson Experimental [Page i] RFC 2523 Schemes and Attributes March 1999

Table of Contents

   1.     Additional Exchange-Schemes ...........................    1
   2.     Additional Key-Generation-Function ....................    5
      2.1       SHA1 Hash .......................................    5
   3.     Additional Privacy-Methods ............................    5
      3.1       DES-CBC over Mask ...............................    5
      3.2       DES-EDE3-CBC over Mask ..........................    6
   4.     Additional Validity-Method ............................    6
      4.1       SHA1-IPMAC Check ................................    6
   5.     Additional Attributes .................................    7
      5.1       SHA1-IPMAC ......................................    7
         5.1.1  Symmetric Identification ........................    8
         5.1.2  Authentication ..................................    9
      5.2       RIPEMD-160-IPMAC ................................    9
         5.2.1  Symmetric Identification ........................   10
         5.2.2  Authentication ..................................   11
      5.3       DES-CBC .........................................   11
      5.4       Invert (Decryption/Encryption) ..................   12
      5.5       XOR Whitening ...................................   13
   APPENDICES ...................................................   15
   A.     Exchange-Scheme Selection .............................   15
      A.1       Responder .......................................   15
      A.2       Initiator .......................................   15
   SECURITY CONSIDERATIONS ......................................   16
   ACKNOWLEDGEMENTS .............................................   16
   REFERENCES ...................................................   17
   CONTACTS .....................................................   18
   COPYRIGHT ....................................................   19

Karn & Simpson Experimental [Page ii] RFC 2523 Schemes and Attributes March 1999

1. Additional Exchange-Schemes

 The packet format and basic facilities are already defined for
 Photuris [RFC-2522].
 These optional Exchange-Schemes are specified separately, and no
 single implementation is expected to support all of them.
 This document defines the following values:
 (3)   Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 3.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       An Exchange-Scheme Size of zero is invalid.
       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "Simple Masking"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
 (4)   Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 2.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Scheme #2.
       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "DES-CBC over Mask"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
 (5)   Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 5.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       An Exchange-Scheme Size of zero is invalid.

Karn & Simpson Experimental [Page 1] RFC 2523 Schemes and Attributes March 1999

       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "Simple Masking"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
 (6)   Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 3.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Scheme #3.
       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "DES-CBC over Mask"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
 (7)   Implementation Optional.  Any modulus (p) with a variable
       generator (g).  When the Exchange-Scheme Size is non-zero, the
       pair [g,p] is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.  Each is encoded in a separate
       Variable Precision Integer (VPI).  The generator VPI is
       followed by (concatenated to) the modulus VPI, and the result
       is nested inside the Exchange-Scheme Value field.
       An Exchange-Scheme Size of zero is invalid.
       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "Simple Masking"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
       When more than one modulus is specified for a given kind of
       Scheme, the Size of the modulus MUST be unique, independent of
       the Size of the generator.
 (8)   Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 2.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in

Karn & Simpson Experimental [Page 2] RFC 2523 Schemes and Attributes March 1999

       the list of Offered-Schemes.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Schemes #2 and #4.
       Key-Generation-Function     "SHA1 Hash"
       Privacy-Method              "DES-EDE3-CBC over Mask"
       Validity-Method             "SHA1-IPMAC Check"
       This combination of features requires a modulus with at least
       112-bits of cryptographic strength.
 (10)  Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 5.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Scheme #5.
       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "DES-CBC over Mask"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
 (12)  Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 3.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Schemes #3 and #6.
       Key-Generation-Function     "SHA1 Hash"
       Privacy-Method              "DES-EDE3-CBC over Mask"
       Validity-Method             "SHA1-IPMAC Check"
       This combination of features requires a modulus with at least
       112-bits of cryptographic strength.
 (14)  Implementation Optional.  Any modulus (p) with a variable
       generator (g).  When the Exchange-Scheme Size is non-zero, the
       pair [g,p] is contained in the Exchange-Scheme Value field in

Karn & Simpson Experimental [Page 3] RFC 2523 Schemes and Attributes March 1999

       the list of Offered-Schemes.  Each is encoded in a separate
       Variable Precision Integer (VPI).  The generator VPI is
       followed by (concatenated to) the modulus VPI, and the result
       is nested inside the Exchange-Scheme Value field.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Scheme #7.
       Key-Generation-Function     "MD5 Hash"
       Privacy-Method              "DES-CBC over Mask"
       Validity-Method             "MD5-IPMAC Check"
       This combination of features requires a modulus with at least
       64-bits of cryptographic strength.
       When more than one modulus is specified for a given kind of
       Scheme, the Size of the modulus MUST be unique, independent of
       the Size of the generator.
 (20)  Implementation Optional.  Any modulus (p) with a recommended
       generator (g) of 5.  When the Exchange-Scheme Size is non-zero,
       the modulus is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Schemes #5 and #10.
       Key-Generation-Function     "SHA1 Hash"
       Privacy-Method              "DES-EDE3-CBC over Mask"
       Validity-Method             "SHA1-IPMAC Check"
       This combination of features requires a modulus with at least
       112-bits of cryptographic strength.
 (28)  Implementation Optional.  Any modulus (p) with a variable
       generator (g).  When the Exchange-Scheme Size is non-zero, the
       pair [g,p] is contained in the Exchange-Scheme Value field in
       the list of Offered-Schemes.  Each is encoded in a separate
       Variable Precision Integer (VPI).  The generator VPI is
       followed by (concatenated to) the modulus VPI, and the result
       is nested inside the Exchange-Scheme Value field.
       When the Exchange-Scheme Size field is zero, includes by
       reference all of the moduli specified in the list of Offered-
       Schemes for Schemes #7 and #14.

Karn & Simpson Experimental [Page 4] RFC 2523 Schemes and Attributes March 1999

       Key-Generation-Function     "SHA1 Hash"
       Privacy-Method              "DES-EDE3-CBC over Mask"
       Validity-Method             "SHA1-IPMAC Check"
       This combination of features requires a modulus with at least
       112-bits of cryptographic strength.
       When more than one modulus is specified for a given kind of
       Scheme, the Size of the modulus MUST be unique, independent of
       the Size of the generator.

2. Additional Key-Generation-Function 2.1. SHA1 Hash

 SHA1 [FIPS-180-1] is used as a pseudo-random-function for generating
 the key(s).  The key(s) begin with the most significant bits of the
 hash.  SHA1 is iterated as needed to generate the requisite length of
 key material.
 When an individual key does not use all 160-bits of the last hash,
 any remaining unused (least significant) bits of the last hash are
 discarded.  When combined with other uses of key generation for the
 same purpose, the next key will begin with a new hash iteration.

3. Additional Privacy-Methods 3.1. DES-CBC over Mask

 As described in [RFC-2522] "Privacy-Key Computation", sufficient
 privacy-key material is generated to match the message length,
 beginning with the next field after the SPI, and including the
 Padding.  The message is masked by XOR with the privacy-key.
 Then, the Key-Generation-Function is iterated to generate a DES key.
 The most significant 64-bits (8 bytes) of the generated hash are used
 for the privacy-key, and the remainder are discarded.  Although
 extremely rare, the 64 weak, semi-weak, and possibly weak keys
 [Schneier95, pages 280-282] are discarded.  The Key-Generation-
 Function is iterated until a valid key is obtained.
 The least significant bit of each key byte is ignored (or set to
 parity when the implementation requires).
 The 64-bit CBC IV is zero.  Message encryption begins with the next
 field after the SPI, and continues to the end of the data indicated

Karn & Simpson Experimental [Page 5] RFC 2523 Schemes and Attributes March 1999

 by the UDP Length.

3.2. DES-EDE3-CBC over Mask

 This is "Triple DES" outer-CBC EDE encryption (and DED decryption)
 with three 56-bit keys [KR96].
 As described in [RFC-2522] "Privacy-Key Computation", sufficient
 privacy-key material is generated to match the message length,
 beginning with the next field after the SPI, and including the
 Padding.  The message is masked by XOR with the privacy-key.
 Then, the Key-Generation-Function is iterated (at least) three times
 to generate the three DES keys.  The most significant 64-bits (8
 bytes) of each generated hash are used for each successive privacy-
 key, and the remainder are discarded.  Each key is examined
 sequentially, in the order used for encryption.  A key that is
 identical to a previous key MUST be discarded.  Although extremely
 rare, the 64 weak, semi-weak, and possibly weak keys [Schneier95,
 pages 280-282] MUST be discarded.  The Key-Generation-Function is
 iterated until a valid key is obtained before generating the next
 key.
 In all three keys, the least significant bit of each key byte is
 ignored (or set to parity when the implementation requires).
 The 64-bit CBC IV is zero.  Message encryption begins with the next
 field after the SPI, and continues to the end of the data indicated
 by the UDP Length.

4. Additional Validity-Method 4.1. SHA1-IPMAC Check

 As described in [RFC-2522] "Validity Verification", the Verification
 field value is the SHA1 [FIPS-180-1] hash over the concatenation of
    SHA1( key, keyfill, data, datafill, key, mdfill )
 where the key is the computed verification-key.
 The keyfill and datafill use the same pad-with-length technique
 defined for mdfill.  This padding and length is implicit, and does
 not appear in the datagram.
 The resulting Verification field is a 160-bit Variable Precision
 Integer (22 bytes including Size).  When used in calculations, the

Karn & Simpson Experimental [Page 6] RFC 2523 Schemes and Attributes March 1999

 Verification data includes both the Size and Value fields.

5. Additional Attributes

 The attribute format and basic facilities are already defined for
 Photuris [RFC-2522].
 These optional attributes are specified separately, and no single
 implementation is expected to support all of them.
 This document defines the following values:
   Use    Type
   AEI      6  SHA1-IPMAC
   AEI      7  RIPEMD-160-IPMAC
    E       8  DES-CBC
    E       9  Invert (Decryption/Encryption)
    E      10  XOR
   A      AH Attribute-Choice
    E     ESP Attribute-Choice
     I    Identity-Choice
      X   dependent on list location

5.1. SHA1-IPMAC

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        6
 Length           0

Karn & Simpson Experimental [Page 7] RFC 2523 Schemes and Attributes March 1999

5.1.1. Symmetric Identification

 When selected as an Identity-Choice, the immediately following
 Identification field contains an unstructured Variable Precision
 Integer.  Valid Identifications and symmetric secret-keys are
 preconfigured by the parties.
 There is no required format or content for the Identification value.
 The value may be a number or string of any kind.  See [RFC-2522] "Use
 of Identification and Secrets" for details.
 The symmetric secret-key (as specified) is selected based on the
 contents of the Identification field.  All implementations MUST
 support at least 62 bytes.  The selected symmetric secret-key SHOULD
 provide at least 80-bits of cryptographic strength.
 As described in [RFC-2522] "Identity Verification", the Verification
 field value is the SHA1 [FIPS-180-1] hash over the concatenation of:
    SHA1( key, keyfill, data, datafill, key, mdfill )
 where the key is the computed verification-key.
 The keyfill and datafill use the same pad-with-length technique
 defined for mdfill.  This padding and length is implicit, and does
 not appear in the datagram.
 The resulting Verification field is a 160-bit Variable Precision
 Integer (22 bytes including Size).  When used in calculations, the
 Verification data includes both the Size and Value fields.
 For both [RFC-2522] "Identity Verification" and "Validity
 Verification", the verification-key is the SHA1 [FIPS-180-1] hash of
 the following concatenated values:
  + the symmetric secret-key,
  + the computed shared-secret.
 For [RFC-2522] "Session-Key Computation", the symmetric secret-key is
 used directly as the generation-key.
 The symmetric secret-key is used in calculations in the same fashion
 as [RFC-2522] "MD5-IPMAC Symmetric Identification".

Karn & Simpson Experimental [Page 8] RFC 2523 Schemes and Attributes March 1999

5.1.2. Authentication

 May be selected as an AH or ESP Attribute-Choice, pursuant to [RFC-
 1852] et sequitur.  The selected Exchange-Scheme SHOULD provide at
 least 80-bits of cryptographic strength.
 As described in [RFC-2522] "Session-Key Computation", the most
 significant 384-bits (48 bytes) of the Key-Generation-Function
 iterations are used for the key.
 Profile:
    When negotiated with Photuris, the transform differs slightly from
    [RFC-1852].
    The form of the authenticated message is:
       SHA1( key, keyfill, datagram, datafill, key, mdfill )
    where the key is the SPI session-key.
    The additional datafill protects against the attack described in
    [PO96].  The keyfill and datafill use the same pad-with-length
    technique defined for mdfill.  This padding and length is
    implicit, and does not appear in the datagram.

5.2. RIPEMD-160-IPMAC

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        7
 Length           0

Karn & Simpson Experimental [Page 9] RFC 2523 Schemes and Attributes March 1999

5.2.1. Symmetric Identification

 When selected as an Identity-Choice, the immediately following
 Identification field contains an unstructured Variable Precision
 Integer.  Valid Identifications and symmetric secret-keys are
 preconfigured by the parties.
 There is no required format or content for the Identification value.
 The value may be a number or string of any kind.  See [RFC-2522] "Use
 of Identification and Secrets" for details.
 The symmetric secret-key (as specified) is selected based on the
 contents of the Identification field.  All implementations MUST
 support at least 62 bytes.  The selected symmetric secret-key SHOULD
 provide at least 80-bits of cryptographic strength.
 As described in [RFC-2522] "Identity Verification", the Verification
 field value is the RIPEMD-160 [DBP96] hash over the concatenation of:
    RIPEMD160( key, keyfill, data, datafill, key, mdfill )
 where the key is the computed verification-key.
 The keyfill and datafill use the same pad-with-length technique
 defined for mdfill.  This padding and length is implicit, and does
 not appear in the datagram.
 The resulting Verification field is a 160-bit Variable Precision
 Integer (22 bytes including Size).  When used in calculations, the
 Verification data includes both the Size and Value fields.
 For both [RFC-2522] "Identity Verification" and "Validity
 Verification", the verification-key is the RIPEMD-160 [DBP96] hash of
 the following concatenated values:
  + the symmetric secret-key,
  + the computed shared-secret.
 For [RFC-2522] "Session-Key Computation", the symmetric secret-key is
 used directly as the generation-key.
 The symmetric secret-key is used in calculations in the same fashion
 as [RFC-2522] "MD5-IPMAC Symmetric Identification".

Karn & Simpson Experimental [Page 10] RFC 2523 Schemes and Attributes March 1999

5.2.2. Authentication

 May be selected as an AH or ESP Attribute-Choice.  The selected
 Exchange-Scheme SHOULD provide at least 80-bits of cryptographic
 strength.
 As described in [RFC-2522] "Session-Key Computation", the most
 significant 384-bits (48 bytes) of the Key-Generation-Function
 iterations are used for the key.
 Profile:
    When negotiated with Photuris, the form of the authenticated
    message is:
       RIPEMD160( key, keyfill, datagram, datafill, key, mdfill )
    where the key is the SPI session-key.
    The additional datafill protects against the attack described in
    [PO96].  The keyfill and datafill use the same pad-with-length
    technique defined for mdfill.  This padding and length is
    implicit, and does not appear in the datagram.

5.3. DES-CBC

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        8
 Length           0
 May be selected as an ESP Attribute-Choice, pursuant to [RFC-1829] et
 sequitur.  The selected Exchange-Scheme SHOULD provide at least 56-
 bits of cryptographic strength.
 As described in [RFC-2522] "Session-Key Computation", the most
 significant 64-bits (8 bytes) of the Key-Generation iteration are
 used for the key, and the remainder are discarded.  Although
 extremely rare, the 64 weak, semi-weak, and possibly weak keys
 [Schneier95, pages 280-282] MUST be discarded.  The Key-Generation-
 Function is iterated until a valid key is obtained.
 The least significant bit of each key byte is ignored (or set to

Karn & Simpson Experimental [Page 11] RFC 2523 Schemes and Attributes March 1999

 parity when the implementation requires).
 Profile:
    When negotiated with Photuris, the transform differs slightly from
    [RFC-1829].
    The 32-bit Security Parameters Index (SPI) field is followed by a
    32-bit Sequence Number (SN).
    The 64-bit CBC IV is generated from the 32-bit Security Parameters
    Index (SPI) field followed by (concatenated with) the 32-bit
    Sequence Number (SN) field.  Then, the bit-wise complement of the
    32-bit Sequence Number (SN) value is XOR'd with the first 32-bits
    (SPI):
       (SPI ^ -SN) || SN
    The Padding values begin with the value 1, and count up to the
    number of padding bytes.  For example, if the plaintext length is
    41, the padding values are 1, 2, 3, 4, 5, 6 and 7, plus any
    additional obscuring padding.
    The PadLength and PayloadType are not appended.  Instead, the
    PayloadType is indicated by the SPI, as specified by the ESP-
    Attributes attribute (#2).
    After decryption, if the padding bytes are not the correct
    sequential values, then the payload is discarded, and a
    "Decryption Failed" error is indicated, as described in [RFC-
    2521].

5.4. Invert (Decryption/Encryption)

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        9
 Length           0
 May be selected as an ESP Attribute-Choice, immediately preceding an
 encryption choice.  This indicates that the following attribute is
 inverted from encryption to decryption (or decryption to encryption)
 as the attributes are processed.

Karn & Simpson Experimental [Page 12] RFC 2523 Schemes and Attributes March 1999

 For example, the combination
    "DES-CBC",
    "Invert",
    "DES-CBC",
    "DES-CBC",
 indicates "Triple DES" outer-CBC EDE encryption (and DED decryption)
 with three keys [KR96] pursuant to [RFC-1851] et sequitur.  The
 selected Exchange-Scheme SHOULD provide at least 112-bits of
 cryptographic strength.
 As described in [RFC-2522] "Session-Key Computation", the Key-
 Generation-Function is iterated (at least) three times to generate
 the three independent keys, in the order used for encryption.  The
 most significant 64-bits (8 bytes) of each iteration are used for
 each successive key, and the remainder are discarded.
 Each key is examined sequentially, in the order used for encryption.
 A key that is identical to any previous key MUST be discarded.  Any
 weak keys indicated for the algorithm MUST be discarded.  The Key-
 Generation-Function is iterated until a valid key is obtained before
 generating the next key.
 Profile:
    When negotiated with Photuris, the "DES-EDE3-CBC" transform
    differs slightly from [RFC-1851], in the same fashion as "DES-CBC"
    (described earlier).

5.5. XOR Whitening

 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 |   Attribute   |    Length     |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Attribute        10
 Length           0
 May be selected as an ESP Attribute-Choice, pursuant to [XEX3] et
 sequitur.  The combination
    "XOR",
    "DES-CBC",
    "XOR",

Karn & Simpson Experimental [Page 13] RFC 2523 Schemes and Attributes March 1999

 indicates "DESX" encryption with three keys [KR96].  The selected
 Exchange-Scheme SHOULD provide at least 104-bits of cryptographic
 strength.
 As described in [RFC-2522] "Session-Key Computation", the Key-
 Generation-Function is iterated (at least) three times to generate
 the three independent keys, in the order used for encryption.  The
 most significant bytes of each iteration are used for each successive
 key, and the remainder are discarded.
 Note that this attribute may appear multiple times in the same ESP
 attribute list, both before and after an encryption transform.  For
 example,
    "XOR",
    "DES-CBC",
    "XOR",
    "Invert",
    "DES-CBC",
    "XOR",
    "DES-CBC",
    "XOR",
 would be one possible combination with Triple DES.

Karn & Simpson Experimental [Page 14] RFC 2523 Schemes and Attributes March 1999

A. Exchange-Scheme Selection

 At first glance, there appear to be a large number of exchange-
 schemes.  In practice, the selection is simple to automate.
 Each scheme indicates a needed strength.  This strength is based upon
 the functions used in protecting the Photuris Exchanges themselves.
 Each keyed attribute also indicates a needed strength.  This strength
 is based upon its cryptographic functions.
 Because the usage of these functions is orthogonal, the same strength
 value can select an appropriate scheme that meets the needs of both
 features.

A.1. Responder

 The attributes to be offered to the particular Initiator are
 examined.  For each level of strength specified, a scheme that meets
 or exceeds the requirements is offered.
 For example, a Responder offering MD5-IPMAC and SHA1-IPMAC might
 offer scheme #2 with a 512-bit modulus and a 1024-bit modulus, and
 scheme #4 with a zero Size (indicating moduli of #2).

A.2. Initiator

 The strength indicated by the application for the Security
 Association, together with the party privacy policy of the system
 operator, is used to select from the offered schemes.  The strength
 indicates the minimal level to be chosen, while the party privacy
 policy indicates whether to choose the minimal or maximal level of
 available protection.
 For example, an application might indicate that it desires 80-bits of
 strength.  In that case, only the 1024-bit modulus would be
 appropriate.  The party privacy policy of the system operator would
 indicate whether to choose scheme #2 with "Simple Masking" or scheme
 #4 with "DES-CBC over Mask".
 Alternatively, an application might indicate that it desires 64-bits
 of strength.  The party privacy policy of the system operator would
 indicate whether to choose scheme #2 with the 512-bit modulus, or
 scheme #4 with the 1024-bit modulus.

Karn & Simpson Experimental [Page 15] RFC 2523 Schemes and Attributes March 1999

Security Considerations

 Provision for multiple generators does not enhance the security of
 the Photuris protocol exchange itself.  Rather, it provides an
 opportunity for novelty of moduli, by allowing more forms of moduli
 to be used.  An abundance of moduli inhibits a determined attacker
 from pre-calculating moduli exchange values, and discourages
 dedication of resources for analysis of any particular modulus.  That
 is, this protects the community of Photuris users.
 In addition to preventing various attacks by protecting verification
 fields, the masking of the message plaintext before encryption is
 intended to obscure the relation of the number of parties and SPIs
 active between two IP nodes.  The privacy mask dependency on the SPI
 and SPILT generates a different initial encrypted block for every SPI
 creation message.
 This obscurement would be less effective when the SPI and SPILT are
 invariant or are not created for a particular exchange direction.
 The number of parties could be revealed by the number of exchanges
 with differences in the initial encrypted blocks.

Acknowledgements

 Phil Karn was principally responsible for the design of party privacy
 protection, and provided much of the design rationale text (now
 removed to a separate document).
 William Simpson was responsible for the packet formats, and
 additional Exchange-Schemes, editing and formatting.  All such
 mistakes are his responsibity.
 Use of encryption for privacy protection is also found in the
 Station-To-Station authentication protocol [DOW92].
 Bart Preneel and Paul C van Oorschot in [PO96] recommended padding
 between the data and trailing key when hashing for authentication.
 Niels Provos developed the first implementation with multiple schemes
 and multiple moduli per scheme (circa July 1997).
 Special thanks to the Center for Information Technology Integration
 (CITI) for providing computing resources.

Karn & Simpson Experimental [Page 16] RFC 2523 Schemes and Attributes March 1999

References

 [DBP96]     Dobbertin, H., Bosselaers, A., and Preneel, B., "RIPEMD-
             160: a strengthened version of RIPEMD", Fast Software
             Encryption, Third International Workshop, Lecture Notes
             in Computer Science 1039 (1996), Springer-Verlag, pages
             71-82.
             See also corrections at
             ftp://ftp.esat.kuleuven.ac.be/pub/COSIC/bosselae/ripemd/.
 [DOW92]     Whitfield Diffie, Paul C van Oorshot, and Michael J
             Wiener, "Authentication and Authenticated Key Exchanges",
             Designs, Codes and Cryptography, v 2 pp 107-125, Kluwer
             Academic Publishers, 1992.
 [FIPS-180-1]
             "Secure Hash Standard", National Institute of Standards
             and Technology, U.S. Department Of Commerce, April 1995.
             Also known as: 59 Fed Reg 35317 (1994).
 [KR96]      Kaliski, B., and Robshaw, M., "Multiple Encryption:
             Weighing Security and Performance", Dr. Dobbs Journal,
             January 1996.
 [PO96]      Bart Preneel, and Paul C van Oorshot, "On the security of
             two MAC algorithms", Advances in Cryptology -- Eurocrypt
             '96, Lecture Notes in Computer Science 1070 (May 1996),
             Springer-Verlag, pages 19-32.
 [RFC-1829]  Karn, P., Metzger, P., Simpson, W., "The ESP DES-CBC
             Transform", July 1995.
 [RFC-1850]  Karn, P., Metzger, P., Simpson, W., "The ESP Triple DES
             Transform", September 1995.
 [RFC-1851]  Metzger, P., Simpson, W., "IP Authentication using Keyed
             SHA", September 1995.
 [RFC-2521]  Karn, P., and Simpson, W., "ICMP Security Failures
             Messages", March 1999.
 [RFC-2522]  Karn, P., and Simpson, W., "Photuris: Session-Key
             Management Protocol", March 1999.
 [XEX3]      Simpson, W., Baldwin, R., "The ESP DES-XEX3-CBC
             Transform", Work In Progress, June 1997.

Karn & Simpson Experimental [Page 17] RFC 2523 Schemes and Attributes March 1999

Contacts

 Comments about this document should be discussed on the
 photuris@adk.gr mailing list.
 Questions about this document can also be directed to:
    Phil Karn
    Qualcomm, Inc.
    6455 Lusk Blvd.
    San Diego, California  92121-2779
        karn@qualcomm.com
        karn@unix.ka9q.ampr.org (preferred)
    William Allen Simpson
    DayDreamer
    Computer Systems Consulting Services
    1384 Fontaine
    Madison Heights, Michigan  48071
        wsimpson@UMich.edu
        wsimpson@GreenDragon.com (preferred)

Karn & Simpson Experimental [Page 18] RFC 2523 Schemes and Attributes March 1999

Full Copyright Statement

 Copyright (C) The Internet Society (1999).  Copyright (C) Philip Karn
 and William Allen Simpson (1994-1999).  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
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 Internet organizations, except as needed for the purpose of
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 followed), or as required to translate it into languages other than
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 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 DISCLAIM 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.

Karn & Simpson Experimental [Page 19]

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