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EXPERIMENTAL
Network Working Group                                            P. KarnRequest for Comments: 2523                                      QualcommCategory: Experimental                                        W. Simpson                                                              DayDreamer                                                              March 1999Photuris: Extended Schemes and AttributesStatus 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 1999Table of Contents1.     Additional Exchange-Schemes ...........................12.     Additional Key-Generation-Function ....................52.1       SHA1 Hash .......................................53.     Additional Privacy-Methods ............................53.1       DES-CBC over Mask ...............................53.2       DES-EDE3-CBC over Mask ..........................64.     Additional Validity-Method ............................64.1       SHA1-IPMAC Check ................................65.     Additional Attributes .................................75.1       SHA1-IPMAC ......................................75.1.1  Symmetric Identification ........................85.1.2  Authentication ..................................95.2       RIPEMD-160-IPMAC ................................95.2.1  Symmetric Identification ........................105.2.2  Authentication ..................................115.3       DES-CBC .........................................115.4       Invert (Decryption/Encryption) ..................125.5       XOR Whitening ...................................13     APPENDICES ...................................................15A.     Exchange-Scheme Selection .............................15A.1       Responder .......................................15A.2       Initiator .......................................15     SECURITY CONSIDERATIONS ......................................16     ACKNOWLEDGEMENTS .............................................16     REFERENCES ...................................................17     CONTACTS .....................................................18     COPYRIGHT ....................................................19Karn & Simpson                Experimental                     [Page ii]

RFC 2523                 Schemes and Attributes               March 19991.  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 inKarn & 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 inKarn & 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-Function2.1.  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-Methods3.1.  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 indicatedKarn & 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-Method4.1.  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, theKarn & 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 location5.1.  SHA1-IPMAC   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   |   Attribute   |    Length     |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+   Attribute        6   Length           0Karn & Simpson                Experimental                      [Page 7]

RFC 2523                 Schemes and Attributes               March 19995.1.1.  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 19995.1.2.  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           0Karn & Simpson                Experimental                      [Page 9]

RFC 2523                 Schemes and Attributes               March 19995.2.1.  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 19995.2.2.  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 toKarn & 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 1999A.  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 1999Security 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 1999References   [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 atftp://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 1999Contacts   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 1999Full 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   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 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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