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Network Working Group                                        J. SchillerRequest for Comments: 4307         Massachusetts Institute of TechnologyCategory: Standards Track                                  December 2005Cryptographic Algorithms for Use in theInternet Key Exchange Version 2 (IKEv2)Status of This Memo   This document specifies an Internet standards track protocol for the   Internet community, and requests discussion and suggestions for   improvements.  Please refer to the current edition of the "Internet   Official Protocol Standards" (STD 1) for the standardization state   and status of this protocol.  Distribution of this memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2005).Abstract   The IPsec series of protocols makes use of various cryptographic   algorithms in order to provide security services.  The Internet Key   Exchange (IKE (RFC 2409) and IKEv2) provide a mechanism to negotiate   which algorithms should be used in any given association.  However,   to ensure interoperability between disparate implementations, it is   necessary to specify a set of mandatory-to-implement algorithms to   ensure that there is at least one algorithm that all implementations   will have available.  This document defines the current set of   algorithms that are mandatory to implement as part of IKEv2, as well   as algorithms that should be implemented because they may be promoted   to mandatory at some future time.1.  Introduction   The Internet Key Exchange protocol provides for the negotiation of   cryptographic algorithms between both endpoints of a cryptographic   association.  Different implementations of IPsec and IKE may provide   different algorithms.  However, the IETF desires that all   implementations should have some way to interoperate.  In particular,   this requires that IKE define a set of mandatory-to-implement   algorithms because IKE itself uses such algorithms as part of its own   negotiations.  This requires that some set of algorithms be specified   as "mandatory-to-implement" for IKE.Schiller                    Standards Track                     [Page 1]

RFC 4307             IKEv2 Cryptographic Algorithms        December 2005   The nature of cryptography is that new algorithms surface   continuously and existing algorithms are continuously attacked.  An   algorithm believed to be strong today may be demonstrated to be weak   tomorrow.  Given this, the choice of mandatory-to-implement algorithm   should be conservative so as to minimize the likelihood of it being   compromised quickly.  Thought should also be given to performance   considerations as many uses of IPsec will be in environments where   performance is a concern.   Finally, we need to recognize that the mandatory-to-implement   algorithm(s) may need to change over time to adapt to the changing   world.  For this reason, the selection of mandatory-to-implement   algorithms was removed from the main IKEv2 specification and placed   in this document.  As the choice of algorithm changes, only this   document should need to be updated.   Ideally, the mandatory-to-implement algorithm of tomorrow should   already be available in most implementations of IPsec by the time it   is made mandatory.  To facilitate this, we will attempt to identify   those algorithms (that are known today) in this document.  There is   no guarantee that the algorithms we believe today may be mandatory in   the future will in fact become so.  All algorithms known today are   subject to cryptographic attack and may be broken in the future.2.  Requirements Terminology   Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT", and   "MAY" that appear in this document are to be interpreted as described   in [RFC2119].   We define some additional terms here:   SHOULD+    This term means the same as SHOULD.  However, it is likely              that an algorithm marked as SHOULD+ will be promoted at              some future time to be a MUST.   SHOULD-    This term means the same as SHOULD.  However, an algorithm              marked as SHOULD- may be deprecated to a MAY in a future              version of this document.   MUST-      This term means the same as MUST.  However, we expect at              some point that this algorithm will no longer be a MUST in              a future document.  Although its status will be determined              at a later time, it is reasonable to expect that if a              future revision of a document alters the status of a MUST-              algorithm, it will remain at least a SHOULD or a SHOULD-.Schiller                    Standards Track                     [Page 2]

RFC 4307             IKEv2 Cryptographic Algorithms        December 20053.  Algorithm Selection3.1.  IKEv2 Algorithm Selection3.1.1.  Encrypted Payload Algorithms   The IKEv2 Encrypted Payload requires both a confidentiality algorithm   and an integrity algorithm.  For confidentiality, implementations   MUST- implement 3DES-CBC and SHOULD+ implement AES-128-CBC.  For   integrity, HMAC-SHA1 MUST be implemented.3.1.2.  Diffie-Hellman Groups   There are several Modular Exponential (MODP) groups that are defined   for use in IKEv2.  They are defined in both the [IKEv2] base document   and in the MODP extensions document.  They are identified by group   number.  Any groups not listed here are considered as "MAY be   implemented".      Group Number        Bit Length            Status     Defined      2                   1024 MODP Group       MUST-      [RFC2409]      14                  2048 MODP Group       SHOULD+    [RFC3526]3.1.3.  IKEv2 Transform Type 1 Algorithms   IKEv2 defines several possible algorithms for Transfer Type 1   (encryption).  These are defined below with their implementation   status.      Name                     Number    Defined In      Status      RESERVED                 0      ENCR_3DES                3         [RFC2451]       MUST-      ENCR_NULL                11        [RFC2410]       MAY      ENCR_AES_CBC             12        [AES-CBC]       SHOULD+      ENCR_AES_CTR             13        [AES-CTR]       SHOULD3.1.4.  IKEv2 Transform Type 2 Algorithms   Transfer Type 2 Algorithms are pseudo-random functions used to   generate random values when needed.      Name                Number     Defined In   Status      RESERVED            0      PRF_HMAC_MD5        1          [RFC2104]    MAY      PRF_HMAC_SHA1       2          [RFC2104]    MUST      PRF_AES128_CBC      4          [AESPRF]     SHOULD+Schiller                    Standards Track                     [Page 3]

RFC 4307             IKEv2 Cryptographic Algorithms        December 20053.1.5.  IKEv2 Transform Type 3 Algorithms   Transfer Type 3 Algorithms are Integrity algorithms used to protect   data against tampering.      Name                     Number       Defined In           Status      NONE                     0      AUTH_HMAC_MD5_96         1            [RFC2403]            MAY      AUTH_HMAC_SHA1_96        2            [RFC2404]            MUST      AUTH_AES_XCBC_96         5            [AES-MAC]            SHOULD+4.  Security Considerations   The security of cryptographic-based systems depends on both the   strength of the cryptographic algorithms chosen and the strength of   the keys used with those algorithms.  The security also depends on   the engineering of the protocol used by the system to ensure that   there are no non-cryptographic ways to bypass the security of the   overall system.   This document concerns itself with the selection of cryptographic   algorithms for the use of IKEv2, specifically with the selection of   "mandatory-to-implement" algorithms.  The algorithms identified in   this document as "MUST implement" or "SHOULD implement" are not known   to be broken at the current time, and cryptographic research so far   leads us to believe that they will likely remain secure into the   foreseeable future.  However, this isn't necessarily forever.  We   would therefore expect that new revisions of this document will be   issued from time to time that reflect the current best practice in   this area.5.  Normative References   [RFC2409]    Harkins, D. and D. Carrel, "The Internet Key Exchange                (IKE)",RFC 2409, November 1998.   [IKEv2]      Kaufman, C., Ed., "Internet Key Exchange (IKEv2)                Protocol",RFC 4306, December 2005.   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3526]    Kivinen, T. and M. Kojo, "More Modular Exponential                (MODP) Diffie-Hellman groups for Internet Key Exchange                (IKE)",RFC 3526, May 2003.   [RFC2451]    Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher                Algorithms",RFC 2451, November 1998.Schiller                    Standards Track                     [Page 4]

RFC 4307             IKEv2 Cryptographic Algorithms        December 2005   [RFC2410]    Glenn, R. and S. Kent, "The NULL Encryption Algorithm                and Its Use With IPsec",RFC 2410, November 1998.   [AES-CBC]    Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC                Cipher Algorithm and Its Use with IPsec",RFC 3602,                September 2003.   [AES-CTR]    Housley, R., "Using Advanced Encryption Standard (AES)                Counter Mode With IPsec Encapsulating Security Payload                (ESP)",RFC 3686, January 2004.   [RFC2104]    Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:                Keyed-Hashing for Message Authentication",RFC 2104,                February 1997.   [AESPRF]     Hoffman, P., "The AES-XCBC-PRF-128 Algorithm for the                Internet Key Exchange Protocol (IKE)",RFC 3664, January                2004.   [RFC2403]    Madson, C. and R. Glenn, "The Use of HMAC-MD5-96 within                ESP and AH",RFC 2403, November 1998.   [RFC2404]    Madson, C. and R. Glenn, "The Use of HMAC-SHA-1-96                within ESP and AH",RFC 2404, November 1998.   [AES-MAC]    Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96                Algorithm and Its Use With IPsec",RFC 3566, September                2003.Author's Address   Jeffrey I. Schiller   Massachusetts Institute of Technology   Room W92-190   77 Massachusetts Avenue   Cambridge, MA 02139-4307   USA   Phone: +1 (617) 253-0161   EMail: jis@mit.eduSchiller                    Standards Track                     [Page 5]

RFC 4307             IKEv2 Cryptographic Algorithms        December 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), 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.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at ietf-   ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Schiller                    Standards Track                     [Page 6]