Network Working Group J. Schiller
Request for Comments: 4307 Massachusetts Institute of Technology
Category: Standards Track December 2005
Cryptographic Algorithms for Use in the
Internet 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.
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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-.
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3. Algorithm Selection
3.1. IKEv2 Algorithm Selection
3.1.1. 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] SHOULD
3.1.4. 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+
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3.1.5. 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.
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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.edu
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RFC 4307 IKEv2 Cryptographic Algorithms December 2005
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Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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