INTERNET-DRAFT Donald E. Eastlake 3rd
Motorola Laboratories
Expires: July 2004 January 2004
Cryptographic Algorithm Implementation Requirements For ESP And AH
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<draft-ietf-ipsec-esp-ah-algorithms-01.txt>
Status of This Document
Distribution of this draft is unlimited. Comments should be sent to
the authors.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
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Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
The IPSEC series of protocols makes use of various cryptographic
algorithms in order to provide security services. The Encapsulating
Security Payload (ESP) and the Authentication Header (AH) provide two
mechanisms for protecting data being sent over an IPSEC Security
Association (SA). To ensure interoperability between disparate
implementations it is necessary to specify a set of mandatory to
implement algorithms to ensure at least one algorithm that all
implementations will have available. This document defines the
current set of mandatory to implement algorithms for ESP and AH as
well as specifying algorithms that should be implemented because they
may be promoted to mandatory at some future time.
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Acknowledgement
Much of the wording herein was adapted from "Cryptographic Algorithms
for use in the Internet Key Exchange Version 2" <draft-ietf-ipsec-
ikev2-algorithms-*.txt> by Jeffrey I. Schiller.
Table of Contents
Status of This Document....................................1
Copyright Notice...........................................1
Abstract...................................................1
Acknowledgement............................................2
Table of Contents..........................................2
1. Introduction............................................3
2. Requirements Terminology................................3
3. Algorithm Selection.....................................4
3.1 Encapsulating Security Payload.........................4
3.1.1 ESP Encryption and Authentication Algorithms.........4
3.1.2 ESP Combined Mode Algorithms.........................5
3.2 Authentication Header..................................5
4. Security Considerations.................................6
5. IANA Considerations.....................................6
Normative References.......................................7
Informative References.....................................7
Author's Address...........................................9
Full Copyright Statement..................................10
Expiration and File Name..................................10
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1. Introduction
The Encapsulating Security Payload (ESP) and the Authentication
Header (AH) provide two mechanisms for protecting data being sent
over an IPSEC Security Association (SA) [IPSEC, ESP, AH]. To ensure
interoperability between disparate implementations it is necessary to
specify a set of mandatory to implement algorithms to ensure at least
one algorithm that all implementations will have available. This
document defines the current set of mandatory to implement algorithms
for ESP and AH as well as specifying algorithms that should be
implemented because they may be promoted to mandatory at some future
time.
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 is not included the main IPSEC, ESP, or AH specifications.
It is instead 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
such algorithms as they 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 [RFC 2119].
In addition we will 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.
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SHOULD- This terms means the same as SHOULD. However it is likely
that an algorithm marked as SHOULD- will be deprecated to
a MAY or worse in a future version of this document.
MUST- This term means the same as MUST. However we expect at
some point in the future this algorithm will no longer be
a MUST.
3. Algorithm Selection
For IPSEC implementations to interoperate, they must support one or
more security algorithms in common. This section specifies the
security algorithm implementation requirements for standards
comformant ESP and AH implementations. The security algorithms
actually used for any particular ESP or AH security association is
determined by a negotiation mechahism, such as the Internet Key
Exchange (IKE [RFC 2409, IKEv2]), or pre-establishment.
Of course, additional standard and proprietary algorithms beyond
those listed below can be implemented.
3.1 Encapsulating Security Payload
The implementation conformance requirements for security algorithms
for ESP are given in the tables below. See section 2 for definitions
of the values in the "Requirement" column.
3.1.1 ESP Encryption and Authentication Algorithms
These tables list encryption and authentication algorithms for the
IPSEC Encapsulating Security Payload protocol.
Requirement Encryption Algorithm
----------- --------------------
MUST NULL (1)
MUST- TripleDES-CBC [RFC 2451]
SHOULD+ AES-CBC with 128-bit keys [RFC 3602]
SHOULD AES-CTR [AES-CTR]
SHOULD NOT DES-CBC [RFC 2405] (3)
Requirement Authentication Algorithm
----------- ------------------------
MUST HMAC-SHA1-96 [RFC 2404]
MUST NULL (1)
SHOULD+ AES-XCBC-MAC-96 [RFC 3566]
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MAY HMAC-MD5-96 [RFC 2403] (2)
Notes:
1. Since ESP encryption and authentication are optional, support for
the two "NULL" algorithms is required to maintain consistency with
the way these services are negotiated. NOTE that while
authentication and encryption can each be "NULL", they MUST NOT
both be "NULL".
2. Weaknesses have become apparent in MD5, however these should not
effect the use of MD5 with HMAC.
3. DES, with its small key size and publicly demonstrated special
purpose cracking hardware, is of questionable security for general
use.
3.1.2 ESP Combined Mode Algorithms
As specified in [ESP], combined mode algorithms are supported which
provide both confidentiality and authentication services. Support of
such algorithms will require proper structuring of ESP
implementations. Under many circumstances, combined mode algorithms
provide significant efficiency and throughput advantages. Although
there are no suggested or required combined algorithms at this time,
AES-CCM [CCM], which is in the process of being adopted as the
prefered mode for security in IEEE 802.11 [802.11i], is expected to
be of interest in the near future.
3.2 Authentication Header
The implementation conformance requirements for security algorithms
for AH are given below. See section 2 for definitions of the values
in the "Requirement" column. As you would suspect, all of these
algorithms are authentication algorithms.
Requirement Algorithm
----------- ---------
MUST HMAC-SHA1-96 [RFC 2404]
SHOULD+ AES-XCBC-MAC-96 [RFC 3566]
MAY HMAC-MD5-96 [RFC 2403] (1)
Notes:
1. Weaknesses have become apparent in MD5, however these should not
effect the use of MD5 with HMAC.
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4. Security Considerations
The security of cryptographic based systems depends on both the
strength of the cryptographic algorithms chosen, the strength of the
keys used with those algorithms and the engineering and
administration 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 ESP and AH, 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. IANA Considerations
This document does not define any new registries nor elements in
existing registries.
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Normative References
[AES CTR] - "Using AES Counter Mode With IPSec ESP", draft-ietf-
ipsec-ciph-aes-ctr-*.txt, R. Housley, July 2003.
[AH] - "IP Authentication Header", draft-ietf-ipsec-rfc2402bis-*.txt,
S. Kent, September 2003.
[ESP] - "IP Encapsulating Security Payload (ESP)", draft-ietf-ipsec-
esp-v3-*.txt, S. Kent, January 2004.
[IPSEC] - "Security Architecture for the Internet Protocol", draft-
ietf-ipsec-rfc2401bis-*.txt, S. Kent, October 2003.
[RFC 2119] - "Key words for use in RFCs to Indicate Requirement
Levels", S. Bradner, March 1997.
[RFC 2403] - "The Use of HMAC-MD5-96 within ESP and AH", C. Madson,
and R. Glenn, November 1998.
[RFC 2404] - "The Use of HMAC-SHA-1-96 within ESP and AH", C. Madson,
and R. Glenn, November 1998.
[RFC 2405] - "The ESP DES-CBC Cipher Algorithm With Explicit IV", C.
Madson, and N. Doraswamy, November 1998.
[RFC 2406] - "IP Encapsulating Security Payload (ESP)", S. Kent, R.
Atkinson, November 1998.
[RFC 3566] - "The AES-XCBC-MAC-96 Algorithm and Its Use With IPSec",
S. Frankel. H. Herbert, September 2003.
[RFC 3602] - "The AES-CBC Cipher Algorithm and Its Use with IPsec",
S. Frankel, R. Glenn, S. Kelly, September 2003.
Informative References
[802.11i] - LAN/MAN Specific Requirements - Part 11: Wireless Medium
Access Control (MAC) and physical layer (PHY) specifications: Medium
Access Control (MAC) Security Enhancements, IEEE Std 802.11i/D7.0,
October 2003.
[AES CCM] - "Using AES CCM Mode With IPsec ESP", draft-ietf-ipsec-
ciph-aes-ccm-05.txt which is in the RFC Editor Queue, R. Housley,
November 2003.
[IKEv2] - "Internet Key Exchange (IKEv2) Protocol", draft-ietf-ipsec-
ikev2-*.txt, C. Kaufman, October 2003.
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[RFC 791] - "Internet Protocol", J. Postel, September 1981.
[RFC 2409] - "The Internet Key Exchange (IKE)", D. Harkins, D.
Carrel, November 1998.
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Author's Address
Donald E. Eastlake 3rd
Motorola Laboratories
155 Beaver Street
Milford, MA 01757 USA
Telephone: +1-508-786-7554 (w)
+1-508-634-2066 (h)
EMail: Donald.Eastlake@Motorola.com
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Full Copyright Statement
Copyright (C) The Internet Society (2004). All Rights Reserved.
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Expiration and File Name
This draft expires July 2004.
Its file name is draft-ietf-ipsec-esp-ah-algorithms-01.txt.
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