Network Working Group V. Manral
Request for Comments: 4835 IP Infusion Inc.
Obsoletes: 4305 April 2007
Category: Standards Track
Cryptographic Algorithm Implementation Requirements for
Encapsulating Security Payload (ESP) and Authentication Header (AH)
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 IETF Trust (2007).
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 that there is 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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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
Table of Contents
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. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
6. Changes from RFC 2402 and RFC 2406 to RFC 4305 . . . . . . . . 7
7. Changes from RFC 4305 . . . . . . . . . . . . . . . . . . . . . 7
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
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) [RFC4301], [RFC4302]. 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
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 in 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 that we (today) believe 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
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
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, 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 that 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-
conformant ESP and AH implementations. The security algorithms
actually used for any particular ESP or AH security association are
determined by a negotiation mechanism, such as the Internet Key
Exchange (IKE [RFC2409], [RFC4306]) 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 (notes)
----------- --------------------------
MUST NULL [RFC2410] (1)
MUST AES-CBC with 128-bit keys [RFC3602]
MUST- TripleDES-CBC [RFC2451]
SHOULD AES-CTR [RFC3686]
SHOULD NOT DES-CBC [RFC2405] (2)
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
Requirement Authentication Algorithm (notes)
----------- -----------------------------
MUST HMAC-SHA1-96 [RFC2404] (3)
SHOULD+ AES-XCBC-MAC-96 [RFC3566]
MAY NULL (1)
MAY HMAC-MD5-96 [RFC2403] (4)
Notes:
(1) Since ESP encryption is optional, support for the "NULL"
algorithm is required to maintain consistency with the way
services are negotiated. Note that while authentication and
encryption can each be "NULL", they MUST NOT both be "NULL"
[RFC4301].
(2) DES, with its small key size and publicly demonstrated and
open-design special-purpose cracking hardware, is of questionable
security for general use.
(3) Weaknesses have become apparent in SHA-1 [SHA1-COLL]; however,
these should not affect the use of SHA1 with HMAC.
(4) Weaknesses have become apparent in MD5 [MD5-COLL]; however,
these should not affect the use of MD5 with HMAC.
3.1.2. ESP Combined Mode Algorithms
As specified in [RFC4303], combined mode algorithms are supported
that 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 [RFC4309] and AES-GCM [RFC4106] are of interest. AES-CCM has
been adopted as the preferred mode in IEEE 802.11 [802.11i], and AES-
GCM has been adopted as the preferred mode in IEEE 802.1ae [802.1ae].
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.
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
Requirement Algorithm (notes)
----------- ----------------
MUST HMAC-SHA1-96 [RFC2404] (1)
SHOULD+ AES-XCBC-MAC-96 [RFC3566]
MAY HMAC-MD5-96 [RFC2403] (2)
Note:
(1) Weaknesses have become apparent in SHA-1 [SHA1-COLL]; however,
these should not affect the use of SHA1 with HMAC.
(2) Weaknesses have become apparent in MD5 [MD5-COLL]; however,
these should not affect the use of MD5 with HMAC.
4. Security Considerations
The security of cryptography-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 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 is not 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. Acknowledgements
Much of the wording herein was adapted from RFC 4305, the parent
document of this document. RFC 4305 itself borrows text from
[RFC4307], "Cryptographic Algorithms for Use in the Internet Key
Exchange Version 2", by Jeffrey I. Schiller.
Thanks to the following people for reporting or responding to reports
of the errors in RFC 4305: Paul Hoffman, Stephen Kent, Paul Koning,
and Lars Volker. Helpful Last-Call comments were received from Russ
Housley, Elwyn Davies, Nicolas Williams, and Alfred Hoenes.
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
6. Changes from RFC 2402 and RFC 2406 to RFC 4305
[RFC2402] and [RFC2406] defined the IPsec Authentication Header and
IPsec Encapsulating Security Payload. Each specified the
implementation requirements for cryptographic algorithms for their
respective protocols. They have now been replaced with [RFC4302] and
[RFC4303], which do not specify cryptographic algorithm
implementation requirements, and this document, which specifies such
requirements for both [RFC4302] and [RFC4303].
The implementation requirements are compared below:
Old Old New
Req. RFC(s) Requirement Algorithm (notes)
---- ------ ----------- -----------------
MUST 2406 SHOULD NOT DES-CBC [RFC2405] (1)
MUST 2402 2406 MAY HMAC-MD5-96 [RFC2403]
MUST 2402 2406 MUST HMAC-SHA1-96 [RFC2404]
Note:
(1) The IETF deprecated the use of single DES years ago and has
not included it in any new standard for some time (see IESG note
on the first page of [RFC2407]). [RFC4305] represented the first
standards-track recognition of that deprecation by specifying that
implementations SHOULD NOT provide single DES. The US Government
National Institute of Standards and Technology (NIST) has formally
recognized the weakness of single DES by a notice published
[DES-WDRAW] proposing to withdraw it as a US Government Standard.
Triple DES remains approved by both the IETF and NIST.
7. Changes from RFC 4305
This document obsoletes [RFC4305]. The document incorporates changes
for the support for the NULL Authentication Algorithm making the
support from a MUST to a MAY. This change is made to make this
document consistent with [RFC4301]. Text for SHA-1 collision attacks
as well as the future use of AES-GCM and AES-CCM is added.
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
The changed implementation requirement resulting from the above
changes is listed below:
Old Old New
Req. RFC(s) Requirement Algorithm (notes)
---- ------ ----------- -----------------
MUST 2406 MAY NULL Authentication
MUST 2406 MUST NULL Encryption
SHOULD+ 4305 MUST AES-CBC Encryption
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP14, RFC2119, March 1997.
[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.
[RFC2405] Madson, C. and N. Doraswamy, "The ESP DES-CBC Cipher
Algorithm With Explicit IV", RFC 2405, November 1998.
[RFC2410] Glenn, R. and S. Kent, "The NULL Encryption Algorithm
and Its Use With IPsec", RFC 2410, November 1998.
[RFC2451] Pereira, R. and R. Adams, "The ESP CBC-Mode Cipher
Algorithms", RFC 2451, November 1998.
[RFC3566] Frankel, S. and H. Herbert, "The AES-XCBC-MAC-96
Algorithm and Its Use With IPsec", RFC 3566,
September 2003.
[RFC3602] Frankel, S., Glenn, R., and S. Kelly, "The AES-CBC
Cipher Algorithm and Its Use with IPsec", RFC 3602,
September 2003.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4305] Eastlake, D., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP)
and Authentication Header (AH)", RFC 4305,
December 2005.
8.2. Informative References
[802.11i] "LAN/MAN Specific Requirements Part 11: Wireless Medium
Access Control (MAC) and physical layer (PHY)
specifications", IEEE Standard Medium Access Control
(MAC) Security, IEEE Std 802.11i, June 2004.
[802.1ae] "Media Access Control (MAC) Security", IEEE
Standard Medium Access Control (MAC) Security, IEEE Std
802.1ae, June 2006.
[DES-WDRAW] "Announcing Proposed Withdrawal of Federal Information
Processing Standard (FIPS) for the Data Encryption
Standard (DES) and Request for Comments", FIPS
Notice Docket No. 040602169-4169-01, July 2004.
[MD5-COLL] Klima, V., "Finding MD5 Collisions - a Toy For a
Notebook", Cryptology ePrint Archive Medium Report 2005/
075, March 2005.
[RFC2402] Kent, S. and R. Atkinson, "IP Authentication Header",
RFC 2402, November 1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2407] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
[RFC4307] Schiller, J., "Cryptographic Algorithms for Use in the
Internet Key Exchange Version 2 (IKEv2)", RFC 4307,
December 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES)
CCM Mode with IPsec Encapsulating Security Payload
(ESP)", RFC 4309, December 2005.
[SHA1-COLL] Rijmen, V. and E. Oswald, "Update on SHA-1", Cryptology
ePrint Archive Report 2005/010, January 2005.
Author's Address
Vishwas Manral
IP Infusion Inc.
Bamankhola, Bansgali,
Almora, Uttarakhand 263601
India
Phone: +91-98456-61911
EMail: vishwas@ipinfusion.com
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RFC 4835 Cryptographic Algorithms ESP and AH April 2007
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Manral Standards Track [Page 11]
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