Internet Engineering Task Force (IETF) D. McGrew
Request for Comments: 7321 Cisco Systems
Obsoletes: 4835 P. Hoffman
Category: Standards Track VPN Consortium
ISSN: 2070-1721 August 2014
Cryptographic Algorithm Implementation Requirements and Usage Guidance
for Encapsulating Security Payload (ESP) and Authentication Header (AH)
Abstract
This document updates the Cryptographic Algorithm Implementation
Requirements for the Encapsulating Security Payload (ESP) and
Authentication Header (AH). It also adds usage guidance to help in
the selection of these algorithms.
ESP and AH protocols make use of various cryptographic algorithms to
provide confidentiality and/or data origin authentication to
protected data communications in the IP Security (IPsec)
architecture. To ensure interoperability between disparate
implementations, the IPsec standard specifies a set of mandatory-to-
implement algorithms. This document specifies the current set of
mandatory-to-implement algorithms for ESP and AH, specifies
algorithms that should be implemented because they may be promoted to
mandatory at some future time, and also recommends against the
implementation of some obsolete algorithms. Usage guidance is also
provided to help the user of ESP and AH best achieve their security
goals through appropriate choices of cryptographic algorithms.
This document obsoletes RFC 4835.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc7321.
McGrew & Hoffman Standards Track [Page 1]
RFC 7321 Requirements for ESP and AH August 2014
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. Implementation Requirements . . . . . . . . . . . . . . . . . 4
2.1. ESP Authenticated Encryption (Combined Mode Algorithms) . 4
2.2. ESP Encryption Algorithms . . . . . . . . . . . . . . . . 4
2.3. ESP Authentication Algorithms . . . . . . . . . . . . . . 5
2.4. AH Authentication Algorithms . . . . . . . . . . . . . . 5
2.5. Summary of Changes from RFC 4835 . . . . . . . . . . . . 5
3. Usage Guidance . . . . . . . . . . . . . . . . . . . . . . . 5
4. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.1. Authenticated Encryption . . . . . . . . . . . . . . . . 7
4.2. Encryption Transforms . . . . . . . . . . . . . . . . . . 7
4.3. Authentication Transforms . . . . . . . . . . . . . . . . 7
5. Algorithm Diversity . . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 10
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1. Introduction
The Encapsulating Security Payload (ESP) [RFC4303] and the
Authentication Header (AH) [RFC4302] are the mechanisms for applying
cryptographic protection to data being sent over an IPsec Security
Association (SA) [RFC4301].
To ensure interoperability between disparate implementations, it is
necessary to specify a set of mandatory-to-implement algorithms.
This ensures that there is at least one algorithm that all
implementations will have in common. This document specifies the
current set of mandatory-to-implement algorithms for ESP and AH,
specifies algorithms that should be implemented because they may be
promoted to mandatory at some future time, and also recommends
against the implementation of some obsolete algorithms. Usage
guidance is also provided to help the user of ESP and AH best achieve
their security goals through appropriate choices of mechanisms.
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.
The ESP and AH mandatory-to-implement algorithm(s) may need to change
over time to adapt to new developments in cryptography. For this
reason, the specification of the mandatory-to-implement algorithms is
not included in the main IPsec, ESP, or AH specifications, but is
instead placed in this document. 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, this document identifies such algorithms, as they
are known today. There is no guarantee that the algorithms that we
predict will be mandatory in the future will actually be so. All
algorithms known today are subject to cryptographic attack and may be
broken in the future.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
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We define some additional keywords here:
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.
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.
2. Implementation Requirements
This section specifies the cryptographic algorithms that MUST be
implemented, and provides guidance about ones that SHOULD or SHOULD
NOT be implemented.
In the following sections, all AES modes are for 128-bit AES. 192-bit
and 256-bit AES MAY be supported for those modes, but the
requirements here are for 128-bit AES.
2.1. ESP Authenticated Encryption (Combined Mode Algorithms)
ESP combined mode algorithms provide both confidentiality and
authentication services; in cryptographic terms, these are
authenticated encryption algorithms [RFC5116]. Authenticated
encryption transforms are listed in the ESP encryption transforms
IANA registry.
Requirement Authenticated Encryption Algorithm
----------- ----------------------------------
SHOULD+ AES-GCM with a 16 octet ICV [RFC4106]
MAY AES-CCM [RFC4309]
2.2. ESP Encryption Algorithms
Requirement Encryption Algorithm
----------- --------------------------
MUST NULL [RFC2410]
MUST AES-CBC [RFC3602]
MAY AES-CTR [RFC3686]
MAY TripleDES-CBC [RFC2451]
MUST NOT DES-CBC [RFC2405]
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2.3. ESP Authentication Algorithms
Requirement Authentication Algorithm (notes)
----------- -----------------------------
MUST HMAC-SHA1-96 [RFC2404]
SHOULD+ AES-GMAC with AES-128 [RFC4543]
SHOULD AES-XCBC-MAC-96 [RFC3566]
MAY NULL [RFC4303]
Note that the requirement level for NULL authentication depends on
the type of encryption used. When using authenticated encryption
from Section 2.1, the requirement for NULL encryption is the same as
the requirement for the authenticated encryption itself. When using
the encryption from Section 2.2, the requirement for NULL encryption
is truly "MAY"; see Section 3 for more detail.
2.4. AH Authentication Algorithms
The requirements for AH are the same as for ESP Authentication
Algorithms, except that NULL authentication is inapplicable.
2.5. Summary of Changes from RFC 4835
The following is a summary of the changes from RFC 4835.
Old New
Requirement Requirement Algorithm (notes)
---- ----------- -----------------
MAY SHOULD+ AES-GCM with a 16 octet ICV [RFC4106]
MAY SHOULD+ AES-GMAC with AES-128 [RFC4543]
MUST- MAY TripleDES-CBC [RFC2451]
SHOULD NOT MUST NOT DES-CBC [RFC2405]
SHOULD+ SHOULD AES-XCBC-MAC-96 [RFC3566]
SHOULD MAY AES-CTR [RFC3686]
3. Usage Guidance
Since ESP and AH can be used in several different ways, this document
provides guidance on the best way to utilize these mechanisms.
ESP can provide confidentiality, data origin authentication, or the
combination of both of those security services. AH provides only
data origin authentication. Background information on those security
services is available [RFC4949]. In the following, we shorten "data
origin authentication" to "authentication".
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Providing both confidentiality and authentication offers the best
security. If confidentiality is not needed, providing authentication
can still be useful. Confidentiality without authentication is not
effective [DP07] and therefore SHOULD NOT be used. We describe each
of these cases in more detail below.
To provide both confidentiality and authentication, an authenticated
encryption transform from Section 2.1 SHOULD be used in ESP, in
conjunction with NULL authentication. Alternatively, an ESP
encryption transform and ESP authentication transform MAY be used
together. It is NOT RECOMMENDED to use ESP with NULL authentication
in conjunction with AH; some configurations of this combination of
services have been shown to be insecure [PD10].
To provide authentication without confidentiality, an authentication
transform MUST be used in either ESP or AH. The IPsec community
generally prefers ESP with NULL encryption over AH. AH is still
required in some protocols and operational environments when there
are security-sensitive options in the IP header, such as source
routing headers; ESP inherently cannot protect those IP options. It
is not possible to provide effective confidentiality without
authentication, because the lack of authentication undermines the
trustworthiness of encryption [B96][V02]. Therefore, an encryption
transform MUST NOT be used with a NULL authentication transform
(unless the encryption transform is an authenticated encryption
transform from Section 2.1).
Triple-DES SHOULD NOT be used in any scenario in which multiple
gigabytes of data will be encrypted with a single key. As a 64-bit
block cipher, it leaks information about plaintexts above that
"birthday bound" [M13]. Triple-DES CBC is listed as a MAY implement
for the sake of backwards compatibility, but its use is discouraged.
4. Rationale
This section explains the principles behind the implementation
requirements described above.
The algorithms listed as "MAY implement" are not meant to be endorsed
over other non-standard alternatives. All of the algorithms that
appeared in [RFC4835] are included in this document, for the sake of
continuity. In some cases, these algorithms have moved from being
"SHOULD implement" to "MAY implement".
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4.1. Authenticated Encryption
This document encourages the use of authenticated encryption
algorithms because they can provide significant efficiency and
throughput advantages, and the tight binding between authentication
and encryption can be a security advantage [RFC5116].
AES-GCM [RFC4106] brings significant performance benefits [KKGEGD],
has been incorporated into IPsec recommendations [RFC6379], and has
emerged as the preferred authenticated encryption method in IPsec and
other standards.
4.2. Encryption Transforms
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] [H10].
AES Counter Mode (AES-CTR) is an efficient encryption method, but it
provides no authentication capability. The AES-GCM authenticated
encryption method has all of the advantages of AES-CTR, while also
providing authentication. Thus, this document moves AES-CTR from a
SHOULD to a MAY.
The Triple Data Encryption Standard (TDES) is obsolete because of its
small block size; as with all 64-bit block ciphers, it SHOULD NOT be
used to encrypt more than one gigabyte of data with a single key
[M13]. Its key size is smaller than that of the Advanced Encryption
Standard (AES), while at the same time its performance and efficiency
are worse. Thus, its use in new implementations is discouraged.
The Data Encryption Standard (DES) is obsolete because of its small
key size and small block size. There have been publicly demonstrated
and open-design special-purpose cracking hardware. Therefore, its
use is has been changed to MUST NOT in this document.
4.3. Authentication Transforms
AES-GMAC provides good security along with performance advantages,
even over HMAC-MD5. In addition, it uses the same internal
components as AES-GCM and is easy to implement in a way that shares
components with that authenticated encryption algorithm.
The MD5 hash function has been found to not meet its goal of
collision resistance; it is so weak that its use in digital
signatures is highly discouraged [RFC6151]. There have been
theoretical results against HMAC-MD5, but that message authentication
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code does not seem to have a practical vulnerability. Thus, it may
not be urgent to remove HMAC-MD5 from the existing protocols.
SHA-1 has been found to not meet its goal of collision resistance.
However, HMAC-SHA-1 does not rely on this property, and HMAC-SHA-1 is
believed to be secure.
HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512 are believed to provide
a good security margin, and they perform adequately on many
platforms. However, these algorithms are not recommended for
implementation in this document, because HMAC-SHA-1 support is
widespread and its security is good, AES-GMAC provides good security
with better performance, and Authenticated Encryption algorithms do
not need any authentication methods.
AES-XCBC has not seen widespread deployment, despite being previously
recommended as a SHOULD+ in RFC 4835. Thus, this document lists it
only as a SHOULD.
5. Algorithm Diversity
When the AES cipher was first adopted, it was decided to continue
encouraging the implementation of Triple-DES, in order to provide
algorithm diversity. But the passage of time has eroded the
viability of Triple-DES as an alternative to AES. As it is a 64-bit
block cipher, its security is inadequate at high data rates (see
Section 4.2). Its performance in software and Field-Programmable
Gate Arrays (FPGAs) is considerably worse than that of AES. Since it
would not be possible to use Triple-DES as an alternative to AES in
high data rate environments, or in environments where its performance
could not keep up the requirements, the rationale of retaining
Triple-DES to provide algorithm diversity is disappearing. (Of
course, this does not change the rationale of retaining Triple-DES in
IPsec implementations for backwards compatibility.)
Recent discussions in the IETF have started considering how to make
the selection of a different cipher that could provide algorithm
diversity in IPsec and other IETF standards. That work is expected
to take a long time and involve discussions among many participants
and organizations.
It is important to bear in mind that it is very unlikely that an
exploitable flaw will be found in AES (e.g., a flaw that required
less than a terabyte of known plaintext, when AES is used in a
conventional mode of operation). The only reason that algorithm
diversity deserves any consideration is because there would be large
problems if such a flaw were found.
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6. Acknowledgements
Some of the wording herein was adapted from [RFC4835], the document
that this one obsoletes. That RFC itself borrows from earlier RFCs,
notably RFC 4305 and 4307. RFC 4835, RFC 4305, and RFC 4307 were
authored by Vishwas Manral, Donald Eastlake, and Jeffrey Schiller
respectively.
Thanks are due to Wajdi Feghali, Brian Weis, Cheryl Madson, Dan
Harkins, Paul Wouters, Ran Atkinson, Scott Fluhrer, Tero Kivinen, and
Valery Smyslov for insightful feedback on this document.
7. Security Considerations
The security of a system that uses cryptography 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 to date
leads us to believe that they will likely remain secure into the
foreseeable future. However, this is not necessarily forever.
Therefore, we expect that revisions of that document will be issued
from time to time to reflect the current best practice in this area.
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC
4303, December 2005.
McGrew & Hoffman Standards Track [Page 9]RFC 7321 Requirements for ESP and AH August 2014
8.2. Informative References
[B96] Bellovin, S., "Problem areas for the IP security
protocols", Proceedings of the Sixth Usenix Unix Security
Symposium, 1996.
[DP07] Degabriele, J. and K. Paterson, "Attacking the IPsec
Standards in Encryption-only Configurations", IEEE
Symposium on Privacy and Security, 2007.
[H10] Hoban, A., "Using Intel AES New Instructions and PCLMULQDQ
to Significantly Improve IPSec Performance on Linux",
Intel White Paper, August 2010.
[KKGEGD] Kounavis, M., Kang, X., Grewal, K., Eszenyi, M., Gueron,
S., and D. Durham, "Encrypting the Internet", SIGCOMM,
2010.
[M13] McGrew, D., "Impossible plaintext cryptanalysis and
probable-plaintext collision attacks of 64-bit block
cipher modes", IACR ePrint, 2012.
[PD10] Paterson, K. and J. Degabriele, "On the (in)security of
IPsec in MAC-then-encrypt configurations", CCS '10, ACM
Conference on Computer and Communications Security, 2010.
[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.
McGrew & Hoffman Standards Track [Page 10]RFC 7321 Requirements for ESP and AH August 2014
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)", RFC
4106, June 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES) CCM
Mode with IPsec Encapsulating Security Payload (ESP)", RFC
4309, December 2005.
[RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message
Authentication Code (GMAC) in IPsec ESP and AH", RFC 4543,
May 2006.
[RFC4835] Manral, V., "Cryptographic Algorithm Implementation
Requirements for Encapsulating Security Payload (ESP) and
Authentication Header (AH)", RFC 4835, April 2007.
[RFC4949] Shirey, R., "Internet Security Glossary, Version 2", RFC
4949, August 2007.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC6151] Turner, S. and L. Chen, "Updated Security Considerations
for the MD5 Message-Digest and the HMAC-MD5 Algorithms",
RFC 6151, March 2011.
[RFC6379] Law, L. and J. Solinas, "Suite B Cryptographic Suites for
IPsec", RFC 6379, October 2011.
[V02] Vaudenay, S., "Security Flaws Induced by CBC Padding -
Applications to SSL, IPSEC, WTLS ...", EUROCRYPT, 2002.
Authors' Addresses
David McGrew
Cisco Systems
EMail: mcgrew@cisco.com
Paul Hoffman
VPN Consortium
EMail: paul.hoffman@vpnc.org
McGrew & Hoffman Standards Track [Page 11]
