MSEC Working Group D. McGrew
Internet-Draft B. Weis
Expires: April 16, 2007 Cisco Systems
October 13, 2006
Using Counter Modes with Encapsulating Security Payload (ESP) and
Authentication Header (AH) to Protect Group Traffic
draft-weis-esp-group-counter-cipher-00
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Copyright (C) The Internet Society (2006).
Abstract
This memo describes the use of Advanced Encryption Standard (AES)
counter modes when applied to the Encapsulating Security Payload
(ESP) and Authentication Header (AH) in multiple-sender group
applications.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 4
3. Recommended IV Formation . . . . . . . . . . . . . . . . . . . 5
4. Group Key Management Conventions . . . . . . . . . . . . . . . 6
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
6. Security Considerations . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . . 9
Appendix A. Rationale for the Recommended IV Formation . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
Intellectual Property and Copyright Statements . . . . . . . . . . 13
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1. Introduction
Several new AES encryption modes of operation have been specified for
the IP Encapsulating Security Payload (ESP) [RFC4303]: ESP: Counter
Mode (CTR) [RFC3686], Galois/Counter Mode (GCM) [RFC4106], Counter
with CBC-MAC Mode (CCM) [RFC4309]; and one that has been specified
for both ESP and the Authentication Header (AH): Galois MAC Mode
(GMAC) [RFC4543]. These new modes offer advantages over traditional
modes of operation. However, they all have restrictions on their use
in situations in which multiple senders are protecting traffic using
the same key. This document addresses this restriction and describes
how these modes can be used with group key management protocols such
as the Group Domain of Interpretation (GDOI) protocol [RFC3547] and
the Group Secure Association Key Management Protocol (GSAKMP)
[RFC4535].
1.1. Requirements notation
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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2. Problem Statement
The counter mode of operation (CTR) [FIPS.800-38A.2001] has become
important because of its performance and implementation advantages.
It is the basis for several modes of operation that combine
encryption, including CCM and GCM. All of the counter-based modes
require that, if a single key is shared by multiple encryption
engines, those engines must coordinate to ensure that every
initialization vector (IV) used with that key is distinct. That is,
for each key, no IV value can be used more than once. This
restriction on IV usage is imposed on ESP CTR, ESP GCM, and ESP CCM.
All ESP transforms using an AES counter mode have a restriction that
an application must not use the same key, IV, and Salt values to
protect two different data payloads. Notwithstanding this security
condition, AES counter mode ESP transforms are often preferred
because of their favorable performance characteristics as compared to
other AES modes.
Each of the ESP transforms using AES counter mode specify the
construction of keying material for point-to-point applications which
are keyed by the Internet Key Exchange version 1 (IKE) [RFC2409] or
version 2 (IKEv2) [RFC4306]. The specified constructions guarantee
that the security condition is not violated by a single sender.
However, group applications, where multiple senders encrypt using a
single IPsec Security Association (SA), also may find AES counter
mode transforms to be valuable. Such groups are possible when IPsec
is used to protect network traffic between members of a multiple
sender IP multicast application, known as a Many-to-Many Application
[RFC3170]. Some Many-to-many Applications are comprised of a large
number of senders such that defining an individual IPsec SA for each
sender is unmanageable.
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3. Recommended IV Formation
This section specifies a particular construction of the IV that
enables a group of senders to safely share a single IPsec SA. A
rationale for this method is given in Appendix A. In the recommended
construction, each IV is formed by concatenating a Sender Identifier
(SID) field with a Sender-Specific IV (SSIV) field. The value of the
SID MUST be unique for each sender, across all of the senders sharing
a particular Security Association. The value of the SSIV field MUST
be unique for each IV constructed by a particular sender for use with
a particular SA. The SSIV MAY be chosen in any manner convenient to
the sender, e.g. successive values of a counter. The leftmost bits
of the IV contain the SID, and the remaining bits contain the SSIV.
The number of bits used by the SID may vary depending on group
policy, though for each particular Security Association, each SID
used with that SA MUST have the same length. Additionally, a
conforming implementation MUST support SID lengths of 8, 12, and 16
bits. It should be noted that the size of the SID associated with an
SA provides a tradeoff between the number of possible senders and the
number of packets that each sending station is able to send using
that SA.
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4. Group Key Management Conventions
A group uses ESP to protect traffic uses a Group Key Management (GKM)
subsystem and protocol [I-D.ietf-msec-ipsec-extensions] to distribute
the ESP transform policy and keying material. This document
RECOMMENDS that the GKM subsystem on a Group Controller Key Sever
(GCKS) both allocate SIDs to group members and distributes them to
group members using a GKM protocol such as GDOI or GSAKMP.
The following requirements apply to a GKM protocol that manages SIDs.
o The GCKS MUST NOT give the same sender identifier to more than one
active group member. If the entire set of sender identifiers has
been exhausted, the GCKS MUST refuse to allow new group members to
join.
o The GCKS SHOULD allocate a single sender identifier for each group
member, and issue this value to the sender for all group SAs for
which that member is a sender. This strategy simplifies the
rekeying process.
o If a GCKS allocates a single sender identifier to be used with all
group SAs for which it is a sender, and it determines that a
particular group member is no longer a part of the group, then it
MAY re-allocate the sender identifier to a new group member. In
this case, the GCKS MUST first delete and replace any active AH or
ESP SAs with which the SID may have been used. This is necessary
to avoid re-use of an IV with the cipher key associated with the
SA.
A GKM subsystem MUST support a group member notifying the GCKS that
its IV space will soon be exhausted and requires a new SA to be
distributed. A group member SHOULD notify the GCKS in advance of its
IV space being exhausted. A GCKS MAY choose to ignore this
notification based on policy (e.g., if the group member appears to be
asking for new SAs so frequent as to negatively affect group
communications).
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5. IANA Considerations
Note to RFC Editor: this section may be removed on publication as an
RFC.
This memo has no IANA considerations.
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6. Security Considerations
This specification provides a method for securely using crypto
algorithms that require a unique IV, such as a block cipher mode of
operation based on counter mode, in a scenario in which there are
multiple encryption devices. When its recommendations are followed,
the security of the crypto algorithms is equivalent to the
conventional case in which there is a single sender.
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7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 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.
7.2. Informative References
[FIPS.800-38A.2001]
National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation",
FIPS PUB 800-38A, December 2001, <http://csrc.nist.gov/
publications/nistpubs/800-38a/sp800-38a.pdf>.
[I-D.ietf-msec-ipsec-extensions]
Weis, B., Gross, G., and D. Ignjatac, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", draft-ietf-msec-ipsec-extensions-02 (work in
progress), June 2006.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC3170] Quinn, B. and K. Almeroth, "IP Multicast Applications:
Challenges and Solutions", RFC 3170, September 2001.
[RFC3547] Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
Group Domain of Interpretation", RFC 3547, July 2003.
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[RFC4306] Kaufman, C., "Internet Key Exchange (IKEv2) Protocol",
RFC 4306, December 2005.
[RFC4535] Harney, H., Meth, U., Colegrove, A., and G. Gross,
"GSAKMP: Group Secure Association Key Management
Protocol", RFC 4535, June 2006.
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Appendix A. Rationale for the Recommended IV Formation
The two main alternatives for ensuring the uniqueness of IVs in a
multi-sender environment are to have each sender include a Sender
Identifier (SID) value in either the Salt value or in the explicit IV
field (recall that the IV used as input to the crypto algorithm is
constructed by concatenating the Salt and the explicit IV). The
explicit IV field was chosen as the location for the SID because it
is explicitly present in the packet. If the SID had been included in
the Salt, then a receiver would need to infer the SID value for a
particular ESP packet by recognizing which sender had sent that
packet. This inference could be made on the IP source address, if
ESP is transported directly over IP. However, if an alternate
transport mechanism such as UDP is being used (e.g. for NAT
traversal), the method used to infer the sender would need to take
that mechanism into account. It is simpler to use the explicit IV
field, and thus avoid the need to infer the sender from the packet at
all.
The normative requirement that all of the SID values used with a
particular Security Association must have the same length is not
strictly necessary, but was added to promote simplicity of
implementation. Alternatively, it would be acceptable to have the
SID values be chosen to be the codewords of a variable-length prefix
free code. This approach preserves security since the distinctness
of the IVs follows from the fact that no SID is a prefix of another;
thus any pair of IVs has a subset of bits that are distinct. If a
Huffman code is used to form the SIDs, then a set of optimal SIDs can
be found, in the sense that the number of SIDs can be maximized for a
given distribution of SID lengths. Additionally, there are simple
methods for generating efficient prefix free codes whose codewords
are octet strings. Nevertheless, these methods were disallowed in
order to favor simplicity over generality.
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Authors' Addresses
David A. McGrew
Cisco Systems
170 W. Tasman Drive
San Jose, California 95134-1706
USA
Phone: +1-408-525-8651
Email: mcgrew@cisco.com
Brian Weis
Cisco Systems
170 W. Tasman Drive
San Jose, California 95134-1706
USA
Phone: +1-408-526-4796
Email: bew@cisco.com
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