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PROPOSED STANDARD
Internet Engineering Task Force (IETF) D. McGrew
Request for Comments: 6054 B. Weis
Category: Standards Track Cisco Systems
ISSN: 2070-1721 November 2010
Using Counter Modes with Encapsulating Security Payload (ESP) and
Authentication Header (AH) to Protect Group Traffic
Abstract
Counter modes have been defined for block ciphers such as the
Advanced Encryption Standard (AES). Counter modes use a counter,
which is typically assumed to be incremented by a single sender.
This memo describes the use of counter modes when applied to the
Encapsulating Security Payload (ESP) and Authentication Header (AH)
in multiple-sender group applications.
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/rfc6054.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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Table of Contents
1. Introduction ....................................................2
1.1. Requirements Notation ......................................2
2. Problem Statement ...............................................2
3. IV Formation for Counter Modes with Group Keys ..................3
4. Group Key Management Conventions ................................4
5. Security Considerations .........................................5
6. Acknowledgements ................................................6
7. References ......................................................6
7.1. Normative References .......................................6
7.2. Informative References .....................................6
Appendix A. Rationale for the IV Formation for Counter Modes
with Group Keys ........................................9
Appendix B. Example ................................................9
1. Introduction
The IP Encapsulating Security Payload (ESP) specification [RFC4303]
and Authentication Header (AH) [RFC4302] are security protocols for
IPsec [RFC4301]. Several new AES encryption modes of operation have
been specified for ESP: Counter Mode (CTR) [RFC3686], Galois/Counter
Mode (GCM) [RFC4106], and Counter with Cipher Block Chaining-Message
Authentication Code (CBC-MAC) Mode (CCM) [RFC4309]; and one that has
been specified for both ESP and AH: the Galois Message Authentication
Code (GMAC) [RFC4543]. A Camellia counter mode [RFC5528] and a GOST
counter mode [RFC4357] have also been specified. 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].
2. Problem Statement
The Counter Mode (CTR) of operation [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
authentication with encryption, including CCM and GCM. All of the
counter-based modes require that, if a single key is shared by
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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.
In cryptographic terms, the IV is a nonce. (Note that CBC mode
[RFC3602] requires IVs that are unpredictable. CTR, GCM, GMAC, and
CCM do not have this restriction.)
All ESP and AH transforms using a block cipher 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, block cipher counter mode transforms are
often preferred because of their favorable performance
characteristics as compared to other modes.
Each of the block cipher counter mode transforms specify the
construction of keying material for point-to-point applications that
are keyed by the Internet Key Exchange version 2 (IKEv2) [RFC5996].
The specified constructions guarantee that the security condition is
not violated by a single sender. Group applications of IPsec
[RFC5374] may also find counter mode transforms to be valuable. Some
group applications can create an IPsec Security Association (SA) per
sender, which meets the security condition, and no further
specification is required. However, IPsec can be used to protect
group applications known as Many-to-Many Applications [RFC3170],
where a single IPsec SA is used to protect network traffic between
members of a multiple-sender IP multicast application. Some Many-to-
Many Applications are comprised of a large number of senders, in
which case defining an individual IPsec SA for each sender is
unmanageable.
3. IV Formation for Counter Modes with Group Keys
This section specifies a particular construction of the IV that
enables a group of senders to safely share a single IPsec SA. This
construction conforms to the recommendations of [RFC5116]. A
rationale for this method is given in Appendix A. In the
construction defined by this specification, 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. By way of example,
Figure 1 shows the correct placement of an 8-bit SID within an
Initialization Vector.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
! SID ! !
+-+-+-+-+-+-+-+-+ SSIV !
! !
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-!
Figure 1. IV with an 8-bit SID
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. To facilitate
interoperability, 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 trade-off between the number
of possible senders and the number of packets that each sending
station is able to send using that SA.
4. Group Key Management Conventions
Group applications use a Group Key Management System (GKMS) composed
of one or more Group Controller and Key Server (GCKS) entities
[RFC3740]. The GKMS distributes IPsec transform policy and
associated keying material to authorized group members. This
document RECOMMENDS that the GKMS both allocate unique SIDs to group
members and distribute them to group members using a GKM protocol
such as GDOI or GSAKMP. The strategy used by the GKMS does not need
to be mandated in order to achieve interoperability; the GKMS is
solely responsible for allocating SIDs for the group. Allocating
SIDs sequentially is acceptable as long as the allocation method
follows the requirements in this section.
The following requirements apply to a GKMS that manages SIDs. One
example of such a GKMS is [GDOI-UPDATE].
o For each SA for which sender identifiers are used, the GKMS MUST
NOT give the same sender identifier to more than one active group
member. If the GKMS is uncertain as to the SID associated with a
group member, it MUST allocate it a new one. If more than one
entity within the GKMS is distributing sender identifiers, then
the sets of identifiers distributed by each entity MUST NOT
overlap.
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o If the entire set of sender identifiers has been exhausted, the
GKMS MUST refuse to allow new group members to join.
Alternatively, the GKMS could distribute replacement ESP or AH
Security Associations to all group members. When replacement SAs
are distributed, the GKMS could also distribute larger SID values
so that more senders can be accommodated.
o The GKMS 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 enables both the
GKMS and the senders to avoid managing SIDs on a per-SA basis. It
also simplifies the rekeying process, since SIDs do not need to be
changed or re-issued along with replacement SAs during a rekey
event.
o When a GKMS determines that a particular group member is no longer
a part of the group, then it MAY re-allocate any sender identifier
associated with that group member for use with a new group member.
In this case, the GKMS 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.
5. Security Considerations
This specification provides a method for securely using cryptographic
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 cryptographic devices that each generate IVs. This is done
by partitioning the set of possible IV values such that each
cryptographic device has exclusive use of a set of IV values. When
the recommendations in this specification are followed, the security
of the cryptographic algorithms is equivalent to the conventional
case in which there is a single sender. Unlike CBC mode, CTR, GCM,
GMAC, and CCM do not require IVs that are unpredictable.
The security of a group depends upon the correct operation of the
group members. Any group member using an SID not allocated to it may
reduce the security of the system.
As is the case with a single sender, a cryptographic device storing
keying material over a reboot is responsible for storing a counter
value such that upon resumption it never re-uses counters. In the
context of this specification, the cryptographic device would need to
store both SID and SSIV values used with a particular IPsec SA in
addition to policy associated with the IPsec SA.
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A group member that reaches the end of its IV space MUST stop sending
data traffic on that SA. This can happen if the group member does
not notify the GKMS in time for the GKMS to remedy the problem (e.g.,
to provide the group member with a new SID or to provide a new SA),
or if the GKMS ignores the notification for some reason. In this
case, the group member should re-register with the GCKS and expect to
receive the SAs that it needs to continue participating in the group.
This specification does not address virtual machine rollbacks that
may cause the cryptographic device to re-use nonce values.
Other security considerations applying to IPsec SAs with multiple
senders are described in [RFC5374].
6. Acknowledgements
The authors wish to thank David Black, Sheela Rowles, and Alfred
Hoenes for their helpful comments and suggestions.
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.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302, December
2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
7.2. Informative References
[FIPS.800-38A.2001]
National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation",
Special Publication FIPS PUB 800-38A, December 2001,
<http://csrc.nist.gov/publications/>.
[GDOI-UPDATE]
Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
of Interpretation", Work in Progress, October 2010.
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[H52] Huffman, D., "A Method for the Construction of Minimum-
Redundancy Codes", Proceedings of the IRE, Volume:40,
Issue:9, On page(s): 1098-1101, ISSN: 0096-8390,
September 1952, <http://ieeexplore.ieee.org/xpl/
freeabs_all.jsp?arnumber=4051119>.
[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.
[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.
[RFC3740] Hardjono, T. and B. Weis, "The Multicast Group Security
Architecture", RFC 3740, March 2004.
[RFC3948] Huttunen, A., Swander, B., Volpe, V., DiBurro, L., and M.
Stenberg, "UDP Encapsulation of IPsec ESP Packets",
RFC 3948, January 2005.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter Mode
(GCM) in IPsec Encapsulating Security Payload (ESP)",
RFC 4106, June 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4309] Housley, R., "Using Advanced Encryption Standard (AES)
CCM Mode with IPsec Encapsulating Security Payload
(ESP)", RFC 4309, December 2005.
[RFC4357] Popov, V., Kurepkin, I., and S. Leontiev, "Additional
Cryptographic Algorithms for Use with GOST 28147-89, GOST
R 34.10-94, GOST R 34.10-2001, and GOST R 34.11-94
Algorithms", RFC 4357, January 2006.
[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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[RFC4543] McGrew, D. and J. Viega, "The Use of Galois Message
Authentication Code (GMAC) in IPsec ESP and AH",
RFC 4543, May 2006.
[RFC5116] McGrew, D., "An Interface and Algorithms for
Authenticated Encryption", RFC 5116, January 2008.
[RFC5374] Weis, B., Gross, G., and D. Ignjatic, "Multicast
Extensions to the Security Architecture for the Internet
Protocol", RFC 5374, November 2008.
[RFC5528] Kato, A., Kanda, M., and S. Kanno, "Camellia Counter Mode
and Camellia Counter with CBC-MAC Mode Algorithms",
RFC 5528, April 2009.
[RFC5996] Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
"Internet Key Exchange Protocol Version 2 (IKEv2)",
RFC 5996, September 2010.
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Appendix A. Rationale for the IV Formation for Counter Modes with Group
Keys
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 AH or ESP packet by recognizing which sender had sent that
packet. This inference could be made on the IP source address, if AH
or ESP is transported directly over IP. However, if an alternate
transport mechanism such as UDP is being used [RFC3948] (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 [H52] 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.
Appendix B. Example
This section provides an example of SID allocation and IV generation,
as defined in this document. A GCKS administrator determines that
the group has one SA that is shared by all senders. The algorithm
for the SA is AES-GCM using an SID of size 8 bits.
When the first sender registers with the GCKS, it is allocated SID 1.
The sender subsequently sends AES-GCM encrypted packets with the
following IVs (shown in network byte order): 0x0100000000000001,
0x0100000000000002, 0x0100000000000003, ... with a final value of
0x01FFFFFFFFFFFFFF. The second sender registering with the GCKS is
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RFC 6054 Group Counter Modes November 2010
allocated SID 2, and begins sending packets with the following IVs:
0x0200000000000001, 0x0200000000000002, 0x0200000000000003, ... with
a final value of 0x02FFFFFFFFFFFFFF.
According to group policy, the GCKS may later distribute policy and
keying material for a replacement SA. When group senders begin
sending AES-GCM packets encrypted with the new SA, each sender
continues to use the SID value previously allocated to it. For
example, the sender allocated SID 2 would be sending on a new SA with
IV values of 0x0200000000000001, 0x0200000000000002,
0x0200000000000003, ... with a final value of 0x02FFFFFFFFFFFFFF.
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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