Network Working Group J. Viega

Network Working Group J. Viega
Request for Comments: 4106 Secure Software, Inc.
Category: Standards Track D. McGrew
Cisco Systems, Inc.
June 2005

The Use of Galois/Counter Mode (GCM)
in IPsec Encapsulating Security Payload (ESP)

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 Internet Society (2005).

Abstract

This memo describes the use of the Advanced Encryption Standard (AES)
in Galois/Counter Mode (GCM) as an IPsec Encapsulating Security
Payload (ESP) mechanism to provide confidentiality and data origin
authentication. This method can be efficiently implemented in
hardware for speeds of 10 gigabits per second and above, and is also
well-suited to software implementations.

Table of Contents

1. Introduction ....................................................2
1.1. Conventions Used in This Document ..........................2
2. AES-GCM .........................................................3
3. ESP Payload Data ................................................3
3.1. Initialization Vector (IV) .................................3
3.2. Ciphertext .................................................4
4. Nonce Format ....................................................4
5. AAD Construction ................................................5
6. Integrity Check Value (ICV) .....................................5
7. Packet Expansion ................................................6
8. IKE Conventions .................................................6
8.1. Keying Material and Salt Values ............................6
8.2. Phase 1 Identifier .........................................6
8.3. Phase 2 Identifier .........................................7
8.4. Key Length Attribute .......................................7

Viega & McGrew Standards Track [Page 1]

RFC 4106 GCM ESP June 2005

9. Test Vectors ....................................................7
10. Security Considerations ........................................7
11. Design Rationale ...............................................8
12. IANA Considerations ............................................8
13. Acknowledgements ...............................................9
14. Normative References ...........................................9
15. Informative References .........................................9

1. Introduction

This document describes the use of AES in GCM mode (AES-GCM) as an
IPsec ESP mechanism for confidentiality and data origin
authentication. We refer to this method as AES-GCM-ESP. This
mechanism is not only efficient and secure, but it also enables
high-speed implementations in hardware. Thus, AES-GCM-ESP allows
IPsec connections that can make effective use of emerging 10-gigabit
and 40-gigabit network devices.

Counter mode (CTR) has emerged as the preferred encryption method for
high-speed implementations. Unlike conventional encryption modes
such as Cipher Block Chaining (CBC) and Cipher Block Chaining Message
Authentication Code (CBC-MAC), CTR can be efficiently implemented at
high data rates because it can be pipelined. The ESP CTR protocol
describes how this mode can be used with IPsec ESP [RFC3686].

Unfortunately, CTR provides no data origin authentication, and thus
the ESP CTR standard requires the use of a data origin authentication
algorithm in conjunction with CTR. This requirement is problematic,
because none of the standard data origin authentication algorithms
can be efficiently implemented for high data rates. GCM solves this
problem, because under the hood, it combines CTR mode with a secure,
parallelizable, and efficient authentication mechanism.

This document does not cover implementation details of GCM. Those
details can be found in [GCM], along with test vectors.

1.1. Conventions Used in This Document

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].

Viega & McGrew Standards Track [Page 2]

RFC 4106 GCM ESP June 2005

2. AES-GCM

GCM is a block cipher mode of operation providing both
confidentiality and data origin authentication. The GCM
authenticated encryption operation has four inputs: a secret key, an
initialization vector (IV), a plaintext, and an input for additional
authenticated data (AAD). It has two outputs, a ciphertext whose
length is identical to the plaintext, and an authentication tag. In
the following, we describe how the IV, plaintext, and AAD are formed
from the ESP fields, and how the ESP packet is formed from the
ciphertext and authentication tag.

ESP also defines an IV. For clarity, we refer to the AES-GCM IV as a
nonce in the context of AES-GCM-ESP. The same nonce and key
combination MUST NOT be used more than once.

Because reusing an nonce/key combination destroys the security
guarantees of AES-GCM mode, it can be difficult to use this mode
securely when using statically configured keys. For safety's sake,
implementations MUST use an automated key management system, such as
the Internet Key Exchange (IKE) [RFC2409], to ensure that this
requirement is met.

3. ESP Payload Data

The ESP Payload Data is comprised of an eight-octet initialization
vector (IV), followed by the ciphertext. The payload field, as
defined in [RFC2406], is structured as shown in Figure 1, along with
the ICV associated with the payload.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| (8 octets) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
~ Ciphertext (variable) ~
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 1: ESP Payload Encrypted with AES-GCM.

3.1. Initialization Vector (IV)

The AES-GCM-ESP IV field MUST be eight octets. For a given key, the
IV MUST NOT repeat. The most natural way to implement this is with a
counter, but anything that guarantees uniqueness can be used, such as

Viega & McGrew Standards Track [Page 3]

RFC 4106 GCM ESP June 2005

a linear feedback shift register (LFSR). Note that the encrypter can
use any IV generation method that meets the uniqueness requirement,
without coordinating with the decrypter.

3.2. Ciphertext

The plaintext input to AES-GCM is formed by concatenating the
plaintext data described by the Next Header field with the Padding,
the Pad Length, and the Next Header field. The Ciphertext field
consists of the ciphertext output from the AES-GCM algorithm. The
length of the ciphertext is identical to that of the plaintext.

Implementations that do not seek to hide the length of the plaintext
SHOULD use the minimum amount of padding required, which will be less
than four octets.

4. Nonce Format

The nonce passed to the GCM-AES encryption algorithm has the
following layout:

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Salt |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Initialization Vector |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 2: Nonce Format

The components of the nonce are as follows:

Salt
The salt field is a four-octet value that is assigned at the
beginning of the security association, and then remains constant
for the life of the security association. The salt SHOULD be
unpredictable (i.e., chosen at random) before it is selected, but
need not be secret. We describe how to set the salt for a
Security Association established via the Internet Key Exchange in
Section 8.1.

Initialization Vector
The IV field is described in Section 3.1.

Viega & McGrew Standards Track [Page 4]

RFC 4106 GCM ESP June 2005

5. AAD Construction

The authentication of data integrity and data origin for the SPI and
(Extended) Sequence Number fields is provided without encryption.
This is done by including those fields in the AES-GCM Additional
Authenticated Data (AAD) field. Two formats of the AAD are defined:
one for 32-bit sequence numbers, and one for 64-bit extended sequence
numbers. The format with 32-bit sequence numbers is shown in Figure
3, and the format with 64-bit extended sequence numbers is shown in
Figure 4.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 32-bit Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 3: AAD Format with 32-bit Sequence Number

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| SPI |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 64-bit Extended Sequence Number |
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Figure 4: AAD Format with 64-bit Extended Sequence Number

6. Integrity Check Value (ICV)

The ICV consists solely of the AES-GCM Authentication Tag.
Implementations MUST support a full-length 16-octet ICV, and MAY
support 8 or 12 octet ICVs, and MUST NOT support other ICV lengths.
Although ESP does not require that an ICV be present, AES-GCM-ESP
intentionally does not allow a zero-length ICV. This is because GCM
provides no integrity protection whatsoever when used with a zero-
length Authentication Tag.

Viega & McGrew Standards Track [Page 5]

RFC 4106 GCM ESP June 2005

7. Packet Expansion

The IV adds an additional eight octets to the packet, and the ICV
adds an additional 8, 12, or 16 octets. These are the only sources
of packet expansion, other than the 10-13 octets taken up by the ESP
SPI, Sequence Number, Padding, Pad Length, and Next Header fields (if
the minimal amount of padding is used).

8. IKE Conventions

This section describes the conventions used to generate keying
material and salt values, for use with AES-GCM-ESP, using the
Internet Key Exchange (IKE) [RFC2409] protocol. The identifiers and
attributes needed to negotiate a security association using AES-GCM-
ESP are also defined.

8.1. Keying Material and Salt Values

IKE makes use of a pseudo-random function (PRF) to derive keying
material. The PRF is used iteratively to derive keying material of
arbitrary size, called KEYMAT. Keying material is extracted from the
output string without regard to boundaries.

The size of the KEYMAT for the AES-GCM-ESP MUST be four octets longer
than is needed for the associated AES key. The keying material is
used as follows:

AES-GCM-ESP with a 128 bit key
The KEYMAT requested for each AES-GCM key is 20 octets. The first
16 octets are the 128-bit AES key, and the remaining four octets
are used as the salt value in the nonce.

AES-GCM-ESP with a 192 bit key
The KEYMAT requested for each AES-GCM key is 28 octets. The first
24 octets are the 192-bit AES key, and the remaining four octets
are used as the salt value in the nonce.

AES-GCM-ESP with a 256 bit key
The KEYMAT requested for each AES GCM key is 36 octets. The first
32 octets are the 256-bit AES key, and the remaining four octets
are used as the salt value in the nonce.

8.2. Phase 1 Identifier

This document does not specify the conventions for using AES-GCM for
IKE Phase 1 negotiations. For AES-GCM to be used in this manner, a
separate specification is needed, and an Encryption Algorithm
Identifier needs to be assigned. Implementations SHOULD use an IKE

Viega & McGrew Standards Track [Page 6]

RFC 4106 GCM ESP June 2005

Phase 1 cipher that is at least as strong as AES-GCM. The use of AES
CBC [RFC3602] with the same key size used by AES-GCM-ESP is
RECOMMENDED.

8.3. Phase 2 Identifier

For IKE Phase 2 negotiations, IANA has assigned three ESP Transform
Identifiers for AES-GCM with an eight-byte explicit IV:

18 for AES-GCM with an 8 octet ICV;
19 for AES-GCM with a 12 octet ICV; and
20 for AES-GCM with a 16 octet ICV.

8.4. Key Length Attribute

Because the AES supports three key lengths, the Key Length attribute
MUST be specified in the IKE Phase 2 exchange [RFC2407]. The Key
Length attribute MUST have a value of 128, 192, or 256.

9. Test Vectors

Appendix B of [GCM] provides test vectors that will assist
implementers with AES-GCM mode.

10. Security Considerations

GCM is provably secure against adversaries that can adaptively choose
plaintexts, ciphertexts, ICVs, and the AAD field, under standard
cryptographic assumptions (roughly, that the output of the underlying
cipher, under a randomly chosen key, is indistinguishable from a
randomly selected output). Essentially, this means that, if used
within its intended parameters, a break of GCM implies a break of the
underlying block cipher. The proof of security for GCM is available
in [GCM].

The most important security consideration is that the IV never repeat
for a given key. In part, this is handled by disallowing the use of
AES-GCM when using statically configured keys, as discussed in
Section 2.

When IKE is used to establish fresh keys between two peer entities,
separate keys are established for the two traffic flows. If a
different mechanism is used to establish fresh keys (one that
establishes only a single key to encrypt packets), then there is a
high probability that the peers will select the same IV values for
some packets. Thus, to avoid counter block collisions, ESP

Viega & McGrew Standards Track [Page 7]

RFC 4106 GCM ESP June 2005

implementations that permit use of the same key for encrypting and
decrypting packets with the same peer MUST ensure that the two peers
assign different salt values to the security association (SA).

The other consideration is that, as with any encryption mode, the
security of all data protected under a given security association
decreases slightly with each message.

To protect against this problem, implementations MUST generate a
fresh key before encrypting 2^64 blocks of data with a given key.
Note that it is impossible to reach this limit when using 32-bit
Sequence Numbers.

Note that, for each message, GCM calls the block cipher once for each
full 16-octet block in the payload, once for any remaining octets in
the payload, and one additional time for computing the ICV.

Clearly, smaller ICV values are more likely to be subject to forgery
attacks. Implementations SHOULD use as large a size as reasonable.

11. Design Rationale

This specification was designed to be as similar to the AES-CCM ESP
[CCM-ESP] and AES-CTR ESP [RFC3686] mechanisms as reasonable, while
promoting simple, efficient implementations in both hardware and
software. We re-use the design and implementation experience from
those standards.

The major difference with CCM is that the CCM ESP mechanism requires
an 11-octet nonce, whereas the GCM ESP mechanism requires using a
12-octet nonce. GCM is specially optimized to handle the 12-octet
nonce case efficiently. Nonces of other lengths would cause
unnecessary, additional complexity and delays, particularly in
hardware implementations. The additional octet of nonce is used to
increase the size of the salt.

12. IANA Considerations

IANA has assigned three ESP Transform Identifiers for AES-GCM with an
eight-byte explicit IV:

18 for AES-GCM with an 8 octet ICV;
19 for AES-GCM with a 12 octet ICV; and
20 for AES-GCM with a 16 octet ICV.

Viega & McGrew Standards Track [Page 8]

RFC 4106 GCM ESP June 2005

13. Acknowledgements

This work is closely modeled after Russ Housley's AES-CCM transform
[CCM-ESP]. Portions of this document are directly copied from that
work in progress. We thank Russ for his support of this work.

Additionally, the GCM mode of operation was originally conceived as
an improvement to Carter-Wegman Counter (CWC) mode [CWC], the first
unencumbered block cipher mode capable of supporting high-speed
authenticated encryption.

14. Normative References

[GCM] McGrew, D. and J. Viega, "The Galois/Counter Mode of
Operation (GCM)", Submission to NIST. http://
csrc.nist.gov/CryptoToolkit/modes/proposedmodes/gcm/
gcm-spec.pdf, January 2004.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[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.

[RFC3602] Frankel, S., Glenn, R. and S. Kelly, "The AES-CBC Cipher
Algorithm and Its Use with IPsec", RFC 3602, September
2003.

15. Informative References

[CCM-ESP] Housley, R., "Using AES CCM Mode With IPsec ESP", Work In
Progress.

[CWC] Kohno, T., Viega, J. and D. Whiting, "CWC: A high-
performance conventional authenticated encryption mode",
Fast Software Encryption. http://eprint.iacr.org/
2003/106.pdf, February 2004.

[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.

[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.

Viega & McGrew Standards Track [Page 9]

RFC 4106 GCM ESP June 2005

Authors' Addresses

John Viega
Secure Software, Inc.
4100 Lafayette Center Dr., Suite 100
Chantilly, VA 20151
US

Phone: (703) 814 4402
EMail: [email protected]

David A. McGrew
Cisco Systems, Inc.
510 McCarthy Blvd.
Milpitas, CA 95035
US

Phone: (408) 525 8651
EMail: [email protected]
URI: http://www.mindspring.com/~dmcgrew/dam.htm

Viega & McGrew Standards Track [Page 10]

RFC 4106 GCM ESP June 2005

Full Copyright Statement

Copyright (C) The Internet Society (2005).

This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.

This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Intellectual Property

The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.

Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
[email protected].

Acknowledgement

Funding for the RFC Editor function is currently provided by the
Internet Society.

Viega & McGrew Standards Track [Page 11]