AES-GCM for TLS: AES Galois Counter Mode (GCM) Cipher Suites for TLS
Twelve TLS 1.2 cipher suites that use AES-GCM for encryption and authentication.
Salowey, J., Choudhury, A., and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, DOI 10.17487/RFC5288, August 2008, <https://www.rfc-editor.org/info/rfc5288>.
@misc{rfc5288,
series = {Request for Comments},
number = 5288,
howpublished = {RFC 5288},
publisher = {RFC Editor},
doi = {10.17487/RFC5288},
url = {https://www.rfc-editor.org/info/rfc5288},
author = {J. Salowey and A. Choudhury and D. McGrew},
title = {{AES Galois Counter Mode (GCM) Cipher Suites for TLS}},
pagetotal = 8,
year = 2008,
month = aug,
} RFC text
Contents
Network Working Group J. Salowey
Request for Comments: 5288 A. Choudhury
Category: Standards Track D. McGrew
Cisco Systems, Inc.
August 2008
AES Galois Counter Mode (GCM) Cipher Suites for TLS
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.
Abstract
This memo describes the use of the Advanced Encryption Standard (AES)
in Galois/Counter Mode (GCM) as a Transport Layer Security (TLS)
authenticated encryption operation. GCM provides both
confidentiality and data origin authentication, can be efficiently
implemented in hardware for speeds of 10 gigabits per second and
above, and is also well-suited to software implementations. This
memo defines TLS cipher suites that use AES-GCM with RSA, DSA, and
Diffie-Hellman-based key exchange mechanisms.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions Used in This Document . . . . . . . . . . . . . . . 2
3. AES-GCM Cipher Suites . . . . . . . . . . . . . . . . . . . . . 2
4. TLS Versions . . . . . . . . . . . . . . . . . . . . . . . . . 3
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 4
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 4
6.1. Counter Reuse . . . . . . . . . . . . . . . . . . . . . . . 4
6.2. Recommendations for Multiple Encryption Processors . . . . 4
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
8.1. Normative References . . . . . . . . . . . . . . . . . . . 6
8.2. Informative References . . . . . . . . . . . . . . . . . . 6
1. Introduction
This document describes the use of AES [AES] in Galois Counter Mode
(GCM) [GCM] (AES-GCM) with various key exchange mechanisms as a
cipher suite for TLS. AES-GCM is an authenticated encryption with
associated data (AEAD) cipher (as defined in TLS 1.2 [RFC5246])
providing both confidentiality and data origin authentication. The
following sections define cipher suites based on RSA, DSA, and
Diffie-Hellman key exchanges; ECC-based (Elliptic Curve Cryptography)
cipher suites are defined in a separate document [RFC5289].
AES-GCM is not only efficient and secure, but hardware
implementations can achieve high speeds with low cost and low
latency, because the mode can be pipelined. Applications that
require high data throughput can benefit from these high-speed
implementations. AES-GCM has been specified as a mode that can be
used with IPsec ESP [RFC4106] and 802.1AE Media Access Control (MAC)
Security [IEEE8021AE].
2. 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].
3. AES-GCM Cipher Suites
The following cipher suites use the new authenticated encryption
modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) [GCM]:
CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}
These cipher suites use the AES-GCM authenticated encryption with
associated data (AEAD) algorithms AEAD_AES_128_GCM and
AEAD_AES_256_GCM described in [RFC5116]. Note that each of these
AEAD algorithms uses a 128-bit authentication tag with GCM (in
particular, as described in Section 3.5 of [RFC4366], the
"truncated_hmac" extension does not have an effect on cipher suites
that do not use HMAC). The "nonce" SHALL be 12 bytes long consisting
of two parts as follows: (this is an example of a "partially
explicit" nonce; see Section 3.2.1 in [RFC5116]).
struct {
opaque salt[4];
opaque nonce_explicit[8];
} GCMNonce;
The salt is the "implicit" part of the nonce and is not sent in the
packet. Instead, the salt is generated as part of the handshake
process: it is either the client_write_IV (when the client is
sending) or the server_write_IV (when the server is sending). The
salt length (SecurityParameters.fixed_iv_length) is 4 octets.
The nonce_explicit is the "explicit" part of the nonce. It is chosen
by the sender and is carried in each TLS record in the
GenericAEADCipher.nonce_explicit field. The nonce_explicit length
(SecurityParameters.record_iv_length) is 8 octets.
Each value of the nonce_explicit MUST be distinct for each distinct
invocation of the GCM encrypt function for any fixed key. Failure to
meet this uniqueness requirement can significantly degrade security.
The nonce_explicit MAY be the 64-bit sequence number.
The RSA, DHE_RSA, DH_RSA, DHE_DSS, DH_DSS, and DH_anon key exchanges
are performed as defined in [RFC5246].
The Pseudo Random Function (PRF) algorithms SHALL be as follows:
For cipher suites ending with _SHA256, the PRF is the TLS PRF
[RFC5246] with SHA-256 as the hash function.
For cipher suites ending with _SHA384, the PRF is the TLS PRF
[RFC5246] with SHA-384 as the hash function.
Implementations MUST send TLS Alert bad_record_mac for all types of
failures encountered in processing the AES-GCM algorithm.
4. TLS Versions
These cipher suites make use of the authenticated encryption with
additional data defined in TLS 1.2 [RFC5246]. They MUST NOT be
negotiated in older versions of TLS. Clients MUST NOT offer these
cipher suites if they do not offer TLS 1.2 or later. Servers that
select an earlier version of TLS MUST NOT select one of these cipher
suites. Because TLS has no way for the client to indicate that it
supports TLS 1.2 but not earlier, a non-compliant server might
potentially negotiate TLS 1.1 or earlier and select one of the cipher
suites in this document. Clients MUST check the TLS version and
generate a fatal "illegal_parameter" alert if they detect an
incorrect version.
5. IANA Considerations
IANA has assigned the following values for the cipher suites defined
in this document:
CipherSuite TLS_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9C}
CipherSuite TLS_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9D}
CipherSuite TLS_DHE_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0x9E}
CipherSuite TLS_DHE_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0x9F}
CipherSuite TLS_DH_RSA_WITH_AES_128_GCM_SHA256 = {0x00,0xA0}
CipherSuite TLS_DH_RSA_WITH_AES_256_GCM_SHA384 = {0x00,0xA1}
CipherSuite TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA2}
CipherSuite TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA3}
CipherSuite TLS_DH_DSS_WITH_AES_128_GCM_SHA256 = {0x00,0xA4}
CipherSuite TLS_DH_DSS_WITH_AES_256_GCM_SHA384 = {0x00,0xA5}
CipherSuite TLS_DH_anon_WITH_AES_128_GCM_SHA256 = {0x00,0xA6}
CipherSuite TLS_DH_anon_WITH_AES_256_GCM_SHA384 = {0x00,0xA7}
6. Security Considerations
The security considerations in [RFC5246] apply to this document as
well. The remainder of this section describes security
considerations specific to the cipher suites described in this
document.
6.1. Counter Reuse
AES-GCM security requires that the counter is never reused. The IV
construction in Section 3 is designed to prevent counter reuse.
Implementers should also understand the practical considerations of
IV handling outlined in Section 9 of [GCM].
6.2. Recommendations for Multiple Encryption Processors
If multiple cryptographic processors are in use by the sender, then
the sender MUST ensure that, for a particular key, each value of the
nonce_explicit used with that key is distinct. In this case, each
encryption processor SHOULD include, in the nonce_explicit, a fixed
value that is distinct for each processor. The recommended format is
nonce_explicit = FixedDistinct || Variable
where the FixedDistinct field is distinct for each encryption
processor, but is fixed for a given processor, and the Variable field
is distinct for each distinct nonce used by a particular encryption
processor. When this method is used, the FixedDistinct fields used
by the different processors MUST have the same length.
In the terms of Figure 2 in [RFC5116], the Salt is the Fixed-Common
part of the nonce (it is fixed, and it is common across all
encryption processors), the FixedDistinct field exactly corresponds
to the Fixed-Distinct field, the Variable field corresponds to the
Counter field, and the explicit part exactly corresponds to the
nonce_explicit.
For clarity, we provide an example for TLS in which there are two
distinct encryption processors, each of which uses a one-byte
FixedDistinct field:
Salt = eedc68dc
FixedDistinct = 01 (for the first encryption processor)
FixedDistinct = 02 (for the second encryption processor)
The GCMnonces generated by the first encryption processor, and their
corresponding nonce_explicit, are:
GCMNonce nonce_explicit
------------------------ ----------------------------
eedc68dc0100000000000000 0100000000000000
eedc68dc0100000000000001 0100000000000001
eedc68dc0100000000000002 0100000000000002
...
The GCMnonces generated by the second encryption processor, and their
corresponding nonce_explicit, are
GCMNonce nonce_explicit
------------------------ ----------------------------
eedc68dc0200000000000000 0200000000000000
eedc68dc0200000000000001 0200000000000001
eedc68dc0200000000000002 0200000000000002
...
7. Acknowledgements
This document borrows heavily from [RFC5289]. The authors would like
to thank Alex Lam, Simon Josefsson, and Pasi Eronen for providing
useful comments during the review of this document.
8. References
8.1. Normative References
[AES] National Institute of Standards and Technology,
"Advanced Encryption Standard (AES)", FIPS 197,
November 2001.
[GCM] Dworkin, M., "Recommendation for Block Cipher Modes of
Operation: Galois/Counter Mode (GCM) and GMAC",
National Institute of Standards and Technology SP 800-
38D, November 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5116] McGrew, D., "An Interface and Algorithms for
Authenticated Encryption", RFC 5116, January 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer
Security (TLS) Protocol Version 1.2", RFC 5246,
August 2008.
8.2. Informative References
[IEEE8021AE] Institute of Electrical and Electronics Engineers,
"Media Access Control Security", IEEE Standard 802.1AE,
August 2006.
[RFC4106] Viega, J. and D. McGrew, "The Use of Galois/Counter
Mode (GCM) in IPsec Encapsulating Security Payload
(ESP)", RFC 4106, June 2005.
[RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen,
J., and T. Wright, "Transport Layer Security (TLS)
Extensions", RFC 4366, April 2006.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with
SHA-256/384 and AES Galois Counter Mode", RFC 5289,
August 2008.
Authors' Addresses
Joseph Salowey
Cisco Systems, Inc.
2901 3rd. Ave
Seattle, WA 98121
USA
EMail: jsalowey@cisco.com
Abhijit Choudhury
Cisco Systems, Inc.
3625 Cisco Way
San Jose, CA 95134
USA
EMail: abhijitc@cisco.com
David McGrew
Cisco Systems, Inc.
170 W Tasman Drive
San Jose, CA 95134
USA
EMail: mcgrew@cisco.com
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
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, THE IETF TRUST 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-ipr@ietf.org.
ISO Plain Language
On this page
§1AES-GCM as a TLS Cipher
RFC 5288 specifies how Transport Layer Security (TLS) uses the Advanced Encryption Standard (AES) in Galois Counter Mode (GCM). GCM is authenticated encryption with associated data (AEAD): it provides confidentiality and data origin authentication.
The RFC focuses on RSA, DSA, and Diffie-Hellman key exchanges. It says the mode can be efficiently implemented in hardware and software, and that elliptic-curve cipher suites are defined in a separate document.
AES-GCM encrypts TLS records and checks their authenticity in one operation. This RFC defines the TLS cipher suites that use it.
§3The Twelve Cipher Suites
The RFC defines twelve suites in six pairs. Each pair uses either a 128-bit or 256-bit AES key; the suite name also identifies a key-exchange method and a hash for the TLS pseudorandom function.
The key-exchange names are RSA, DHE_RSA, DH_RSA, DHE_DSS, DH_DSS, and DH_anon. The 128-bit suites use SHA-256 for the TLS PRF, and the 256-bit suites use SHA-384. Each suite uses a 128-bit authentication tag.
IANA assigned the suite values from 0x00,0x9C through 0x00,0xA7, in the order listed in the RFC.
The suite list offers six key-exchange choices, each with a 128-bit and a 256-bit AES-GCM version. The suite name also determines which hash the TLS pseudorandom function uses.
§3.1The GCM Nonce in Each Record
AES-GCM uses a nonce, a value that must not be repeated with the same key. RFC 5288 defines a 12-byte nonce with two parts: a 4-byte salt and an 8-byte nonce_explicit.
The salt comes from the TLS handshake and is not sent in each packet. The sender chooses the explicit part and carries it in every TLS record. It may use the 64-bit record sequence number for that value.
For each fixed key, every invocation of GCM encryption must use a distinct explicit nonce. Reusing one can significantly weaken security.
Each TLS record needs a nonce that is unique for its encryption key. The handshake supplies the salt, and the sender includes the explicit part in the record.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SALT | salt | 4 bytes | Implicit part from the TLS handshake; not sent in the packet. | |
NONCE-EXPLICIT | nonce_explicit | 8 bytes | Chosen by the sender and carried in each TLS record. |
§3.2The TLS Pseudorandom Function and Errors
The suite's suffix selects the hash used by the TLS pseudorandom function (PRF). A suite ending in _SHA256 uses the TLS PRF with SHA-256; a suite ending in _SHA384 uses it with SHA-384.
If any failure occurs while processing AES-GCM, an implementation must send the TLS alert bad_record_mac. The RFC does not define separate alerts for different GCM processing failures.
The suite name tells the peers which hash to use for the TLS PRF. If AES-GCM processing fails, the implementation reports the same TLS alert for every failure type.
§4TLS 1.2 or Later Is Required
These cipher suites rely on authenticated encryption defined in TLS 1.2. They must not be negotiated with an earlier TLS version. A client must not offer them unless it also offers TLS 1.2 or later, and a server selecting an earlier version must not select one.
The RFC notes that a client cannot indicate support for TLS 1.2 without also permitting earlier versions. Therefore, if a server selects an earlier version with one of these suites, the client must send a fatal illegal_parameter alert.
These suites work only with TLS 1.2 or later. A client must stop if a server selects one while negotiating an earlier version.
§6Avoiding Counter Reuse
The main additional security concern is reuse of a GCM counter value. The nonce construction is intended to prevent reuse, but implementations still need to ensure uniqueness under each key.
If a sender uses multiple encryption processors, each processor must use distinct explicit nonce values for a given key. The RFC recommends assigning each processor a fixed distinct part and combining it with a variable part that changes for each nonce. Those fixed parts must have equal lengths.
Two encryption operations must not reuse the same nonce with one key. When several processors encrypt records, each needs a way to produce distinct nonce values.