AES-GCM for TLS: AES Galois Counter Mode (GCM) Cipher Suites for TLS
Twelve TLS 1.2 cipher suites that encrypt and authenticate each record in one step with AES in Galois Counter Mode.
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
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AI ELI5
On this page
§1Why AES-GCM for TLS
RFC 5288 describes how to use the Advanced Encryption Standard (AES) in Galois Counter Mode (GCM) as a cipher suite for TLS. AES-GCM is an authenticated encryption with associated data (AEAD) cipher, as defined in TLS 1.2 (RFC 5246). It gives both confidentiality and data origin authentication in a single operation.
The RFC says the mode is efficient and secure, and that hardware implementations can reach high speeds with low cost and low latency because the mode can be pipelined. It also notes that AES-GCM is already specified for IPsec ESP (RFC 4106) and for 802.1AE MAC Security. Cipher suites based on elliptic curve cryptography are left to a separate document, RFC 5289.
Instead of one tool to hide a message and another to check it was not changed, this uses one fast tool that does both at once.
§3The Twelve Cipher Suites
Section 3 defines twelve cipher suites, six pairs. In each pair one suite uses AES with a 128-bit key and the SHA-256 hash, and the other uses a 256-bit key and SHA-384. The pairs differ in the key exchange: RSA, DHE_RSA, DH_RSA, DHE_DSS, DH_DSS and DH_anon. The key exchanges themselves work as defined in RFC 5246.
The suites use the AEAD algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM from RFC 5116. Each uses a 128-bit authentication tag, and the RFC points out that the truncated_hmac extension has no effect on cipher suites that do not use HMAC.
IANA assigned the values {0x00,0x9C} through {0x00,0xA7}, in the order listed in the table below.
It is a menu of twelve ready-made recipes. Each one picks a lock size and a way for the two sides to agree on a secret.
- 0x00,0x9CTLS_RSA_WITH_AES_128_GCM_SHA256 — RSA key exchange, 128-bit AES-GCM.
- 0x00,0x9DTLS_RSA_WITH_AES_256_GCM_SHA384 — RSA key exchange, 256-bit AES-GCM.
- 0x00,0x9ETLS_DHE_RSA_WITH_AES_128_GCM_SHA256 — DHE_RSA key exchange, 128-bit.
- 0x00,0x9FTLS_DHE_RSA_WITH_AES_256_GCM_SHA384 — DHE_RSA key exchange, 256-bit.
- 0x00,0xA0TLS_DH_RSA_WITH_AES_128_GCM_SHA256 — DH_RSA key exchange, 128-bit.
- 0x00,0xA1TLS_DH_RSA_WITH_AES_256_GCM_SHA384 — DH_RSA key exchange, 256-bit.
- 0x00,0xA2TLS_DHE_DSS_WITH_AES_128_GCM_SHA256 — DHE_DSS key exchange, 128-bit.
- 0x00,0xA3TLS_DHE_DSS_WITH_AES_256_GCM_SHA384 — DHE_DSS key exchange, 256-bit.
- 0x00,0xA4TLS_DH_DSS_WITH_AES_128_GCM_SHA256 — DH_DSS key exchange, 128-bit.
- 0x00,0xA5TLS_DH_DSS_WITH_AES_256_GCM_SHA384 — DH_DSS key exchange, 256-bit.
- 0x00,0xA6TLS_DH_anon_WITH_AES_128_GCM_SHA256 — DH_anon key exchange, 128-bit.
- 0x00,0xA7TLS_DH_anon_WITH_AES_256_GCM_SHA384 — DH_anon key exchange, 256-bit.
§3.1The GCM Nonce
AES-GCM needs a nonce, a value used once per encryption. Here the nonce SHALL be 12 bytes long and is made of two parts, a "partially explicit" nonce in the terms of RFC 5116: a salt of 4 bytes and a nonce_explicit of 8 bytes.
The salt is the implicit part. It is not sent in the packet. It comes from the handshake: it is the client_write_IV when the client is sending and the server_write_IV when the server is sending. The nonce_explicit is chosen by the sender and carried in each TLS record in the GenericAEADCipher.nonce_explicit field. It MAY be the 64-bit sequence number.
Each nonce_explicit value MUST be distinct for each invocation of the GCM encrypt function under a given key. Failing this can significantly degrade security.
Every message needs its own one-time number, like a ticket stub. Part of the number both sides already know, and the sender writes the rest on each message.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
SALT | salt | 4 bytes | Implicit part, not sent; taken from the write IV set up in the handshake. | |
NONCE-EXPLICIT | nonce_explicit | 8 bytes | Explicit part, chosen by the sender and carried in each TLS record. |
§3.2PRF and Error Handling
The Pseudo Random Function (PRF) depends on the suite name. For suites ending in _SHA256 it is the TLS PRF from RFC 5246 with SHA-256 as the hash. For suites ending in _SHA384 it is the same PRF with SHA-384.
On errors, implementations MUST send the TLS alert bad_record_mac for all types of failures encountered while processing the AES-GCM algorithm. The RFC does not distinguish between kinds of failure.
The name of the recipe says which mixing method to use, and if anything goes wrong with a message, everyone says the same thing: it did not check out.
§4TLS Versions
These suites rely on the authenticated encryption defined in TLS 1.2, so they MUST NOT be negotiated in older TLS versions. Clients MUST NOT offer them unless they also offer TLS 1.2 or later, and servers that select an earlier version MUST NOT select one of them.
TLS gives a client no way to say it supports TLS 1.2 but not earlier versions, so a non-compliant server might negotiate TLS 1.1 or earlier and still pick one of these suites. Clients MUST check the version and generate a fatal illegal_parameter alert if they see an incorrect one.
These recipes only work in the newer kitchen. If the old kitchen is picked by mistake, the client has to stop and say so.
§6Counter Reuse and Multiple Processors
The security considerations of RFC 5246 apply too. The main extra risk is that AES-GCM security requires the counter never be reused. The nonce construction in section 3 is designed to prevent that, and implementers should also read section 9 of the GCM specification on practical IV handling.
If a sender uses several cryptographic processors, it MUST make sure each nonce_explicit value under a given key is distinct. Each processor SHOULD put a fixed value that is distinct for that processor into the nonce_explicit, in the format FixedDistinct || Variable. All FixedDistinct fields MUST have the same length. The RFC's example uses salt eedc68dc with FixedDistinct values 01 and 02 for two processors.
If two helpers stamp tickets, give each helper their own first digit so they never print the same ticket number.