RFC 5487 PROPOSED STANDARD TLS · CIPHER SUITES · CRYPTOGRAPHY
PSK TLS GCM suites: Pre-Shared Key Cipher Suites for TLS with SHA-256/384 and AES Galois Counter Mode
New TLS pre-shared-key suites use AES-GCM or replace SHA-1 with SHA-256 or SHA-384.
RFC EDITOR
Badra, M., "Pre-Shared Key Cipher Suites for TLS with SHA-256/384 and AES Galois Counter Mode", RFC 5487, DOI 10.17487/RFC5487, March 2009, <https://www.rfc-editor.org/info/rfc5487>.
BIBTEX
@misc{rfc5487,
series = {Request for Comments},
number = 5487,
howpublished = {RFC 5487},
publisher = {RFC Editor},
doi = {10.17487/RFC5487},
url = {https://www.rfc-editor.org/info/rfc5487},
author = {M. Badra},
title = {{Pre-Shared Key Cipher Suites for TLS with SHA-256/384 and AES Galois Counter Mode}},
pagetotal = 7,
year = 2009,
month = mar,
} RFC text
Contents
Network Working Group M. Badra
Request for Comments: 5487 CNRS/LIMOS Laboratory
Category: Standards Track March 2009
Pre-Shared Key Cipher Suites for TLS with
SHA-256/384 and AES Galois Counter Mode
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) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Without obtaining an adequate license from the person(s) controlling
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than English.
Abstract
RFC 4279 and RFC 4785 describe pre-shared key cipher suites for
Transport Layer Security (TLS). However, all those cipher suites use
SHA-1 in their Message Authentication Code (MAC) algorithm. This
document describes a set of pre-shared key cipher suites for TLS that
uses stronger digest algorithms (i.e., SHA-256 or SHA-384) and
another set that uses the Advanced Encryption Standard (AES) in
Galois Counter Mode (GCM).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Applicability Statement . . . . . . . . . . . . . . . . . . 3
1.2. Conventions Used in This Document . . . . . . . . . . . . . 3
2. PSK, DHE_PSK, and RSA_PSK Key Exchange Algorithms with
AES-GCM . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. PSK, DHE_PSK, and RSA_PSK Key Exchange with SHA-256/384 . . . . 4
3.1. PSK Key Exchange Algorithm with SHA-256/384 . . . . . . . . 4
3.2. DHE_PSK Key Exchange Algorithm with SHA-256/384 . . . . . . 5
3.3. RSA_PSK Key Exchange Algorithm with SHA-256/384 . . . . . . 5
4. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 6
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
7.1. Normative References . . . . . . . . . . . . . . . . . . . 6
7.2. Informative References . . . . . . . . . . . . . . . . . . 7
1. Introduction
The benefits of pre-shared symmetric-key vs. public-/private-key pair
based authentication for the key exchange in TLS have been explained
in the Introduction of [RFC4279]. This document leverages the
already defined algorithms for the application of newer, generally
regarded stronger, cryptographic primitives and building blocks.
TLS 1.2 [RFC5246] adds support for authenticated encryption with
additional data (AEAD) cipher modes [RFC5116]. This document
describes the use of Advanced Encryption Standard [AES] in Galois
Counter Mode [GCM] (AES-GCM) with various pre-shared key (PSK)
authenticated key exchange mechanisms ([RFC4279] and [RFC4785]) in
cipher suites for TLS.
This document also specifies PSK cipher suites for TLS that replace
SHA-1 by SHA-256 or SHA-384 [SHS]. RFC 4279 [RFC4279] and RFC 4785
[RFC4785] describe PSK cipher suites for TLS. However, all of the
RFC 4279 and the RFC 4785 cipher suites use HMAC-SHA1 as their MAC
algorithm. Due to recent analytic work on SHA-1 [Wang05], the IETF
is gradually moving away from SHA-1 and towards stronger hash
algorithms.
Related TLS cipher suites with key exchange algorithms that are
authenticated using public/private key pairs have recently been
specified:
o RSA-, DSS-, and Diffie-Hellman-based cipher suites in [RFC5288],
and
o ECC-based cipher suites with SHA-256/384 and AES-GCM in [RFC5289].
The reader is expected to become familiar with these two memos prior
to studying this document.
1.1. Applicability Statement
The cipher suites defined in Section 3 can be negotiated, whatever
the negotiated TLS version is.
The cipher suites defined in Section 2 can be negotiated in TLS
version 1.2 or higher.
The applicability statement in [RFC4279] applies to this document as
well.
1.2. 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].
2. PSK, DHE_PSK, and RSA_PSK Key Exchange Algorithms with AES-GCM
The following six cipher suites use the new authenticated encryption
modes defined in TLS 1.2 with AES in Galois Counter Mode [GCM]. The
cipher suites with the DHE_PSK key exchange algorithm
(TLS_DHE_PSK_WITH_AES_128_GCM_SHA256 and
TLS_DHE_PSK_WITH_AES_256_GCM_SHA348) provide Perfect Forward Secrecy
(PFS).
CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA256 = {0x00,0xA8};
CipherSuite TLS_PSK_WITH_AES_256_GCM_SHA384 = {0x00,0xA9};
CipherSuite TLS_DHE_PSK_WITH_AES_128_GCM_SHA256 = {0x00,0xAA};
CipherSuite TLS_DHE_PSK_WITH_AES_256_GCM_SHA384 = {0x00,0xAB};
CipherSuite TLS_RSA_PSK_WITH_AES_128_GCM_SHA256 = {0x00,0xAC};
CipherSuite TLS_RSA_PSK_WITH_AES_256_GCM_SHA384 = {0x00,0xAD};
These cipher suites use authenticated encryption with additional data
(AEAD) algorithms, AEAD_AES_128_GCM and AEAD_AES_256_GCM, as
described in RFC 5116. GCM is used as described in [RFC5288].
The PSK, DHE_PSK, and RSA_PSK key exchanges are performed as defined
in [RFC4279].
The Pseudo-Random Function (PRF) algorithms SHALL be as follows:
o For cipher suites ending with _SHA256, the PRF is the TLS PRF
[RFC5246] with SHA-256 as the hash function.
o For cipher suites ending with _SHA384, the PRF is the TLS PRF
[RFC5246] with SHA-384 as the hash function.
Implementations MUST send a TLS Alert 'bad_record_mac' for all types
of failures encountered in processing the AES-GCM algorithm.
3. PSK, DHE_PSK, and RSA_PSK Key Exchange with SHA-256/384
The first two cipher suites described in each of the following three
sections use AES [AES] in Cipher Block Chaining (CBC) mode [MODES]
for data confidentiality, whereas the other two cipher suites do not
provide data confidentiality; all cipher suites provide integrity
protection and authentication using HMAC-based MACs.
3.1. PSK Key Exchange Algorithm with SHA-256/384
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA256 = {0x00,0xAE};
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA384 = {0x00,0xAF};
CipherSuite TLS_PSK_WITH_NULL_SHA256 = {0x00,0xB0};
CipherSuite TLS_PSK_WITH_NULL_SHA384 = {0x00,0xB1};
The above four cipher suites are the same as the corresponding cipher
suites in RFC 4279 and RFC 4785 (with names ending in "_SHA" in place
of "_SHA256" or "_SHA384"), except for the hash and PRF algorithms,
as explained below.
o For cipher suites with names ending in "_SHA256":
* The MAC is HMAC [RFC2104] with SHA-256 as the hash function.
* When negotiated in a version of TLS prior to 1.2, the PRF from
that version is used; otherwise, the PRF is the TLS PRF
[RFC5246] with SHA-256 as the hash function.
o For cipher suites with names ending in "_SHA384":
* The MAC is HMAC [RFC2104] with SHA-384 as the hash function.
* When negotiated in a version of TLS prior to 1.2, the PRF from
that version is used; otherwise, the PRF is the TLS PRF
[RFC5246] with SHA-384 as the hash function.
3.2. DHE_PSK Key Exchange Algorithm with SHA-256/384
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA256 = {0x00,0xB2};
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA384 = {0x00,0xB3};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA256 = {0x00,0xB4};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA384 = {0x00,0xB5};
The above four cipher suites are the same as the corresponding cipher
suites in RFC 4279 and RFC 4785 (with names ending in "_SHA" in place
of "_SHA256" or "_SHA384"), except for the hash and PRF algorithms,
as explained in Section 3.1.
3.3. RSA_PSK Key Exchange Algorithm with SHA-256/384
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA256 = {0x00,0xB6};
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA384 = {0x00,0xB7};
CipherSuite TLS_RSA_PSK_WITH_NULL_SHA256 = {0x00,0xB8};
CipherSuite TLS_RSA_PSK_WITH_NULL_SHA384 = {0x00,0xB9};
The above four cipher suites are the same as the corresponding cipher
suites in RFC 4279 and RFC 4785 (with names ending in "_SHA" in place
of "_SHA256" or "_SHA384"), except for the hash and PRF algorithms,
as explained in Section 3.1.
4. Security Considerations
The security considerations in [RFC4279], [RFC4785], and [RFC5288]
apply to this document as well. In particular, as authentication-
only cipher suites (with no encryption) defined here do not support
confidentiality, care should be taken not to send sensitive
information (such as passwords) over connections protected with one
of the cipher suites with NULL encryption defined in this document.
5. IANA Considerations
IANA has assigned the following values for the cipher suites defined
in this document:
CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA256 = {0x00,0xA8};
CipherSuite TLS_PSK_WITH_AES_256_GCM_SHA384 = {0x00,0xA9};
CipherSuite TLS_DHE_PSK_WITH_AES_128_GCM_SHA256 = {0x00,0xAA};
CipherSuite TLS_DHE_PSK_WITH_AES_256_GCM_SHA384 = {0x00,0xAB};
CipherSuite TLS_RSA_PSK_WITH_AES_128_GCM_SHA256 = {0x00,0xAC};
CipherSuite TLS_RSA_PSK_WITH_AES_256_GCM_SHA384 = {0x00,0xAD};
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA256 = {0x00,0xAE};
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA384 = {0x00,0xAF};
CipherSuite TLS_PSK_WITH_NULL_SHA256 = {0x00,0xB0};
CipherSuite TLS_PSK_WITH_NULL_SHA384 = {0x00,0xB1};
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA256 = {0x00,0xB2};
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA384 = {0x00,0xB3};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA256 = {0x00,0xB4};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA384 = {0x00,0xB5};
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA256 = {0x00,0xB6};
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA384 = {0x00,0xB7};
CipherSuite TLS_RSA_PSK_WITH_NULL_SHA256 = {0x00,0xB8};
CipherSuite TLS_RSA_PSK_WITH_NULL_SHA384 = {0x00,0xB9};
6. Acknowledgments
This document borrows from [RFC5289]. The author appreciates Alfred
Hoenes for his detailed review and effort on resolving issues in
discussion. The author would like also to acknowledge Ibrahim
Hajjeh, Simon Josefsson, Hassnaa Moustafa, Joseph Salowey, and Pascal
Urien for their reviews of the content of the document.
7. References
7.1. Normative References
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard
(AES)", FIPS 197, November 2001.
[GCM] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation:
Galois/Counter Mode (GCM) for Confidentiality and
Authentication", SP 800-38D, November 2007.
[MODES] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation -
Methods and Techniques", SP 800-38A, December 2001.
[RFC2104] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
Hashing for Message Authentication", RFC 2104,
February 1997.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279,
December 2005.
[RFC4785] Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK)
Ciphersuites with NULL Encryption for Transport Layer
Security (TLS)", RFC 4785, January 2007.
[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.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-2, August 2002.
7.2. Informative References
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
August 2008.
[Wang05] Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
Full SHA-1", CRYPTO 2005, August 2005.
Author's Address
Mohamad Badra
CNRS/LIMOS Laboratory
Campus de cezeaux, Bat. ISIMA
Aubiere 63170
France
EMail: badra@isima.fr
ISO Plain Language
On this page
PROBLEMThe earlier TLS pre-shared-key cipher suites used HMAC-SHA1, while the IETF was moving toward stronger hash algorithms.
MECHANISMThe RFC defines six AES-GCM suites for TLS 1.2 or later and twelve suites that use SHA-256 or SHA-384 with existing PSK key exchanges.
WHY IT MATTERSTLS can negotiate pre-shared-key suites using these updated algorithms. RFC 5487 is updated by RFC 10015.
§1Why Add Pre-Shared-Key Suites
A pre-shared-key (PSK) cipher suite lets TLS endpoints authenticate with a symmetric secret they already share. RFC 4279 and RFC 4785 defined earlier PSK suites, but all of those suites used HMAC-SHA1 as their message authentication code.
TLS 1.2 added authenticated-encryption modes, including AES-GCM. RFC 5487 defines PSK suites using AES-GCM and suites that use SHA-256 or SHA-384 in place of SHA-1.
These suites let two TLS endpoints use a shared secret instead of public and private key pairs. They add newer encryption and hash choices to the existing PSK methods.
§1.1TLS Versions for the Suites
The RFC defines two groups of suites with different version rules. The AES-GCM suites in Section 2 can be negotiated only with TLS 1.2 or higher. The SHA-256 and SHA-384 suites in Section 3 can be negotiated regardless of the negotiated TLS version.
The applicability statement in RFC 4279 also applies to these suites. The key exchanges themselves continue to follow the procedures defined for PSK, DHE_PSK, and RSA_PSK.
AES-GCM suites require TLS 1.2 or later. The other suites use the version rules of the earlier PSK specifications.
§2AES-GCM Cipher Suites
Six cipher suites combine PSK, DHE_PSK, or RSA_PSK key exchange with AES-128-GCM or AES-256-GCM. They use the AEAD algorithms named in RFC 5116 and apply GCM as described in RFC 5288. The DHE_PSK suites provide Perfect Forward Secrecy.
Suites ending in _SHA256 use the TLS PRF with SHA-256, while those ending in _SHA384 use SHA-384. Implementations MUST send the TLS alert bad_record_mac for any failure while processing AES-GCM.
AES-GCM combines encryption and authentication for a record. The cipher-suite name also identifies the hash used by the TLS key-derivation function.
§3SHA-256 and SHA-384 Suites
Twelve additional suites keep the PSK, DHE_PSK, and RSA_PSK key exchanges while changing the hash and PRF. Some use AES in CBC mode for confidentiality; others use NULL encryption and therefore provide integrity and authentication without confidentiality.
For a suite ending in _SHA256, HMAC uses SHA-256. For a suite ending in _SHA384, HMAC uses SHA-384. In TLS versions before 1.2, the PRF from that TLS version is used; otherwise the matching SHA-256 or SHA-384 TLS PRF applies.
These suites retain earlier key exchanges but use a different hash. The NULL-encryption options authenticate data without hiding it.
§5Cipher-Suite Identifiers
IANA assigned two-byte identifiers to all eighteen suites. The six AES-GCM suites use values from 0x00,0xA8 through 0x00,0xAD. The twelve SHA-256 and SHA-384 suites use values from 0x00,0xAE through 0x00,0xB9.
Each endpoint uses these identifiers to select the same cipher suite during TLS negotiation. The RFC lists the mapping between each identifier and its suite name.
Each suite has an identifier that the endpoints can use to select matching TLS settings. The assigned values cover all eighteen suites.
§4Confidentiality and Security
The security considerations of RFC 4279, RFC 4785, and RFC 5288 apply. In particular, the suites with NULL encryption do not provide confidentiality, even though they provide integrity and authentication.
The RFC warns against sending sensitive information, such as passwords, over a connection using one of those NULL-encryption suites.
Some suites verify data without hiding it. Do not send passwords or other sensitive information over a connection that uses NULL encryption.
§7References
[RFC4279]RFC 4279Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites for Transport Layer Security (TLS)", RFC 4279, December 2005.December 2005[RFC4785]RFC 4785Blumenthal, U. and P. Goel, "Pre-Shared Key (PSK) Ciphersuites with NULL Encryption for Transport Layer Security (TLS)", RFC 4785, January 2007.January 2007[RFC5116]RFC 5116McGrew, D., "An Interface and Algorithms for Authenticated Encryption", RFC 5116, January 2008.January 2008[RFC5246]RFC 5246Dierks, T. and E. Rescorla, "The Transport Layer Security (TLS) Protocol Version 1.2", RFC 5246, August 2008.August 2008[RFC5288]RFC 5288Salowey, J., Choudhury, A., and D. McGrew, "AES Galois Counter Mode (GCM) Cipher Suites for TLS", RFC 5288, August 2008.August 2008