TLS Working Group Mohamad Badra
Internet Draft LIMOS Laboratory
Intended status: Standards Track March 29, 2008
Expires: September 2008
Pre-Shared Key Cipher Suites for Transport Layer Security
with SHA-256/384 and AES Galois Counter Mode
draft-badra-tls-psk-new-mac-aes-gcm-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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 as their MAC algorithm. This document describes a set of
cipher suites for TLS/DTLS which uses stronger digest algorithms
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(i.e. SHA-256 or SHA-384) and another which uses AES in Galois
Counter Mode (GCM).
Table of Contents
1. Introduction...................................................3
1.1. 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. TLS Versions...................................................6
5. Security Considerations........................................6
5.1. Counter Reuse with GCM....................................6
5.2. Recommendations for Multiple Encryption Processors........6
6. IANA Considerations............................................7
7. Acknowledgments................................................8
8. References.....................................................8
8.1. Normative References......................................8
8.2. Informative References....................................9
Author's Addresses................................................9
Intellectual Property Statement..................................10
Disclaimer of Validity...........................................10
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1. Introduction
This document describes the use of AES [AES] in Galois Counter Mode
(GCM) [GCM] (AES-GCM) with various pre-shared key (PSK) key exchange
mechanisms ([RFC4279] and [RFC4785]) as a ciphersuite for Transport
Layer Security (TLS). 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.
This document also specifies PSK cipher suites for TLS which replace
SHA-256 and SHA-384 rather than SHA-1. RFC 4279 [RFC4279] and RFC
4785 [RFC4785] describe pre-shared key (PSK) cipher suites for TLS.
However, all of the RFC 4279 and the RFC 4785 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.
[I-D.ietf-tls-ecc-new-mac] and [I-D.ietf-tls-rsa-aes-gcm] provide
support for GCM with other key establishment methods.
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].
2. PSK, DHE_PSK and RSA_PSK Key Exchange Algorithms with AES-GCM
The following eight cipher suites use the new authenticated
encryption modes defined in TLS 1.2 with AES in Galois Counter Mode
(GCM) [GCM]:
CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_258_GCM_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA384 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_256_GCM_SHA384 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_128_GCM_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_256_GCM_SHA384 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_128_GCM_SHA256 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_256_GCM_SHA384 = {0xXX,0xXX};
These cipher suites use authenticated encryption with additional
data algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM described in
RFC 5116. The "nonce" input to the AEAD algorithm SHALL be 12 bytes
long, and is "partially implicit" (see Section 3.2.1 of RFC 5116).
Part of the nonce is generated as part of the handshake process and
is static for the entire session and part is carried in each packet.
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struct {
opaque salt[4];
opaque explicit_nonce_part[8];
} GCMNonce.
The salt value is either the client_write_IV if the client is
sending or the server_write_IV if the server is sending. These IVs
SHALL be 4 bytes long. Therefore, for all the algorithms defined in
this section, SecurityParameters.fixed_iv_length=4.
The explicit_nonce_part is chosen by the sender and included in the
packet. Each value of the explicit_nonce_part MUST be distinct from
all other values, for any fixed key. Failure to meet this
uniqueness requirement can significantly degrade security. The
explicit_nonce_part is carried in the IV field of the
GenericAEADCipher structure. Therefore, for all the algorithms
defined in this section, SecurityParameters.record_iv_length=8.
In the case of TLS the counter MAY be the 64-bit sequence number.
In the case of Datagram TLS [RFC4347] the counter MAY be formed from
the concatenation of the 16-bit epoch with the 48-bit sequence
number.
The PRF algorithms SHALL be as follows:
For ciphersuites ending in _SHA256 the hash function is SHA256.
For ciphersuites ending in _SHA384 the hash function is SHA384.
3. PSK, DHE_PSK and RSA_PSK Key Exchange with SHA-256/384
The cipher suites described in this section use AES [AES] in CBC
[CBC] mode with an HMAC-based MAC.
3.1. PSK Key Exchange Algorithm with SHA-256/384
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_NULL_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_NULL_SHA384 = {0xXX,0xXX};
The above six cipher suites are the same as the corresponding cipher
suites in RFC 4279 and RFC 4785 (TLS_PSK_WITH_AES_128_CBC_SHA,
TLS_PSK_WITH_AES_256_CBC_SHA, and TLS_PSK_WITH_NULL_SHA) except for
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the hash and PRF algorithms, which are SHA-256 and SHA-384 [SHS] as
follows.
Cipher Suite MAC PRF
------------ --- ---
TLS_PSK_WITH_AES_128_CBC_SHA256 HMAC-SHA-256 P_SHA-256
TLS_PSK_WITH_AES_128_CBC_SHA384 HMAC-SHA-384 P_SHA-384
TLS_PSK_WITH_AES_256_CBC_SHA256 HMAC-SHA-256 P_SHA-256
TLS_PSK_WITH_AES_256_CBC_SHA384 HMAC-SHA-384 P_SHA-384
TLS_PSK_WITH_NULL_SHA256 HMAC-SHA-256 P_SHA-256
TLS_PSK_WITH_NULL_SHA384 HMAC-SHA-384 P_SHA-384
3.2. DHE_PSK Key Exchange Algorithm with SHA-256/384
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA384 = {0xXX,0xXX};
The above six cipher suites are the same as the corresponding cipher
suites in RFC 4279 and RFC 4785 (TLS_DHE_PSK_WITH_AES_128_CBC_SHA,
TLS_DHE_PSK_WITH_AES_256_CBC_SHA, and TLS_DHE_PSK_WITH_NULL_SHA)
except for the hash and PRF algorithms, which are SHA-256 and SHA-
384 [SHS] as follows.
Cipher Suite MAC PRF
------------ --- ---
TLS_DHE_PSK_WITH_AES_128_CBC_SHA256 HMAC-SHA-256 P_SHA-256
TLS_DHE_PSK_WITH_AES_128_CBC_SHA384 HMAC-SHA-384 P_SHA-384
TLS_DHE_PSK_WITH_AES_256_CBC_SHA256 HMAC-SHA-256 P_SHA-256
TLS_DHE_PSK_WITH_AES_256_CBC_SHA384 HMAC-SHA-384 P_SHA-384
3.3. RSA_PSK Key Exchange Algorithm with SHA-256/384
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
The above four cipher suites are the same as the corresponding
cipher suites in RFC 4279 and RFC 4785
(TLS_RSA_PSK_WITH_AES_128_CBC_SHA, TLS_RSA_PSK_WITH_AES_256_CBC_SHA,
and TLS_RSA_PSK_WITH_NULL_SHA) except for the hash and PRF
algorithms, which are SHA-256 and SHA-384 [SHS] as follows.
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Cipher Suite MAC PRF
------------ --- ---
TLS_RSA_PSK_WITH_AES_128_CBC_SHA256 HMAC-SHA-256 P_SHA-256
TLS_RSA_PSK_WITH_AES_128_CBC_SHA384 HMAC-SHA-384 P_SHA-384
TLS_RSA_PSK_WITH_AES_256_CBC_SHA256 HMAC-SHA-256 P_SHA-256
TLS_RSA_PSK_WITH_AES_256_CBC_SHA384 HMAC-SHA-384 P_SHA-384
4. TLS Versions
Because these cipher suites depend on features available only in TLS
1.2 (PRF flexibility and combined authenticated encryption cipher
modes), they MUST NOT be negotiated by older versions of TLS.
Clients MUST NOT offer these cipher suites if they do not offer TLS
1.2 or later. Servers which 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. Security Considerations
The security considerations in [I-D.ietf-tls-rfc4346-bis], RFC 4279
and RFC 4785 apply to this document as well. The remainder of this
section describes security considerations specific to the cipher
suites described in this document.
5.1. Counter Reuse with GCM
AES-GCM is only secure if the counter is never reused. The IV
construction algorithm above is designed to ensure that this cannot
happen.
5.2. 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
explicit_nonce_part used with that key is distinct. In this case
each encryption processor SHOULD include in the explicit_nonce_part
a fixed value that is distinct for each processor. The recommended
format is
explicit_nonce_part = FixedDistinct || Variable
where the FixedDistinct field is distinct for each encryption
processor, but is fixed for a given processor, and the Variable
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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, and the Variable field corresponds to
the Counter field, and the explicit part exactly corresponds to the
explicit_nonce_part.
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 explicit_nonce_parts, are:
GCMNonce explicit_nonce_part
------------------------ --------------------
eedc68dc0100000000000000 0100000000000000
eedc68dc0100000000000001 0100000000000001
eedc68dc0100000000000002 0100000000000002
...
The GCMnonces generated by the second encryption processor, and
their corresponding explicit_nonce_parts, are
GCMNonce explicit_nonce_part
------------------------ --------------------
eedc68dc0200000000000000 0200000000000000
eedc68dc0200000000000001 0200000000000001
eedc68dc0200000000000002 0200000000000002
...
6. IANA Considerations
IANA has assigned the following values for the cipher suites defined
in this document:
CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_258_GCM_SHA256 = {0xXX,0xXX};
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CipherSuite TLS_PSK_WITH_AES_128_GCM_SHA384 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_256_GCM_SHA384 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_128_GCM_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_256_GCM_SHA384 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_128_GCM_SHA256 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_256_GCM_SHA384 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_128_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_NULL_SHA256 = {0xXX,0xXX};
CipherSuite TLS_PSK_WITH_NULL_SHA384 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_128_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA256 = {0xXX,0xXX};
CipherSuite TLS_DHE_PSK_WITH_NULL_SHA384 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_128_CBC_SHA384 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA256 = {0xXX,0xXX};
CipherSuite TLS_RSA_PSK_WITH_AES_256_CBC_SHA384 = {0xXX,0xXX};
7. Acknowledgments
This draft borrows heavily from [I-D.ietf-tls-ecc-new-mac] and [I-
D.ietf-tls-rsa-aes-gcm].
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[I-D.ietf-tls-rfc4346-bis]
Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", draft-ietf-tls-rfc4346-bis-
10, work in progress, March 2008.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, January 2008.
[RFC4279] Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
for Transport Layer Security (TLS)", RFC 4279, December
2005.
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[RFC4785] Blumenthal, U., Goel, P., "Pre-Shared Key (PSK)
Ciphersuites with NULL Encryption for Transport Layer
Security (TLS)", RFC 4785, January 2007.
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard
(AES)", FIPS 197, November 2001.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-2, August 2002.
[CBC] National Institute of Standards and Technology,
"Recommendation for Block Cipher Modes of Operation -
Methods and Techniques", SP 800-38A, December 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.
8.2. Informative References
[Wang05] Wang, X., Yin, Y., and H. Yu, "Finding Collisions in the
Full SHA-1", CRYPTO 2005, August 2005.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[I-D.ietf-tls-ecc-new-mac]
Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode", draft-ietf-tls-ecc-
new-mac-04 (work in progress), February 2008.
[I-D.ietf-tls-rsa-aes-gcm]
Salowey, J., A. Choudhury, and C. McGrew, "RSA based AES-
GCM Cipher Suites for TLS", draft-ietf-tls-rsa-aes-gcm-02
(work in progress), February 2008.
Author's Addresses
Mohamad Badra
LIMOS Laboratory - UMR6158, CNRS
France
Email: badra@isima.fr
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