Network Working Group M. Salter
Internet-Draft National Security Agency
Expires: June 16, 2007 E. Rescorla
Network Resonance
December 13, 2006
SuiteB CipherSuites for TLS
draft-rescorla-tls-suiteb-00.txt
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Copyright (C) The Internet Society (2006).
Abstract
RFC 4492 describes elliptic curve cipher suites for Transport Layer
Security (TLS). However, all those cipher suites use SHA-1 as their
MAC algorithm, which makes them unsuitable for some applications.
This document describes eight new CipherSuites for TLS/DTLS which
specify stronger digest algorithms and therefore are suitable for use
in applications which require compliance with the United States
Government's guidelines for "NSA Suite B Cryptography" dated July,
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2005.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. SuiteB Requirements . . . . . . . . . . . . . . . . . . . . . 3
4. Cipher Suites . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. HMAC-based Cipher Suites . . . . . . . . . . . . . . . . . 4
4.2. Galois Counter Mode-based Cipher Suites . . . . . . . . . 5
5. Suite B Compliance Requirements . . . . . . . . . . . . . . . 6
5.1. Security Levels . . . . . . . . . . . . . . . . . . . . . 6
5.2. Acceptable Curves . . . . . . . . . . . . . . . . . . . . 6
6. TLS Versions . . . . . . . . . . . . . . . . . . . . . . . . . 7
7. Security Considerations . . . . . . . . . . . . . . . . . . . 7
7.1. Downgrade Attack . . . . . . . . . . . . . . . . . . . . . 7
7.2. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . 8
7.3. Counter Reuse with GCM . . . . . . . . . . . . . . . . . . 8
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
10.1. Normative References . . . . . . . . . . . . . . . . . . . 8
10.2. Informative References . . . . . . . . . . . . . . . . . . 9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
Intellectual Property and Copyright Statements . . . . . . . . . . 11
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1. Introduction
In July, 2005 the National Security Agency posted "Fact Sheet, NSA
Suite B Cryptography" which stated:
To complement the existing policy for the use of the Advanced
Encryption Standard (AES) to protect national security systems
and information as specified in The National Policy on the use of
the Advanced Encryption Standard (AES) to Protect National
Security Systems and National Security Information (CNSSP-15),
the National Security Agency (NSA) announced Suite B Cryptography
at the 2005 RSA Conference. In addition to the AES, Suite B
includes cryptographic algorithms for hashing, digital
signatures, and key exchange.
Suite B only specifies the cryptographic algorithms to be
used. Many other factors need to be addressed in determining
whether a particular device implementing a particular set of
cryptographic algorithms should be used to satisfy a particular
requirement.
Among those factors are "requirements for interoperability both
domestically and internationally".
This document is intended to address those requirements in the
particular case of TLS [RFC4346] and Datagram TLS [RFC4347]. Much of
what is needed to define the Suite B CipherSuites is specified in RFC
4492 "ECC for TLS" [RFC4492]. We use the terminology, notation, and
details from that document to the extent possible. A key ingredient
of SuiteB not found in RFC4492--or the definition of TLS itself prior
to 1.2--is the use of SHA256 or SHA384 for key derivation. TLS 1.2
[I-D.ietf-tls-rfc4346-bis] allows for the use of these hash in the
pseudo-random function (PRF) used to derive the keys.
Unless specifically stated herein, details of the protocol are
identical to those given in [I-D.ietf-tls-rfc4346-bis] and [RFC4492]
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. SuiteB Requirements
The "Suite B Fact Sheet" requires that key establishment and
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authentication algorithms be based on Elliptic Curve Cryptography,
that the encryption algorithm be AES [AES], and that the function
used for key derivation and data integrity be SHA [SHS]. It defines
two security levels, of 128 and 192 bits.
In particular it states:
SUITE B includes:
Encryption: Advanced Encryption Standard (AES) -
FIPS 197 with keys sizes of 128 and 256
bits)
Digital Signature: Elliptic Curve Digital Signature Algorithm -
FIPS 186-2 (using the curves with 256 and
384-bit prime moduli)
Key Exchange: Elliptic Curve Diffie-Hellman or Elliptic
Curve MQV Draft NIST Special Publication
800-56 (using the curves with 256 and
384-bit prime moduli)
Hashing: Secure Hash Algorithm - FIPS 180-2
(using SHA-256 and SHA-384)
All implementations of Suite B must, at a minimum, include AES
with 128-bit keys, the 256-bit prime modulus elliptic curve and
SHA-256 as a common mode for widespread interoperability.
The 128-bit security level corresponds to an elliptic curve size of
256 bits, AES-128, and SHA-256. The 192-bit security level
corresponds to an elliptic curve size of 384 bits, AES-256, and SHA-
384. Because both settings require a digest algorithm other than
SHA-1, new cipher suites are required and defined in this document.
4. Cipher Suites
This document defines 8 new cipher suites to be added to TLS. All
use Elliptic Curve Cryptography for key exchange and digital
signature, as defined in RFC 4492.
4.1. HMAC-based Cipher Suites
The first four cipher suites use AES in CBC mode with an HMAC-based
MAC:
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CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX}
These four cipher suites are the same as the corresponding cipher
suites in RFC 4492 (TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA,
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA,
TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA, and
TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA) except that SHA-256 is used for
the MAC with AES-128 and SHA-384 is used for the MAC with AES-256.
As described in TLS 1.2, the digest used for the MAC MUST also be
used in the PRF.
4.2. Galois Counter Mode-based Cipher Suites
The second four cipher suites use the new authenticated encryption
modes defined in TLS 1.2 with AES in Galois Counter Mode (GCM) [GCM]:
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX}
These cipher suites use the authenticated encryption with additional
data algorithms AEAD_AES_128_GCM and AEAD_AES_256_GCM described in
[I-D.mcgrew-auth-enc]. The "nonce" input to the AEAD algorithm SHALL
be 12 bytes long, and constructed as follows:
struct {
case client:
uint32 client_write_IV; // low order 32-bits
case server:
uint32 server_write_IV; // low order 32-bits
uint64 seq_num;
} GCMNonce.
In DTLS, the 64-bit seq_num is the 16-bit epoch concatenated with the
48-bit seq_num.
This construction allows the internal counter to be 32-bits long,
which is the most convenient size for use with GCM.
Note: the role played by the client_write_IV and server_write_IV is
often called "salt" in counter mode specifications [RFC3686].
Because GCM does not use HMAC as a MAC function, the hash function
for the TLS PRF must be explicitly specified.
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For TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 it SHALL be SHA-256.
For TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 it SHALL be SHA-384.
5. Suite B Compliance Requirements
The following requirements apply only to SuiteB compliant
implementations. However, ordinary TLS implementations MAY use these
cipher suites even if they do not comply with the requirements in
this section.
To be considered "SuiteB compatible" at least one of the CipherSuites
defined in this document MUST be negotiated. In compliance with the
guidance in the Suite B Fact Sheet every TLS implementation of SuiteB
SHOULD implement TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256.
5.1. Security Levels
As described in Section 1, Suite B specifies two security levels, 128
and 192 bit. The following table lists the security levels for each
cipher suite:
+-----------------------------------------+----------------+
| Cipher Suite | Security Level |
+-----------------------------------------+----------------+
| TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 | 128 |
| TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 | 192 |
| TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 | 128 |
| TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 | 192 |
| TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 | 128 |
| TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 | 192 |
| TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 | 128 |
| TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 | 192 |
+-----------------------------------------+----------------+
5.2. Acceptable Curves
RFC 4492 defines a variety of elliptic curves. For cipher suites
defined in this specification, only secp256r1 (23) or secp384r1 (24)
may be used. (These are the same curves that appear in FIPS 186-2
and ANSI X9.62 as P256 and P384, respectively). For cipher suites at
the 128-bit security level, secp256r1 MUST be used. For cipher
suites at the 192-bit security level, secp256r MUST be used. Both
uncompressed (0) and ansiX962_compressed_prime(1) point formats
SHOULD be supported.
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Clients desiring to negotiate only a SuiteB-compliant connection MUST
generate a "Supported Elliptic Curves Extension" containing only the
allowed curves. These curves MUST match the cipher suite security
levels being offered. Clients which are willing to do both SuiteB-
compliant and non-SuiteB-compliant connections MAY omit the extension
or send the extension but offer other curves as well as the
appropriate SuiteB ones.
Servers desiring to negotiate a SuiteB-compliant connection SHOULD
check for the presence of the extension, but MUST NOT negotiate
inappropriate curves even if they are offered by the client. This
allows a Client which is willing to do either SuiteB-compliant or
non-SuiteB-compliant modes to interoperate with a server which will
only do SuiteB-compliant modes. If the client does not advertise an
acceptable curve, the server MUST generate a fatal
"handshake_failure" alert and terminate the connection. Clients
SHOULD check the chosen curve to make sure it is acceptable.
6. 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 in 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.
7. Security Considerations
The security considerations in RFC 4346 and RFC 4492 apply to this
document as well. The remainder of this section describes security
considerations specific to the cipher suites described in this
document.
7.1. Downgrade Attack
TLS negotiation is only as secure as the weakest cipher suite that is
supported. For instance, an implementation which supports both 160-
bit and 256-bit elliptic curves can be subject to an active downgrade
attack to the 160-bit security level. An attacker who can attack
that can then forge the Finished handshake check and successfully
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mount a man-in-the-middle attack.
In environments where there is a concern about this form of attack,
implementations SHOULD only offer cipher suites which are as strong
as their minimum acceptable security level.
7.2. Perfect Forward Secrecy
The static ECDH cipher suites specified in this document do not
provide perfect forward secrecy (PFS). Thus, compromise of a single
static key leads to potential decryption of all traffic protected
using that key. Implementors of this specification SHOULD provide at
least one ECDHE mode of operation.
7.3. 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 that cannot
happen.
8. IANA Considerations
IANA has assigned the following values for the SuiteB cipher suites:
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_CBC_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_CBC_SHA384 = {0xXX,XX}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_128_GCM_SHA256 = {0xXX,XX}
CipherSuite TLS_ECDH_ECDSA_WITH_AES_256_GCM_SHA384 = {0xXX,XX}
9. Acknowledgements
This work was supported by the US Department of Defense.
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3686] Housley, R., "Using Advanced Encryption Standard (AES)
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Counter Mode With IPsec Encapsulating Security Payload
(ESP)", RFC 3686, January 2004.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC4347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security", RFC 4347, April 2006.
[RFC4492] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
for Transport Layer Security (TLS)", RFC 4492, May 2006.
[I-D.mcgrew-auth-enc]
McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", draft-mcgrew-auth-enc-01 (work in progress),
October 2006.
[I-D.ietf-tls-rfc4346-bis]
Dierks, T. and E. Rescorla, "The TLS Protocol Version
1.2", draft-ietf-tls-rfc4346-bis-02 (work in progress),
October 2006.
[I-D.ietf-tls-ctr]
Modadugu, N. and E. Rescorla, "AES Counter Mode Cipher
Suites for TLS and DTLS", draft-ietf-tls-ctr-01 (work in
progress), June 2006.
[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.
[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, April 2006.
10.2. Informative References
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Authors' Addresses
Margaret Salter
National Security Agency
9800 Savage Rd.
Fort Meade 20755-6709
USA
Email: msalter@restarea.ncsc.mil
Eric Rescorla
Network Resonance
2483 E. Bayshore #212
Palo Alto 94303
USA
Email: ekr@networkresonance.com
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