rfc:rfc5430
Network Working Group M. Salter Request for Comments: 5430 National Security Agency Category: Informational E. Rescorla
Network Resonance
R. Housley
Vigil Security
March 2009
Suite B Profile for Transport Layer Security (TLS)
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Salter, et al. Informational [Page 1] RFC 5430 Suite B for TLS March 2009
Abstract
The United States government has published guidelines for "NSA Suite B Cryptography", which defines cryptographic algorithm policy for national security applications. This document defines a profile of Transport Layer Security (TLS) version 1.2 that is fully conformant with Suite B. This document also defines a transitional profile for use with TLS version 1.0 and TLS version 1.1 which employs Suite B algorithms to the greatest extent possible.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. Suite B Requirements ............................................3
4. Suite B Compliance and Interoperability Requirements ............4
4.1. Security Levels ............................................7
4.2. Acceptable Curves ..........................................8
4.3. Certificates ...............................................8
4.4. signature_algorithms Extension .............................9
4.5. CertificateRequest Message .................................9
4.6. CertificateVerify Message .................................10
4.7. ServerKeyExchange Message Signature .......................10
5. Security Considerations ........................................10
6. Acknowledgements ...............................................10
7. References .....................................................11
7.1. Normative References ......................................11
7.2. Informative References ....................................11
1. Introduction
The United States government has posted the Fact Sheet on National Security Agency (NSA) Suite B Cryptography [NSA], and at the time of writing, it states:
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
Salter, et al. Informational [Page 2] RFC 5430 Suite B for TLS March 2009
cryptographic algorithms should be used to satisfy a particular
requirement.
Among those factors are "requirements for interoperability both domestically and internationally".
This document does not define any new cipher suites; instead, it defines two profiles:
o A Suite B compliant profile for use with TLS version 1.2 [RFC5246]
and the cipher suites defined in [RFC5289]. This profile uses
only Suite B algorithms.
o A transitional profile for use with TLS version 1.0 [RFC2246] or
TLS version 1.1 [RFC4346] and the cipher suites defined in
[RFC4492]. This profile uses the Suite B cryptographic algorithms
to the greatest extent possible and provides backward
compatibility. While the transitional profile is not Suite B
compliant, it provides a transition path towards the Suite B
compliant profile.
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. Suite B Requirements
The Fact Sheet on Suite B Cryptography requires that key establishment and authentication algorithms be based on Elliptic Curve Cryptography, and that the encryption algorithm be AES [AES]. Suite B defines two security levels, of 128 and 192 bits.
In particular, Suite B includes:
Encryption: Advanced Encryption Standard (AES) [AES] --
FIPS 197 (with key sizes of 128 and 256 bits)
Digital Signature: Elliptic Curve Digital Signature Algorithm
(ECDSA) [DSS] - FIPS 186-2 (using the
curves with 256- and 384-bit prime moduli)
Key Exchange: Elliptic Curve Diffie-Hellman (ECDH) - NIST
Special Publication 800-56A [PWKE] (using the
curves with 256- and 384-bit prime moduli)
Salter, et al. Informational [Page 3] RFC 5430 Suite B for TLS March 2009
The 128-bit security level corresponds to an elliptic curve size of 256 bits and AES-128; it also makes use of SHA-256 [SHS]. The 192- bit security level corresponds to an elliptic curve size of 384 bits and AES-256; it also makes use of SHA-384 [SHS].
Note: Some people refer to the two security levels based on the AES key size that is employed instead of the overall security provided by the combination of Suite B algorithms. At the 128-bit security level, an AES key size of 128 bits is used, which does not lead to any confusion. However, at the 192-bit security level, an AES key size of 256 bits is used, which sometimes leads to an expectation of more security than is offered by the combination of Suite B algorithms.
To accommodate backward compatibility, a Suite B compliant client or server can be configured to accept a cipher suite that is not part of Suite B. However, whenever a Suite B compliant client and a Suite B compliant server establish a TLS version 1.2 session, only Suite B algorithms are employed.
4. Suite B Compliance and Interoperability Requirements
TLS version 1.1 [RFC4346] and earlier do not support Galois Counter Mode (GCM) cipher suites [RFC5289]. However, TLS version 1.2 [RFC5246] and later do support GCM. For Suite B TLS compliance, GCM cipher suites are REQUIRED to be used whenever both the client and the server support the necessary cipher suites. Also, for Suite B TLS compliance, Cipher Block Chaining (CBC) cipher suites are employed when GCM cipher suites cannot be employed.
For a client to implement the Suite B compliant profile, it MUST implement TLS version 1.2 or later, and the following cipher suite rules apply:
o A Suite B compliant TLS version 1.2 or later client MUST offer at
least two cipher suites for each supported security level. For
the 128-bit security level,
TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 and
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 MUST be offered in this
order in the ClientHello message. For the 192-bit security level,
TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 and
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 MUST be offered in this
order in the ClientHello message. One of these cipher suites MUST
be the first (most preferred) cipher suite in the ClientHello
message.
Salter, et al. Informational [Page 4] RFC 5430 Suite B for TLS March 2009
o A Suite B compliant TLS version 1.2 or later client that offers
backward compatibility with TLS version 1.1 or earlier servers MAY
offer an additional cipher suite for each supported security
level. If these cipher suites are offered, they MUST appear after
the ones discussed above. For the 128-bit security level,
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA MAY be offered in the
ClientHello message. For the 192-bit security level,
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA MAY be offered in the
ClientHello message.
o A Suite B compliant TLS version 1.2 or later client that offers
interoperability with non-Suite B compliant servers MAY offer
additional cipher suites. If any additional cipher suites are
offered, they MUST appear after the ones discussed above in the
ClientHello message.
For a client to implement the Suite B transitional profile, it MUST implement TLS version 1.1 or earlier and the following cipher suite rules apply:
o A Suite B transitional TLS version 1.1 or earlier client MUST
offer the cipher suite for the 128-bit security level, the cipher
suite for the 192-bit security level, or both. For the 128-bit
security level, TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA MUST be
offered in the ClientHello message. For the 192-bit security
level, TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA MUST be offered in the
ClientHello message. One of these cipher suites MUST be the first
(most preferred) cipher suite in the ClientHello message.
o A Suite B transitional TLS version 1.1 or earlier client that
offers interoperability with non-Suite B compliant servers MAY
offer additional cipher suites. If any additional cipher suites
are offered, they MUST appear after the ones discussed above in
the ClientHello message.
A Suite B compliant TLS server MUST be configured to support the 128- bit security level, the 192-bit security level, or both security levels. The cipher suite rules for each of these security levels is described below. If a Suite B compliant TLS server is configured to support both security levels, then the configuration MUST prefer one security level over the other. In practice, this means that the cipher suite rules associated with the cipher suites listed in Section 4.1 for the preferred security level are processed before the cipher suite rules for the less preferred security level.
Salter, et al. Informational [Page 5] RFC 5430 Suite B for TLS March 2009
For a server to implement the Suite B conformant profile at the 128- bit security level, the following cipher suite rules apply:
o A Suite B compliant TLS version 1.2 or later server MUST accept
the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite if it is
offered.
o If the preceding cipher suite is not offered, then a Suite B
compliant TLS version 1.2 or later server MUST accept the
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 cipher suite if it is
offered.
o If neither of the preceding two cipher suites is offered, then a
Suite B compliant TLS version 1.2 or later server that offers
backward compatibility with TLS version 1.1 or earlier clients MAY
accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it
is offered.
o If the server is not offered any of the preceding three cipher
suites and interoperability with clients that are not compliant or
interoperable with Suite B is desired, then the server MAY accept
another offered cipher suite that is considered acceptable by the
server administrator.
For a server to implement the Suite B transitional profile at the 128-bit security level, the following cipher suite rules apply:
o A Suite B transitional TLS version 1.1 or earlier server MUST
accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it
is offered.
o If the server is not offered the preceding cipher suite and
interoperability with clients that are not Suite B transitional is
desired, then the server MAY accept another offered cipher suite
that is considered acceptable by the server administrator.
For a server to implement the Suite B conformant profile at the 192- bit security level, the following cipher suite rules apply:
o A Suite B compliant TLS version 1.2 or later server MUST accept
the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite if it is
offered.
o If the preceding cipher suite is not offered, then a Suite B
compliant TLS version 1.2 or later server MUST accept the
TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 cipher suite if it is
offered.
Salter, et al. Informational [Page 6] RFC 5430 Suite B for TLS March 2009
o If neither of the preceding two cipher suites is offered, then a
Suite B compliant TLS version 1.2 or later server that offers
backward compatibility with TLS version 1.1 or earlier clients MAY
accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it
is offered.
o If the server is not offered any of the preceding three cipher
suites and interoperability with clients that are not compliant or
interoperable with Suite B is desired, then the server MAY accept
another offered cipher suite that is considered acceptable by the
server administrator.
For a server to implement the Suite B transitional profile at the 192-bit security level, the following cipher suite rules apply:
o A Suite B transitional TLS version 1.1 or earlier server MUST
accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it
is offered.
o If the server is not offered the preceding cipher suite and
interoperability with clients that are not Suite B transitional is
desired, then the server MAY accept another offered cipher suite
that is considered acceptable by the server administrator.
Note that these rules explicitly permit the use of CBC cipher suites in TLS version 1.2 connections in order to permit operation between Suite B compliant and non-Suite B compliant implementations. For instance, a Suite B compliant TLS version 1.2 client might offer TLS version 1.2 with both GCM and CBC cipher suites when communicating with a non-Suite B TLS version 1.2 server, which then selected the CBC cipher suites. This connection would nevertheless meet the requirements of this specification. However, any two Suite B compliant implementations will negotiate a GCM cipher suite when doing TLS version 1.2.
4.1. Security Levels
As described in Section 1, Suite B specifies two security levels: 128-bit and 192-bit. The following table lists the cipher suites for each security level. Within each security level, the cipher suites are listed in their preferred order for selection by a TLS version 1.2 implementation.
Salter, et al. Informational [Page 7] RFC 5430 Suite B for TLS March 2009
+-----------------------------------------+----------------+
| Cipher Suite | Security Level |
+-----------------------------------------+----------------+
| TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 | 128 |
| TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 | 128 |
| TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA | 128 |
| TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 | 192 |
| TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 | 192 |
| TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA | 192 |
+-----------------------------------------+----------------+
4.2. 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 [DSS] as P-256 and P-384, respectively. For cipher suites at the 128-bit security level, secp256r1 MUST be used. For cipher suites at the 192-bit security level, secp384r1 MUST be used. RFC 4492 requires that the uncompressed(0) form be supported. The ansiX962_compressed_prime(1) point formats MAY also be supported.
Clients desiring to negotiate only a Suite B 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 that are willing to do both Suite B compliant and non-Suite B compliant connections MAY omit the extension or send the extension but offer other curves as well as the appropriate Suite B ones.
Servers desiring to negotiate a Suite B 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 that is willing to do either Suite B compliant or non-Suite B compliant modes to interoperate with a server that will only do Suite B 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 MUST check the chosen curve to make sure it is acceptable.
4.3. Certificates
Server and client certificates used to establish a Suite B compliant connection MUST be signed with ECDSA. Digital signatures MUST be calculated using either the P-256 curve along with the SHA-256 hash algorithm or calculated using the P-384 curve along with the SHA-384 hash algorithm. For certificates used at the 128-bit security level, the subject public key MUST use the P-256 curve and be signed with
Salter, et al. Informational [Page 8] RFC 5430 Suite B for TLS March 2009
either the P-384 curve or the P-256 curve. For certificates used at the 192-bit security level, the subject public key MUST use the P-384 curve and be signed with the P-384 curve.
In TLS version 1.0 and TLS version 1.1, the key exchange algorithm
used in the TLS_ECDHE_ECDSA-collection of cipher suites requires the
server's certificate to be signed with a particular signature scheme.
TLS version 1.2 offers more flexibility. This specification does not
impose any additional restrictions on the server certificate
signature or the signature schemes used elsewhere in the
certification path. (Often such restrictions will be useful, and it
is expected that this will be taken into account in practices of
certification authorities. However, such restrictions are not
strictly required, even if it is beyond the capabilities of a client
to completely validate a given certification path, the client may be
able to validate the server's certificate by relying on a trusted
certification authority whose certificate appears as one of the
intermediate certificates in the certification path.)
Likewise, this specification does not impose restrictions on signature schemes used in the certification path for the client's certificate when mutual authentication is employed.
4.4. signature_algorithms Extension
The signature_algorithms extension is defined in Section 7.4.1.4.1 of TLS version 1.2 [RFC5246]. A Suite B compliant TLS version 1.2 or later client MUST include the signature_algorithms extension. For the 128-bit security level, SHA-256 with ECDSA MUST be offered in the signature_algorithms extension. For the 192-bit security level, SHA- 384 with ECDSA MUST be offered in the signature_algorithms extension. Other offerings MAY be included to indicate the signature algorithms that are acceptable in cipher suites that are offered for interoperability with servers that are not compliant with Suite B and to indicate the signature algorithms that are acceptable for certification path validation.
4.5. CertificateRequest Message
A Suite B compliant TLS version 1.2 or later server MUST include SHA- 256 with ECDSA and/or SHA-384 with ECDSA in the supported_signature_algorithms field of the CertificateRequest message. For the 128-bit security level, SHA-256 with ECDSA MUST appear in the supported_signature_algorithms field. For the 192-bit security level, SHA-384 with ECDSA MUST appear in the supported_signature_algorithms field.
Salter, et al. Informational [Page 9] RFC 5430 Suite B for TLS March 2009
4.6. CertificateVerify Message
A Suite B compliant TLS version 1.2 or later client MUST use SHA-256 with ECDSA or SHA-384 with ECDSA for the signature in the CertificateVerify message. For the 128-bit security level, SHA-256 with ECDSA MUST be used. For the 192-bit security level, SHA-384 with ECDSA MUST be used.
4.7. ServerKeyExchange Message Signature
In the TLS_ECDHE_ECDSA-collection of cipher suites, the server sends its ephemeral ECDH public key and a specification of the corresponding curve in the ServerKeyExchange message. These parameters MUST be signed with ECDSA using the private key corresponding to the public key in the server's certificate.
A TLS version 1.1 or earlier server MUST sign the ServerKeyExchange message using SHA-1 with ECDSA.
A Suite B compliant TLS version 1.2 or later server MUST sign the ServerKeyExchange message using either SHA-256 with ECDSA or SHA-384 with ECDSA. For the 128-bit security level, SHA-256 with ECDSA MUST be used. For the 192-bit security level, SHA-384 with ECDSA MUST be used.
5. Security Considerations
Most of the security considerations for this document are described in "The Transport Layer Security (TLS) Protocol Version 1.2" [RFC5246], "Elliptic Curve Cryptography (ECC) Cipher Suites for Transport Layer Security (TLS)" [RFC4492], "AES Galois Counter Mode (GCM) Cipher Suites for TLS" [RFC5288], and "TLS Elliptic Curve Cipher Suites with SHA-256/384 and AES Galois Counter Mode (GCM)" [RFC5289]. Readers should consult those documents.
In order to meet the goal of a consistent security level for the entire cipher suite, in Suite B mode TLS implementations MUST ONLY use the curves defined in Section 4.2. Otherwise, it is possible to have a set of symmetric algorithms with much weaker or stronger security properties than the asymmetric (ECC) algorithms.
6. Acknowledgements
Thanks to Pasi Eronen, Steve Hanna, and Paul Hoffman for their review, comments, and insightful suggestions.
This work was supported by the US Department of Defense.
Salter, et al. Informational [Page 10] RFC 5430 Suite B for TLS March 2009
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
[RFC5289] Rescorla, E., "TLS Elliptic Curve Cipher Suites with SHA-
256/384 and AES Galois Counter Mode (GCM)", RFC 5289,
August 2008.
[AES] National Institute of Standards and Technology,
"Specification for the Advanced Encryption Standard
(AES)", FIPS 197, November 2001.
[DSS] National Institute of Standards and Technology, "Digital
Signature Standard", FIPS 186-2, January 2000.
[PWKE] National Institute of Standards and Technology,
"Recommendation for Pair-Wise Key Establishment Schemes
Using Discrete Logarithm Cryptography (Revised)", NIST
Special Publication 800-56A, March 2007.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-2, August 2002.
7.2. Informative References
[RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[RFC4346] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.1", RFC 4346, April 2006.
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
August 2008.
[NSA] National Security Agency, "Fact Sheet NSA Suite B
Cryptography",
<http://www.nsa.gov/ia/Industry/crypto_suite_b.cfm>.
Salter, et al. Informational [Page 11] RFC 5430 Suite B for TLS March 2009
Authors' Addresses
Margaret Salter National Security Agency 9800 Savage Rd. Fort Meade 20755-6709 USA
EMail: msalter@restarea.ncsc.mil
Eric Rescorla Network Resonance 2064 Edgewood Drive Palo Alto 94303 USA
EMail: ekr@rtfm.com
Russ Housley Vigil Security 918 Spring Knoll Drive Herndon 21070 USA
EMail: housley@vigilsec.com
Salter, et al. Informational [Page 12]
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