Network Working Group M. Salter
Internet-Draft National Security Agency
Intended status: Informational E. Rescorla
Expires: March 20, 2009 Network Resonance
R. Housley
Vigil Security
September 16, 2008
Suite B Cipher Suites for TLS
draft-rescorla-tls-suiteb-05.txt
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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
TLS which is conformant with Suite B.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions Used In This Document . . . . . . . . . . . . . . 3
3. Suite B Requirements . . . . . . . . . . . . . . . . . . . . . 3
4. Suite B Compliance Requirements . . . . . . . . . . . . . . . 4
4.1. Security Levels . . . . . . . . . . . . . . . . . . . . . 6
4.2. Acceptable Curves . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8.1. Normative References . . . . . . . . . . . . . . . . . . . 8
8.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
National Security Agency posted a Fact Sheet on Suite B Cryptography,
and at the time of this 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
cryptographic algorithms should be used to satisfy a particular
requirement.
Among those factors are "requirements for interoperability both
domestically and internationally".
This document is a profile of TLS 1.2 [RFC5246] and of the cipher
suites defined in [RFC5289], but does not itself define any new
cipher suites.
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, 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, Suite B includes:
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Encryption: Advanced Encryption Standard (AES) -
FIPS 197 (with keys sizes of 128 and 256
bits)
Digital Signature: Elliptic Curve Digital Signature Algorithm
(ECDSA) - FIPS 186-2 (using the curves with
256 and 384-bit prime moduli)
Key Exchange: Elliptic Curve Diffie-Hellman (ECDH) - 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)
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.
Note: Some people refer to the two security levels based on the AES
key size that is employed. At the 128-bit security level, an AES key
length of 128 bits is used. However, at the 192-bit security level,
an AES key length of 256 bits is used.
4. Suite B Compliance Requirements
TLS version 1.1 and earlier does not support Galois Counter Mode
(GCM) cipher suites [RFC5289]. However, TLS version 1.2 and later
does support GCM. For Suite B TLS compliance, GCM cipher suites are
REQUIRED to be used whenever possible. 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 be considered Suite B compliant, the following cipher
suite rules apply:
o A Suite B compliant TLS version 1.1 or earlier client MUST offer
one cipher suite for each supported security level. 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 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,
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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.
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 (of any version) 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 server that is configured to support the 128-bit security level
to be considered Suite B compliant, the following cipher suite rules
apply:
o A Suite B compliant TLS version 1.1 or earlier server MUST accept
the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it is
offered. If the server is not offered this cipher suite and
interoperability with clients that are not compliant with Suite B
is desired, then the server MAY accept another offered cipher
suite that is considered acceptable by the server administrator.
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. If this cipher suite is not offered, then the server
MUST accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 cipher
suite if it is offered. If neither of these cipher suites is
offered, then the server MUST accept the
TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite if it is
offered. If the server is not offered any of those three cipher
suites and interoperability with clients that are not compliant
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 that is configured to support the 192-bit security level
to be considered Suite B compliant, the following cipher suite rules
apply:
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o A Suite B compliant TLS version 1.1 or earlier server MUST accept
the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if it is
offered. If the server is not offered this cipher suite and
interoperability with clients that are not compliant with Suite B
is desired, then the server MAY accept another offered cipher
suite that is considered acceptable by the server administrator.
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. If this cipher suite is offered, then the server MUST
accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 cipher suite if
it is offered. If neither of these cipher suites is offered, then
the server MUST accept the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA
cipher suite if it is offered. If the server is not offered any
of those three cipher suites and interoperability with clients
that are not compliant with Suite B 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. Withing each security level, the cipher suites
are listed in their preferred order for selection by a TLS version
1.2 implementation.
+-----------------------------------------+----------------+
| 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 |
+-----------------------------------------+----------------+
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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 uncompressed(0) form be supported.
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 which 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 which is willing to do either Suite B-compliant or
non-Suite B-compliant modes to interoperate with a server which 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.
Server and client certificates used to establish a Suite B-compliant
connection MUST be signed with ECDSA. For certificates used at the
128-bit security level, the subject public key MUST use the P-256
curve, and the digital signature MUST be calculated using the P-256
curve and the SHA-256 hash algorithm. For certificates used at the
192-bit security level, the subject public key MUST use the P-384
curve, and the digital signature MUST be calculated using the P-384
curve and the SHA-384 hash algorithm.
5. Security Considerations
Most of the security considerations for this document are described
in TLS 1.2 [RFC5246], Elliptic Curve Cryptography (ECC) Cipher Suites
for TLS [RFC4492], AES-GCM Cipher Suites for TLS [RFC5288], and TLS
ECC Cipher Suites with SHA-256/384 and AES-GCM [RFC5289]. Readers
should consult those documents.
In order to meet the goal of a consistent security level for the
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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. IANA Considerations
This document defines no actions for IANA.
7. Acknowledgements
This work was supported by the US Department of Defense.
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.
[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.
[SHS] National Institute of Standards and Technology, "Secure
Hash Standard", FIPS 180-2, August 2002.
[DSS] National Institute of Standards and Technology, "Digital
Signature Standard", FIPS 186-2, January 2000.
8.2. Informative References
[RFC5288] Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
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August 2008.
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
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