RFC 6460
Suite B Profile for Transport Layer Security (TLS)
Internet Engineering Task Force (IETF) M. Salter Request for Comments: 6460 National Security Agency Obsoletes: 5430 R. Housley Category: Informational Vigil Security ISSN: 2070-1721 January 2012 Suite B Profile for Transport Layer Security (TLS)Abstract
The United States government has published guidelines for "NSA Suite B Cryptography" that define cryptographic algorithm policy for national security applications. This document defines a profile of Transport Layer Security (TLS) version 1.2 that is fully compliant with Suite B. Status of This Memo This document is not an Internet Standards Track specification; it is published for informational purposes. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Not all documents approved by the IESG are a candidate for any level of Internet Standard; see Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6460. Copyright Notice Copyright (c) 2012 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.
This document may contain material from IETF Documents or IETF Contributions published or made publicly available before November 10, 2008. The person(s) controlling the copyright in some of this material may not have granted the IETF Trust the right to allow modifications of such material outside the IETF Standards Process. Without obtaining an adequate license from the person(s) controlling the copyright in such materials, this document may not be modified outside the IETF Standards Process, and derivative works of it may not be created outside the IETF Standards Process, except to format it for publication as an RFC or to translate it into languages other than English.Table of Contents
1. Introduction ....................................................2 2. Conventions Used in This Document ...............................3 3. Suite B Requirements ............................................3 3.1. Minimum Levels of Security (minLOS) for Suite B TLS ........4 3.2. Suite B TLS Authentication .................................5 4. Suite B Compliance and Interoperability Requirements ............5 4.1. Acceptable Curves ..........................................6 4.2. Certificates ...............................................7 4.3. signature_algorithms Extension .............................7 4.4. CertificateRequest Message .................................8 4.5. CertificateVerify Message ..................................8 4.6. ServerKeyExchange Message Signature ........................8 5. Security Considerations .........................................8 6. Acknowledgments .................................................9 7. References ......................................................9 7.1. Normative References .......................................9 7.2. Informative References ....................................10 Annex A. A Transitional Suite B Profile for TLS 1.1 and 1.0 .......11 Annex B. Changes since RFC 5430 ...................................131. Introduction
This document specifies the conventions for using National Security Agency (NSA) Suite B Cryptography [SuiteB] with the Transport Layer Security (TLS) protocol, and the Datagram Transport Layer Security (DTLS) protocol. This document does not define any new cipher suites; instead, it defines a Suite B compliant profile for use with TLS version 1.2 [RFC5246], DTLS version 1.2 [RFC6347], and the cipher suites defined in [RFC5289]. This profile uses only Suite B algorithms.
RFC 5430 defined an additional transitional profile for use with TLS versions 1.0 [RFC2246] and 1.1 [RFC4346] or with DTLS version 1.0 [RFC4347] and the cipher suites defined in [RFC4492]. When either the client or the server does not support TLS version 1.2 and DTLS version 1.2, the transitional profile can be used to achieve interoperability that is not Suite B compliant. The description for the transitional profile appears in Annex A of this document.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]. We will use the notation "ECDSA-256" to represent the use of the Elliptic Curve Digital Signature Algorithm (ECDSA) with the P-256 curve and the SHA-256 hash function. Similarly, "ECDSA-384" will represent the use of the ECDSA with the P-384 curve and the SHA-384 hash function.3. Suite B Requirements
The Fact Sheet on Suite B Cryptography requires key establishment and authentication algorithms based on Elliptic Curve Cryptography and encryption using AES [AES]. Suite B algorithms are defined to support two minimum levels of security: 128 and 192 bits. In particular, Suite B includes the following: 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 186-3 (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) The two elliptic curves used in Suite B each appear in the literature under two different names. For sake of clarity, we list both names below: Curve NIST name [SECG] name -------------------------------- P-256 nistp256 secp256r1 P-384 nistp384 secp384r1
The purpose of this document is to specify the requirements for a Suite B compliant implementation of TLS (hereafter referred to as "Suite B TLS").3.1. Minimum Levels of Security (minLOS) for Suite B TLS
Suite B provides two levels of cryptographic security, namely a 128-bit minimum level of security (minLOS_128) and a 192-bit minimum level of security (minLOS_192). Each level defines a minimum strength that all cryptographic algorithms must provide. The following combination of algorithms and key sizes are used in Suite B TLS: Suite B Combination 1 Suite B Combination 2 -------------------------------- -------------------------------- AES with 128-bit key in GCM mode AES with 256-bit key in GCM mode ECDH using the 256-bit prime ECDH using the 384-bit prime modulus curve P-256 [DSS] modulus curve P-384 [DSS] TLS PRF with SHA-256 [SHS] TLS PRF with SHA-384 [SHS] Suite B TLS configured at a minimum level of security of 128 bits MUST use a TLS cipher suite satisfying either SuiteB_Combination_1 in its entirety or SuiteB_Combination_2 in its entirety. Suite B TLS configured at a minimum level of security of 192 bits MUST use a TLS cipher suite satisfying SuiteB_Combination_2 in its entirety. The specific Suite B compliant cipher suites for each combination are listed in Section 4. For Suite B TLS, ECDH uses the Ephemeral Unified Model Scheme with cofactor set to 1 (see Section 6.1.2.2 in [PWKE]). To accommodate backward compatibility, a Suite B TLS client or server MAY be configured to accept a cipher suite that is not part of Suite B. However, whenever a Suite B TLS client and a Suite B TLS server establish a TLS version 1.2 session, Suite B algorithms MUST be employed.
3.2 Suite B TLS Authentication
Suite B TLS MUST use ECDSA for digital signatures; authentication methods other than ECDSA-256 and ECDSA-384 MUST NOT be used for TLS authentication. If a relying party receives a signature based on any other authentication method, it MUST return a TLS error and stop the TLS handshake. A system compliant with the Suite B TLS and configured at a minimum level of security of 128 bits MUST use either ECDSA-256 or ECDSA-384 for client or server authentication. One party can authenticate with ECDSA-256 when the other party authenticates with ECDSA-384. This flexibility allows interoperation between a client and a server that have ECDSA authentication keys of different sizes. Clients and servers in a system configured at a minimum level of security of 128 bits MUST be able to verify ECDSA-256 signatures and SHOULD be able to verify ECDSA-384 signatures unless it is absolutely certain that the implementation will never need to verify certificates originating from an authority that uses an ECDSA-384 signing key. A system compliant with the Suite B TLS and configured at a minimum level of security of 192 bits MUST use ECDSA-384 for client and server authentication. Clients and servers in a system configured at a minimum level of security of 192 bits MUST be able to verify ECDSA-384 signatures. In all cases, the client MUST authenticate the server. The server MAY authenticate the client, as needed by the specific application.4. Suite B Compliance and Interoperability Requirements
TLS versions 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, GCM cipher suites MUST be used; therefore, a Suite B TLS client MUST implement TLS version 1.2 or later. A Suite B TLS client configured at a minimum level of security of 128 bits MUST offer the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 or the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite in the ClientHello message. The TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite is preferred; if offered, it MUST appear before the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite.
If configured at a minimum level of security of 192 bits, the client MUST offer the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite and MUST NOT offer the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite. One of these two cipher suites MUST be the first (most preferred) cipher suites in the ClientHello message. A Suite B TLS client that offers interoperability with servers that are not Suite B compliant MAY offer additional cipher suites, but any additional cipher suites MUST appear after the two Suite B compliant cipher suites in the ClientHello message. A Suite B TLS server MUST implement TLS version 1.2 or later. A Suite B TLS server configured at a minimum level of security of 128 bits MUST accept either the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite or the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite if it is offered in the ClientHello message, with the TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 cipher suite being preferred. A Suite B TLS server configured at a minimum level of security of 192 bits MUST accept the TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384 cipher suite if it is offered in the ClientHello message. If the server is not offered either of the Suite B cipher suites, and interoperability with clients that are not Suite B compliant is desired, then the Suite B TLS server MAY accept another offered cipher suite that is considered acceptable by the server administrator.4.1. Acceptable Curves
RFC 4492 defines a variety of elliptic curves. Suite B TLS connections MUST use secp256r1(23) or secp384r1(24). These are the same curves that appear in FIPS 186-3 [DSS] as P-256 and P-384, respectively. Secp256r1 MUST be used for the key exchange in all cipher suites in this specification using AES-128; secp384r1 MUST be used for the key exchange in all cipher suites in this specification using AES-256. RFC 4492 requires that the uncompressed(0) form be supported. The ansiX962_compressed_prime(1) point format MAY also be supported. Clients desiring to negotiate only a Suite B TLS connection MUST generate a "Supported Elliptic Curves Extension" containing only the allowed curves. Clients operating at a minimum level of security of 128 bits MUST include secp256r1 and SHOULD include secp384r1 in the extension. Clients operating at a minimum level of security of 192 bits MUST include secp384r1 in the extension. In order to be able to
verify ECDSA signatures, a client and server in a system configured at a minimum level of security of 128 bits MUST support secp256r1 and SHOULD support secp384r1 unless it is absolutely certain that the client and server will never need to use or verify certificates originating from an authority which uses an ECDSA-384 signing key. A client and server in a system configured at a minimum level of 192 bits MUST support secp384r1. TLS connections that offer options that are both compliant and non- compliant with Suite B MAY omit the extension, or they MAY send the extension but offer other curves as well as the appropriate Suite B ones. Servers desiring to negotiate a Suite B TLS connection SHOULD check for the presence of the extension, but they MUST NOT select a curve that is not Suite B even if it is offered by the client. This allows a client that is willing to do either Suite B or non-Suite B TLS connections to interoperate with a server that will only do Suite B TLS. 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 that it is one of the Suite B curves.4.2. Certificates
Server and client certificates used to establish a Suite B TLS connection MUST be signed with ECDSA and MUST be compliant with the "Suite B Certificate and Certificate Revocation List (CRL) Profile", [RFC5759].4.3. 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 TLS version 1.2 or later client MUST include the signature_algorithms extension. A Suite B TLS client configured at a minimum level of security of 128 bits MUST offer SHA-256 with ECDSA and SHOULD offer ECDSA with SHA-384 in the signature_algorithms extension unless it is absolutely certain that a client will never need to use or verify certificates originating from an authority that uses an ECDSA-384 signing key. A Suite B TLS client configured at a minimum level of 192 bits MUST offer ECDSA with SHA-384 in the signature_algorithms extension. Following the guidance in [RFC5759], Suite B TLS connections MUST only accept signature algorithms ECDSA with either SHA-256 or SHA-384 for certification path validation. (Note that this is a change from [RFC5430].)
Other offerings MAY be included to indicate the acceptable signature algorithms in cipher suites that are offered for interoperability with servers not compliant with Suite B and to indicate the signature algorithms that are acceptable for certification path validation in non-compliant Suite B TLS connections.4.4. CertificateRequest Message
A Suite B TLS server configured at a minimum level of security of 128 bits MUST include ECDSA with SHA-256 and SHOULD include ECDSA with SHA-384 in the supported_signature_algorithms field of the CertificateRequest message unless it is absolutely certain that a server will never need to verify certificates originating from an authority that uses an ECDSA-384 signing key. A Suite B TLS server configured at a minimum level of security of 192 bits MUST include ECDSA with SHA-384 in the supported_signature_algorithms field.4.5. CertificateVerify Message
Using the definitions found in Section 3.2, a Suite B TLS client MUST use ECDSA-256 or ECDSA-384 for the signature in the CertificateVerify message. A Suite B TLS client configured at a minimum security level of 128 bits MUST use ECDSA-256 or ECDSA-384. A Suite B TLS client configured at a minimum security level of 192 bits MUST use ECDSA-384.4.6. 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 server's private key, which corresponds to the public key in the server's certificate. A Suite B TLS server MUST sign the ServerKeyExchange message using either ECDSA-256 or ECDSA-384. A system configured at a minimum level of security of 128 bits MUST use either ECDSA-256 or ECDSA-384. A system configured at a minimum level of security of 192-bits MUST use ECDSA-384.5. 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, Suite B TLS implementations MUST ONLY use the curves defined in Section 4.1. Otherwise, it is possible to have a set of symmetric algorithms with much weaker or stronger security properties than the asymmetric (ECC) algorithms.6. Acknowledgments
The authors would like to thank Eric Rescorla for his work on the original RFC 5430. This work was supported by the US Department of Defense.7. References
7.1. Normative References
[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 186-3, June 2009. [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. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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. [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. [RFC5759] Solinas, J. and L. Zieglar, "Suite B Certificate and Certificate Revocation List (CRL) Profile", RFC 5759, January 2010. [SHS] National Institute of Standards and Technology, "Secure Hash Standard", FIPS 180-3, October 2008. [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, January 2012.7.2. 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. [RFC5430] Salter, M., Rescorla, E., and R. Housley, "Suite B Profile for Transport Layer Security (TLS)", RFC 5430, March 2009. [SECG] Brown, D., "SEC 2: Recommended Elliptic Curve Domain Parameters", http://www.secg.org/download/aid-784/sec2-v2.pdf, February 2010. [SuiteB] National Security Agency, "Fact Sheet NSA Suite B Cryptography", November 2010, http://www.nsa.gov/ia/programs/suiteb_cryptography/.
Annex A. A Transitional Suite B Profile for TLS 1.1 and 1.0
A transitional profile is described for use with TLS version 1.0 [RFC2246], TLS version 1.1 [RFC4346], or DTLS version 1.0 [RFC4347] 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 a Suite B Compliant implementation of TLS, it provides a transitional path towards the Suite B compliant Profile. The following combination of algorithms and key sizes are defined for use with the Suite B TLS transitional profile: Transitional Suite B Combination 1 Transitional Suite B Combination 2 ---------------------------------- ---------------------------------- AES with 128-bit key in CBC mode AES with 256-bit key in CBC mode ECDH using the 256-bit prime ECDH using the 384-bit prime modulus curve P-256 [DSS] modulus curve P-384 [DSS] Standard TLS PRF Standard TLS PRF (with SHA-1 and MD5) (with SHA-1 and MD5) HMAC with SHA-1 for message HMAC with SHA-1 for message authentication authentication A Transitional Suite B TLS system configured at a minimum level of security of 128 bits MUST use a TLS cipher suite satisfying either Transitional Suite B Combination 1 in its entirety or Transitional Suite B Combination 2 in its entirety. A Transitional Suite B TLS system configured at a minimum level of security of 192 bits MUST use a TLS cipher suite satisfying Transitional Suite B Combination 2 in its entirety. TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA and TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA satisfy the requirements of Transitional Suite B Combination 1 and Transitional Suite B Combination 2, respectively. A Transitional Suite B TLS client MUST implement TLS version 1.1 or earlier. A Transitional Suite B TLS system configured at a minimum level of security of 128 bits, MUST offer the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite and/or the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite in the
ClientHello message. The TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite is preferred; if it is offered, it MUST appear before the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite (if present). A Transitional Suite B TLS system configured at a minimum level of security of 192 bits MUST offer the TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA cipher suite in the ClientHello message. One of these Transitional Suite B cipher suites MUST be the first (most preferred) in the ClientHello message. A Transitional Suite B client that offers interoperability with servers that are not Suite B transitional MAY offer additional cipher suites. If any additional cipher suites are offered, they MUST appear after the Transitional Suite B cipher suites in the ClientHello message. A Transitional Suite B TLS server MUST implement TLS version 1.1 or earlier. A Transitional Suite B TLS server configured at a minimum level of security of 128 bits MUST accept the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA cipher suite (preferred) or the TLS_ECHDE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if offered in the ClientHello message. A Transitional Suite B TLS server configured at a minimum level of security of 192 bits MUST accept the TLS_ECHDE_ECDSA_WITH_AES_256_CBC_SHA cipher suite if offered in the ClientHello message. If a Transitional Suite B TLS server is not offered the Transitional Suite B cipher suites and interoperability with non-Transitional Suite B clients is desired, then the server MAY accept another offered cipher suite that is considered acceptable by the server administrator. A Transitional Suite B TLS server MUST sign the ServerKeyExchange message using ECDSA with SHA-1. The Transitional Suite B profile does not impose any additional restrictions on the server certificate signature or the signature schemes used elsewhere in the certification path. Likewise, the Transitional Suite B Profile does not impose restrictions on signature schemes used in the certification path for the client's certificate when mutual authentication is employed.
Annex B. Changes since RFC 5430
The changes from RFC 5430 [RFC5430] are as follows: - The transitional profile for use with TLS version 1.0, TLS version 1.1, and DTLS version 1.0 was moved to an annex. - The requirement of Section 4 of RFC 5430 that a Suite B TLS 1.2 Client offer the TLS_ECDHE_ECDSA_WITH_AES_128_CBC_SHA256 or TLS_ECDHE_ECDSA_WITH_AES_256_CBC_SHA384 cipher suites was removed. - A Suite B TLS system configured at a minimum level of security of 128 bits MUST use either TLS_ECDHE_ECDSA_WITH_AES_128_GCM_SHA256 or TLS_ECDHE_ECDSA_WITH_AES_256_GCM_SHA384, with the first being preferred. - A Suite B TLS system configured at a minimum level of security of 128 bits MUST use either ECDSA on the secp256r1 curve with SHA-256 or ECDSA on the secp384r1 curve with SHA-384. One party can authenticate with ECDSA on the secp256r1 curve and SHA-256 when the other party authenticates with ECDSA on the secp384r1 curve and SHA-384. - A system desiring to negotiate a Suite B TLS connection at a minimum level of security of 128 bits MUST generate a "Supported Elliptic Curves Extension", MUST include secp256r1 in the extension, and SHOULD include secp384r1 in the extension. - A client and server, in order to verify digital signatures in a Suite B TLS system configured at a minimum level of security of 128 bits, MUST support secp256r1 and SHOULD support secp384r1. - A Suite B TLS client configured at a minimum level of security of 128 bits MUST offer SHA-256 with ECDSA and SHOULD offer SHA-384 with ECDSA in the signature_algorithms extension. - Certification path validation MUST only include certificates containing an ECDSA public key on the secp256r1 curve or on the secp384r1 curve. The ECDSA public keys used in the certification path MUST be in non-descending order of size from the end entity public key to the root public key. - A Suite B TLS server configured at a minimum level of security of 128 bits MUST include ECDSA with SHA-256 and SHOULD include ECDSA with SHA-384 in the supported_signature_algorithms field of the CertificateRequest message.
- A Suite B TLS client configured at a minimum level of security of
128 bits MUST use ECDSA on the secp256r1 curve and SHA-256 or
ECDSA on the secp384r1 curve and SHA-384.
- A Suite B TLS server configured at a minimum level of security of
128 bits MUST use either ECDSA on the secp256r1 curve and SHA-256
or ECDSA on the secp384r1 curve and SHA-384 when signing the
ServerKeyExchange message.
Authors' Addresses
Margaret Salter
National Security Agency
9800 Savage Rd.
Fort Meade 20755-6709
USA
EMail: misalte@nsa.gov
Russ Housley
Vigil Security
918 Spring Knoll Drive
Herndon 21070
USA
EMail: housley@vigilsec.com