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IPSec Working Group                        S. Blake-Wilson and P. Fahn
INTERNET-DRAFT                                           Certicom Corp.
Expires: April 14, 2001                               October 15, 2000


                     IKE Authentication Using ECDSA
                 <draft-ietf-ipsec-ike-auth-ecdsa-01.txt>

Status of this Memo

This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC2026. Internet-Drafts are working
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Abstract

   This document describes how the Elliptic Curve Digital Signature
   Algorithm (ECDSA) may be used as the authentication method within
   the Internet Key Exchange (IKE) protocol. ECDSA provides
   authentication and non-repudiation with benefits of computational
   efficiency, small signature sizes, and minimal bandwidth, compared
   to other available digital signature methods. This document adds
   ECDSA capability to IKE without introducing any changes to existing
   IKE operation.

1. Introduction

The Internet Key Exchange, or IKE [RFC2409, IKE], is a key agreement
and security negotiation protocol; it is used for key establishment in
IPSec. In Phase 1 of IKE, both parties must authenticate each other
using a negotiated authentication method. One option for the
authentication method is digital signatures using public key
cryptography. Currently, there are two digital signature methods
defined for use within Phase 1: RSA signatures and DSA (DSS)
signatures. This document introduces ECDSA signatures as a third
method.

For any given level of security, ECDSA signatures are smaller than RSA
signatures and ECDSA keys require less bandwidth than DSA keys; there
are also advantages of computational speed and efficiency in many
settings. Additional efficiency may be gained by simultaneously using
ECDSA for IKE authentication and using elliptic curve groups for the
IKE key exchange. Implementers of IPSec and IKE may therefore find it
desirable to use ECDSA as the Phase 1 authentication method.

2. ECDSA

The Elliptic Curve Digital Signature Algorithm (ECDSA) is the elliptic
curve analogue of the DSA (also called DSS) signature method
[FIPS-186]. The Elliptic Curve Digital Signature Algorithm (ECDSA) is
defined in the ANSI X9.62 standard [X9.62]; a compatible
specification, along with test vectors, can be found in the documents
of the Standards for Efficient Cryptography Group at
<http://www.secg.org>. A profile for the use of ECDSA in X.509
certificates [EPKIX] describes the means to carry ECDSA keys in X.509
certificates.

ECDSA signatures are smaller than RSA signatures of similar
cryptographic strength; see [KEYS] for a security analysis of key
sizes across public key algorithms. ECDSA public keys (and
certificates) are smaller than similar strength DSA keys, resulting in
improved communications efficiency. Furthermore, on many platforms
ECDSA operations can be computed faster than similar strength RSA or
DSA operations. These advantages of signature size, bandwidth, and
computational efficiency make ECDSA an attractive choice for many IKE
implementions.

Recommended elliptic curve domain parameters for use with ECDSA are
given in [SEC2]. A subset of these are recommended in [ECC-GR] for use
in the IKE key exchange.

Like DSA, ECDSA incorporates the use of a hash function; currently,
the only hash function defined for use with ECDSA is the SHA-1 message
digest algorithm [FIPS-180].

3. Specifying ECDSA within IKE

The IKE key negotiation protocol consists of two phases, Phase 1 and
Phase 2. Within Phase 1, the two negotiating parties authenticate each
other, using either pre-shared keys, digital signatures, or public-key
encryption. For digital signatures and public-key encryption methods,
there are multiple options. The authentication method is specified as
an attribute in the negotiated Phase 1 Security Association (SA).

Until now, there have been a total of seven specific authentication
methods in Phase 1. We now add an eighth option: ECDSA signatures,
specified by attribute value 8 in the SA. The new list of
IANA-assigned attribute numbers for Phase 1 authentication is:

   - Authentication Method
      pre-shared key                      1
      DSS signatures                      2
      RSA signatures                      3
      Encryption with RSA                 4
      Revised encryption with RSA         5
      Encryption with El-Gamal            6
      Revised encryption with El-Gamal    7
      ECDSA signatures                    8

      values 9-65000 are reserved to IANA. Values 65001-65535 are for
      private use among mutually consenting parties.

Phase 1 can be either Main Mode or Agressive Mode. The use and
specification of ECDSA signatures as the authentication method applies
to both modes. The sequence of Phase 1 message payloads is the same
with ECDSA signatures as with DSS or RSA signatures.

When ECDSA is used in IKE, the signature payload shall contain an
encoding of the computed signature, consisting of a pair of integers r
and s, encoded using the ASN.1 syntax "ECDSA-Sig-Value" as specified
in ANSI X9.62 [X9.62] and PKIX [EPKIX].

Note also that, like the other digital signature methods, ECDSA
authentication requires the parties to know and trust each other's
public key. This can be done by exchanging certificates, possibly
within the Phase 1 negotiation, if the public keys of the parties are
not already known to each other. The use of Internet X.509 public key
infrastructure certificates [RFC 2459] is recommended; the
representation of ECDSA keys in X.509 certificates is specified in
[EPKIX].

Since ECDSA requires the use of the SHA-1 hash function,
implemententers may find it convenient to specify SHA-1 as the value
of the hash algorithm attribute when using ECDSA as the authentication
method. Implementers may also find it convenient to use ECDSA
authentication in conjunction with an elliptic curve group for the IKE
Diffie-Hellman key agreement; see [ECC-GR] for specific curves for the
key agreement.

Security Considerations

Implementors should ensure that appropriate security measures are in
place when they deploy ECDSA within IKE. In particular, the security
of ECDSA requires the careful selection of both key sizes and elliptic
curve domain parameters. Selection guidelines for these parameters and
some specific recommended curves that are considered safe are provided
in [X9.62], [NIST-ECC], and [SEC2].

Intellectual Property Rights

To be provided.

[NOTE: The readers should be aware of the possibility that
implementation of this draft may require use of inventions covered by
patent rights.]

Acknowledgments

The author would like to thank Simon Blake-Wilson, Prakash Panjwani,
and Paul Lambert for their comments and suggestions.

References

   [IKE] Harkins, D. and Carrel, D., "The Internet Key Exchange",
         draft-ietf-ipsec-ike-01.txt, May 1999.

   [RFC2409] Harkins, D. and Carrel, D., "The Internet Key Exchange"
             (RFC 2409).  November, 1998.

   [X9.62] American National Standards Institute. ANSI X9.62-1998,
           "Public Key Cryptography for the Financial Services
           Industry: The Elliptic Curve Digital Signature
           Algorithm". January, 1999.

   [KEYS] Lenstra, A.K. and Verheul, E.R., "Selecting Cryptographic
          Key Sizes", October 1999. Presented at Public Key
          Cryptography Conference, Melbourne, Australia, January,
          2000. Available at <http://www.cryptosavvy.com/>.

   [FIPS-180]  Federal Information Processing Standards Publication
               (FIPS PUB) 180-1, "Secure Hash Standard", April 17,
               1995.

   [FIPS-186]  Federal Information Processing Standards Publication
               (FIPS PUB) 186, "Digital Signature Standard", May 19,
               1994.

   [NIST-ECC] National Institute for Standards and Technology,
              "Recommended Elliptic Curves for Federal Government Use",
              July 1999, <http://csrc.nist.gov/encryption/NISTReCur.pdf>

   [SEC1]     Standards for Efficient Cryptography Group, "SEC 1:
              Elliptic Curve Cryptography", Version 0.5, September,
              1999. <http://www.secg.org>

   [SEC2]     Standards for Efficient Cryptography Group, "SEC 2:
              Recommended Elliptic Curve Domain Parameters",
              Version 0.6, October, 1999. <http://www.secg.org>

   [ECC-GR]  Panjwani, P. and Poeluev, Y., "Additional ECC Groups for
             IKE", draft-ietf-ipsec-ike-ecc-groups-01.txt. September,
             1999.

   [RFC 2459] R. Housley, W. Ford, W. Polk and D. Solo "Internet X.509
              Public Key Infrastructure: Certificate and CRL Profile",
              January, 1999.

   [EPKIX]  Bassham, L., Johnson, D., and Polk, W., "Representation of
            Elliptic Curve Digital Signature Algorithm (ECDSA) Keys
            and Signatures in Internet X.509 Public Key Infrastructure
            Certificates", draft-ietf-pkix-ipki-ecdsa-02.txt. October,
            1999.


Author's Address

   Paul Fahn
   Certicom Corp.
   25801 Industrial Blvd.
   Hayward, CA 94545

   e-mail: pfahn@certicom.com


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