IPSec Working Group J. Solinas, NSA
INTERNET-DRAFT
Expires October 2, 2005 March 31, 2005
IKE Authentication Using ECDSA
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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 may provide benefits
including 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.
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1. Introduction
The Internet Key Exchange, or IKE [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 against the best attacks known, 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.
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].
2. ECDSA
The Elliptic Curve Digital Signature Algorithm (ECDSA) is the
elliptic curve analogue of the DSA (DSS) signature method [DSS]. It
is defined in the ANSI X9.62 standard [X9.62]. Other compatible
specifications include FIPS 186-2 [DSS], IEEE 1363 [IEEE-1363], IEEE
1363A [IEEE-1363A], and SEC1 [SEC1].
Like DSA, ECDSA incorporates the use of a hash function. [SHS]
specifies hash functions that are appropriate for use with ECDSA.
Implementations of IKE using ECDSA SHOULD use one of these hash
functions.
ECDSA signatures are smaller than RSA signatures of similar
cryptographic strength. 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 more quickly than similar strength RSA or
DSA operations (see [LV] for a security analysis of key sizes across
public key algorithms). These advantages of signature size,
bandwidth, and computational efficiency may make ECDSA an attractive
choice for many IKE implementations.
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Recommended elliptic curve domain parameters for use with ECDSA are
given in FIPS 186-2 [DSS], ANSI X9.62 [X9.62], and SEC 2 [SEC2].
Implementations of IKE using ECDSA MAY use one of these domain
parameters. A subset of these parameters are recommended in
[IKE-ECP] for use in the IKE key exchange. These parameters MAY
be used for ECDSA as well.
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 IANA-assigned
attribute number for Phase 1 authentication using ECDSA is 8 (see
[IANA]).
Phase 1 can be either Main Mode or Aggressive 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 as a byte string using the ASN.1 syntax
"ECDSA-Sig-Value" with DER encoding rules as specified in ANSI X9.62
[X9.62].
As with 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-3280] is recommended; the representation of ECDSA
keys in X.509 certificates is specified in [RFC-3279]. This
representation SHOULD be used if X.509 certificates are used.
Implemententers may find it convenient, when using ECDSA as the
authentication method, to specify the hash used by ECDSA as the
value of the hash algorithm attribute. 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
[IKE-ECP] for some specific curves for the key agreement.
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4. 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 ANSI X9.62 [X9.62], FIPS 186-2 [DSS], and SEC 2
[SEC2].
5. IANA Considerations
This document has no actions for IANA.
6. References
6.1 Normative
[IKE] D. Harkins and D. Carrel, The Internet Key Exchange, RFC 2409,
November 1998.
[RFC-3279] Bassham, L., Housley, R., and Polk, W., RFC 3279,
Algorithms and Identifiers for the Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List (CRL)
Profile, 2002. (http://www.ietf.org/rfc/rfc3279.txt)
[RFC-3280] Housley, R., Polk, W., Ford, W. and D. Solo, RFC 3280,
Internet X.509 Public Key Infrastructure Certificate and
Certificate Revocation List (CRL) Profile, 2002.
(http://www.ietf.org/rfc/rfc3279.txt)
[X9.62] American National Standards Institute, ANS X9.62-1998:
Public Key Cryptography for the Financial Services Industry: The
Elliptic Curve Digital Signature Algorithm. January 1999.
6.2 Informative
[DSS] U.S. Department of Commerce/National Institute of Standards
and Technology, Digital Signature Standard (DSS), FIPS PUB 186-2,
January 2000. (http://csrc.nist.gov/publications/fips/index.html)
[IANA] Internet Assigned Numbers Authority, Internet Key Exchange
(IKE) Attributes. (http://www.iana.org/assignments/ipsec-registry)
[IEEE-1363] Institute of Electrical and Electronics Engineers.
IEEE 1363-2000, Standard for Public Key Cryptography.
(http://grouper.ieee.org/groups/1363/index.html)
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[IEEE-1363A] Institute of Electrical and Electronics Engineers.
IEEE 1363A-2004, Standard for Public Key Cryptography -
Amendment 1: Additional Techniques.
(http://grouper.ieee.org/groups/1363/index.html)
[IKE-ECP] J. Solinas, ECP Groups For IKE, 2005.
(draft-ietf-ipsec-ike-ecc-groups-05.txt)
[LV] A. Lenstra and E. Verheul, "Selecting Cryptographic Key
Sizes", Journal of Cryptology 14 (2001), pp. 255-293.
[SEC1] Standards for Efficient Cryptography Group. SEC 1 - Elliptic
Curve Cryptography, v. 1.0, 2000. (http://www.secg.org)
[SEC2] Standards for Efficient Cryptography Group. SEC 2 -
Recommended Elliptic Curve Domain Parameters, v. 1.0, 2000.
(http://www.secg.org)
[SHS] FIPS 180-2, "Secure Hash Standard", National Institute of
Standards and Technology, 2002.
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7. Author's Address
Jerome A. Solinas
National Security Agency
jsolinas@orion.ncsc.mil
Comments are solicited and should be addressed to the author.
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