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rfc:rfc5349

Network Working Group L. Zhu Request for Comments: 5349 K. Jaganathan Category: Informational K. Lauter

                                                 Microsoft Corporation
                                                        September 2008

Elliptic Curve Cryptography (ECC) Support for Public Key Cryptography

          for Initial Authentication in Kerberos (PKINIT)

Status of This Memo

 This memo provides information for the Internet community.  It does
 not specify an Internet standard of any kind.  Distribution of this
 memo is unlimited.

Abstract

 This document describes the use of Elliptic Curve certificates,
 Elliptic Curve signature schemes and Elliptic Curve Diffie-Hellman
 (ECDH) key agreement within the framework of PKINIT -- the Kerberos
 Version 5 extension that provides for the use of public key
 cryptography.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
 2.  Conventions Used in This Document . . . . . . . . . . . . . . . 2
 3.  Using Elliptic Curve Certificates and Elliptic Curve
     Signature Schemes . . . . . . . . . . . . . . . . . . . . . . . 2
 4.  Using the ECDH Key Exchange . . . . . . . . . . . . . . . . . . 3
 5.  Choosing the Domain Parameters and the Key Size . . . . . . . . 4
 6.  Interoperability Requirements . . . . . . . . . . . . . . . . . 6
 7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
 8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
   9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
   9.2.  Informative References  . . . . . . . . . . . . . . . . . . 8

Zhu, et al. Informational [Page 1] RFC 5349 ECC Support for PKINIT September 2008

1. Introduction

 Elliptic Curve Cryptography (ECC) is emerging as an attractive
 public-key cryptosystem that provides security equivalent to
 currently popular public-key mechanisms such as RSA and DSA with
 smaller key sizes [LENSTRA] [NISTSP80057].
 Currently, [RFC4556] permits the use of ECC algorithms but it does
 not specify how ECC parameters are chosen or how to derive the shared
 key for key delivery using Elliptic Curve Diffie-Hellman (ECDH)
 [IEEE1363] [X9.63].
 This document describes how to use Elliptic Curve certificates,
 Elliptic Curve signature schemes, and ECDH with [RFC4556].  However,
 it should be noted that there is no syntactic or semantic change to
 the existing [RFC4556] messages.  Both the client and the Key
 Distribution Center (KDC) contribute one ECDH key pair using the key
 agreement protocol described in 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].

3. Using Elliptic Curve Certificates and Elliptic Curve Signature

  Schemes
 ECC certificates and signature schemes can be used in the
 Cryptographic Message Syntax (CMS) [RFC3852] [RFC3278] content type
 'SignedData'.
 X.509 certificates [RFC5280] that contain ECC public keys or are
 signed using ECC signature schemes MUST comply with [RFC3279].
 The signatureAlgorithm field of the CMS data type 'SignerInfo' can
 contain one of the following ECC signature algorithm identifiers:
    ecdsa-with-Sha1   [RFC3279]
    ecdsa-with-Sha256 [X9.62]
    ecdsa-with-Sha384 [X9.62]
    ecdsa-with-Sha512 [X9.62]
 The corresponding digestAlgorithm field contains one of the following
 hash algorithm identifiers respectively:

Zhu, et al. Informational [Page 2] RFC 5349 ECC Support for PKINIT September 2008

    id-sha1           [RFC3279]
    id-sha256         [X9.62]
    id-sha384         [X9.62]
    id-sha512         [X9.62]
 Namely, id-sha1 MUST be used in conjunction with ecdsa-with-Sha1,
 id-sha256 MUST be used in conjunction with ecdsa-with-Sha256,
 id-sha384 MUST be used in conjunction with ecdsa-with-Sha384, and
 id-sha512 MUST be used in conjunction with ecdsa-with-Sha512.
 Implementations of this specification MUST support ecdsa-with-Sha256
 and SHOULD support ecdsa-with-Sha1.

4. Using the ECDH Key Exchange

 This section describes how ECDH can be used as the Authentication
 Service (AS) reply key delivery method [RFC4556].  Note that the
 protocol description here is similar to that of Modular Exponential
 Diffie-Hellman (MODP DH), as described in [RFC4556].
 If the client wishes to use the ECDH key agreement method, it encodes
 its ECDH public key value and the key's domain parameters [IEEE1363]
 [X9.63] in clientPublicValue of the PA-PK-AS-REQ message [RFC4556].
 As described in [RFC4556], the ECDH domain parameters for the
 client's public key are specified in the algorithm field of the type
 SubjectPublicKeyInfo [RFC3279] and the client's ECDH public key value
 is mapped to a subjectPublicKey (a BIT STRING) according to
 [RFC3279].
 The following algorithm identifier is used to identify the client's
 choice of the ECDH key agreement method for key delivery.
      id-ecPublicKey  (Elliptic Curve Diffie-Hellman [RFC3279])
 If the domain parameters are not accepted by the KDC, the KDC sends
 back an error message [RFC4120] with the code
 KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED [RFC4556].  This error message
 contains the list of domain parameters acceptable to the KDC.  This
 list is encoded as TD-DH-PARAMETERS [RFC4556], and it is in the KDC's
 decreasing preference order.  The client can then pick a set of
 domain parameters from the list and retry the authentication.
 Both the client and the KDC MUST have local policy that specifies
 which set of domain parameters are acceptable if they do not have a
 priori knowledge of the chosen domain parameters.  The need for such
 local policy is explained in Section 7.

Zhu, et al. Informational [Page 3] RFC 5349 ECC Support for PKINIT September 2008

 If the ECDH domain parameters are accepted by the KDC, the KDC sends
 back its ECDH public key value in the subjectPublicKey field of the
 PA-PK-AS-REP message [RFC4556].
 As described in [RFC4556], the KDC's ECDH public key value is encoded
 as a BIT STRING according to [RFC3279].
 Note that in the steps above, the client can indicate to the KDC that
 it wishes to reuse ECDH keys or it can allow the KDC to do so, by
 including the clientDHNonce field in the request [RFC4556]; the KDC
 can then reuse the ECDH keys and include the serverDHNonce field in
 the reply [RFC4556].  This logic is the same as that of the Modular
 Exponential Diffie-Hellman key agreement method [RFC4556].
 If ECDH is negotiated as the key delivery method, then the
 PA-PK-AS-REP and AS reply key are generated as in Section 3.2.3.1 of
 [RFC4556] with the following difference: The ECDH shared secret value
 (an elliptic curve point) is calculated using operation ECSVDP-DH as
 described in Section 7.2.1 of [IEEE1363].  The x-coordinate of this
 point is converted to an octet string using operation FE2OSP as
 described in Section 5.5.4 of [IEEE1363].  This octet string is the
 DHSharedSecret.
 Both the client and KDC then proceed as described in [RFC4556] and
 [RFC4120].
 Lastly, it should be noted that ECDH can be used with any
 certificates and signature schemes.  However, a significant advantage
 of using ECDH together with ECC certificates and signature schemes is
 that the ECC domain parameters in the client certificates or the KDC
 certificates can be used.  This obviates the need of locally
 preconfigured domain parameters as described in Section 7.

5. Choosing the Domain Parameters and the Key Size

 The domain parameters and the key size should be chosen so as to
 provide sufficient cryptographic security [RFC3766].  The following
 table, based on table 2 on page 63 of NIST SP800-57 part 1
 [NISTSP80057], gives approximate comparable key sizes for symmetric-
 and asymmetric-key cryptosystems based on the best-known algorithms
 for attacking them.

Zhu, et al. Informational [Page 4] RFC 5349 ECC Support for PKINIT September 2008

               Symmetric    |  ECC       |   RSA
               -------------+----------- +------------
                  80        |  160 - 223 |   1024
                 112        |  224 - 255 |   2048
                 128        |  256 - 383 |   3072
                 192        |  384 - 511 |   7680
                 256        |  512+      |  15360
              Table 1: Comparable key sizes (in bits)
 Thus, for example, when securing a 128-bit symmetric key, it is
 prudent to use 256-bit Elliptic Curve Cryptography (ECC), e.g., group
 P-256 (secp256r1) as described below.
 A set of ECDH domain parameters is also known as a "curve".  A curve
 is a "named curve" if the domain parameters are well known and can be
 identified by an Object Identifier; otherwise, it is called a "custom
 curve".  [RFC4556] supports both named curves and custom curves, see
 Section 7 on the tradeoffs of choosing between named curves and
 custom curves.
 The named curves recommended in this document are also recommended by
 the National Institute of Standards and Technology (NIST)[FIPS186-2].
 These fifteen ECC curves are given in the following table [FIPS186-2]
 [SEC2].
            Description                      SEC 2 OID
            -----------------                ---------
            ECPRGF192Random  group P-192     secp192r1
            EC2NGF163Random  group B-163     sect163r2
            EC2NGF163Koblitz group K-163     sect163k1
            ECPRGF224Random  group P-224     secp224r1
            EC2NGF233Random  group B-233     sect233r1
            EC2NGF233Koblitz group K-233     sect233k1
            ECPRGF256Random  group P-256     secp256r1
            EC2NGF283Random  group B-283     sect283r1
            EC2NGF283Koblitz group K-283     sect283k1
            ECPRGF384Random  group P-384     secp384r1
            EC2NGF409Random  group B-409     sect409r1
            EC2NGF409Koblitz group K-409     sect409k1
            ECPRGF521Random  group P-521     secp521r1
            EC2NGF571Random  group B-571     sect571r1
            EC2NGF571Koblitz group K-571     sect571k1

Zhu, et al. Informational [Page 5] RFC 5349 ECC Support for PKINIT September 2008

6. Interoperability Requirements

 Implementations conforming to this specification MUST support curve
 P-256 and P-384.

7. Security Considerations

 When using ECDH key agreement, the recipient of an elliptic curve
 public key should perform the checks described in IEEE P1363, Section
 A16.10 [IEEE1363].  It is especially important, if the recipient is
 using a long-term ECDH private key, to check that the sender's public
 key is a valid point on the correct elliptic curve; otherwise,
 information may be leaked about the recipient's private key, and
 iterating the attack will eventually completely expose the
 recipient's private key.
 Kerberos error messages are not integrity protected; as a result, the
 domain parameters sent by the KDC as TD-DH-PARAMETERS can be tampered
 with by an attacker so that the set of domain parameters selected
 could be either weaker or not mutually preferred.  Local policy can
 configure sets of domain parameters that are acceptable locally or
 can disallow the negotiation of ECDH domain parameters.
 Beyond elliptic curve size, the main issue is elliptic curve
 structure.  As a general principle, it is more conservative to use
 elliptic curves with as little algebraic structure as possible.
 Thus, random curves are more conservative than special curves (such
 as Koblitz curves), and curves over F_p with p random are more
 conservative than curves over F_p with p of a special form.  (Also,
 curves over F_p with p random might be considered more conservative
 than curves over F_2^m, as there is no choice between multiple fields
 of similar size for characteristic 2.)  Note, however, that algebraic
 structure can also lead to implementation efficiencies, and
 implementors and users may, therefore, need to balance conservatism
 against a need for efficiency.  Concrete attacks are known against
 only very few special classes of curves, such as supersingular
 curves, and these classes are excluded from the ECC standards such as
 [IEEE1363] and [X9.62].
 Another issue is the potential for catastrophic failures when a
 single elliptic curve is widely used.  In this case, an attack on the
 elliptic curve might result in the compromise of a large number of
 keys.  Again, this concern may need to be balanced against efficiency
 and interoperability improvements associated with widely used curves.
 Substantial additional information on elliptic curve choice can be
 found in [IEEE1363], [X9.62], and [FIPS186-2].

Zhu, et al. Informational [Page 6] RFC 5349 ECC Support for PKINIT September 2008

8. Acknowledgements

 The following people have made significant contributions to this
 document: Paul Leach, Dan Simon, Kelvin Yiu, David Cross, Sam
 Hartman, Tolga Acar, and Stefan Santesson.

9. References

9.1. Normative References

 [FIPS186-2]    NIST, "Digital Signature Standard", FIPS 186-2, 2000.
 [IEEE1363]     IEEE, "Standard Specifications for Public Key
                Cryptography", IEEE 1363, 2000.
 [NISTSP80057]  NIST, "Recommendation on Key Management", SP 800-57,
                August 2005,
                <http://csrc.nist.gov/publications/nistpubs/>.
 [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC3278]      Blake-Wilson, S., Brown, D., and P. Lambert, "Use of
                Elliptic Curve Cryptography (ECC) Algorithms in
                Cryptographic Message Syntax (CMS)", RFC 3278,
                April 2002.
 [RFC3279]      Bassham, L., Polk, W., and R. Housley, "Algorithms and
                Identifiers for the Internet X.509 Public Key
                Infrastructure Certificate and Certificate Revocation
                List (CRL) Profile", RFC 3279, April 2002.
 [RFC3766]      Orman, H. and P. Hoffman, "Determining Strengths For
                Public Keys Used For Exchanging Symmetric Keys",
                BCP 86, RFC 3766, April 2004.
 [RFC3852]      Housley, R., "Cryptographic Message Syntax (CMS)",
                RFC 3852, July 2004.
 [RFC4120]      Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
                Kerberos Network Authentication Service (V5)",
                RFC 4120, July 2005.
 [RFC4556]      Zhu, L. and B. Tung, "Public Key Cryptography for
                Initial Authentication in Kerberos (PKINIT)",
                RFC 4556, June 2006.

Zhu, et al. Informational [Page 7] RFC 5349 ECC Support for PKINIT September 2008

 [RFC5280]      Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                Housley, R., and W. Polk, "Internet X.509 Public Key
                Infrastructure Certificate and Certificate Revocation
                List (CRL) Profile", RFC 5280, May 2008.
 [X9.62]        ANSI, "Public Key Cryptography For The Financial
                Services Industry: The Elliptic Curve Digital
                Signature Algorithm (ECDSA)", ANSI X9.62, 2005.
 [X9.63]        ANSI, "Public Key Cryptography for the Financial
                Services Industry: Key Agreement and Key Transport
                using Elliptic Curve Cryptography", ANSI X9.63, 2001.

9.2. Informative References

 [LENSTRA]      Lenstra, A. and E. Verheul, "Selecting Cryptographic
                Key Sizes", Journal of Cryptography 14, 255-293, 2001.
 [SEC2]         Standards for Efficient Cryptography Group, "SEC 2 -
                Recommended Elliptic Curve Domain Parameters",
                Ver. 1.0, 2000, <http://www.secg.org>.

Zhu, et al. Informational [Page 8] RFC 5349 ECC Support for PKINIT September 2008

Authors' Addresses

 Larry Zhu
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 US
 EMail: lzhu@microsoft.com
 Karthik Jaganathan
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 US
 EMail: karthikj@microsoft.com
 Kristin Lauter
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA  98052
 US
 EMail: klauter@microsoft.com

Zhu, et al. Informational [Page 9] RFC 5349 ECC Support for PKINIT September 2008

Full Copyright Statement

 Copyright (C) The IETF Trust (2008).
 This document is subject to the rights, licenses and restrictions
 contained in BCP 78, and except as set forth therein, the authors
 retain all their rights.
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 THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
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Zhu, et al. Informational [Page 10]

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