NETWORK WORKING GROUP L. Zhu Internet-Draft K. Jaganathan Expires: March 17, 2006 K. Lauter Microsoft Corporation September 13, 2005 ECC Support for PKINIT draft-zhu-pkinit-ecc-00 Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on March 17, 2006. Copyright Notice Copyright (C) The Internet Society (2005). 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. Zhu, et al. Expires March 17, 2006 [Page 1] Internet-Draft ECC Support for PKINIT September 2005 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Conventions Used in This Document . . . . . . . . . . . . . . 3 3. Using Elliptic Curve Certificates and Elliptic Curve Signature Schemes . . . . . . . . . . . . . . . . . . . . . . 3 4. Using ECDH Key Exchange . . . . . . . . . . . . . . . . . . . 4 5. Choosing the Domain Parameters and the Key Size . . . . . . . 6 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 9.1. Normative References . . . . . . . . . . . . . . . . . . . 8 9.2. Informative References . . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 11 Zhu, et al. Expires March 17, 2006 [Page 2] Internet-Draft ECC Support for PKINIT September 2005 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 [PKINIT] permits the use of ECC algorithms but it does not specify how ECC parameters are chosen and how to derive the shared key for key delivery using Elliptic Curve Diffie-Hellman (ECDH) [IEEE1363]. This document describes how to use Elliptic Curve certificates, Elliptic Curve signature schemes, and ECDH with [PKINIT]. However, it should be noted that there is no syntactic or semantic change to the existing [PKINIT] messages. Both the client and the KDC contribute one ECDH key pair using the key agrement 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) [RFC3369] content type 'SignedData'. X.509 certificates [RFC3280] containing ECC public keys or signed using ECC signature schemes MUST comply with [RFC3279]. The elliptic curve domain parameters recommended in [X9.62], [FIPS186-2], and [SECG] SHOULD be used. The signatureAlgorithm field of the CMS data type SignerInfo can contain one of the following ECC signature algorithm identifiers: ecdsa-with-Sha1 [ECCPKALGS] ecdsa-with-Sha256 [ECCPKALGS] ecdsa-with-Sha384 [ECCPKALGS] ecdsa-with-Sha512 [ECCPKALGS] Zhu, et al. Expires March 17, 2006 [Page 3] Internet-Draft ECC Support for PKINIT September 2005 The corresponding digestAlgorithm field contains one of the following hash algorithm identifiers respectively: id-sha1 [RFC3279] id-sha256 [ECCPKALGS] id-sha384 [ECCPKALGS] id-sha512 [ECCPKALGS] 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 specfication MUST support ecdsa-with-Sha256 and SHOULD support ecdsa-with-Sha1. 4. Using ECDH Key Exchange This section describes how ECDH can be used as the AS reply key delivery method [PKINIT]. Note that the protocol description is similar to that of Modular Exponential Diffie-Hellman (MODP DH), as described in [PKINIT]. If the client wishes to use ECDH key agreement method, it encodes its ECDH public key value and the domain parameters [IEEE1363] for its ECDH public key in clientPublicValue of the PA-PK-AS-REQ message [PKINIT]. As described in [PKINIT], 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 [IEEE1363]) 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 [PKINIT]. This error message contains the list of domain parameters acceptable to the KDC. This list is encoded as TD-DH-PARAMETERS [PKINIT], 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. Zhu, et al. Expires March 17, 2006 [Page 4] Internet-Draft ECC Support for PKINIT September 2005 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 6. 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 [PKINIT]. As described in [PKINIT], 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 to allow the KDC to do so, by including the clientDHNonce field in the request [PKINIT], and the KDC can then reuse the ECDH keys and include serverDHNonce field in the reply [PKINIT]. This logic is the same as that of the Modular Exponential Diffie-Hellman key agreement method [PKINIT]. If ECDH is negotiated as the key delivery method, both the KDC and the client calculate the shared secret value and derive the reply key as follows: 1) Let DHSharedSecret be the x-coordinate of the shared secret value (an elliptic curve point). DHSharedSecret is the output of operation ECSVDP-DH as described in Section 7.2.1 of [IEEE1363]. 2) DHSharedSecret is first padded with leading zeros such that the size of DHSharedSecret in octets is the same as that of the modulus, then represented as a string of octets in big-endian order. 3) The DHSharedSecret value derived above is used as input to the octetstring2key() function to derive the AS reply key k, as described in Section 3.2.3.1 of [PKINIT]. Both the client and KDC then proceed as described in [PKINIT] 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 or KDC certificates can be used. This obviates the need of locally preconfigured domain parameters as described in Section 6. Zhu, et al. Expires March 17, 2006 [Page 5] Internet-Draft ECC Support for PKINIT September 2005 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. 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 paratmeters are well known and can be identified by an Object Identifier, otherwise it is called a custom curve. [PKINIT] supports both named curves and custom curves, see Section 6 on the tradeoff of choosing between named curves and custom curves. The named curves recommended in this document are also recommended by NIST [FIPS186-2]. These fifteen ECC curves are given in the following table [FIPS186-2] [SECG]. Zhu, et al. Expires March 17, 2006 [Page 6] Internet-Draft ECC Support for PKINIT September 2005 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 6. Security Considerations 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 acceptable locally, or 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 (and 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]. Zhu, et al. Expires March 17, 2006 [Page 7] Internet-Draft ECC Support for PKINIT September 2005 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]. 7. IANA Considerations No IANA actions are required for this document. 8. Acknowledgements The following people have made significant contributions to this draft: Paul Leach, Dan Simon, Kelvin Yiu, David Cross and Sam Hartman. 9. References 9.1. Normative References [ECCPKALGS] RFC-Editor: To be replaced by RFC number for draft-ietf- pkix-ecc-pkalgs. Work in Progress. [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", http://csrc.nist.gov/publications/nistpubs/, SP 800-57, August 2005. [PKINIT] RFC-Editor: To be replaced by RFC number for draft-ietf- cat-kerberos-pk-init. Work in Progress. Zhu, et al. Expires March 17, 2006 [Page 8] Internet-Draft ECC Support for PKINIT September 2005 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [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. [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", RFC 3280, April 2002. [RFC3369] Housley, R., "Cryptographic Message Syntax (CMS)", RFC 3369, August 2002. [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For Public Keys Used For Exchanging Symmetric Keys", BCP 86, RFC 3766, April 2004. [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The Kerberos Network Authentication Service (V5)", RFC 4120, July 2005. [X9.62] ANSI, "Public Key Cryptography For The Financial Services Industry: The Elliptic Curve Digital Signature Algorithm (ECDSA)", ANSI X9.62, 1998. 9.2. Informative References [LENSTRA] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key Sizes", Journal of Cryptology 14 (2001) 255-293. [SECG] SECG, "Elliptic Curve Cryptography", SEC 1, 2000, <http://www.secg.org/>. Zhu, et al. Expires March 17, 2006 [Page 9] Internet-Draft ECC Support for PKINIT September 2005 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. Expires March 17, 2006 [Page 10] Internet-Draft ECC Support for PKINIT September 2005 Intellectual Property Statement The IETF takes no position regarding the validity or scope of any Intellectual Property Rights or other rights that might be claimed to pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; nor does it represent that it has made any independent effort to identify any such rights. 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Disclaimer of Validity This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Copyright Statement Copyright (C) The Internet Society (2005). 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. Acknowledgment Funding for the RFC Editor function is currently provided by the Internet Society. Zhu, et al. Expires March 17, 2006 [Page 11]