Elliptic Curve Cryptography (ECC) Support for Public Key Cryptography for Initial Authentication in Kerberos (PKINIT)
draftzhupkinitecc04
The information below is for an old version of the document that is already published as an RFC.
Document  Type 
This is an older version of an InternetDraft that was ultimately published as RFC 5349.



Authors  Karthik Jaganathan , Kristin Lauter , Larry Zhu  
Last updated  20151014 (Latest revision 20071024)  
RFC stream  Internet Engineering Task Force (IETF)  
Intended RFC status  Informational  
Formats  
Reviews  
Additional resources  Mailing list discussion  
Stream  WG state  (None)  
Document shepherd  (None)  
IESG  IESG state  Became RFC 5349 (Informational)  
Action Holders 
(None)


Consensus boilerplate  Unknown  
Telechat date  (None)  
Responsible AD  Tim Polk  
Send notices to  (None) 
draftzhupkinitecc04
NETWORK WORKING GROUP L. Zhu InternetDraft K. Jaganathan Intended status: Informational K. Lauter Expires: April 26, 2008 Microsoft Corporation October 24, 2007 ECC Support for PKINIT draftzhupkinitecc04 Status of this Memo By submitting this InternetDraft, 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. InternetDrafts 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. InternetDrafts 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 InternetDrafts as reference material or to cite them other than as "work in progress." The list of current InternetDrafts can be accessed at http://www.ietf.org/ietf/1idabstracts.txt. The list of InternetDraft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This InternetDraft will expire on April 26, 2008. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract This document describes the use of Elliptic Curve certificates, Elliptic Curve signature schemes and Elliptic Curve DiffieHellman (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 April 26, 2008 [Page 1] InternetDraft ECC Support for PKINIT October 2007 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 . . . . . . . 5 6. Interoperability Requirements . . . . . . . . . . . . . . . . 7 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9 Intellectual Property and Copyright Statements . . . . . . . . . . 11 Zhu, et al. Expires April 26, 2008 [Page 2] InternetDraft ECC Support for PKINIT October 2007 1. Introduction Elliptic Curve Cryptography (ECC) is emerging as an attractive publickey cryptosystem that provides security equivalent to currently popular publickey 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 and how to derive the shared key for key delivery using Elliptic Curve DiffieHellman (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 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) [RFC3852] [RFC3278] content type 'SignedData'. X.509 certificates [RFC3280] containing ECC public keys or 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: ecdsawithSha1 [RFC3279] ecdsawithSha256 [X9.62] ecdsawithSha384 [X9.62] ecdsawithSha512 [X9.62] The corresponding digestAlgorithm field contains one of the following hash algorithm identifiers respectively: Zhu, et al. Expires April 26, 2008 [Page 3] InternetDraft ECC Support for PKINIT October 2007 idsha1 [RFC3279] idsha256 [X9.62] idsha384 [X9.62] idsha512 [X9.62] Namely idsha1 MUST be used in conjunction with ecdsawithSha1, id sha256 MUST be used in conjunction with ecdsawithSha256, idsha384 MUST be used in conjunction with ecdsawithSha384, and idsha512 MUST be used in conjunction with ecdsawithSha512. Implementations of this specfication MUST support ecdsawithSha256 and SHOULD support ecdsawithSha1. 4. Using ECDH Key Exchange This section describes how ECDH can be used as the AS reply key delivery method [RFC4556]. Note that the protocol description here is similar to that of Modular Exponential DiffieHellman (MODP DH), as described in [RFC4556]. If the client wishes to use ECDH key agreement method, it encodes its ECDH public key value and the domain parameters [IEEE1363] [X9.63] for its ECDH public key in clientPublicValue of the PAPKASREQ 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. idecPublicKey (Elliptic Curve DiffieHellman [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 TDDHPARAMETERS [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 Zhu, et al. Expires April 26, 2008 [Page 4] InternetDraft ECC Support for PKINIT October 2007 local policy is explained in Section 7. 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 PAPKASREP 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 to allow the KDC to do so, by including the clientDHNonce field in the request [RFC4556], and the KDC can then reuse the ECDH keys and include serverDHNonce field in the reply [RFC4556]. This logic is the same as that of the Modular Exponential DiffieHellman key agreement method [RFC4556]. If ECDH is negotiated as the key delivery method, then the PAPKAS REP and AS reply key are generated as in Section 3.2.3.1 of [RFC4556] with the following difference: The DHSharedSecret is the xcoordinate of the shared secret value (an elliptic curve point); DHSharedSecret is the output of operation ECSVDPDH as described in Section 7.2.1 of [IEEE1363]. 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 or 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 SP80057 part 1 [NISTSP80057], gives approximate comparable key sizes for symmetric and asymmetrickey cryptosystems based on the bestknown algorithms for attacking them. Zhu, et al. Expires April 26, 2008 [Page 5] InternetDraft ECC Support for PKINIT October 2007 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 128bit symmetric key, it is prudent to use 256bit Elliptic Curve Cryptography (ECC), e.g. group P256 (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. [RFC4556] supports both named curves and custom curves, see Section 7 on the tradeoff of choosing between named curves and custom curves. The named curves recommended in this document are also recommended by NIST [FIPS1862]. These fifteen ECC curves are given in the following table [FIPS1862] [SEC2]. Description SEC 2 OID   ECPRGF192Random group P192 secp192r1 EC2NGF163Random group B163 sect163r2 EC2NGF163Koblitz group K163 sect163k1 ECPRGF224Random group P224 secp224r1 EC2NGF233Random group B233 sect233r1 EC2NGF233Koblitz group K233 sect233k1 ECPRGF256Random group P256 secp256r1 EC2NGF283Random group B283 sect283r1 EC2NGF283Koblitz group K283 sect283k1 ECPRGF384Random group P384 secp384r1 EC2NGF409Random group B409 sect409r1 EC2NGF409Koblitz group K409 sect409k1 ECPRGF521Random group P521 secp521r1 EC2NGF571Random group B571 sect571r1 EC2NGF571Koblitz group K571 sect571k1 Zhu, et al. Expires April 26, 2008 [Page 6] InternetDraft ECC Support for PKINIT October 2007 6. Interoperability Requirements Implementations conforming to this specification MUST support curve P256 and P384. 7. Security Considerations When using ECDH key agreement, the recipient of an elliptic curve public key should perform certain checks to avoid the attacks described in [ECCValidation]. It is especially important if the recipient is using a longterm 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 TDDHPARAMETERS 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]. 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 widelyused curves. Substantial additional information on elliptic curve choice can be found in [IEEE1363], [X9.62] and [FIPS1862]. Zhu, et al. Expires April 26, 2008 [Page 7] InternetDraft ECC Support for PKINIT October 2007 8. IANA Considerations No IANA actions are required for this document. 9. Acknowledgements The following people have made significant contributions to this draft: Paul Leach, Dan Simon, Kelvin Yiu, David Cross, Sam Hartman, Tolga Acar, and Stefan Santesson. 10. References 10.1. Normative References [ECCValidation] A. Antipa, D. Brown, A. Menezes, R. Struik and S. Vanstone, "Validation of Elliptic Curve Public Keys", Public Key Crytpography  PKC 2003, pp. 211 223, LNCS 2567, Springer, 2003.. [FIPS1862] NIST, "Digital Signature Standard", FIPS 1862, 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 80057, August 2005. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC3278] BlakeWilson, 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. Zhu, et al. Expires April 26, 2008 [Page 8] InternetDraft ECC Support for PKINIT October 2007 [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. [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. [SEC2] Standards for Efficient Cryptography Group. SEC 2  Recommended Elliptic Curve Domain Parameters. Ver. 1.0., 2000. See: http://www.secg.org [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. 10.2. Informative References [LENSTRA] Tung, B., Neuman, B., and S. Medvinsky, "Public Key Cryptography for Initial Authentication in Kerberos", August 2004. Authors' Addresses Larry Zhu Microsoft Corporation One Microsoft Way Redmond, WA 98052 US Email: lzhu@microsoft.com Zhu, et al. Expires April 26, 2008 [Page 9] InternetDraft ECC Support for PKINIT October 2007 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 April 26, 2008 [Page 10] InternetDraft ECC Support for PKINIT October 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). 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The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights that may cover technology that may be required to implement this standard. Please address the information to the IETF at ietfipr@ietf.org. Acknowledgment Funding for the RFC Editor function is provided by the IETF Administrative Support Activity (IASA). Zhu, et al. Expires April 26, 2008 [Page 11]