4 NETWORK WORKING GROUP L. Zhu
5 Internet-Draft K. Jaganathan
6 Expires: March 17, 2006 K. Lauter
11 ECC Support for PKINIT
12 draft-zhu-pkinit-ecc-00
16 By submitting this Internet-Draft, each author represents that any
17 applicable patent or other IPR claims of which he or she is aware
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21 Internet-Drafts are working documents of the Internet Engineering
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34 The list of Internet-Draft Shadow Directories can be accessed at
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37 This Internet-Draft will expire on March 17, 2006.
41 Copyright (C) The Internet Society (2005).
45 This document describes the use of Elliptic Curve certificates,
46 Elliptic Curve signature schemes and Elliptic Curve Diffie-Hellman
47 (ECDH) key agreement within the framework of PKINIT - the Kerberos
48 Version 5 extension that provides for the use of public key
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62 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
63 2. Conventions Used in This Document . . . . . . . . . . . . . . 3
64 3. Using Elliptic Curve Certificates and Elliptic Curve
65 Signature Schemes . . . . . . . . . . . . . . . . . . . . . . 3
66 4. Using ECDH Key Exchange . . . . . . . . . . . . . . . . . . . 4
67 5. Choosing the Domain Parameters and the Key Size . . . . . . . 6
68 6. Security Considerations . . . . . . . . . . . . . . . . . . . 7
69 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
70 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
71 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
72 9.1. Normative References . . . . . . . . . . . . . . . . . . . 8
73 9.2. Informative References . . . . . . . . . . . . . . . . . . 9
74 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10
75 Intellectual Property and Copyright Statements . . . . . . . . . . 11
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118 Elliptic Curve Cryptography (ECC) is emerging as an attractive
119 public-key cryptosystem that provides security equivalent to
120 currently popular public-key mechanisms such as RSA and DSA with
121 smaller key sizes [LENSTRA] [NISTSP80057].
123 Currently [PKINIT] permits the use of ECC algorithms but it does not
124 specify how ECC parameters are chosen and how to derive the shared
125 key for key delivery using Elliptic Curve Diffie-Hellman (ECDH)
128 This document describes how to use Elliptic Curve certificates,
129 Elliptic Curve signature schemes, and ECDH with [PKINIT]. However,
130 it should be noted that there is no syntactic or semantic change to
131 the existing [PKINIT] messages. Both the client and the KDC
132 contribute one ECDH key pair using the key agrement protocol
133 described in this document.
136 2. Conventions Used in This Document
138 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
139 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
140 document are to be interpreted as described in [RFC2119].
143 3. Using Elliptic Curve Certificates and Elliptic Curve Signature
146 ECC certificates and signature schemes can be used in the
147 Cryptographic Message Syntax (CMS) [RFC3369] content type
150 X.509 certificates [RFC3280] containing ECC public keys or signed
151 using ECC signature schemes MUST comply with [RFC3279].
153 The elliptic curve domain parameters recommended in [X9.62],
154 [FIPS186-2], and [SECG] SHOULD be used.
156 The signatureAlgorithm field of the CMS data type SignerInfo can
157 contain one of the following ECC signature algorithm identifiers:
159 ecdsa-with-Sha1 [ECCPKALGS]
160 ecdsa-with-Sha256 [ECCPKALGS]
161 ecdsa-with-Sha384 [ECCPKALGS]
162 ecdsa-with-Sha512 [ECCPKALGS]
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172 The corresponding digestAlgorithm field contains one of the following
173 hash algorithm identifiers respectively:
176 id-sha256 [ECCPKALGS]
177 id-sha384 [ECCPKALGS]
178 id-sha512 [ECCPKALGS]
180 Namely id-sha1 MUST be used in conjunction with ecdsa-with-Sha1, id-
181 sha256 MUST be used in conjunction with ecdsa-with-Sha256, id-sha384
182 MUST be used in conjunction with ecdsa-with-Sha384, and id-sha512
183 MUST be used in conjunction with ecdsa-with-Sha512.
185 Implementations of this specfication MUST support ecdsa-with-Sha256
186 and SHOULD support ecdsa-with-Sha1.
189 4. Using ECDH Key Exchange
191 This section describes how ECDH can be used as the AS reply key
192 delivery method [PKINIT]. Note that the protocol description is
193 similar to that of Modular Exponential Diffie-Hellman (MODP DH), as
194 described in [PKINIT].
196 If the client wishes to use ECDH key agreement method, it encodes its
197 ECDH public key value and the domain parameters [IEEE1363] for its
198 ECDH public key in clientPublicValue of the PA-PK-AS-REQ message
201 As described in [PKINIT], the ECDH domain parameters for the client's
202 public key are specified in the algorithm field of the type
203 SubjectPublicKeyInfo [RFC3279] and the client's ECDH public key value
204 is mapped to a subjectPublicKey (a BIT STRING) according to
207 The following algorithm identifier is used to identify the client's
208 choice of the ECDH key agreement method for key delivery.
210 id-ecPublicKey (Elliptic Curve Diffie-Hellman [IEEE1363])
212 If the domain parameters are not accepted by the KDC, the KDC sends
213 back an error message [RFC4120] with the code
214 KDC_ERR_DH_KEY_PARAMETERS_NOT_ACCEPTED [PKINIT]. This error message
215 contains the list of domain parameters acceptable to the KDC. This
216 list is encoded as TD-DH-PARAMETERS [PKINIT], and it is in the KDC's
217 decreasing preference order. The client can then pick a set of
218 domain parameters from the list and retry the authentication.
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228 Both the client and the KDC MUST have local policy that specifies
229 which set of domain parameters are acceptable if they do not have a
230 priori knowledge of the chosen domain parameters. The need for such
231 local policy is explained in Section 6.
233 If the ECDH domain parameters are accepted by the KDC, the KDC sends
234 back its ECDH public key value in the subjectPublicKey field of the
235 PA-PK-AS-REP message [PKINIT].
237 As described in [PKINIT], the KDC's ECDH public key value is encoded
238 as a BIT STRING according to [RFC3279].
240 Note that in the steps above, the client can indicate to the KDC that
241 it wishes to reuse ECDH keys or to allow the KDC to do so, by
242 including the clientDHNonce field in the request [PKINIT], and the
243 KDC can then reuse the ECDH keys and include serverDHNonce field in
244 the reply [PKINIT]. This logic is the same as that of the Modular
245 Exponential Diffie-Hellman key agreement method [PKINIT].
247 If ECDH is negotiated as the key delivery method, both the KDC and
248 the client calculate the shared secret value and derive the reply key
251 1) Let DHSharedSecret be the x-coordinate of the shared secret value
252 (an elliptic curve point). DHSharedSecret is the output of
253 operation ECSVDP-DH as described in Section 7.2.1 of [IEEE1363].
255 2) DHSharedSecret is first padded with leading zeros such that the
256 size of DHSharedSecret in octets is the same as that of the
257 modulus, then represented as a string of octets in big-endian
260 3) The DHSharedSecret value derived above is used as input to the
261 octetstring2key() function to derive the AS reply key k, as
262 described in Section 3.2.3.1 of [PKINIT].
264 Both the client and KDC then proceed as described in [PKINIT] and
267 Lastly it should be noted that ECDH can be used with any certificates
268 and signature schemes. However, a significant advantage of using
269 ECDH together with ECC certificates and signature schemes is that the
270 ECC domain parameters in the client or KDC certificates can be used.
271 This obviates the need of locally preconfigured domain parameters as
272 described in Section 6.
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284 5. Choosing the Domain Parameters and the Key Size
286 The domain parameters and the key size should be chosen so as to
287 provide sufficient cryptographic security [RFC3766]. The following
288 table, based on table 2 on page 63 of NIST SP800-57 part 1
289 [NISTSP80057], gives approximate comparable key sizes for symmetric-
290 and asymmetric-key cryptosystems based on the best-known algorithms
294 Symmetric | ECC | RSA
295 -------------+----------- +------------
296 80 | 160 - 223 | 1024
297 112 | 224 - 255 | 2048
298 128 | 256 - 383 | 3072
299 192 | 384 - 511 | 7680
302 Table 1: Comparable key sizes (in bits)
304 Thus, for example, when securing a 128-bit symmetric key, it is
305 prudent to use 256-bit Elliptic Curve Cryptography (ECC), e.g. group
306 P-256 (secp256r1) as described below.
308 A set of ECDH domain parameters is also known as a curve. A curve is
309 a named curve if the domain paratmeters are well known and can be
310 identified by an Object Identifier, otherwise it is called a custom
311 curve. [PKINIT] supports both named curves and custom curves, see
312 Section 6 on the tradeoff of choosing between named curves and custom
315 The named curves recommended in this document are also recommended by
316 NIST [FIPS186-2]. These fifteen ECC curves are given in the
317 following table [FIPS186-2] [SECG].
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340 Description SEC 2 OID
341 ----------------- ---------
343 ECPRGF192Random group P-192 secp192r1
344 EC2NGF163Random group B-163 sect163r2
345 EC2NGF163Koblitz group K-163 sect163k1
347 ECPRGF224Random group P-224 secp224r1
348 EC2NGF233Random group B-233 sect233r1
349 EC2NGF233Koblitz group K-233 sect233k1
351 ECPRGF256Random group P-256 secp256r1
352 EC2NGF283Random group B-283 sect283r1
353 EC2NGF283Koblitz group K-283 sect283k1
355 ECPRGF384Random group P-384 secp384r1
356 EC2NGF409Random group B-409 sect409r1
357 EC2NGF409Koblitz group K-409 sect409k1
359 ECPRGF521Random group P-521 secp521r1
360 EC2NGF571Random group B-571 sect571r1
361 EC2NGF571Koblitz group K-571 sect571k1
364 6. Security Considerations
366 Kerberos error messages are not integrity protected, as a result, the
367 domain parameters sent by the KDC as TD-DH-PARAMETERS can be tampered
368 with by an attacker so that the set of domain parameters selected
369 could be either weaker or not mutually preferred. Local policy can
370 configure sets of domain parameters acceptable locally, or disallow
371 the negotiation of ECDH domain parameters.
373 Beyond elliptic curve size, the main issue is elliptic curve
374 structure. As a general principle, it is more conservative to use
375 elliptic curves with as little algebraic structure as possible - thus
376 random curves are more conservative than special curves such as
377 Koblitz curves, and curves over F_p with p random are more
378 conservative than curves over F_p with p of a special form (and
379 curves over F_p with p random might be considered more conservative
380 than curves over F_2^m as there is no choice between multiple fields
381 of similar size for characteristic 2). Note, however, that algebraic
382 structure can also lead to implementation efficiencies and
383 implementors and users may, therefore, need to balance conservatism
384 against a need for efficiency. Concrete attacks are known against
385 only very few special classes of curves, such as supersingular
386 curves, and these classes are excluded from the ECC standards such as
387 [IEEE1363] and [X9.62].
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396 Another issue is the potential for catastrophic failures when a
397 single elliptic curve is widely used. In this case, an attack on the
398 elliptic curve might result in the compromise of a large number of
399 keys. Again, this concern may need to be balanced against efficiency
400 and interoperability improvements associated with widely-used curves.
401 Substantial additional information on elliptic curve choice can be
402 found in [IEEE1363], [X9.62] and [FIPS186-2].
405 7. IANA Considerations
407 No IANA actions are required for this document.
412 The following people have made significant contributions to this
413 draft: Paul Leach, Dan Simon, Kelvin Yiu, David Cross and Sam
419 9.1. Normative References
422 RFC-Editor: To be replaced by RFC number for draft-ietf-
423 pkix-ecc-pkalgs. Work in Progress.
426 NIST, "Digital Signature Standard", FIPS 186-2, 2000.
429 IEEE, "Standard Specifications for Public Key Cryptography",
433 NIST, "Recommendation on Key Management",
434 http://csrc.nist.gov/publications/nistpubs/, SP 800-57,
437 [PKINIT] RFC-Editor: To be replaced by RFC number for draft-ietf-
438 cat-kerberos-pk-init. Work in Progress.
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443 Internet-Draft ECC Support for PKINIT September 2005
447 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
448 Requirement Levels", BCP 14, RFC 2119, March 1997.
450 [RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and
451 Identifiers for the Internet X.509 Public Key
452 Infrastructure Certificate and Certificate Revocation List
453 (CRL) Profile", RFC 3279, April 2002.
455 [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
456 X.509 Public Key Infrastructure Certificate and
457 Certificate Revocation List (CRL) Profile", RFC 3280,
460 [RFC3369] Housley, R., "Cryptographic Message Syntax (CMS)",
461 RFC 3369, August 2002.
463 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For
464 Public Keys Used For Exchanging Symmetric Keys", BCP 86,
465 RFC 3766, April 2004.
467 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
468 Kerberos Network Authentication Service (V5)", RFC 4120,
471 [X9.62] ANSI, "Public Key Cryptography For The Financial Services
472 Industry: The Elliptic Curve Digital Signature Algorithm
473 (ECDSA)", ANSI X9.62, 1998.
475 9.2. Informative References
477 [LENSTRA] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key
478 Sizes", Journal of Cryptology 14 (2001) 255-293.
480 [SECG] SECG, "Elliptic Curve Cryptography", SEC 1, 2000,
481 <http://www.secg.org/>.
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504 Microsoft Corporation
509 Email: lzhu@microsoft.com
513 Microsoft Corporation
518 Email: karthikj@microsoft.com
522 Microsoft Corporation
527 Email: klauter@microsoft.com
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608 Zhu, et al. Expires March 17, 2006 [Page 11]