1 2 3 4NETWORK WORKING GROUP L. Zhu 5Internet-Draft K. Jaganathan 6Expires: September 3, 2006 K. Lauter 7 Microsoft Corporation 8 March 2, 2006 9 10 11 ECC Support for PKINIT 12 draft-zhu-pkinit-ecc-01 13 14Status of this Memo 15 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 18 have been or will be disclosed, and any of which he or she becomes 19 aware will be disclosed, in accordance with Section 6 of BCP 79. 20 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF), its areas, and its working groups. Note that 23 other groups may also distribute working documents as Internet- 24 Drafts. 25 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 30 31 The list of current Internet-Drafts can be accessed at 32 http://www.ietf.org/ietf/1id-abstracts.txt. 33 34 The list of Internet-Draft Shadow Directories can be accessed at 35 http://www.ietf.org/shadow.html. 36 37 This Internet-Draft will expire on September 3, 2006. 38 39Copyright Notice 40 41 Copyright (C) The Internet Society (2006). 42 43Abstract 44 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 49 cryptography. 50 51 52 53 54 55Zhu, et al. Expires September 3, 2006 [Page 1] 56 57Internet-Draft ECC Support for PKINIT March 2006 58 59 60Table of Contents 61 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 . . . . . . . 5 68 6. Interoperability Requirements . . . . . . . . . . . . . . . . 7 69 7. Security Considerations . . . . . . . . . . . . . . . . . . . 7 70 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7 71 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7 72 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 73 10.1. Normative References . . . . . . . . . . . . . . . . . . 8 74 10.2. Informative References . . . . . . . . . . . . . . . . . 9 75 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 76 Intellectual Property and Copyright Statements . . . . . . . . . . 11 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111Zhu, et al. Expires September 3, 2006 [Page 2] 112 113Internet-Draft ECC Support for PKINIT March 2006 114 115 1161. Introduction 117 118 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]. 122 123 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) 126 [IEEE1363] [X9.63]. 127 128 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. 134 135 1362. Conventions Used in This Document 137 138 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]. 141 142 1433. Using Elliptic Curve Certificates and Elliptic Curve Signature 144 Schemes 145 146 ECC certificates and signature schemes can be used in the 147 Cryptographic Message Syntax (CMS) [RFC3369] content type 148 'SignedData'. 149 150 X.509 certificates [RFC3280] containing ECC public keys or signed 151 using ECC signature schemes MUST comply with [RFC3279]. 152 153 The elliptic curve domain parameters recommended in [X9.62], 154 [FIPS186-2], and [SECG] SHOULD be used. 155 156 The signatureAlgorithm field of the CMS data type SignerInfo can 157 contain one of the following ECC signature algorithm identifiers: 158 159 ecdsa-with-Sha1 [ECCPKALGS] 160 ecdsa-with-Sha256 [ECCPKALGS] 161 ecdsa-with-Sha384 [ECCPKALGS] 162 ecdsa-with-Sha512 [ECCPKALGS] 163 164 165 166 167Zhu, et al. Expires September 3, 2006 [Page 3] 168 169Internet-Draft ECC Support for PKINIT March 2006 170 171 172 The corresponding digestAlgorithm field contains one of the following 173 hash algorithm identifiers respectively: 174 175 id-sha1 [RFC3279] 176 id-sha256 [ECCPKALGS] 177 id-sha384 [ECCPKALGS] 178 id-sha512 [ECCPKALGS] 179 180 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. 184 185 Implementations of this specfication MUST support ecdsa-with-Sha256 186 and SHOULD support ecdsa-with-Sha1. 187 188 1894. Using ECDH Key Exchange 190 191 This section describes how ECDH can be used as the AS reply key 192 delivery method [PKINIT]. Note that the protocol description here is 193 similar to that of Modular Exponential Diffie-Hellman (MODP DH), as 194 described in [PKINIT]. 195 196 If the client wishes to use ECDH key agreement method, it encodes its 197 ECDH public key value and the domain parameters [IEEE1363] [X9.63] 198 for its ECDH public key in clientPublicValue of the PA-PK-AS-REQ 199 message [PKINIT]. 200 201 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 205 [RFC3279]. 206 207 The following algorithm identifier is used to identify the client's 208 choice of the ECDH key agreement method for key delivery. 209 210 id-ecPublicKey (Elliptic Curve Diffie-Hellman [ECCPKALGS]) 211 212 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. 219 220 221 222 223Zhu, et al. Expires September 3, 2006 [Page 4] 224 225Internet-Draft ECC Support for PKINIT March 2006 226 227 228 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 7. 232 233 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]. 236 237 As described in [PKINIT], the KDC's ECDH public key value is encoded 238 as a BIT STRING according to [RFC3279]. 239 240 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]. 246 247 If ECDH is negotiated as the key delivery method, then the PA-PK-AS- 248 REP and AS reply key are generated as in Section 3.2.3.1 of [PKINIT] 249 with the following difference: The DHSharedSecret is the x-coordinate 250 of the shared secret value (an elliptic curve point); DHSharedSecret 251 is the output of operation ECSVDP-DH as described in Section 7.2.1 of 252 [IEEE1363]. 253 254 Both the client and KDC then proceed as described in [PKINIT] and 255 [RFC4120]. 256 257 Lastly it should be noted that ECDH can be used with any certificates 258 and signature schemes. However, a significant advantage of using 259 ECDH together with ECC certificates and signature schemes is that the 260 ECC domain parameters in the client or KDC certificates can be used. 261 This obviates the need of locally preconfigured domain parameters as 262 described in Section 7. 263 264 2655. Choosing the Domain Parameters and the Key Size 266 267 The domain parameters and the key size should be chosen so as to 268 provide sufficient cryptographic security [RFC3766]. The following 269 table, based on table 2 on page 63 of NIST SP800-57 part 1 270 [NISTSP80057], gives approximate comparable key sizes for symmetric- 271 and asymmetric-key cryptosystems based on the best-known algorithms 272 for attacking them. 273 274 275 276 277 278 279Zhu, et al. Expires September 3, 2006 [Page 5] 280 281Internet-Draft ECC Support for PKINIT March 2006 282 283 284 Symmetric | ECC | RSA 285 -------------+----------- +------------ 286 80 | 160 - 223 | 1024 287 112 | 224 - 255 | 2048 288 128 | 256 - 383 | 3072 289 192 | 384 - 511 | 7680 290 256 | 512+ | 15360 291 292 Table 1: Comparable key sizes (in bits) 293 294 Thus, for example, when securing a 128-bit symmetric key, it is 295 prudent to use 256-bit Elliptic Curve Cryptography (ECC), e.g. group 296 P-256 (secp256r1) as described below. 297 298 A set of ECDH domain parameters is also known as a curve. A curve is 299 a named curve if the domain paratmeters are well known and can be 300 identified by an Object Identifier, otherwise it is called a custom 301 curve. [PKINIT] supports both named curves and custom curves, see 302 Section 7 on the tradeoff of choosing between named curves and custom 303 curves. 304 305 The named curves recommended in this document are also recommended by 306 NIST [FIPS186-2]. These fifteen ECC curves are given in the 307 following table [FIPS186-2] [SECG]. 308 309 Description SEC 2 OID 310 ----------------- --------- 311 312 ECPRGF192Random group P-192 secp192r1 313 EC2NGF163Random group B-163 sect163r2 314 EC2NGF163Koblitz group K-163 sect163k1 315 316 ECPRGF224Random group P-224 secp224r1 317 EC2NGF233Random group B-233 sect233r1 318 EC2NGF233Koblitz group K-233 sect233k1 319 320 ECPRGF256Random group P-256 secp256r1 321 EC2NGF283Random group B-283 sect283r1 322 EC2NGF283Koblitz group K-283 sect283k1 323 324 ECPRGF384Random group P-384 secp384r1 325 EC2NGF409Random group B-409 sect409r1 326 EC2NGF409Koblitz group K-409 sect409k1 327 328 ECPRGF521Random group P-521 secp521r1 329 EC2NGF571Random group B-571 sect571r1 330 EC2NGF571Koblitz group K-571 sect571k1 331 332 333 334 335Zhu, et al. Expires September 3, 2006 [Page 6] 336 337Internet-Draft ECC Support for PKINIT March 2006 338 339 3406. Interoperability Requirements 341 342 Implementations conforming to this specification MUST support curve 343 P-256 and P-384. 344 345 3467. Security Considerations 347 348 Kerberos error messages are not integrity protected, as a result, the 349 domain parameters sent by the KDC as TD-DH-PARAMETERS can be tampered 350 with by an attacker so that the set of domain parameters selected 351 could be either weaker or not mutually preferred. Local policy can 352 configure sets of domain parameters acceptable locally, or disallow 353 the negotiation of ECDH domain parameters. 354 355 Beyond elliptic curve size, the main issue is elliptic curve 356 structure. As a general principle, it is more conservative to use 357 elliptic curves with as little algebraic structure as possible - thus 358 random curves are more conservative than special curves such as 359 Koblitz curves, and curves over F_p with p random are more 360 conservative than curves over F_p with p of a special form (and 361 curves over F_p with p random might be considered more conservative 362 than curves over F_2^m as there is no choice between multiple fields 363 of similar size for characteristic 2). Note, however, that algebraic 364 structure can also lead to implementation efficiencies and 365 implementors and users may, therefore, need to balance conservatism 366 against a need for efficiency. Concrete attacks are known against 367 only very few special classes of curves, such as supersingular 368 curves, and these classes are excluded from the ECC standards such as 369 [IEEE1363] and [X9.62]. 370 371 Another issue is the potential for catastrophic failures when a 372 single elliptic curve is widely used. In this case, an attack on the 373 elliptic curve might result in the compromise of a large number of 374 keys. Again, this concern may need to be balanced against efficiency 375 and interoperability improvements associated with widely-used curves. 376 Substantial additional information on elliptic curve choice can be 377 found in [IEEE1363], [X9.62] and [FIPS186-2]. 378 379 3808. IANA Considerations 381 382 No IANA actions are required for this document. 383 384 3859. Acknowledgements 386 387 The following people have made significant contributions to this 388 389 390 391Zhu, et al. Expires September 3, 2006 [Page 7] 392 393Internet-Draft ECC Support for PKINIT March 2006 394 395 396 draft: Paul Leach, Dan Simon, Kelvin Yiu, David Cross, Sam Hartman, 397 Tolga Acar, and Stefan Santesson. 398 399 40010. References 401 40210.1. Normative References 403 404 [ECCPKALGS] 405 RFC-Editor: To be replaced by RFC number for draft-ietf- 406 pkix-ecc-pkalgs. Work in Progress. 407 408 [FIPS186-2] 409 NIST, "Digital Signature Standard", FIPS 186-2, 2000. 410 411 [IEEE1363] 412 IEEE, "Standard Specifications for Public Key Cryptography", 413 IEEE 1363, 2000. 414 415 [NISTSP80057] 416 NIST, "Recommendation on Key Management", 417 http://csrc.nist.gov/publications/nistpubs/, SP 800-57, 418 August 2005. 419 420 421 422Zhu, et al. Expires September 3, 2006 [Page 8] 423 424Internet-Draft ECC Support for PKINIT March 2006 425 426 427 428 [PKINIT] RFC-Editor: To be replaced by RFC number for draft-ietf- 429 cat-kerberos-pk-init. Work in Progress. 430 431 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 432 Requirement Levels", BCP 14, RFC 2119, March 1997. 433 434 [RFC3279] Bassham, L., Polk, W., and R. Housley, "Algorithms and 435 Identifiers for the Internet X.509 Public Key 436 Infrastructure Certificate and Certificate Revocation List 437 (CRL) Profile", RFC 3279, April 2002. 438 439 [RFC3280] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet 440 X.509 Public Key Infrastructure Certificate and 441 Certificate Revocation List (CRL) Profile", RFC 3280, 442 April 2002. 443 444 [RFC3369] Housley, R., "Cryptographic Message Syntax (CMS)", 445 RFC 3369, August 2002. 446 447 [RFC3766] Orman, H. and P. Hoffman, "Determining Strengths For 448 Public Keys Used For Exchanging Symmetric Keys", BCP 86, 449 RFC 3766, April 2004. 450 451 [RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The 452 Kerberos Network Authentication Service (V5)", RFC 4120, 453 July 2005. 454 455 [X9.62] ANSI, "Public Key Cryptography For The Financial Services 456 Industry: The Elliptic Curve Digital Signature Algorithm 457 (ECDSA)", ANSI X9.62, 1998. 458 459 [X9.63] ANSI, "Public Key Cryptography for the Financial Services 460 Industry: Key Agreement and Key Transport using Elliptic 461 Curve Cryptography", ANSI X9.63, 2001. 462 463 4649.2. Informative References 465 466 [LENSTRA] Lenstra, A. and E. Verheul, "Selecting Cryptographic Key 467 Sizes", Journal of Cryptology 14 (2001) 255-293. 468 469 [SECG] SECG, "Elliptic Curve Cryptography", SEC 1, 2000, 470 <http://www.secg.org/>. 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497Zhu, et al. Expires September 3, 2006 [Page 9] 498 499Internet-Draft ECC Support for PKINIT March 2006 500 501 502Authors' Addresses 503 504 Larry Zhu 505 Microsoft Corporation 506 One Microsoft Way 507 Redmond, WA 98052 508 US 509 510 Email: lzhu@microsoft.com 511 512 513 Karthik Jaganathan 514 Microsoft Corporation 515 One Microsoft Way 516 Redmond, WA 98052 517 US 518 519 Email: karthikj@microsoft.com 520 521 522 Kristin Lauter 523 Microsoft Corporation 524 One Microsoft Way 525 Redmond, WA 98052 526 US 527 528 Email: klauter@microsoft.com 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553Zhu, et al. Expires September 3, 2006 [Page 10] 554 555Internet-Draft ECC Support for PKINIT March 2006 556 557 558Intellectual Property Statement 559 560 The IETF takes no position regarding the validity or scope of any 561 Intellectual Property Rights or other rights that might be claimed to 562 pertain to the implementation or use of the technology described in 563 this document or the extent to which any license under such rights 564 might or might not be available; nor does it represent that it has 565 made any independent effort to identify any such rights. 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This document is subject 597 to the rights, licenses and restrictions contained in BCP 78, and 598 except as set forth therein, the authors retain all their rights. 599 600 601Acknowledgment 602 603 Funding for the RFC Editor function is currently provided by the 604 Internet Society. 605 606 607 608 609Zhu, et al. Expires September 3, 2006 [Page 11] 610 611 612