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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 Internet Engineering Task Force S. Sorce 3 Internet-Draft H. Kario 4 Updates: 4462 (if approved) Red Hat, Inc. 5 Intended status: Standards Track Jul 2, 2018 6 Expires: January 3, 2019 8 GSS-API Key Exchange with SHA2 9 draft-ietf-curdle-gss-keyex-sha2-06 11 Abstract 13 This document specifies additions and amendments to RFC4462. It 14 defines a new key exchange method that uses SHA-2 for integrity and 15 deprecates weak DH groups. The purpose of this specification is to 16 modernize the cryptographic primitives used by GSS Key Exchanges. 18 Status of This Memo 20 This Internet-Draft is submitted in full conformance with the 21 provisions of BCP 78 and BCP 79. 23 Internet-Drafts are working documents of the Internet Engineering 24 Task Force (IETF). Note that other groups may also distribute 25 working documents as Internet-Drafts. The list of current Internet- 26 Drafts is at https://datatracker.ietf.org/drafts/current/. 28 Internet-Drafts are draft documents valid for a maximum of six months 29 and may be updated, replaced, or obsoleted by other documents at any 30 time. It is inappropriate to use Internet-Drafts as reference 31 material or to cite them other than as "work in progress." 33 This Internet-Draft will expire on January 3, 2019. 35 Copyright Notice 37 Copyright (c) 2018 IETF Trust and the persons identified as the 38 document authors. All rights reserved. 40 This document is subject to BCP 78 and the IETF Trust's Legal 41 Provisions Relating to IETF Documents 42 (https://trustee.ietf.org/license-info) in effect on the date of 43 publication of this document. Please review these documents 44 carefully, as they describe your rights and restrictions with respect 45 to this document. Code Components extracted from this document must 46 include Simplified BSD License text as described in Section 4.e of 47 the Trust Legal Provisions and are provided without warranty as 48 described in the Simplified BSD License. 50 Table of Contents 52 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 53 2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 2 54 3. Document Conventions . . . . . . . . . . . . . . . . . . . . 2 55 4. New Diffie-Hellman Key Exchange methods . . . . . . . . . . . 3 56 5. New Elliptic Curve Diffie-Hellman Key Exchange methods . . . 4 57 5.1. Generic GSS-API Key Exchange with ECDH . . . . . . . . . 4 58 5.2. ECDH Key Exchange Methods . . . . . . . . . . . . . . . . 8 59 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 60 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 61 7.1. New Finite Field DH mechanisms . . . . . . . . . . . . . 9 62 7.2. New Elliptic Curve DH mechanisms . . . . . . . . . . . . 10 63 7.3. GSSAPI Delegation . . . . . . . . . . . . . . . . . . . . 10 64 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10 65 8.1. Normative References . . . . . . . . . . . . . . . . . . 10 66 8.2. Informative References . . . . . . . . . . . . . . . . . 11 67 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12 69 1. Introduction 71 SSH GSS-API Methods [RFC4462] allows the use of GSSAPI for 72 authentication and key exchange in SSH. It defines three exchange 73 methods all based on DH groups and SHA-1. This document updates 74 RFC4462 with new methods intended to support environments that desire 75 to use the SHA-2 cryptographic hash functions. 77 2. Rationale 79 Due to security concerns with SHA-1 [RFC6194] and with MODP groups 80 with less than 2048 bits [NIST-SP-800-131Ar1] we propose the use of 81 the SHA-2 [RFC6234] based hashes with DH group14, group15, group16, 82 group17 and group18 [RFC3526]. Additionally we add support for key 83 exchange based on Elliptic Curve Diffie Hellman with the NIST P-256, 84 P-384 and P-521 as well as the X25519 and X448 curves. Following the 85 rationale of [RFC8268] only SHA-256 and SHA-512 hashes are used for 86 DH groups. For NIST curves the same curve-to-hashing algorithm 87 pairing used in [RFC5656] is adopted for consistency. 89 3. Document Conventions 91 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 92 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 93 document are to be interpreted as described in RFC 2119 [RFC2119]. 95 4. New Diffie-Hellman Key Exchange methods 97 This document adopts the same naming convention defined in [RFC4462] 98 to define families of methods that cover any GSS-API mechanism used 99 with a specific Diffie-Hellman group and SHA-2 Hash combination. 101 The following new key exchange algorithms are defined: 103 +--------------------------+--------------------------------+ 104 | Key Exchange Method Name | Implementation Recommendations | 105 +--------------------------+--------------------------------+ 106 | gss-group14-sha256-* | SHOULD/RECOMMENDED | 107 | gss-group15-sha512-* | MAY/OPTIONAL | 108 | gss-group16-sha512-* | SHOULD/RECOMMENDED | 109 | gss-group17-sha512-* | MAY/OPTIONAL | 110 | gss-group18-sha512-* | MAY/OPTIONAL | 111 +--------------------------+--------------------------------+ 113 Each key exchange method is implicitly registered by this document. 114 The IESG is considered to be the owner of all these key exchange 115 methods; this does NOT imply that the IESG is considered to be the 116 owner of the underlying GSS-API mechanism. 118 Each method in any family of methods specifies GSS-API-authenticated 119 Diffie-Hellman key exchanges as described in Section 2.1 of 120 [RFC4462]. The method name for each method is the concatenation of 121 the family method name with the Base64 encoding of the MD5 hash 122 [RFC1321] of the ASN.1 DER encoding [ISO-IEC-8825-1] of the 123 underlying GSS-API mechanism's OID. Base64 encoding is described in 124 Section 6.8 of [RFC2045]. 126 Family method refences 128 +---------------------+-------------+-------------+-----------------+ 129 | Family Name prefix | Hash | Group | Reference | 130 | | Function | | | 131 +---------------------+-------------+-------------+-----------------+ 132 | gss-group14-sha256- | SHA-256 | 2048-bit | Section 3 of | 133 | | | MODP | [RFC3526] | 134 | gss-group15-sha512- | SHA-512 | 3072-bit | Section 4 of | 135 | | | MODP | [RFC3526] | 136 | gss-group16-sha512- | SHA-512 | 4096-bit | Section 5 of | 137 | | | MODP | [RFC3526] | 138 | gss-group17-sha512- | SHA-512 | 6144-bit | Section 6 of | 139 | | | MODP | [RFC3526] | 140 | gss-group18-sha512- | SHA-512 | 8192-bit | Section 7 of | 141 | | | MODP | [RFC3526] | 142 +---------------------+-------------+-------------+-----------------+ 144 5. New Elliptic Curve Diffie-Hellman Key Exchange methods 146 In [RFC5656] new SSH key exchange algorithms based on Elliptic Curve 147 Cryptography are introduced. We reuse much of section 4 to define 148 GSS-API-authenticated ECDH Key Exchanges. 150 Additionally we utilize also the curves defined in 151 [I-D.ietf-curdle-ssh-curves] to complement the 3 classic NIST defined 152 curves required by [RFC5656]. 154 5.1. Generic GSS-API Key Exchange with ECDH 156 This section reuses much of the scheme defined in Section 2.1 of 157 [RFC4462] and combines it with the scheme defined in Section 4 of 158 [RFC5656]; in particular, all checks and verification steps 159 prescribed in Section 4 of [RFC5656] apply here as well. 161 For curve25519 and curve448 related computations see Section 6 of 162 [RFC7748]. 164 This section defers to [RFC7546] as the source of information on GSS- 165 API context establishment operations, Section 3 being the most 166 relevant. All Security Considerations described in [RFC7546] apply 167 here too. 169 The parties generate each an ephemeral key pair, according to 170 Section 3.2.1 of [SEC1v2]. Keys are verified upon receipt by the 171 parties according to Section 3.2.3.1 of [SEC1v2]. 173 For NIST Curves keys use uncompressed point representation and must 174 be converted using the algorithm in Section 2.3.4 of [SEC1v2]. If 175 the conversion fails or the point is trasmitted using compressed 176 representation, the key exchange MUST fail. 178 A GSS Context is established according to Section 4 of [RFC5656]; The 179 client initiates the establishment using GSS_Init_sec_context() and 180 the server completes it using GSS_Accept_sec_context(). For the 181 negotiation, the client MUST set mutual_req_flag and integ_req_flag 182 to "true". In addition, deleg_req_flag MAY be set to "true" to 183 request access delegation, if requested by the user. Since the key 184 exchange process authenticates only the host, the setting of 185 anon_req_flag is immaterial to this process. If the client does not 186 support the "gssapi-keyex" user authentication method described in 187 Section 4 of [RFC4462], or does not intend to use that method in 188 conjunction with the GSS-API context established during key exchange, 189 then anon_req_flag SHOULD be set to "true". Otherwise, this flag MAY 190 be set to true if the client wishes to hide its identity. This key 191 exchange process will exchange only a single token once the context 192 has been established, therefore the replay_det_req_flag and 193 sequence_req_flag SHOULD be set to "false". 195 The client MUST include its public key with the first message it 196 sends to the server during this process; if the server receives more 197 than one key or none at all, the key exchange MUST fail. 199 During GSS Context estalishment multiple tokens may be exchanged by 200 the client and the server. When the GSS Context is established 201 (major_status is GSS_S_COMPLETE) the parties check that mutual_state 202 and integ_avail are both "true". If not the key exchange MUST fail. 204 Once a party receives the peer's public key it proceeds to compute a 205 shared secret K. For NIST Curves the computation is done according 206 to Section 3.3.1 of [SEC1v2] and the resulting value z is converted 207 to the octet string K using the conversion defined in Section 2.3.5 208 of [SEC1v2]. For curve25519 and curve448 the algorithm in Section 6 209 of [RFC7748] is used instead. 211 To verify the integrity of the handshake, peers use the Hash Function 212 defined by the selected Key Exchange method to calculate H: 214 H = hash(V_C || V_S || I_C || I_S || K_S || Q_C || Q_S || K). 216 The GSS_GetMIC() call is used by the server with H as the payload and 217 generates a MIC. The GSS_VerifyMIC() call is used by the client to 218 verify the MIC. 220 If any GSS_Init_sec_context() or GSS_Accept_sec_context() returns a 221 major_status other than GSS_S_COMPLETE or GSS_S_CONTINUE_NEEDED, or 222 any other GSS-API call returns a major_status other than 223 GSS_S_COMPLETE, the key exchange MUST fail. The same recommendations 224 expressed in Section 2.1 of [RFC4462] are followed with regards to 225 error reporting. 227 The following is an overview of the key exchange process: 229 Client Server 230 ------ ------ 231 Generate ephemeral key pair. 232 Calls GSS_Init_sec_context(). 233 SSH_MSG_KEXGSS_INIT ---------------> 235 Verify received key is valid. 236 (Optional) <------------- SSH_MSG_KEXGSS_HOSTKEY 238 (Loop) 239 | Calls GSS_Accept_sec_context(). 240 | <------------ SSH_MSG_KEXGSS_CONTINUE 241 | Calls GSS_Init_sec_context(). 242 | SSH_MSG_KEXGSS_CONTINUE ------------> 244 Calls GSS_Accept_sec_context(). 245 Generate ephemeral key pair. 246 Compute shared secret. 247 Computes hash H. 248 Calls GSS_GetMIC( H ) = MIC. 249 <------------ SSH_MSG_KEXGSS_COMPLETE 251 Verify received key is valid. 252 Compute shared secret. 253 Compute hash = H 254 Calls GSS_VerifyMIC( MIC, H ) 256 This is implemented with the following messages: 258 The client sends: 260 byte SSH_MSG_KEXGSS_INIT 261 string output_token (from GSS_Init_sec_context()) 262 string Q_C, client's ephemeral public key octet string 264 The server may responds with: 266 byte SSH_MSG_KEXGSS_HOSTKEY 267 string server public host key and certificates (K_S) 269 The server sends: 271 byte SSH_MSG_KEXGSS_CONTINUE 272 string output_token (from GSS_Accept_sec_context()) 274 Each time the client receives the message described above, it makes 275 another call to GSS_Init_sec_context(). 277 The client sends: 279 byte SSH_MSG_KEXGSS_CONTINUE 280 string output_token (from GSS_Init_sec_context()) 282 As the final message the server sends either: 284 byte SSH_MSG_KEXGSS_COMPLETE 285 string Q_S, server's ephemeral public key octet string 286 string mic_token (MIC of H) 287 boolean TRUE 288 string output_token (from GSS_Accept_sec_context()) 290 Or the following if no output_token is available: 292 byte SSH_MSG_KEXGSS_COMPLETE 293 string Q_S, server's ephemeral public key octet string 294 string mic_token (MIC of H) 295 boolean FALSE 297 The hash H is computed as the HASH hash of the concatenation of the 298 following: 300 string V_C, the client's version string (CR, NL excluded) 301 string V_S, server's version string (CR, NL excluded) 302 string I_C, payload of the client's SSH_MSG_KEXINIT 303 string I_S, payload of the server's SSH_MSG_KEXINIT 304 string K_S, server's public host key 305 string Q_C, client's ephemeral public key octet string 306 string Q_S, server's ephemeral public key octet string 307 mpint K, shared secret 309 This value is called the exchange hash, and it is used to 310 authenticate the key exchange. The exchange hash SHOULD be kept 311 secret. If no SSH_MSG_KEXGSS_HOSTKEY message has been sent by the 312 server or received by the client, then the empty string is used in 313 place of K_S when computing the exchange hash. 315 Since this key exchange method does not require the host key to be 316 used for any encryption operations, the SSH_MSG_KEXGSS_HOSTKEY 317 message is OPTIONAL. If the "null" host key algorithm described in 318 Section 5 of [RFC4462] is used, this message MUST NOT be sent. 320 If the client receives a SSH_MSG_KEXGSS_CONTINUE message after a call 321 to GSS_Init_sec_context() has returned a major_status code of 322 GSS_S_COMPLETE, a protocol error has occurred and the key exchange 323 MUST fail. 325 If the client receives a SSH_MSG_KEXGSS_COMPLETE message and a call 326 to GSS_Init_sec_context() does not result in a major_status code of 327 GSS_S_COMPLETE, a protocol error has occurred and the key exchange 328 MUST fail. 330 5.2. ECDH Key Exchange Methods 332 The following new key exchange methods are defined: 334 +--------------------------+--------------------------------+ 335 | Key Exchange Method Name | Implementation Recommendations | 336 +--------------------------+--------------------------------+ 337 | gss-nistp256-sha256-* | SHOULD/RECOMMENDED | 338 | gss-nistp384-sha384-* | MAY/OPTIONAL | 339 | gss-nistp521-sha512-* | MAY/OPTIONAL | 340 | gss-curve25519-sha256-* | SHOULD/RECOMMENDED | 341 | gss-curve448-sha512-* | MAY/OPTIONAL | 342 +--------------------------+--------------------------------+ 344 Each key exchange method is implicitly registered by this document. 345 The IESG is considered to be the owner of all these key exchange 346 methods; this does NOT imply that the IESG is considered to be the 347 owner of the underlying GSS-API mechanism. 349 Each method in any family of methods specifies GSS-API-authenticated 350 Elliptic Curve Diffie-Hellman key exchanges as described in 351 Section 5.1. The method name for each method is the concatenation of 352 the family method name with the Base64 encoding of the MD5 hash 353 [RFC1321] of the ASN.1 DER encoding [ISO-IEC-8825-1] of the 354 underlying GSS-API mechanism's OID. Base64 encoding is described in 355 Section 6.8 of [RFC2045]. 357 Family method refences 359 +------------------------+----------+---------------+---------------+ 360 | Family Name prefix | Hash | Parameters / | Definition | 361 | | Function | Function Name | | 362 +------------------------+----------+---------------+---------------+ 363 | gss-nistp256-sha256- | SHA-256 | secp256r1 | Section 2.4.2 | 364 | | | | of [SEC2v2] | 365 | gss-nistp384-sha384- | SHA-384 | secp384r1 | Section 2.5.1 | 366 | | | | of [SEC2v2] | 367 | gss-nistp521-sha512- | SHA-512 | secp521r1 | Section 2.6.1 | 368 | | | | of [SEC2v2] | 369 | gss-curve25519-sha256- | SHA-256 | X22519 | Section 5 of | 370 | | | | [RFC7748] | 371 | gss-curve448-sha512- | SHA-512 | X448 | Section 5 of | 372 | | | | [RFC7748] | 373 +------------------------+----------+---------------+---------------+ 375 6. IANA Considerations 377 This document augments the SSH Key Exchange Method Names in 378 [RFC4462]. 380 IANA is requested to update the SSH Protocol Parameters 381 [IANA-KEX-NAMES] registry with the following entries: 383 +--------------------------+------------+------------------------+ 384 | Key Exchange Method Name | Reference | Implementation Support | 385 +--------------------------+------------+------------------------+ 386 | gss-group14-sha256-* | This draft | SHOULD | 387 | gss-group15-sha512-* | This draft | MAY | 388 | gss-group16-sha512-* | This draft | SHOULD | 389 | gss-group17-sha512-* | This draft | MAY | 390 | gss-group18-sha512-* | This draft | MAY | 391 | gss-nistp256-sha256-* | This draft | SHOULD | 392 | gss-nistp384-sha384-* | This draft | MAY | 393 | gss-nistp521-sha512-* | This draft | MAY | 394 | gss-curve25519-sha256-* | This draft | SHOULD | 395 | gss-curve448-sha512-* | This draft | MAY | 396 +--------------------------+------------+------------------------+ 398 7. Security Considerations 400 7.1. New Finite Field DH mechanisms 402 Except for the use of a different secure hash function and larger DH 403 groups, no significant changes has been made to the protocol 404 described by [RFC4462]; therefore all the original Security 405 Considerations apply. 407 7.2. New Elliptic Curve DH mechanisms 409 Although a new cryptographic primitive is used with these methods the 410 actual key exchange closely follows the key exchange defined in 411 [RFC5656]; therefore all the original Security Considerations as well 412 as those expressed in [RFC5656] apply. 414 7.3. GSSAPI Delegation 416 Some GSSAPI mechanisms can optionally delegate credentials to the 417 target host by setting the deleg_ret_flag. In this case extra care 418 must be taken to ensure that the acceptor being authenticated matches 419 the target the user intended. Some mechanisms implementations (like 420 commonly used krb5 libraries) may use insecure DNS resolution to 421 canonicalize the target name; in these cases spoofing a DNS response 422 that points to an attacker-controlled machine may results in the user 423 silently delegating credentials to the attacker, who can then 424 impersonate the user at will. 426 8. References 428 8.1. Normative References 430 [I-D.ietf-curdle-ssh-curves] 431 Adamantiadis, A., Josefsson, S., and M. Baushke, "Secure 432 Shell (SSH) Key Exchange Method using Curve25519 and 433 Curve448", draft-ietf-curdle-ssh-curves-07 (work in 434 progress), January 2018. 436 [RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, 437 DOI 10.17487/RFC1321, April 1992, 438 . 440 [RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail 441 Extensions (MIME) Part One: Format of Internet Message 442 Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996, 443 . 445 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 446 Requirement Levels", BCP 14, RFC 2119, 447 DOI 10.17487/RFC2119, March 1997, 448 . 450 [RFC3526] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) 451 Diffie-Hellman groups for Internet Key Exchange (IKE)", 452 RFC 3526, DOI 10.17487/RFC3526, May 2003, 453 . 455 [RFC4462] Hutzelman, J., Salowey, J., Galbraith, J., and V. Welch, 456 "Generic Security Service Application Program Interface 457 (GSS-API) Authentication and Key Exchange for the Secure 458 Shell (SSH) Protocol", RFC 4462, DOI 10.17487/RFC4462, May 459 2006, . 461 [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm 462 Integration in the Secure Shell Transport Layer", 463 RFC 5656, DOI 10.17487/RFC5656, December 2009, 464 . 466 [RFC7546] Kaduk, B., "Structure of the Generic Security Service 467 (GSS) Negotiation Loop", RFC 7546, DOI 10.17487/RFC7546, 468 May 2015, . 470 [RFC7748] Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves 471 for Security", RFC 7748, DOI 10.17487/RFC7748, January 472 2016, . 474 [SEC1v2] Certicom Research, "SEC 1: Elliptic Curve Cryptography", 475 Standards for Efficient Cryptography SEC 1, Version 2.0, 476 2009. 478 [SEC2v2] Certicom Research, "SEC 2: Recommended Elliptic Curve 479 Domain Parameters", Standards for Efficient 480 Cryptography SEC 2, Version 2.0, 2010. 482 8.2. Informative References 484 [IANA-KEX-NAMES] 485 Internet Assigned Numbers Authority, "Secure Shell (SSH) 486 Protocol Parameters: Key Exchange Method Names", June 487 2005, . 490 [ISO-IEC-8825-1] 491 International Organization for Standardization / 492 International Electrotechnical Commission, "ASN.1 encoding 493 rules: Specification of Basic Encoding Rules (BER), 494 Canonical Encoding Rules (CER) and Distinguished Encoding 495 Rules (DER)", ISO/IEC 8825-1, November 2015, 496 . 499 [NIST-SP-800-131Ar1] 500 National Institute of Standards and Technology, 501 "Transitions: Recommendation for Transitioning of the Use 502 of Cryptographic Algorithms and Key Lengths", NIST Special 503 Publication 800-131A Revision 1, November 2015, 504 . 507 [RFC6194] Polk, T., Chen, L., Turner, S., and P. Hoffman, "Security 508 Considerations for the SHA-0 and SHA-1 Message-Digest 509 Algorithms", RFC 6194, DOI 10.17487/RFC6194, March 2011, 510 . 512 [RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms 513 (SHA and SHA-based HMAC and HKDF)", RFC 6234, 514 DOI 10.17487/RFC6234, May 2011, 515 . 517 [RFC8268] Baushke, M., "More Modular Exponentiation (MODP) Diffie- 518 Hellman (DH) Key Exchange (KEX) Groups for Secure Shell 519 (SSH)", RFC 8268, DOI 10.17487/RFC8268, December 2017, 520 . 522 Authors' Addresses 524 Simo Sorce 525 Red Hat, Inc. 526 140 Broadway 527 24th Floor 528 New York, NY 10025 529 USA 531 Email: simo@redhat.com 533 Hubert Kario 534 Red Hat, Inc. 535 Purkynova 115 536 Brno 612 00 537 Czech Republic 539 Email: hkario@redhat.com