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Sury 3 Internet-Draft CZ.NIC 4 Intended status: Standards Track January 27, 2012 5 Expires: July 30, 2012 7 Use of SHA-256 Algorithm with RSA, DSA and ECDSA in SSHFP Resource 8 Records 9 draft-os-ietf-sshfp-ecdsa-sha2-07 11 Abstract 13 This document updates IANA registries defined in RFC4255, which 14 defines a DNS resource record - SSHFP that contains a standard SSH 15 key fingerprint used to verify Secure Shell (SSH) host keys using 16 Domain Name System Security (DNSSEC). This document defines 17 additional options supporting Secure Shell (SSH) public keys using 18 the Elliptic Curve Digital Signature Algorithm (ECDSA) and the use of 19 fingerprints computed using the SHA-256 message digest algorithm in 20 SSHFP resource records (SSHFP RR). 22 Status of This Memo 24 This Internet-Draft is submitted in full conformance with the 25 provisions of BCP 78 and BCP 79. 27 Internet-Drafts are working documents of the Internet Engineering 28 Task Force (IETF). Note that other groups may also distribute 29 working documents as Internet-Drafts. The list of current Internet- 30 Drafts is at http://datatracker.ietf.org/drafts/current/. 32 Internet-Drafts are draft documents valid for a maximum of six months 33 and may be updated, replaced, or obsoleted by other documents at any 34 time. It is inappropriate to use Internet-Drafts as reference 35 material or to cite them other than as "work in progress." 37 This Internet-Draft will expire on July 30, 2012. 39 Copyright Notice 41 Copyright (c) 2012 IETF Trust and the persons identified as the 42 document authors. All rights reserved. 44 This document is subject to BCP 78 and the IETF Trust's Legal 45 Provisions Relating to IETF Documents 46 (http://trustee.ietf.org/license-info) in effect on the date of 47 publication of this document. Please review these documents 48 carefully, as they describe your rights and restrictions with respect 49 to this document. Code Components extracted from this document must 50 include Simplified BSD License text as described in Section 4.e of 51 the Trust Legal Provisions and are provided without warranty as 52 described in the Simplified BSD License. 54 Table of Contents 56 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 57 2. Requirements Language . . . . . . . . . . . . . . . . . . . . . 3 58 3. SSHFP Resource Records . . . . . . . . . . . . . . . . . . . . 4 59 3.1. SSHFP Fingerprint Type Specification . . . . . . . . . . . 4 60 3.1.1. SHA-256 SSHFP Fingerprint Type Specification . . . . . 4 61 3.2. SSHFP Algorithm Number Specification . . . . . . . . . . . 4 62 3.2.1. ECDSA SSHFP Algorithm Number Specification . . . . . . 4 63 4. Implementation Considerations . . . . . . . . . . . . . . . . . 4 64 4.1. Support for SHA-256 fingerprints . . . . . . . . . . . . . 4 65 4.2. Support for ECDSA . . . . . . . . . . . . . . . . . . . . . 4 66 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 67 5.1. RSA public key . . . . . . . . . . . . . . . . . . . . . . 5 68 5.1.1. RSA public key with SHA1 fingerprint . . . . . . . . . 5 69 5.1.2. RSA public key with SHA-256 fingerprint . . . . . . . . 5 70 5.2. DSA public key . . . . . . . . . . . . . . . . . . . . . . 5 71 5.2.1. DSA public key with SHA1 fingerprint . . . . . . . . . 6 72 5.2.2. DSA public key with SHA-256 fingerprint . . . . . . . . 6 73 5.3. ECDSA public key . . . . . . . . . . . . . . . . . . . . . 6 74 5.3.1. ECDSA public key with SHA1 fingerprint . . . . . . . . 6 75 5.3.2. ECDSA public key with SHA-256 fingerprint . . . . . . . 6 76 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6 77 6.1. SSHFP RR Types for public key algorithms . . . . . . . . . 7 78 6.2. SSHFP RR types for fingerprint types . . . . . . . . . . . 7 79 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 7 80 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8 81 8.1. Normative References . . . . . . . . . . . . . . . . . . . 8 82 8.2. Informative References . . . . . . . . . . . . . . . . . . 8 84 1. Introduction 86 The Domain Name System (DNS) is the global, hierarchical distributed 87 database for Internet Naming. The Secure Shell (SSH) is a protocol 88 for secure remote login and other secure network services over an 89 insecure network. RFC 4253 [RFC4253] defines Public Key Algorithms 90 for the Secure Shell server public keys. 92 The DNS has been extended to store fingerprints in a DNS resource 93 record named SSHFP [RFC4255], which provides out-of-band verification 94 by looking up a fingerprint of the server public key in the DNS 95 [RFC1034], [RFC1035] and using Domain Name System Security Extensions 96 (DNSSEC) [RFC4033], [RFC4034], [RFC4035] to verify the lookup. 98 RFC 4255 [RFC4255] describes how to store the cryptographic 99 fingerprint of SSH public keys in SSHFP resource records. SSHFP 100 records contain the fingerprint and two index numbers identifying the 101 cryptographic algorithms used: 102 1. to link the fingerprinted public key with the corresponding 103 private key, and 104 2. to derive the message digest stored as the fingerprint in the 105 record. 106 RFC 4255 [RFC4255] then specifies lists of cryptographic algorithms 107 and the corresponding index numbers used to identify them in SSHFP 108 records. 110 This document updates the IANA registry "SSHFP RR Types for public 111 key algorithms" and "SSHFP RR types for fingerprint types" 112 [SSHFPVALS] by adding a new option in each list: 113 o the Elliptic Curve Digital Signature Algorithm (ECDSA)[RFC6090] 114 which has been added to the Secure Shell Public Key list by RFC 115 5656 [RFC5656] in the public key algorithms list; 116 o the SHA-256 algorithm [FIPS.180-3.2008] in the SSHFP Fingerprint 117 Type list. 119 Familiarity with DNSSEC, SSH Protocol [RFC4251], [RFC4253], 120 [RFC4250], SSHFP [RFC4255], and the SHA-2 [FIPS.180-3.2008] family of 121 algorithms is assumed in this document. 123 2. Requirements Language 125 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 126 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 127 document are to be interpreted as described in RFC 2119 [RFC2119]. 129 3. SSHFP Resource Records 131 The format of the SSHFP RR can be found in RFC 4255 [RFC4255]. 133 3.1. SSHFP Fingerprint Type Specification 135 The fingerprint type octet identifies the message-digest algorithm 136 used to calculate the fingerprint of the public key. 138 3.1.1. SHA-256 SSHFP Fingerprint Type Specification 140 SHA-256 fingerprints of the public keys are stored in SSHFP Resource 141 Record with the fingerprint type 2. 143 3.2. SSHFP Algorithm Number Specification 145 The SSHFP Resource Record algorithm number octet describes the 146 algorithm of the public key. 148 3.2.1. ECDSA SSHFP Algorithm Number Specification 150 ECDSA public keys are stored in SSHFP Resource Records with the 151 algorithm number 3. 153 4. Implementation Considerations 155 4.1. Support for SHA-256 fingerprints 157 SSHFP-aware Secure Shell implementations SHOULD support the SHA-256 158 fingerprints for verification of the public key. Secure Shell 159 implementations which support SHA-256 fingerprints MUST prefer a SHA- 160 256 fingerprint over SHA-1 if both are available for a server. If 161 the SHA-256 fingerprint is tested and does not match the key SSH 162 public key received from the SSH server, then the key MUST be 163 rejected rather than testing the alternative SHA-1 fingerprint. 165 4.2. Support for ECDSA 167 SSHFP-aware Secure Shell implementations which also implement ECDSA 168 algorithm for the public key SHOULD support SSHFP fingerprints for 169 ECDSA public keys. 171 5. Examples 173 The following examples provide reference for both the newly defined 174 ECDSA algorithm number and the use of the SHA-256 fingerprint 175 combined with both the new and the existing algorithm numbers. 177 5.1. RSA public key 179 Given a public key with the following value in OpenSSH format 180 [RFC4716]: 182 ---- BEGIN SSH2 PUBLIC KEY ---- 183 AAAAB3NzaC1yc2EAAAADAQABAAABAQDCUR4JOhxTinzq7QO3bQXW4jmPCCulFsnh 184 8Yi7MKwpMnd96+T7uV7nEwy+6+GWYu98IxFJByIjFXX/a6BXDp3878wezH1DZ2tN 185 D/tu/eudz6ErpTFYmnVLyEDARYSzVBNQuIK1UDqvvB6KffJcyt78FpwW27euGkqE 186 kam7GaurPRAgwXehDB/gMwRtXVRZ+13zYWkAmAY+5OAWVmdXuQVm5kjlvcNzto2H 187 3m3nqJtD4J9L1lKPuSVVqwJr4/6hibXJkQEvWpUvdOAUw3frKpNwa932fXFk3ke4 188 rsDjQ/W8GyleMtK3Tx8tE4z1wuowXtYe6Ba8q3LAPs/m2S4pUscx 189 ---- END SSH2 PUBLIC KEY ---- 191 5.1.1. RSA public key with SHA1 fingerprint 193 The SSHFP Resource Record for this key would be: 195 server.example.net IN SSHFP 1 1 ( dd465c09cfa51fb45020cc83316fff 196 21b9ec74ac ) 198 5.1.2. RSA public key with SHA-256 fingerprint 200 The SSHFP Resource Record for this key would be: 202 server.example.net IN SSHFP 1 2 ( b049f950d1397b8fee6a61e4d14a9a 203 cdc4721e084eff5460bbed80cfaa2c 204 e2cb ) 206 5.2. DSA public key 208 Given a public key with the following value in OpenSSH format: 210 ---- BEGIN SSH2 PUBLIC KEY ---- 211 AAAAB3NzaC1kc3MAAACBAPVFrc0U36gWaywbfJzjcv8ef13qAX4EJl8Na6xqvXh1 212 t+aCJEdS7soRjtvK4KsNhk78DjdtnfhEhyFKHHNz3i6/c/s9lP0UjV7mRAo6nA7A 213 3Gs6iQElb6O9Fqm6iVSC6bYWilTSB0tYencEEJUoaAua8YQF/uxRzPrReXxGqHnj 214 AAAAFQDC9M/pli8VIVmEGOO0wC1TeUTN4wAAAIEAgA2Fbkbbeo0+u/qw8mQFOFWZ 215 pTaqNo7d7jov3majbh5LqEVD7yT3MS1GSGhjgvvhus/ehMTqzYbjTc0szUM9JnwT 216 7xq15P2ZYDK98IVxrw31jMtsUUEmBqB4DUjTurtcaWmJ9LNaP1/k4bMo0/hotnOc 217 OVnIPsTLBFWVvdNRxUAAAACAOZcDcK01NTM1qIIYbBqCffrwjQ+9PmsuSKI6nUzf 218 S4NysXHkdbW5u5VxeXLcwWj5PGbRfoS2P3vwYAmakqgq502wigam18u9nAczUYl+ 219 2kOeOiIRrtSmLfpV7thLOAb8k1ESjIlkbn35jKmTcoMFRXbFmkKRTK8OEnWQ8AVg 220 6w8= 221 ---- END SSH2 PUBLIC KEY ---- 223 5.2.1. DSA public key with SHA1 fingerprint 225 The SSHFP Resource Record for this key would be: 227 server.example.net IN SSHFP 2 1 ( 3b6ba6110f5ffcd29469fc1ec2ee25 228 d61718badd ) 230 5.2.2. DSA public key with SHA-256 fingerprint 232 The SSHFP Resource Record for this key would be: 234 server.example.net IN SSHFP 2 2 ( f9b8a6a460639306f1b38910456a6a 235 e1018a253c47ecec12db77d7a0878b 236 4d83 ) 238 5.3. ECDSA public key 240 Given a public key with the following value in OpenSSH format: 242 ---- BEGIN SSH2 PUBLIC KEY ---- 243 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBAD+9COUiX7W 244 YgcvIOdI8+djdoFDVUTxNrcog8sSYdbIzeG+bYdsssvcyy/nRfVhXC5QBCk8IThq 245 s7D4/lFxX5g= 246 ---- END SSH2 PUBLIC KEY ---- 248 5.3.1. ECDSA public key with SHA1 fingerprint 250 The SSHFP Resource Record for this key would be: 252 server.example.net IN SSHFP 3 1 ( c64607a28c5300fec1180b6e417b92 253 2943cffcdd ) 255 5.3.2. ECDSA public key with SHA-256 fingerprint 257 The SSHFP Resource Record for this key would be: 259 server.example.net IN SSHFP 3 2 ( 821eb6c1c98d9cc827ab7f456304c0 260 f14785b7008d9e8646a8519de80849 261 afc7 ) 263 6. IANA Considerations 265 This document updates the IANA registry "SSHFP RR Types for public 266 key algorithms" and "SSHFP RR types for fingerprint types" 267 [SSHFPVALS]. 269 6.1. SSHFP RR Types for public key algorithms 271 The following entries are added to the "SSHFP RR Types for public key 272 algorithms" registry: 274 +-------+-------------+------------+ 275 | Value | Description | Reference | 276 +-------+-------------+------------+ 277 | 3 | ECDSA | [This doc] | 278 +-------+-------------+------------+ 280 Table 1 282 6.2. SSHFP RR types for fingerprint types 284 The following entries are added to the "SSHFP RR types for 285 fingerprint types" registry: 287 +-------+-------------+------------+ 288 | Value | Description | Reference | 289 +-------+-------------+------------+ 290 | 2 | SHA-256 | [This doc] | 291 +-------+-------------+------------+ 293 Table 2 295 7. Security Considerations 297 Please see the security considerations in [RFC4255] for SSHFP record 298 and [RFC5656] for ECDSA algorithm. 300 Users of SSHFP are encouraged to deploy SHA-256 as soon as 301 implementations allow for it. SHA-2 family of algorithms is widely 302 believed to be more resilient to attack than SHA-1, and confidence in 303 SHA-1's strength is being eroded by recently announced attacks [IACR 304 2007/474]. Regardless of whether or not the attacks on SHA-1 will 305 affect SSHFP, it is believed (at the time of this writing) that SHA- 306 256 is the better choice for use in SSHFP records. 308 SHA-256 is considered sufficiently strong for the immediate future, 309 but predictions about future development in cryptography and 310 cryptanalysis are beyond the scope of this document. 312 8. References 313 8.1. Normative References 315 [FIPS.180-3.2008] 316 National Institute of Standards and Technology, "Secure 317 Hash Standard", FIPS PUB 180-3, October 2008, . 321 [RFC1034] Mockapetris, P., "Domain names - concepts and facilities", 322 STD 13, RFC 1034, November 1987. 324 [RFC1035] Mockapetris, P., "Domain names - implementation and 325 specification", STD 13, RFC 1035, November 1987. 327 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 328 Requirement Levels", BCP 14, RFC 2119, March 1997. 330 [RFC4250] Lehtinen, S. and C. Lonvick, "The Secure Shell (SSH) 331 Protocol Assigned Numbers", RFC 4250, January 2006. 333 [RFC4251] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) 334 Protocol Architecture", RFC 4251, January 2006. 336 [RFC4253] Ylonen, T. and C. Lonvick, "The Secure Shell (SSH) 337 Transport Layer Protocol", RFC 4253, January 2006. 339 [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely 340 Publish Secure Shell (SSH) Key Fingerprints", RFC 4255, 341 January 2006. 343 [RFC5656] Stebila, D. and J. Green, "Elliptic Curve Algorithm 344 Integration in the Secure Shell Transport Layer", 345 RFC 5656, December 2009. 347 8.2. Informative References 349 [IACR 2007/474] 350 Cochran, M., "Notes on the Wang et al. 2^63 SHA-1 351 Differential Path", IACR 2007/474, 352 . 354 [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. 355 Rose, "DNS Security Introduction and Requirements", 356 RFC 4033, March 2005. 358 [RFC4034] Arends, R., Austein, R., Larson, M., Massey, D., and S. 359 Rose, "Resource Records for the DNS Security Extensions", 360 RFC 4034, March 2005. 362 [RFC4035] Arends, R., Austein, R., Larson, M., Massey, D., and S. 363 Rose, "Protocol Modifications for the DNS Security 364 Extensions", RFC 4035, March 2005. 366 [RFC4716] Galbraith, J. and R. Thayer, "The Secure Shell (SSH) 367 Public Key File Format", RFC 4716, November 2006. 369 [RFC6090] McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic 370 Curve Cryptography Algorithms", RFC 6090, February 2011. 372 [SSHFPVALS] 373 IANA, "DNS SSHFP Resource Records Parameters", IANA 374 registry available at:, . 377 Author's Address 379 Ondrej Sury 380 CZ.NIC 381 Americka 23 382 120 00 Praha 2 383 Czech Republic 385 Phone: +420 222 745 110 386 Email: ondrej.sury@nic.cz