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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 2 INTERNET DRAFT Craig Metz 3 draft-ietf-otp-ext-00.txt Kaman Sciences 4 September 25, 1996 6 OTP Extended Responses 8 STATUS OF THIS MEMO 10 This document is an Internet Draft. Internet Drafts are working 11 documents of the Internet Engineering Task Force (IETF), its Areas 12 and Working Groups. Note that other groups may also distribute 13 working documents as Internet Drafts. 15 Internet Drafts are draft documents valid for a maximum of six 16 months. Drafts may be updated, replaced, or obsoleted by other 17 documents at any time. It is not appropriate to use Internet Drafts 18 as reference material or to cite them other than as a "working 19 draft" or "work in progress." 21 To learn the current status of any Internet Draft, please check the 22 1id-abstracts.txt listing contained in the Internet-Drafts Shadow 23 Directories on ftp.is.co.za (Africa), ds.internic.net (US East 24 Coast), nic.nordu.net (Europe), ftp.isi.com (US West Coast), or 25 munnari.oz.au (Pacific Rim). 27 The distribution of this Internet Draft is unlimited. It is filed as 28 and it expires on March 25, 1997. 30 1.0 ABSTRACT 32 This document provides a specification for a type of response to an 33 OTP [RFC 1938] challenge that carries explicit indication of the 34 response's encoding. Codings for the two mandatory OTP data formats 35 using this new type of response are presented. This document also 36 provides a specification for a response that allows OTP generator to 37 request that a server re-initialize a sequence and change parameters 38 such as the secret pass phrase. 40 2.0 CONVENTIONS, TERMS, and NOTATION 42 This document specifies the data formats and software behaviors 43 needed to use OTP extended responses. The data formats are described 44 three ways: using an ad-hoc UNIX manual page style syntax, using 45 augmented BNF described in sections two and three of RFC 46 822[RFC822], and by examples. Should there be any conflict between 47 these descriptions, the augmented BNF takes precedence. The software 48 behaviors are described in words, and specific behavior compliance 49 requirements are itemized using the requirements terminology 50 described in section four of RFC 1938. 52 3.0 EXTENDED RESPONSES 54 This document builds on the protocol and terminology specified in 55 RFC 1938 and assumes that you have already read this document and 56 understand its contents. 58 An extended response is a single line of printable text terminated 59 by a new line sequence appropriate for the context of its use (e.g., 60 ASCII CR followed by ASCII LF). It contains two or more tokens that 61 are separated with a single colon (':') character. The first token 62 contains a type specifier that indicates the format of the rest of 63 the response. The tokens that follow are argument data for the OTP 64 extended response. At least one token of data MUST be present. 66 Syntax 68 In UNIX manual page like syntax, the general form of an extended 69 response could be described as: 71 :[:[:...]] 73 In augmented BNF syntax, the syntax of the general form of an 74 extended response is: 76 extended-response = type 1*(":" argument) newline 77 type = token 78 argument = token 79 token = 1* 80 newline = CRLF / CR / LF / 82 An example of the extended response using a mythical type named 83 "foo" is: 85 foo:some data:some more data:12345 87 Requirements 89 A server compliant with this specification: 91 1. MUST be able to receive and parse the general form of an extended 92 response 93 2. MUST process the type field in a case-insensitive manner 94 3. MUST reject any authentication attempt using an extended response 95 if it does not support that type of response 96 4. SHOULD provide an appropriate indication to the generator if the 97 response was rejected because of (3) 98 5. MUST limit the length of the input reasonably 99 6. MUST accept otherwise arbitrary amounts of whitespace wherever a 100 response allows it 101 7. SHOULD be able to receive and correctly process standard OTP 102 responses 104 A generator compliant with this specification: 106 1. SHOULD have an option that selects whether standard or extended 107 responses are generated 108 2. SHOULD make (1) easily available to the end user 109 3. SHOULD be configurable on a per-server or per-seed basis 110 4. SHOULD be able to generate standard OTP responses 111 5. SHOULD initially default to using standard responses 112 6. MUST generate the type field in lower case 114 4.0 THE "HEX" AND "WORD" RESPONSES 116 There exists a very rare case in which a standard OTP response could 117 be a valid coding in both the hexadecimal and six-word formats. An 118 example of this is the response "ABE ACE ADA ADD BAD A." The 119 solution to this problem mandated by the OTP specification is that 120 compliant servers MUST attempt to parse and verify a standard 121 response in both hexadecimal and six-word formats and must consider 122 the authentication successful if either succeeds. 124 This problem can be solved easily using extended responses. The 125 "hex" response and the "word" response are two response types that 126 encode an OTP in an extended response that explicitly describes the 127 encoding. These responses start with a type label of "hex" for a 128 hexadecimal OTP and "word" for a six-word coded OTP. These responses 129 contain one argument field that contains a standard OTP response 130 coded in the indicated format. 132 Syntax 134 In UNIX manual page like syntax, the format of these responses could 135 be described as: 137 hex: 138 word: 140 In augmented BNF syntax and with the definitions already provided, 141 the syntax of these responses is: 143 hex-response = "hex:" hex-64bit newline 144 hex-64bit = 16(hex-char *LWSP-char) 145 hex-char = ("A" / "B" / "C" / "D" / "E" / "F" / 146 "a" / "b" / "c" / "d" / "e" / "f" / 147 "0" / "1" / "2" / "3" / "4" / "5" / 148 "6" / "7" / "8" / "9") 150 word-response = "word:" word-64bit newline 151 word-64bit = 6(otp-word 1*LWSP-char) 152 otp-word = 154 Examples of these responses are: 156 hex:8720 33d4 6202 9172 word:VAST SAUL TAKE SODA SUCH BOLT 158 Requirements 160 A server compliant with this specification: 162 1. MUST treat all arguments in a case-insensitive manner 164 A generator compliant with this specification: 166 1. MUST generate otp-word tokens in upper case with single spaces 167 separating them 168 2. MUST generate hexadecimal numbers using only lower case for 169 letters 171 5.0 THE "INIT" AND "INIT-WORD" RESPONSES 173 The OTP specification requires that implementations provide a means 174 for a client to re-initialize or change its OTP information with a 175 server but does not require any specific protocol for doing it. 176 Implementations that support the OTP extended responses described in 177 this document MUST support the response with the "init" and "init- 178 word" type specifiers, which provide a standard way for a client to 179 re-initialize its OTP information with a server. This response is 180 intended to be used only by automated clients. Because of this, the 181 primary form of this reponse uses the hexadecimal encoding for 182 binary data. It is possible for a user to type in an "init" or 183 "init-word" response. However, there is enough data that would need 184 to be typed that the six-word coding does not make typing this 185 response much easier. 187 Syntax 189 In UNIX manual page like syntax, the format of these responses could 190 be described as: 192 init:::[::] 193 init-word:::[::] 196 In augmented BNF syntax and with the definitions already provided, 197 the syntax of the "init" response is: 199 init-response = "init:" old-OTP ":" new-params ":" new-OTP 200 1*0(":" check-update ":" check-value) newline 202 old-OTP = hex-64bit 203 new-OTP = hex-64bit 204 check-update = hex-64bit 205 check-value = hex-64bit 207 new-params = algorithm SPACE sequence-number SPACE seed 208 algorithm = "md4" / "md5" / "sha1" 209 sequence-number = 4*3DIGIT 210 seed = 16*1(ALPHA / DIGIT) 212 In augmented BNF syntax and with the definitions already provided, 213 the syntax of the "init-word" response is: 215 init-word-response = "init-word:" old-OTP ":" new-params ":" new-OTP 216 1*0(":" check-update ":" check-value) newline 218 old-OTP = word-64bit 219 new-OTP = word-64bit 220 check-update = word-64bit 221 check-value = word-64bit 223 new-params = algorithm SPACE sequence-number SPACE seed 224 algorithm = "md4" / "md5" / "sha1" 225 sequence-number = 4*3DIGIT 226 seed = 16*1(ALPHA / DIGIT) 228 Note that all appropriate fields for the "init" response MUST be 229 hexadecimally coded and that all appropriate fields for the "init- 230 word" response MUST be six-word coded. 232 Examples of these responses are: 234 init:f6bd 6b33 89b8 7203:md5 499 ke6118:23d1 b253 5ae0 2b7e 235 init:c9b2 12bb 6425 5a0f:md5 499 ke0986:fd17 cef1 b4df 093e: 236 6e1e faa6 b7d1 a43f:4c72 33b7 101a 7e62 238 init-word:MOOD SOFT POP COMB BOLO LIFE:md5 499 ke1235: 239 ARTY WEAR TAD RUG HALO GIVE 240 init-word:END KERN BALM NICK EROS WAVY:md5 499 ke1235: 241 BABY FAIN OILY NIL TIDY DADE:DIG DIVE SUNG HORN SWAG GAP: 242 GUT RODE CAKE ROY DATA GOER 244 (Note that all of these responses are one line. Due to their length, 245 they had to be split into multiple lines in order to be included 246 here. These responses MUST NOT span more than one line in actual 247 use) 249 Description of Fields 251 The old-OTP field contains a hexadecimally coded response to the OTP 252 challenge. The new-params field contains the parameters for the 253 client's new requested challenge and the new-OTP field contains a 254 hexadecimally coded response to that challenge. If the re- 255 initialization is successful, a server MUST store the new OTP in its 256 database as the last successful OTP received and the sequence number 257 in the next challenge presented by the server MUST be one less than 258 the sequence number specified in the new-params field. 260 The check-update and check-value fields provide a simple defense 261 against active attacks. This is not intended to provide a high level 262 of security. It is intended to make it significantly more difficult 263 for an active attacker to interfere with the OTP re-initialization 264 process and thus set the OTP sequence and/or secret to a value 265 chosen by the attacker. The check fields are optional. They require 266 that a server store data that is secret for purposes of the re- 267 initialization. This is unacceptable in some circumstances. Also, 268 some sites may already employ other security solutions that already 269 eliminate active attacks (e.g., IP Security [RFC1825]). 271 These fields use a "check key" that is derived from a seed and a 272 secret. The OTP system prepares a secret and seed for iteration by 273 concatenating the secret and seed and running these through a hash 274 function. The check key is generated the same way, except that the 275 order of the secret and seed is reversed. If the secret and seed are 276 the same, the hash result could be used to generate OTP responses. 277 Therefore, servers and generators MUST specifically disallow the 278 secret to be the same as the seed. For re-initialization methods 279 that only supply the server with an OTP result based on the secret, 280 this can be accomplished by generating an OTP using the same 281 sequence number and seed as the generator used and setting the 282 secret equal to the seed. If the resulting OTP is the same as the 283 OTP supplied by the generator, then the server MUST reject it. For 284 re-initialization on the console or with another secure channel that 285 allows the user to supply the server with the secret as cleartext, 286 this check can be made by a simple string comparison. 288 The check-update field contains the exclusive-OR (XOR) of the check 289 key generated from the old seed and secret with the check key 290 generated from the new seed and secret. This allows any party that 291 knows the old check key -- which is considered secret data -- to 292 derive the new check key. 294 The check-value field contains the result of a standard envelope 295 message authentication code using the OTP folded hash function. 296 This is the result of running the folded hash function over the 297 concatenation of: 299 1. old check key 300 2. old-OTP 301 3. new-params (string) 302 4. new-OTP 303 5. check-update 304 6. old check key 306 The new-params field is hashed as a string the same way that a seed 307 or secret pass phrase would be. All other field values are hashed in 308 their uncoded binary forms, in network byte order and without any 309 padding. 311 Requirements 313 A server compliant with this specification: 315 1. MAY refuse to accept any init responses that do not carry the 316 active attack protection values 317 2. MAY refuse to accept any init responses for a user if it does 318 have an old check value in its database 319 3. MUST remove the init response check value from its database if 320 the secret or seed change using any mechanism that does not 321 implement check values exactly the same way as the init 322 response 323 4. MUST NOT allow a user to use the same value for their seed and 324 secret 325 5. MUST make the above check for all (re-)initialization methods 326 they support 327 6. MUST disable all OTP access to any principal whose sequence 328 number would be less than one 329 7. MUST handle the case where the old-OTP is valid but the active 330 attack protection check fails by acting as if the old-OTP 331 were provided as a standard response. That is, the next 332 challenge will contain the old seed and one less than the 333 old sequence number. 334 8. SHOULD allow a user to use the simple active attack protection 335 provided by the check-update and check-value fields 336 9. SHOULD require that this protection be used if it has a check 337 value in its database for the old secret and seed 338 10. MUST perform the verification steps for the active attack 339 protection if it has a check value in its database for the old 340 secret and seed and active attack protection information is 341 provided in the response 342 11. MUST NOT re-initialize the sequence if (10) fails, even if the 343 use of active attack protection is not mandatory 345 A generator compliant with this specification: 347 1. MUST NOT allow a user to use the same value for their seed and 348 secret 349 2. MUST take specific steps to prevent infinite loops of 350 re-initialization attempts in case of failure 351 3. SHOULD provide the user with some indication that the 352 re-initialization is taking place 353 4. SHOULD support the simple active attack protection 354 5. SHOULD NOT do a re-initialization without the user's permission, 355 either for that specific instance or as a configuration option 356 6. SHOULD NOT retry a failed re-initialization without a user's 357 permission 358 7. MUST refuse to generate OTPs with a sequence number below one 360 6.0 SECURITY CONSIDERATIONS 362 All of the security considerations for the OTP system also apply to 363 the OTP system with extended responses. 365 The re-initialization response provides a weak means of protection 366 against active attacks. It is not meant to defeat well-funded and 367 well-skilled adversaries. This protection is not a substitute for 368 stronger measures such as IP Security. 370 The active attack protection requires that shared secret information 371 be stored on the server. It is the responsibility of the server to 372 keep that information secret. Disclosure of the secret data reduces 373 the security of the re-initialization response with active attack 374 protection to that of the response without it (the user's secret 375 pass phrase is not compromised, but an attacker could substitute a 376 different secret pass phrase). 378 7.0 ACKNOWLEDGEMENTS 380 Like rfc 1938, the protocol described in this document was created 381 by contributors in the IETF OTP working group. Specific 382 contributions were made by Neil Haller, who provided input on the 383 overall design requirements of a re-initialization protocol, Denis 384 Pinkas, who suggested an active attack defense mechanism that is 385 used here with minor modifications, and Phil Servita, who opened the 386 debate with the first real protocol proposal and provided lots of 387 specific input on the design of this and earlier protocols. 389 Randall Atkinson and Ted T'so also contributed their views to 390 discussions about details of the protocol extensions in this 391 document. 393 8.0 REFERENCES 395 [RFC 822] David H. Crocker, Standard for the Format of ARPA 396 Internet Text Messages, "Request for Comments (RFC) 397 822", August 13, 1982. 399 [RFC 1825] R. Atkinson, Security Architecture for the Internet 400 Protocol, "Request for Comments (RFC) 1825", August 9, 401 1995. 403 [RFC1938] N. Haller and C. Metz, A One-Time Password System, 404 "Request for Comments (RFC) 1938", Bellcore and Kaman 405 Sciences Corporation, May 1996. 407 9.0 Author's Address 409 Craig Metz 410 Kaman Sciences Corporation 411 For NRL Code 5544 412 4555 Overlook Avenue, S.W. 413 Washington, DC, 20375-5337, USA 415 Email: cmetz@itd.nrl.navy.mil 417 Appendix - Reference Responses 419 The following responses were generated by the One-Time Passwords in 420 Everything (OPIE) 2.3 implementation of the extended responses 421 described here. 423 All of these are responses to the challenge: 425 otp-md5 499 ke1234 427 Note that the re-initialization responses use the same secret pass 428 phrase for new and old and a new seed of "ke1235". Also, these 429 responses have been split for formatting purposes into multiple lines; 430 they MUST NOT be multiple lines in actual use. 432 The secret pass phrase for these responses is: 434 This is a test. 436 The OTP standard hexadecimal response is: 438 5bf0 75d9 959d 036d 440 The OTP standard six-word response is: 442 BOND FOGY DRAB NE RISE MART 444 The OTP extended "hex" response is: 446 hex:5bf0 75d9 959d 036f 448 The OTP extended "word" response is: 450 word:BOND FOGY DRAB NE RISE MART 452 The OTP extended "init" response without active attack protection is: 454 init:5BF0 75D9 959D 036F:md5 499 ke1235:3712 DCB4 AA53 16C1 456 The OTP extended "init-word" response without active attack protection 457 is: 459 init-word:BOND FOGY DRAB NE RISE MART:md5 499 ke1235: 460 RED HERD NOW BEAN PA BURG 462 The OTP extended "init" response with active attack protection is: 464 init:5BF0 75D9 959D 036F:md5 499 ke1235:3712 DCB4 AA53 16C1: 465 6E00 18AF 5582 73DE:E69E 1812 2A85 9DB6 467 The OTP extended "init-word" response with active attack protection 468 is: 470 init-word:BOND FOGY DRAB NE RISE MART:md5 499 ke1235: 471 RED HERD NOW BEAN PA BURG:CURL ADD NIB LATE MAO TRAY: 472 SWUM TUFT AVE BEAU BLUE SICK