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Run idnits with the --verbose option for more detailed information about the items above. -------------------------------------------------------------------------------- 1 INTERNET DRAFT Craig Metz 2 draft-ietf-otp-ext-02.txt Kaman Sciences 3 April 18, 1997 5 OTP Extended Responses 7 STATUS OF THIS MEMO 9 This document is an Internet Draft. Internet Drafts are working 10 documents of the Internet Engineering Task Force (IETF), its Areas 11 and Working Groups. Note that other groups may also distribute 12 working documents as Internet Drafts. 14 Internet Drafts are draft documents valid for a maximum of six 15 months. Drafts may be updated, replaced, or obsoleted by other 16 documents at any time. It is not appropriate to use Internet Drafts 17 as reference material or to cite them other than as a "working 18 draft" or "work in progress." 20 To learn the current status of any Internet Draft, please check the 21 1id-abstracts.txt listing contained in the Internet-Drafts Shadow 22 Directories on ftp.is.co.za (Africa), ds.internic.net (US East 23 Coast), nic.nordu.net (Europe), ftp.isi.com (US West Coast), or 24 munnari.oz.au (Pacific Rim). 26 The distribution of this Internet Draft is unlimited. It is filed as 27 and it expires on October 18, 1997. 29 1.0 ABSTRACT 31 This document provides a specification for a type of response to an 32 OTP [RFC 1938] challenge that carries explicit indication of the 33 response's encoding. Codings for the two mandatory OTP data formats 34 using this new type of response are presented. 36 This document also provides a specification for a response that 37 allows OTP generator to request that a server re-initialize a 38 sequence and change parameters 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 822, and by 46 examples. Should there be any conflict between these descriptions, 47 the augmented BNF takes precedence. The software behaviors are 48 described in words, and specific behavior compliance requirements 49 are itemized using the requirements terminology described in section 50 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) NL 77 type = token 78 argument = token 79 token = 1* 80 NL = 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. MUST 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. MUST 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 NL 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 NL 151 word-64bit = 6(otp-word 1*LWSP-char) 152 otp-word = 154 Examples of these responses are: 156 hex:8720 33d4 6202 9172 157 word:VAST SAUL TAKE SODA SUCH BOLT 159 Requirements 161 A server compliant with this specification: 163 1. MUST process all arguments in a case-insensitive manner 165 A generator compliant with this specification: 167 1. MUST generate otp-word tokens in upper case with single spaces 168 separating them 169 2. MUST generate hexadecimal numbers using only lower case for 170 letters 172 5.0 THE "INIT-HEX" AND "INIT-WORD" RESPONSES 174 The OTP specification requires that implementations provide a means 175 for a client to re-initialize or change its OTP information with a 176 server but does not require any specific protocol for doing it. 177 Implementations that support the OTP extended responses described in 178 this document MUST support the response with the "init-hex" and 179 "init-word" type specifiers, which provide a standard way for a 180 client to re-initialize its OTP information with a server. This 181 response is intended to be used only by automated clients. Because 182 of this, the recommended form of this response uses the hexadecimal 183 encoding for binary data. It is possible for a user to type an 184 "init-hex" or "init-word" response. 186 Syntax 188 In UNIX manual page like syntax, the format of these responses could 189 be described as: 191 init-hex::: 192 init-word::: 194 In augmented BNF syntax and with the definitions already provided, 195 the syntax of the "init-hex" response is: 197 init-hex-response = "init-hex:" current-OTP ":" new-params ":" new-OTP NL 199 current-OTP = hex-64bit 200 new-OTP = hex-64bit 202 new-params = algorithm SPACE sequence-number SPACE seed 203 algorithm = "md4" / "md5" / "sha1" 204 sequence-number = 4*3DIGIT 205 seed = 16*1(ALPHA / DIGIT) 207 In augmented BNF syntax and with the definitions already provided, 208 the syntax of the "init-word" response is: 210 init-word-response = "init-word:" current-OTP ":" new-params ":" new-OTP NL 212 current-OTP = word-64bit 213 new-OTP = word-64bit 215 new-params = algorithm SPACE sequence-number SPACE seed 216 algorithm = "md4" / "md5" / "sha1" 217 sequence-number = 4*3DIGIT 218 seed = 16*1(ALPHA / DIGIT) 220 Note that all appropriate fields for the "init-hex" response MUST be 221 hexadecimally coded and that all appropriate fields for the "init- 222 word" response MUST be six-word coded. 224 Examples of these responses are: 226 init-hex:f6bd 6b33 89b8 7203:md5 499 ke6118:23d1 b253 5ae0 2b7e 227 init-hex:c9b2 12bb 6425 5a0f:md5 499 ke0986:fd17 cef1 b4df 093e 229 init-word:MOOD SOFT POP COMB BOLO LIFE:md5 499 ke1235: 230 ARTY WEAR TAD RUG HALO GIVE 231 init-word:END KERN BALM NICK EROS WAVY:md5 499 ke1235: 232 BABY FAIN OILY NIL TIDY DADE 234 (Note that all of these responses are one line. Due to their length, 235 they had to be split into multiple lines in order to be included 236 here. These responses MUST NOT span more than one line in actual 237 use) 239 Description of Fields 241 The current-OTP field contains the (RFC 1938) response to the OTP 242 challenge. The new-params field contains the parameters for the 243 client's new requested challenge and the new-OTP field contains a 244 response to that challenge. If the re-initialization is successful, 245 a server MUST store the new OTP in its database as the last 246 successful OTP received and the sequence number in the next 247 challenge presented by the server MUST be one less than the sequence 248 number specified in the new-params field. 250 The new-params field is hashed as a string the same way that a seed 251 or secret pass phrase would be. All other field values are hashed in 252 their uncoded binary forms, in network byte order and without any 253 padding. 255 Requirements 257 A server compliant with this specification: 259 1. SHOULD NOT allow a user to use the same value for their seed and 260 secret pass phrase. 261 2. MUST disable all OTP access to any principal whose sequence 262 number would be less than one 264 A generator compliant with this specification: 266 1. SHOULD NOT allow a user to use the same value for their seed and 267 secret pass phrase 268 2. MUST take specific steps to prevent infinite loops of 269 re-initialization attempts in case of failure 270 3. SHOULD provide the user with some indication that the 271 re-initialization is taking place 272 4. SHOULD NOT do a re-initialization without the user's permission, 273 either for that specific instance or as a configuration option 274 5. SHOULD NOT retry a failed re-initialization without a user's 275 permission 276 6. SHOULD warn the user if the sequence number falls below ten 277 7. MUST refuse to generate OTPs with a sequence number below one 279 6.0 SECURITY CONSIDERATIONS 281 All of the security considerations for the OTP system also apply to 282 the OTP system with extended responses. 284 These extended responses, like the OTP system itself, do not protect 285 the user against active attackers. The IPsec Authentication Header 286 should be used to provide this protection. 288 The consequences of a successful active attack on the re- 289 initialization response may be more severe than simply hijacking a 290 single session. An attacker could substitute his own response for 291 that of a legitimate user. The attacker may then be able to use the 292 OTP system to authenticate himself as the user at will (at least 293 until detected). 295 7.0 ACKNOWLEDGMENTS 297 Like RFC 1938, the protocol described in this document was created 298 by contributors in the IETF OTP working group. Specific 299 contributions were made by Neil Haller, who provided input on the 300 overall design requirements of a re-initialization protocol, Denis 301 Pinkas, who suggested several modifications to the originally 302 proposed re-initialization protocol, and Phil Servita, who opened 303 the debate with the first real protocol proposal and provided lots 304 of specific input on the design of this and earlier protocols. 306 Randall Atkinson and Ted T'so also contributed their views to 307 discussions about details of the protocol extensions in this 308 document. 310 8.0 REFERENCES 312 [RFC 822] David H. Crocker, Standard for the Format of ARPA 313 Internet Text Messages, "Request for Comments (RFC) 314 822", August 13, 1982. 316 [RFC 1825] R. Atkinson, Security Architecture for the Internet 317 Protocol, "Request for Comments (RFC) 1825", August 9, 318 1995. 320 [RFC 1938] N. Haller and C. Metz, A One-Time Password System, 321 "Request for Comments (RFC) 1938", Bellcore and Kaman 322 Sciences Corporation, May 1996. 324 9.0 Author's Address 326 Craig Metz 327 Kaman Sciences Corporation 328 For NRL Code 5544 329 4555 Overlook Avenue, S.W. 330 Washington, DC, 20375-5337, USA 332 Email: cmetz@itd.nrl.navy.mil 333 Appendix - Reference Responses 335 The following responses were generated by a development version of the 336 One-Time Passwords in Everything (OPIE) implementation of this 337 specification. 339 All of these are responses to the challenge: 341 otp-md5 499 ke1234 343 Note that the re-initialization responses use the same secret pass 344 phrase for new and current and a new seed of "ke1235". Also, these 345 responses have been split for formatting purposes into multiple lines; 346 they MUST NOT be multiple lines in actual use. 348 The secret pass phrase for these responses is: 350 This is a test. 352 The OTP standard hexadecimal response is: 354 5bf0 75d9 959d 036f 356 The OTP standard six-word response is: 358 BOND FOGY DRAB NE RISE MART 360 The OTP extended "hex" response is: 362 hex:5Bf0 75d9 959d 036f 364 The OTP extended "word" response is: 366 word:BOND FOGY DRAB NE RISE MART 368 The OTP extended "init-hex" response is: 370 init-hex:5bf0 75d9 959d 036f:md5 499 ke1235:3712 dcb4 aa53 16c1 372 The OTP extended "init-word" response is: 374 init-word:BOND FOGY DRAB NE RISE MART:md5 499 ke1235: 375 RED HERD NOW BEAN PA BURG