INTERNET DRAFT Craig Metz draft-ietf-otp-ext-02.txt Kaman Sciences April 18, 1997 OTP Extended Responses STATUS OF THIS MEMO This document is an Internet Draft. Internet Drafts are working documents of the Internet Engineering Task Force (IETF), its Areas and Working Groups. Note that other groups may also distribute working documents as Internet Drafts. Internet Drafts are draft documents valid for a maximum of six months. Drafts may be updated, replaced, or obsoleted by other documents at any time. It is not appropriate to use Internet Drafts as reference material or to cite them other than as a "working draft" or "work in progress." To learn the current status of any Internet Draft, please check the 1id-abstracts.txt listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), ds.internic.net (US East Coast), nic.nordu.net (Europe), ftp.isi.com (US West Coast), or munnari.oz.au (Pacific Rim). The distribution of this Internet Draft is unlimited. It is filed as <draft-ietf-otp-ext-02.txt> and it expires on October 18, 1997. 1.0 ABSTRACT This document provides a specification for a type of response to an OTP [RFC 1938] challenge that carries explicit indication of the response's encoding. Codings for the two mandatory OTP data formats using this new type of response are presented. This document also provides a specification for a response that allows OTP generator to request that a server re-initialize a sequence and change parameters such as the secret pass phrase. 2.0 CONVENTIONS, TERMS, and NOTATION This document specifies the data formats and software behaviors needed to use OTP extended responses. The data formats are described three ways: using an ad-hoc UNIX manual page style syntax, using augmented BNF described in sections two and three of RFC 822, and by examples. Should there be any conflict between these descriptions, the augmented BNF takes precedence. The software behaviors are described in words, and specific behavior compliance requirements are itemized using the requirements terminology described in section four of RFC 1938. C. Metz [Page 1]
INTERNET DRAFT OTP Extended Responses April 18, 1997 3.0 EXTENDED RESPONSES This document builds on the protocol and terminology specified in RFC 1938 and assumes that you have already read this document and understand its contents. An extended response is a single line of printable text terminated by a new line sequence appropriate for the context of its use (e.g., ASCII CR followed by ASCII LF). It contains two or more tokens that are separated with a single colon (':') character. The first token contains a type specifier that indicates the format of the rest of the response. The tokens that follow are argument data for the OTP extended response. At least one token of data MUST be present. Syntax In UNIX manual page like syntax, the general form of an extended response could be described as: <type-specifier>:<arg1>[:<arg2>[:...]] In augmented BNF syntax, the syntax of the general form of an extended response is: extended-response = type 1*(":" argument) NL type = token argument = token token = 1*<any CHAR except ":" and CTLs> NL = CRLF / CR / LF / <other appropriate new line sequence> An example of the extended response using a mythical type named "foo" is: foo:some data:some more data:12345 Requirements A server compliant with this specification: 1. MUST be able to receive and parse the general form of an extended response 2. MUST process the type field in a case-insensitive manner 3. MUST reject any authentication attempt using an extended response if it does not support that type of response 4. SHOULD provide an appropriate indication to the generator if the response was rejected because of (3) 5. MUST limit the length of the input reasonably 6. MUST accept otherwise arbitrary amounts of whitespace wherever a response allows it 7. MUST be able to receive and correctly process standard OTP responses C. Metz [Page 2]
INTERNET DRAFT OTP Extended Responses April 18, 1997 A generator compliant with this specification: 1. SHOULD have an option that selects whether standard or extended responses are generated 2. SHOULD make (1) easily available to the end user 3. SHOULD be configurable on a per-server or per-seed basis 4. MUST be able to generate standard OTP responses 5. SHOULD initially default to using standard responses 6. MUST generate the type field in lower case 4.0 THE "HEX" AND "WORD" RESPONSES There exists a very rare case in which a standard OTP response could be a valid coding in both the hexadecimal and six-word formats. An example of this is the response "ABE ACE ADA ADD BAD A." The solution to this problem mandated by the OTP specification is that compliant servers MUST attempt to parse and verify a standard response in both hexadecimal and six-word formats and must consider the authentication successful if either succeeds. This problem can be solved easily using extended responses. The "hex" response and the "word" response are two response types that encode an OTP in an extended response that explicitly describes the encoding. These responses start with a type label of "hex" for a hexadecimal OTP and "word" for a six-word coded OTP. These responses contain one argument field that contains a standard OTP response coded in the indicated format. Syntax In UNIX manual page like syntax, the format of these responses could be described as: hex:<hexadecimal number> word:<six dictionary words> In augmented BNF syntax and with the definitions already provided, the syntax of these responses is: hex-response = "hex:" hex-64bit NL hex-64bit = 16(hex-char *LWSP-char) hex-char = ("A" / "B" / "C" / "D" / "E" / "F" / "a" / "b" / "c" / "d" / "e" / "f" / "0" / "1" / "2" / "3" / "4" / "5" / "6" / "7" / "8" / "9") word-response = "word:" word-64bit NL word-64bit = 6(otp-word 1*LWSP-char) otp-word = <any valid word in the standard OTP coding dictionary> C. Metz [Page 3]
INTERNET DRAFT OTP Extended Responses April 18, 1997 Examples of these responses are: hex:8720 33d4 6202 9172 word:VAST SAUL TAKE SODA SUCH BOLT Requirements A server compliant with this specification: 1. MUST process all arguments in a case-insensitive manner A generator compliant with this specification: 1. MUST generate otp-word tokens in upper case with single spaces separating them 2. MUST generate hexadecimal numbers using only lower case for letters 5.0 THE "INIT-HEX" AND "INIT-WORD" RESPONSES The OTP specification requires that implementations provide a means for a client to re-initialize or change its OTP information with a server but does not require any specific protocol for doing it. Implementations that support the OTP extended responses described in this document MUST support the response with the "init-hex" and "init-word" type specifiers, which provide a standard way for a client to re-initialize its OTP information with a server. This response is intended to be used only by automated clients. Because of this, the recommended form of this response uses the hexadecimal encoding for binary data. It is possible for a user to type an "init-hex" or "init-word" response. Syntax In UNIX manual page like syntax, the format of these responses could be described as: init-hex:<current-OTP>:<new-params>:<new-OTP> init-word:<current-OTP>:<new-params>:<new-OTP> In augmented BNF syntax and with the definitions already provided, the syntax of the "init-hex" response is: init-hex-response = "init-hex:" current-OTP ":" new-params ":" new-OTP NL current-OTP = hex-64bit new-OTP = hex-64bit new-params = algorithm SPACE sequence-number SPACE seed algorithm = "md4" / "md5" / "sha1" C. Metz [Page 4]
INTERNET DRAFT OTP Extended Responses April 18, 1997 sequence-number = 4*3DIGIT seed = 16*1(ALPHA / DIGIT) In augmented BNF syntax and with the definitions already provided, the syntax of the "init-word" response is: init-word-response = "init-word:" current-OTP ":" new-params ":" new-OTP NL current-OTP = word-64bit new-OTP = word-64bit new-params = algorithm SPACE sequence-number SPACE seed algorithm = "md4" / "md5" / "sha1" sequence-number = 4*3DIGIT seed = 16*1(ALPHA / DIGIT) Note that all appropriate fields for the "init-hex" response MUST be hexadecimally coded and that all appropriate fields for the "init- word" response MUST be six-word coded. Examples of these responses are: init-hex:f6bd 6b33 89b8 7203:md5 499 ke6118:23d1 b253 5ae0 2b7e init-hex:c9b2 12bb 6425 5a0f:md5 499 ke0986:fd17 cef1 b4df 093e init-word:MOOD SOFT POP COMB BOLO LIFE:md5 499 ke1235: ARTY WEAR TAD RUG HALO GIVE init-word:END KERN BALM NICK EROS WAVY:md5 499 ke1235: BABY FAIN OILY NIL TIDY DADE (Note that all of these responses are one line. Due to their length, they had to be split into multiple lines in order to be included here. These responses MUST NOT span more than one line in actual use) Description of Fields The current-OTP field contains the (RFC 1938) response to the OTP challenge. The new-params field contains the parameters for the client's new requested challenge and the new-OTP field contains a response to that challenge. If the re-initialization is successful, a server MUST store the new OTP in its database as the last successful OTP received and the sequence number in the next challenge presented by the server MUST be one less than the sequence number specified in the new-params field. The new-params field is hashed as a string the same way that a seed or secret pass phrase would be. All other field values are hashed in their uncoded binary forms, in network byte order and without any padding. C. Metz [Page 5]
INTERNET DRAFT OTP Extended Responses April 18, 1997 Requirements A server compliant with this specification: 1. SHOULD NOT allow a user to use the same value for their seed and secret pass phrase. 2. MUST disable all OTP access to any principal whose sequence number would be less than one A generator compliant with this specification: 1. SHOULD NOT allow a user to use the same value for their seed and secret pass phrase 2. MUST take specific steps to prevent infinite loops of re-initialization attempts in case of failure 3. SHOULD provide the user with some indication that the re-initialization is taking place 4. SHOULD NOT do a re-initialization without the user's permission, either for that specific instance or as a configuration option 5. SHOULD NOT retry a failed re-initialization without a user's permission 6. SHOULD warn the user if the sequence number falls below ten 7. MUST refuse to generate OTPs with a sequence number below one 6.0 SECURITY CONSIDERATIONS All of the security considerations for the OTP system also apply to the OTP system with extended responses. These extended responses, like the OTP system itself, do not protect the user against active attackers. The IPsec Authentication Header should be used to provide this protection. The consequences of a successful active attack on the re- initialization response may be more severe than simply hijacking a single session. An attacker could substitute his own response for that of a legitimate user. The attacker may then be able to use the OTP system to authenticate himself as the user at will (at least until detected). 7.0 ACKNOWLEDGMENTS Like RFC 1938, the protocol described in this document was created by contributors in the IETF OTP working group. Specific contributions were made by Neil Haller, who provided input on the overall design requirements of a re-initialization protocol, Denis Pinkas, who suggested several modifications to the originally proposed re-initialization protocol, and Phil Servita, who opened the debate with the first real protocol proposal and provided lots of specific input on the design of this and earlier protocols. C. Metz [Page 6]
INTERNET DRAFT OTP Extended Responses April 18, 1997 Randall Atkinson and Ted T'so also contributed their views to discussions about details of the protocol extensions in this document. 8.0 REFERENCES [RFC 822] David H. Crocker, Standard for the Format of ARPA Internet Text Messages, "Request for Comments (RFC) 822", August 13, 1982. [RFC 1825] R. Atkinson, Security Architecture for the Internet Protocol, "Request for Comments (RFC) 1825", August 9, 1995. [RFC 1938] N. Haller and C. Metz, A One-Time Password System, "Request for Comments (RFC) 1938", Bellcore and Kaman Sciences Corporation, May 1996. 9.0 Author's Address Craig Metz Kaman Sciences Corporation For NRL Code 5544 4555 Overlook Avenue, S.W. Washington, DC, 20375-5337, USA Email: cmetz@itd.nrl.navy.mil C. Metz [Page 7]
INTERNET DRAFT OTP Extended Responses April 18, 1997 Appendix - Reference Responses The following responses were generated by a development version of the One-Time Passwords in Everything (OPIE) implementation of this specification. All of these are responses to the challenge: otp-md5 499 ke1234 Note that the re-initialization responses use the same secret pass phrase for new and current and a new seed of "ke1235". Also, these responses have been split for formatting purposes into multiple lines; they MUST NOT be multiple lines in actual use. The secret pass phrase for these responses is: This is a test. The OTP standard hexadecimal response is: 5bf0 75d9 959d 036f The OTP standard six-word response is: BOND FOGY DRAB NE RISE MART The OTP extended "hex" response is: hex:5Bf0 75d9 959d 036f The OTP extended "word" response is: word:BOND FOGY DRAB NE RISE MART The OTP extended "init-hex" response is: init-hex:5bf0 75d9 959d 036f:md5 499 ke1235:3712 dcb4 aa53 16c1 The OTP extended "init-word" response is: init-word:BOND FOGY DRAB NE RISE MART:md5 499 ke1235: RED HERD NOW BEAN PA BURG C. Metz [Page 8]
