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rfc:rfc2243

Network Working Group C. Metz Request for Comments: 2243 The Inner Net Category: Standards Track November 1997

                       OTP Extended Responses

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1997).  All Rights Reserved.

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 an OTP generator to request that a server re-initialize a
 sequence and change parameters such as the secret pass phrase.

1. 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 (specifically, the words
 MUST, SHOULD, and MAY) defined in RFC 2119.

Metz Standards Track [Page 1] RFC 2243 OTP Extended Responses November 1997

2. Extended Challenges and 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 challenge is a single line of printable text terminated
 by either a new line sequence appropriate for the context of its use
 (e.g., ASCII CR followed by ASCII LF) or a whitespace character. It
 contains a standard OTP challenge, a whitespace character, and a list
 that generators use to determine which extended responses are
 supported by a server.
 An extended response is a single line of printable text terminated by
 a new line sequence appropriate for the context of its use. 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.

2.1. Syntax

 In UNIX manual page like syntax, the general form of an extended
 challenge could be described as:
    <standard OTP challenge> ext[,<extension set id>[, ...]]
 And 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 challenge and an extended response is:
 extended-challenge = otp-challenge 1*LWSP-char capability-list
                      (NL / *LWSP-char)
 otp-challenge     = <a standard OTP challenge>
 capability-list   = "ext" *("," extension-set-id)
 extension-set-id  = *<any CHAR except LWSP, CTLs, or ",">
 extended-response = type 1*(":" argument) NL
 type              = token
 argument          = token
 token             = 1*<any CHAR except ":" and CTLs>
 NL                = <new line sequence appropriate for the context
                      in which OTP is being used>

Metz Standards Track [Page 2] RFC 2243 OTP Extended Responses November 1997

 An example of an extended challenge indicating support for OTP
 extended responses and for a mythical response set "foo" is:
    otp-md5 123 mi1234 ext,foo
 An example of an extended response using a mythical type named "foo"
 is:
    foo:some data:some more data:12345

2.2. Requirements

 A server compliant with this specification:
    1. MUST be able to receive and parse the general form of an
       extended response
    2. MUST be able to receive, parse, and correctly process all
       extended responses specified in this document
    3. MUST process the type field in a case-insensitive manner
    4. MUST reject any authentication attempt using an extended
       response if it does not support that type of response
    5. SHOULD provide an appropriate indication to the generator
       if the response was rejected because of (4)
    6. MUST limit the length of the input reasonably
    7. MUST accept otherwise arbitrary amounts of whitespace
       wherever a response allows it
    8. MUST be able to receive and correctly process standard OTP
       responses
 A generator compliant with this specification:
    1. MUST be able to generate standard OTP responses
    2. MUST use standard responses unless an extended challenge
       has been received for the particular server AND seed
    3. MUST generate the type field in lower case
    4. MUST NOT send a response type for which the server has not
       indicated support through an extended challenge
 Extension set identifiers and extension type identifiers named with
 the prefix "x-" are reserved for private use among mutually
 consenting implementations. Implementations that do not recognise a
 particular "x-" extension MUST ignore that extension. This means that
 all "x-" extensions are likely to be non-interoperable with other
 extensions. Careful consideration should be given to the possibility
 of a server interacting with with a generator implementation which,
 although it recognizes a given "x-" extension, uses it for a
 different purpose. All of the remaining extension namespace is
 reserved to IANA, which will only officially assign the extension

Metz Standards Track [Page 3] RFC 2243 OTP Extended Responses November 1997

 into this namespace after the IESG approves of such an assignment.
 During the lifetime of the OTP WG, it is recommended that the IESG
 consult with the OTP WG prior to approving such an assignment.

3. 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.

3.1. 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>

Metz Standards Track [Page 4] RFC 2243 OTP Extended Responses November 1997

 Examples of these responses are:
    hex:8720 33d4 6202 9172
    word:VAST SAUL TAKE SODA SUCH BOLT

3.2. Requirements

 A server compliant with this specification:
    1. MUST process all arguments in a case-insensitive manner
 A generator compliant with this specification:
    1. SHOULD generate otp-word tokens in upper case with single
       spaces separating them
    2. SHOULD generate hexadecimal numbers using only lower case
       for letters

4. 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.

4.1. 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

Metz Standards Track [Page 5] RFC 2243 OTP Extended Responses November 1997

 new-params      = algorithm SPACE sequence-number SPACE seed
 algorithm       = "md4" / "md5" / "sha1"
 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)

4.2. 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.

Metz Standards Track [Page 6] RFC 2243 OTP Extended Responses November 1997

 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.

4.3. 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
    3. MUST decrement the sequence number if a reinitialization
       response includes a valid current-OTP, but the server is
       unable to successfully process the new-params or new-OTP for
       any reason.
 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

5. Security Considerations

 All of the security considerations for the OTP system also apply to
 the OTP system with extended responses.
 These extended responses, like OTP itself, do not protect the user
 against active attacks. The IPsec Authentication Header (RFC-1826)
 (or another technique with at least as much strength as IPsec AH)
 SHOULD be used to protect against such attacks.
 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

Metz Standards Track [Page 7] RFC 2243 OTP Extended Responses November 1997

 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).
 Failure to implement server requirement 3 in section 4.3 opens an
 implementation to an attack based on replay of the current-OTP part
 of the response.

6. 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. The extensions to the
 OTP challenge were suggested by Chris Newman and John Valdes.
 Randall Atkinson and Ted T'so also contributed their views to
 discussions about details of the protocol extensions in this
 document.

References

 [RFC 822]   Crocker, D., "Standard for the Format of ARPA Internet
             Text Messages," RFC 822, August 1982.
 [RFC 1825]  Atkinson, R., "Security Architecture for the Internet
             Protocol," RFC 1825, August 1995.
 [RFC 1938]  Haller, N. and C. Metz, "A One-Time Password System,"
             RFC 1938, May 1996.
 [RFC 2119]  Bradner, S., "Key words for use in RFCs to
             Indicate Requirement Level," RFC 2119,
             March 1997.

Author's Address

 Craig Metz
 The Inner Net
 Box 10314-1936
 Blacksburg, VA 24062-0314
 (DSN) 354-8590
 cmetz@inner.net

Metz Standards Track [Page 8] RFC 2243 OTP Extended Responses November 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 ext
 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

Metz Standards Track [Page 9] RFC 2243 OTP Extended Responses November 1997

Full Copyright Statement

 Copyright (C) The Internet Society (1997).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Metz Standards Track [Page 10]

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