OTP Extended Responses
RFC 2243

quot;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

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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>

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   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

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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.

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   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

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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

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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

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