rfc2243.txt

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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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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




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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
   [e-mail]




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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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