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One-Time Passwords Working Group                             Craig Metz
Internet Draft                                            The Inner Net
draft-ietf-otp-ext-03.txt(when posted)               September 12, 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
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   The distribution of this Internet Draft is unlimited. It is filed  as
   <draft-ietf-otp-ext-03.txt> and it expires on March 12, 1998.

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



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   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.

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 single space 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 ",">



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   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>

   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
     9. MUST NOT advertise an extension set ID that has not been
           assigned by the Internet Assigned Numbers Authority (IANA).
           That is, "private" or "extension" values are forbidden.

   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
     5. MUST NOT send a response type value that has not been



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           assigned by the Internet Assigned Numbers Authority (IANA).
           That is, "private" or "extension" values are forbidden.

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>

   Examples of these responses are:

           hex:8720 33d4 6202 9172
           word:VAST SAUL TAKE SODA SUCH BOLT



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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

   new-params      = algorithm SPACE sequence-number SPACE seed
   algorithm       = "md4" / "md5" / "sha1"
   sequence-number = 4*3DIGIT
   seed            = 16*1(ALPHA / DIGIT)



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   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.

   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.




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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
   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.




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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]   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.

Author's Address

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

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



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   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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