[Docs] [txt|pdf] [Tracker] [WG] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02 03 04 RFC 2243

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]


Html markup produced by rfcmarkup 1.129d, available from https://tools.ietf.org/tools/rfcmarkup/