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Versions: 02 RFC 2444

Network Working Group                                          C. Newman
Internet Draft: OTP SASL Mechanism                              Innosoft
Updates: 2222                                                   May 1998
Document: draft-ietf-otp-sasl-02.txt               Expires in six months


                  The One-Time-Password SASL Mechanism


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 its working groups.  Note that other groups may also distribute
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     West Coast).

Abstract

     OTP [OTP] provides a useful authentication mechanism for situations
     where there is limited client or server trust.  Currently, OTP is
     added to protocols in an ad-hoc fashion with heuristic parsing.
     This specification defines an OTP SASL [SASL] mechanism so it can
     be easily and formally integrated into many application protocols.

1. How to Read This Document

     The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD
     NOT", "RECOMMENDED" and "MAY" in this document are to be
     interpreted as defined in "Key words for use in RFCs to Indicate
     Requirement Levels" [KEYWORDS].

     This memo assumes the reader is familiar with OTP [OTP], OTP
     extended responses [OTP-EXT] and SASL [SASL].





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Internet Draft             OTP SASL Mechanism                   May 1998


2. Intended Use

     The OTP SASL mechanism replaces the SKEY SASL mechanism [SASL].
     OTP is a good choice for usage scenarios where the client is
     untrusted (e.g., a kiosk client), as a one-time password will only
     give the client a single opportunity to act on behalf of the user.
     OTP is also a good choice for situations where interactive logins
     are permitted to the server, as a compromised OTP authentication
     database is only subject to dictionary attacks, unlike
     authentication databases for other simple mechanisms such as
     CRAM-MD5 [CRAM-MD5].  It is important to note that each use of the
     OTP mechanism causes the authentication database entry for a user
     to be updated.

     This SASL mechanism provides a formal way to integrate OTP into
     SASL-enabled protocols including IMAP [IMAP4], ACAP [ACAP], POP3
     [POP-AUTH] and LDAPv3 [LDAPv3].

3. Profiling OTP for SASL

     OTP [OTP] and OTP extended responses [OTP-EXT] offer a number of
     options.  However, for authentication to succeed, the client and
     server need compatible option sets.  This specification defines a
     single SASL mechanism: OTP.  The following rules apply to this
     mechanism:

     o   The extended response syntax MUST be used.

     o   Servers MUST support the following four OTP extended responses:
         "hex", "word", "init-hex" and "init-word".  Servers MUST
         support the "word" and "init-word" responses for the standard
         dictionary and SHOULD support alternate dictionaries.  Servers
         MUST NOT require use of any additional OTP extensions or
         options.

     o   Clients SHOULD support display of the OTP challenge to the user
         and entry of an OTP in multi-word format.  Clients MAY also
         support direct entry of the pass phrase and compute the "hex"
         or "word" response.

     o   Clients MUST indicate when authentication fails due to the
         sequence number getting too low and SHOULD offer the user the
         option to reset the sequence using the "init-hex" or
         "init-word" response.

     Support for the MD5 algorithm is REQUIRED, and support for the SHA1
     algorithm is RECOMMENDED.




Newman                                                          [Page 2]

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4. OTP Authentication Mechanism

     The mechanism does not provide any security layer.

     The client begins by sending a message to the server containing the
     following two pieces of information.

     (1) An authorization identity.  When the empty string is used, this
     defaults to the authentication identity.  This is used by system
     administrators or proxy servers to login with a different user
     identity.  This field may be up to 255 octets and is terminated by
     a NUL (0) octet.  US-ASCII printable characters are preferred,
     although UTF-8 [UTF-8] printable characters are permitted to
     support international names.  Use of character sets other than
     US-ASCII and UTF-8 is forbidden.

     (2) An authentication identity.  The identity whose pass phrase
     will be used.  This field may be up to 255 octets.  US-ASCII
     printable characters are preferred, although UTF-8 [UTF-8]
     printable characters are permitted to support international names.
     Use of character sets other than US-ASCII and UTF-8 is forbidden.

     The server responds by sending a message containing the OTP
     challenge as described in OTP [OTP] and OTP extended responses
     [OTP-EXT].

     If a client sees an unknown hash algorithm name it will not be able
     to process a pass phrase input by the user.  In this situation the
     client MAY prompt for the six-word format, issue the cancel
     sequence as specified by the SASL profile for the protocol in use
     and try a different SASL mechanism, or close the connection and
     refuse to authenticate.  As a result of this behavior, a server is
     restricted to one OTP hash algorithm per user.

     On success, the client generates an extended response in the "hex",
     "word", "init-hex" or "init-word" format.  The client is not
     required to terminate the response with a space or a newline and
     SHOULD NOT include unnecessary whitespace.

     Servers MUST tolerate input of arbitrary length, but MAY fail the
     authentication if the length of client input exceeds reasonable
     size.

5. Examples

     In these example, "C:" represents lines sent from the client to the
     server and "S:" represents lines sent from the server to the
     client.  The user name is "tim" and no authorization identity is



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Internet Draft             OTP SASL Mechanism                   May 1998


     provided.  The "<NUL>" below represents an ASCII NUL octet.

     The following is an example of the OTP mechanism using the ACAP
     [ACAP] profile of SASL.  The pass phrase used in this example is:
          This is a test.

       C: a001 AUTHENTICATE "OTP" {4}
       C: <NUL>tim
       S: + "otp-md5 499 ke1234 ext"
       C: "hex:5bf075d9959d036f"
       S: a001 OK "AUTHENTICATE completed"

     Here is the same example using the six-words response:

       C: a001 AUTHENTICATE "OTP" {4}
       C: <NUL>tim
       S: + "otp-md5 499 ke1234 ext"
       C: "word:BOND FOGY DRAB NE RISE MART"
       S: a001 OK "AUTHENTICATE completed"

     Here is the same example using the OTP-SHA1 mechanism:

       C: a001 AUTHENTICATE "OTP" {4}
       C: <NUL>tim
       S: + "otp-sha1 499 ke1234 ext"
       C: "hex:c90fc02cc488df5e"
       S: a001 OK "AUTHENTICATE completed"

     Here is the same example with the init-hex extended response

       C: a001 AUTHENTICATE "OTP" {4}
       C: <NUL>tim
       S: + "otp-md5 499 ke1234 ext"
       C: "init-hex:5bf075d9959d036f:md5 499 ke1235:3712dcb4aa5316c1"
       S: a001 OK "OTP sequence reset, authentication complete"

     The following is an example of the OTP mechanism using the IMAP
     [IMAP4] profile of SASL.  The pass phrase used in this example is:
          this is a test

       C: a001 AUTHENTICATE OTP
       S: +
       C: AHRpbQ==
       S: + b3RwLW1kNSAxMjMga2UxMjM0IGV4dA==
       C: aGV4OjExZDRjMTQ3ZTIyN2MxZjE=
       S: a001 OK AUTHENTICATE completed

     Note that the lack of an initial client response and the base64



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     encoding are characteristics of the IMAP profile of SASL.  The
     server challenge is "otp-md5 123 ke1234 ext" and the client
     response is "hex:11d4c147e227c1f1".

6. Security Considerations

     This specification introduces no security considerations beyond
     those those described in SASL [SASL], OTP [OTP] and OTP extended
     responses [OTP-EXT].  A brief summary of these considerations
     follows:

     This mechanism does not provide session privacy, server
     authentication or protection from active attacks.

     This mechanism is subject to passive dictionary attacks.  The
     severity of this attack can be reduced by choosing pass phrases
     well.

     The server authentication database necessary for use with OTP need
     not be plaintext-equivalent.

     Server implementations MUST protect against the race attack [OTP].

7. Multinational Considerations

     As remote access is a crucial service, users are encouraged to
     restrict user names and pass phrases to the US-ASCII character set.
     However, if characters outside the US-ASCII chracter set are used
     in user names and pass phrases, then they are interpreted according
     to UTF-8 [UTF-8].

     Server support for alternate dictionaries is strongly RECOMMENDED
     to permit use of the six-word format with non-English words.

8. IANA Considerations

     Here is the registration template for the OTP SASL mechanism:

     SASL mechanism name: OTP
     Security Considerations: See section 6 of this memo
     Published specification: this memo
     Person & email address to contact for futher information:
       see author's address section below
     Intended usage: COMMON
     Author/Change controller: see author's address section below

     This memo also amends the SKEY SASL mechanism registration [SASL]
     by changing its intended usage to OBSOLETE.



Newman                                                          [Page 5]

Internet Draft             OTP SASL Mechanism                   May 1998


9. References

     [ACAP] Newman, Myers, "ACAP -- Application Configuration Access
     Protocol", RFC 2244, Innosoft, Netscape, November 1997.

     [CRAM-MD5] Klensin, Catoe, Krumviede, "IMAP/POP AUTHorize Extension
     for Simple Challenge/Response", RFC 2195, MCI, September 1997.

     [IMAP4] Crispin, M., "Internet Message Access Protocol - Version
     4rev1", RFC 2060, University of Washington, December 1996.

     [KEYWORDS] Bradner, "Key words for use in RFCs to Indicate
     Requirement Levels", RFC 2119, Harvard University, March 1997.

     [LDAPv3] Wahl, M., Howes, T., Kille, S., "Lightweight Directory
     Access Protocol (v3)", RFC 2251, Critical Angle Inc., Netscape
     Communications Corp, Isode Limited, December 1997.

     [MD5] Rivest, "The MD5 Message Digest Algorithm", RFC 1321, MIT
     Laboratory for Computer Science, April 1992.

     [OTP] Haller, Metz, Nesser, Straw, "A One-Time Password System",
     RFC 2289, Bellcore, Kaman Sciences Corporation, Nesser & Nesser
     Consulting, February 1998.

     [OTP-EXT] Metz, "OTP Extended Responses", RFC 2243, The Inner Net,
     November 1997.

     [POP-AUTH] Myers, "POP3 AUTHentication command", RFC 1734, Carnegie
     Mellon, December 1994.

     [SASL] Myers, "Simple Authentication and Security Layer (SASL)",
     RFC 2222, Netscape Communications, October 1997.

     [UTF-8] Yergeau, F. "UTF-8, a transformation format of ISO 10646",
     RFC 2279, Alis Technologies, January 1998.

10. Author's Address

     Chris Newman
     Innosoft International, Inc.
     1050 Lakes Drive
     West Covina, CA 91790 USA

     Email: chris.newman@innosoft.com






Newman                                                          [Page 6]


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