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RFC 2444 - The One-Time-Password SASL Mechanism

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Network Working Group                                          C. Newman
Request for Comments: 2444                                      Innosoft
Updates: 2222                                               October 1998
Category: Standards Track

                  The One-Time-Password SASL Mechanism

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1998).  All Rights Reserved.


   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"

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

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

   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"

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

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 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

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

   Note that the lack of an initial client response and the base64
   encoding are characteristics of the IMAP profile of SASL.  The server
   challenge is "otp-md5 123 ke1234 ext" and the client response is

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.

9. References

   [ACAP]     Newman, C. and J. Myers, "ACAP -- Application
              Configuration Access Protocol", RFC 2244, November 1997.

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

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

   [KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [LDAPv3]   Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
              Access Protocol (v3)", RFC 2251, December 1997.

   [MD5]      Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
              April 1992.

   [OTP]      Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time
              Password System", RFC 2289, February 1998.

   [OTP-EXT]  Metz, C., "OTP Extended Responses", RFC 2243, November

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

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

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

10. Author's Address

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

   EMail: chris.newman@innosoft.com

11.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
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   followed, or as required to translate it into languages other than

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an


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