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. 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]. 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]. Newman Standards Track [Page 1]  RFC 2444 OTP SASL Mechanism October 1998 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. 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 Newman Standards Track [Page 2]  RFC 2444 OTP SASL Mechanism October 1998 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 "" 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: tim S: + "otp-md5 499 ke1234 ext" C: "hex:5bf075d9959d036f" S: a001 OK "AUTHENTICATE completed" Newman Standards Track [Page 3]  RFC 2444 OTP SASL Mechanism October 1998 Here is the same example using the six-words response: C: a001 AUTHENTICATE "OTP" {4} C: 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: 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: 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 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. Newman Standards Track [Page 4]  RFC 2444 OTP SASL Mechanism October 1998 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. Newman Standards Track [Page 5]  RFC 2444 OTP SASL Mechanism October 1998 [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 1997. [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 Newman Standards Track [Page 6]  RFC 2444 OTP SASL Mechanism October 1998 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 copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. 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 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Newman Standards Track [Page 7]