RFC2444 - 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.

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].

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

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

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.

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", RFC2244, 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", RFC2060, December 1996.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[LDAPv3] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory
Access Protocol (v3)", RFC2251, December 1997.

[MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC1321,
April 1992.

[OTP] Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time
Password System", RFC2289, February 1998.

[OTP-EXT] Metz, C., "OTP Extended Responses", RFC2243, November
1997.

[POP-AUTH] Myers, J., "POP3 AUTHentication command", RFC1734,
December 1994.

[SASL] Myers, J., "Simple Authentication and Security Layer
(SASL)", RFC2222, October 1997.

[UTF-8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC2279, 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.

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