Internet DRAFT - draft-newman-sasl-otp
draft-newman-sasl-otp
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Network Working Group C. Newman
Internet Draft: OTP SASL Mechanism Innosoft
Updates: 2222 May 1998
Document: draft-ietf-otp-sasl-01.txt Expires in six months
The One-Time-Password SASL Mechanism
Status of this memo
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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", "MANDATORY", "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] and OTP
extended responses [OTP-EXT].
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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 both standard and alternate dictionaries for the "word"
and "init-word" responses. 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 MANDATORY, and support for the
SHA1 algorithm is RECOMMENDED.
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Internet Draft OTP SASL Mechanism May 1998
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-MD5" {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-MD5" {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-SHA1" {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-MD5" {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-MD5
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 MANDATORY 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.
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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
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