Network Working Group M. Nystrom
Request for Comments: 2808 RSA Laboratories
Category: Informational April 2000

The SecurID(r) SASL Mechanism

Status of this Memo

This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2000). All Rights Reserved.

Abstract

SecurID is a hardware token card prodUCt (or software emulation
thereof) produced by RSA Security Inc., which is used for end-user
authentication. This document defines a SASL [RFC2222] authentication
mechanism using these tokens, thereby providing a means for such
tokens to be used in SASL environments. This mechanism is only for
authentication, and has no effect on the protocol encoding and is not
designed to provide integrity or confidentiality services.

This memo assumes the reader has basic familiarity with the SecurID
token, its associated authentication protocol and SASL.

How to read this document

The key Words "MUST", "MUST NOT", "SHALL", "SHOULD" and "MAY" in this
document are to be interpreted as defined in [RFC2119].

In examples, "C:" and "S:" indicate messages sent by the client and
server respectively.

1. Introduction

The SECURID SASL mechanism is a good choice for usage scenarios where
a client, acting on behalf of a user, is untrusted, as a one-time
passcode will only give the client a single opportunity to act
maliciously. This mechanism provides authentication only.

The SECURID SASL mechanism provides a formal way to integrate the
existing SecurID authentication method into SASL-enabled protocols
including IMAP [RFC2060], ACAP [RFC2244], POP3 [RFC1734] and LDAPv3
[RFC2251].

2. Authentication Model

The SECURID SASL mechanism provides two-factor based user
authentication as defined below.

There are basically three entities in the authentication mechanism
described here: A user, possessing a SecurID token, an application
server, to which the user wants to connect, and an authentication
server, capable of authenticating the user. Even though the
application server in practice may function as a client with respect
to the authentication server, relaying authentication credentials
etc. as needed, both servers are, unless eXPlicitly mentioned,
collectively termed "the server" here. The protocol used between the
application server and the authentication server is outside the scope
of this memo. The application client, acting on behalf of the user,
is termed "the client".

The mechanism is based on the use of a shared secret key, or "seed",
and a personal identification number (PIN), which is known both by
the user and the authentication server. The secret seed is stored on
a token that the user possesses, as well as on the authentication
server. Hence the term "two-factor authentication", a user needs not
only physical Access to the token but also knowledge about the PIN in
order to perform an authentication. Given the seed, current time of
day, and the PIN, a "PASSCODE(r)" is generated by the user's token
and sent to the server.

The SECURID SASL mechanism provides one service:

- User authentication where the user provides information to the
server, so that the server can authenticate the user.

This mechanism is identified with the SASL key "SECURID".

3. Authentication Procedure

a) The client generates the credentials using local information
(seed, current time and user PIN/password).

b) If the underlying protocol permits, the client sends credentials
to the server in an initial response message. Otherwise, the
client sends a request to the server to initiate the
authentication mechanism, and sends credentials after the server's
response (see [RFC2222] section 5.1 for more information regarding
the initial response option).

Unless the server requests a new PIN (see below), the contents of
the client's initial response SHALL be as follows:

(1) An authorization identity. When this field is empty, it
defaults to the authentication identity. This field MAY be used
by system administrators or proxy servers to login with a
different user identity. This field MUST NOT be longer than 255
octets, SHALL be terminated by a NUL (0) octet, and MUST consist
of UTF-8-encoded [RFC2279] printable characters only (US-ASCII
[X3.4] is a subset of UTF-8).

(2) An authentication identity. The identity whose passcode will
be used. If this field is empty, it is assumed to have been
transferred by other means (e.g. if the underlying protocol has
support for this, like [RFC2251]). This field MUST NOT be longer
than 255 octets, SHALL be terminated by a NUL (0) octet, and MUST
consist of UTF-8-encoded printable characters only.

(3) A passcode. The one-time password that will be used to grant
access. This field MUST NOT be shorter than 4 octets, MUST NOT be
longer than 32 octets, SHALL be terminated by a NUL (0) octet, and
MUST consist of UTF-8-encoded printable characters only.
Passcodes usually consist of 4-8 digits.

The ABNF [RFC2234] form of this message is as follows:

credential-pdu = authorization-id authentication-id passcode [pin]

authorization-id = 0*255VUTF8 %x00

authentication-id = 0*255VUTF8 %x00

passcode = 4*32VUTF8 %x00

pin ::= 4*32VUTF8 %x00

VUTF8 = <Visible (printable) UTF8-encoded characters>

Regarding the <pin> rule, see d) below.

c) The server verifies these credentials using its own information.
If the verification succeeds, the server sends back a response
indicating success to the client. After receiving this response,
the client is authenticated. Otherwise, the verification either
failed or the server needs an additional set of credentials from
the client in order to authenticate the user.

d) If the server needs an additional set of credentials, it requests
them now. This request has the following format, described in ABNF
notation:

server-request = passcode pin

passcode = "passcode" %x00

pin = "pin" %x00 [suggested-pin]

suggested-pin = 4*32VUTF8 %x00 ; Between 4 and 32 UTF-8 characters

The 'passcode' choice will be sent when the server requests
another passcode. The 'pin' choice will be sent when the server
requests a new user PIN. The server will either send an empty
string or suggest a new user PIN in this message.

e) The client generates a new set of credentials using local
information and depending on the server's request and sends them
to the server. Authentication now continues as in c) above.

Note 1: Case d) above may occur e.g. when the clocks on which the
server and the client relies are not synchronized.

Note 2: If the server requests a new user PIN, the client MUST
respond with a new user PIN (together with a passcode), encoded as a
UTF-8 string. If the server supplies the client with a suggested PIN,
the client accepts this by replying with the same PIN, but MAY
replace it with another one. The length of the PIN is application-
dependent as are any other requirements for the PIN, e.g. allowed
characters. If the server for some reason does not accept the
received PIN, the client MUST be prepared to receive either a message
indicating the failure of the authentication or a repeated request
for a new PIN. Mechanisms for transferring knowledge about PIN
requirements from the server to the client are outside the scope of
this memo. However, some information MAY be provided in error
messages transferred from the server to the client when applicable.

4. Examples

4.1 IMAP4

The following example shows the use of the SECURID SASL mechanism
with IMAP4. The example is only designed to illustrate the protocol
interaction but do provide valid encoding examples.

The base64 encoding of the last client response, as well as the "+ "
preceding the response, is part of the IMAP4 profile, and not a part
of this specification itself.

S: * OK IMAP4 server ready
C: A001 CAPABILITY
S: * CAPABILITY IMAP4 AUTH=CRAM-MD5 AUTH=SECURID
S: A001 OK done
C: A002 AUTHENTICATE SECURID
S: +
C: AG1hZ251cwAxMjM0NTY3OAA=
S: A002 OK Welcome, SECURID authenticated user: magnus

4.2 LDAPv3

The following examples show the use of the SECURID SASL mechanism
with LDAPv3. The examples are only designed to illustrate the
protocol interaction, but do provide valid encoding examples.
Usernames, passcodes and PINs are of course fictitious. For
readability, all messages are shown in the value-notation defined in
[X680]. <credential-pdu> values are shown hex-encoded in the
'credentials' field of LDAP's 'BindRequest' and <server-request>
values are shown hex-encoded in the 'serverSaslCreds' field of LDAP's
'BindResponse'.

4.2.1 LDAPv3 Example 1

Initial response message, successful authentication.

C: { messageID 1,
protocolOp bindRequest :
{ version 1,
name '434E3D4D41474E5553'H, -- "CN=MAGNUS"
authentication sasl :
{ mechanism '53454355524944'H, -- "SECURID"
credentials '006d61676e757300313233343536373800'H
}
}
}

S: { messageID 1,
protocolOp bindResponse :
{ resultCode success,
matchedDN ''H,
errorMessage ''H,
}
}

4.2.2 LDAPv3 Example 2

Initial response message, server requires second passcode.

C: {
messageID 1,
protocolOp bindRequest : {
version 1,
name '434E3D4D41474E5553'H, -- "CN=MAGNUS"
authentication sasl : {
mechanism '53454355524944'H, -- "SECURID"
credentials '006d61676e757300313233343536373800'H
}
}
}

S: {
messageID 1,
protocolOp bindResponse : {
resultCode saslBindInProgress,
matchedDN ''H,
errorMessage ''H,
serverSaslCreds '70617373636f646500'H
}
}

C: {
messageID 1,
protocolOp bindRequest : {
version 1,
name '434E3D4D41474E5553'H, -- "CN=MAGNUS"
authentication sasl : {
mechanism '53454355524944'H, -- "SECURID"
credentials '006d61676e757300383736353433323100'H
}
}
}

S: {
messageID 1,

protocolOp bindResponse : {
resultCode success,
matchedDN ''H,
errorMessage ''H,
}
}

4.2.3 LDAPv3 Example 3

Initial response message, server requires new PIN and passcode, and
supplies client with a suggested new PIN (which the client accepts).

C: {
messageID 1,
protocolOp bindRequest : {
version 1,
name '434E3D4D41474E5553'H, -- "CN=MAGNUS"
authentication sasl : {
mechanism '53454355524944'H, -- "SECURID"
credentials '006d61676e757300313233343536373800'H
}
}
}

S: {
messageID 1,
protocolOp bindResponse : {
resultCode saslBindInProgress,
matchedDN ''H,
errorMessage ''H,
serverSaslCreds '70696e006b616c6c6500'H
}
}

C: {
messageID 1,
protocolOp bindRequest : {
version 1,
name '434E3D4D41474E5553'H, -- "CN=MAGNUS"
authentication sasl : {
mechanism '53454355524944'H, -- "SECURID"
credentials '006d61676e7573003837343434363734006b616c6c6500'H
}
}
}

S: {
messageID 1,

protocolOp bindResponse : {
resultCode success,
matchedDN ''H,
errorMessage ''H,
}
}

5. Security Considerations

This mechanism only provides protection against passive eavesdropping
attacks. It does not provide session privacy, server authentication
or protection from active attacks. In particular, man-in-the-middle
attacks, were an attacker acts as an application server in order to
acquire a valid passcode are possible.

In order to protect against such attacks, the client SHOULD make sure
that the server is properly authenticated. When user PINs are
transmitted, user authentication SHOULD take place on a server-
authenticated and confidentiality-protected connection.

Server implementations MUST protect against replay attacks, since an
attacker could otherwise gain access by replaying a previous, valid
request. Clients MUST also protect against replay of PIN-change
messages.

5.1 The Race Attack

It is possible for an attacker to listen to most of a passcode, guess
the remainder, and then race the legitimate user to complete the
authentication. As for OTP [RFC2289], conforming server
implementations MUST protect against this race condition. One defense
against this attack is outlined below and borrowed from [RFC2289];
implementations MAY use this approach or MAY select an alternative
defense.

One possible defense is to prevent a user from starting multiple
simultaneous authentication sessions. This means that once the
legitimate user has initiated authentication, an attacker would be
blocked until the first authentication process has completed. In
this approach, a timeout is necessary to thwart a denial of service
attack.

6. IANA Considerations

By registering the SecurID protocol as a SASL mechanism, implementers
will have a well-defined way of adding this authentication mechanism
to their product. Here is the registration template for the SECURID
SASL mechanism:

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

7. Intellectual Property Considerations

RSA Security Inc. does not make any claims on the general
constructions described in this memo, although underlying techniques
may be covered. Among the underlying techniques, the SecurID
technology is covered by a number of US patents (and foreign
counterparts), in particular US patent no. 4,885,778, no. 5,097,505,
no. 5,168,520, and 5,657,388.

SecurID is a registered trademark, and PASSCODE is a trademark, of
RSA Security Inc.

8. References

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

[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC2026, October 1996.

[RFC2060] Crispin, M., "Internet Message Access Protocol - Version
4rev1", RFC2060, December 1996.

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

[RFC2222] Myers, J., "Simple Authentication and Security Layer", RFC
2222, October 1997.

[RFC2234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC2234, November 1997.

[RFC2244] Newman, C. and J. Myers, "ACAP -- Application Configuration
Access Protocol", RFC2244, November 1997.

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

[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646",
RFC2279, January 1998.

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

[X3.4] ANSI, "ANSI X3.4: Information Systems - Coded Character
Sets - 7-Bit American National Standard Code for
Information Interchange (7-Bit ASCII)," American National
Standards Institute.

[X680] ITU-T, "Information Technology - Abstract Syntax Notation
One (ASN.1): Specification of Basic Notation,"
International Telecommunication Union, 1997.

9. Acknowledgements

The author gratefully acknowledges the contributions of various
reviewers of this memo, in particular the ones from John Myers. They
have significantly clarified and improved the utility of this
specification.

10. Author's Address

Magnus Nystrom
RSA Laboratories
Box 10704
121 29 Stockholm
Sweden

Phone: +46 8 725 0900
EMail: magnus@rsasecurity.com

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