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.
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.
Nystrom Informational [Page 1]
RFC 2808 The SecurID(r) SASL Mechanism April 2000
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].
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".
a) The client generates the credentials using local information (seed, current time and user PIN/password).
Nystrom Informational [Page 2]
RFC 2808 The SecurID(r) SASL Mechanism April 2000
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:
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.
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
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'.
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.
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.
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:
Nystrom Informational [Page 8]
RFC 2808 The SecurID(r) SASL Mechanism April 2000
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
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.
[RFC1734] Myers, J., "POP3 AUTHentication command", RFC 1734, December 1994.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision 3", BCP 9, RFC 2026, October 1996.
[RFC2060] Crispin, M., "Internet Message Access Protocol - Version 4rev1", RFC 2060, December 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, 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", RFC 2234, November 1997.
[RFC2244] Newman, C. and J. Myers, "ACAP -- Application Configuration Access Protocol", RFC 2244, November 1997.
[RFC2251] Wahl, M., Howes, T. and S. Kille, "Lightweight Directory Access Protocol (v3)", RFC 2251, December 1997.
Nystrom Informational [Page 9]
RFC 2808 The SecurID(r) SASL Mechanism April 2000
[RFC2279] Yergeau, F., "UTF-8, a transformation format of ISO 10646", RFC 2279, January 1998.
[RFC2289] Haller, N., Metz, C., Nesser, P. and M. Straw, "A One-Time Password System", RFC 2289, 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.
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.
Copyright (C) The Internet Society (2000). 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.
Acknowledgement
Funding for the RFC Editor function is currently provided by the Internet Society.