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rfc:rfc2808

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.

Nystrom Informational [Page 1] RFC 2808 The SecurID® 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].

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:
  1. 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).

Nystrom Informational [Page 2] RFC 2808 The SecurID® 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:
    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.

Nystrom Informational [Page 3] RFC 2808 The SecurID® SASL Mechanism April 2000

 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.

Nystrom Informational [Page 4] RFC 2808 The SecurID® SASL Mechanism April 2000

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
            }
        }
    }

Nystrom Informational [Page 5] RFC 2808 The SecurID® SASL Mechanism April 2000

 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,

Nystrom Informational [Page 6] RFC 2808 The SecurID® SASL Mechanism April 2000

     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,

Nystrom Informational [Page 7] RFC 2808 The SecurID® SASL Mechanism April 2000

     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:

Nystrom Informational [Page 8] RFC 2808 The SecurID® 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

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", 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® 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.

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

Nystrom Informational [Page 10] RFC 2808 The SecurID® SASL Mechanism April 2000

11. Full Copyright Statement

 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.

Nystrom Informational [Page 11]

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