rfc2808.txt

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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.







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   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).








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   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.





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   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.







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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
              }
          }
      }




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   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
           }
       }
   }