SASL(n) 1.0.0 sasl "Simple Authentication and Security Layer (SASL)"

NAME

SASL - Implementation of SASL mechanisms for Tcl

TABLE OF CONTENTS

    TABLE OF CONTENTS
    SYNOPSIS
    DESCRIPTION
    COMMANDS
    OPTIONS
    CALLBACK PROCEDURE
    MECHANISMS
    EXAMPLES
    REFERENCES
    AUTHORS
    KEYWORDS
    COPYRIGHT

SYNOPSIS

package require Tcl 8.2
package require SASL ?1.0?

::SASL::new option value ?...?
::SASL::configure option value ?...?
::SASL::step context challenge ?...?
::SASL::response context
::SASL::reset context
::SASL::cleanup context
::SASL::mechanisms
::SASL::register mechanism preference clientproc ?serverproc?

DESCRIPTION

The Simple Authentication and Security Layer (SASL) is a framework for providing authentication and authorization to comunications protocols. The SASL framework is structured to permit negotiation among a number of authentication mechanisms. SASL may be used in SMTP, IMAP and HTTP authentication. It is also in use in XMPP, LDAP and BEEP. See MECHANISMS for the set of available SASL mechanisms provided with tcllib.

The SASL framework operates using a simple multi-step challenge response mechanism. All the mechanisms work the same way although the number of steps may vary. In this implementation a callback procedure must be provided from which the SASL framework will obtain users details. See CALLBACK PROCEDURE for details of this procedure.

COMMANDS

::SASL::new option value ?...?
Contruct a new SASL context. See OPTIONS for details of the possible options to this command. A context token is required for most of the SASL procedures.

::SASL::configure option value ?...?
Modify and inspect the SASL context option. See OPTIONS for further details.

::SASL::step context challenge ?...?
This is the core procedure for useing the SASL framework. The step procedure should be called until it returns 0. Each step takes a server challenge string and the response is calculated and stored in the context. Each mechanism may require one or more steps. For some steps there may be no server challenge required in which case an empty string should be provided for this parameter.

::SASL::response context
Returns the next response string that should be sent to the server.

::SASL::reset context
Re-initialize the SASL context. Discards any internal state and permits the token to be reused.

::SASL::cleanup context
Release all resources associated with the SASL context. The context token may not be used again after this procedure has been called.

::SASL::mechanisms
Returns a list of all the available SASL mechanisms. The list is sorted by the mechanism preference value (see register) with the preferred mechanisms and the head of the list.

::SASL::register mechanism preference clientproc ?serverproc?
New mechanisms can be added to the package by registering the mechanism name and the implementing procedures. The server procedure is optional. The preference value is an integer that is used to order the list returned by the mechanisms command. Higher values indicate a preferred mechanism.

OPTIONS

-callback
Specify a command to be evaluated when the SASL mechanism requires information about the user. The command is called with the current SASL context and a name specifying the information desired. See EXAMPLES.

-mechanism
Set the SASL mechanism to be used. See mechanisms for a list of supported authentication mechanisms.

-service
Set the service type for this context. Some mechanisms may make use of this parameter (eg DIGEST-MD5, GSSAPI and Kerberos). If not set it defaults to an empty string. If the -type is set to 'server' then this option should be set to a valid service identity. Some examples of valid service names are smtp, ldap, beep and xmpp.

-server
This option is used to set the server name used in SASL challenges when operating as a SASL server.

-type
The context type may be one of 'client' or 'server'. The default is to operate as a client application and respond to server challenges. Mechanisms may be written to support server-side SASL and setting this option will cause each step to issue the next challenge. A new context must be created for each incoming client connection when in server mode.

CALLBACK PROCEDURE

When the SASL framework requires any user details it will call the procedure provided when the context was created with an argument that specfies the item of information required.

In all cases a single response string should be returned.

login
The callback procedure should return the users authorization identity. Return an empty string unless this is to be different to the authentication identity. Read [1] for a discussion about the specific meaning of authorization and authentication identities within SASL.

username
The callback procedure should return the users authentication identity. Read [1] for a discussion about the specific meaning of authorization and authentication identities within SASL.

password
The callback procedure should return the password that matches the authentication identity as used within the current realm.

realm
Some SASL mechanisms use realms to partition authentication identities. The realm string is protocol dependent and is often the current DNS domain or in NTLM the Windows domain name.

hostname
Returns the client host name - typically [info host].

MECHANISMS

ANONYMOUS
As used in FTP this mechanism only passes an email address for authentication. The ANONYMOUS mechanism is specified in [2].

PLAIN
This is the simplest mechanism. The users authentication details are transmitted in plain text. This mechanism should not be provided unless an encrypted link is in use - typically after SSL or TLS has been negotiated.

LOGIN
The LOGIN [1] mechanism transmits the users details with base64 encoding. This is no more secure than PLAIN and likewise should not be used without a secure link.

CRAM-MD5
This mechanism avoids sending the users password over the network in plain text by hashing the password with a server provided random value (known as a nonce). A disadvantage of this mechanism is that the server must maintain a database of plaintext passwords for comparison. CRAM-MD5 was defined in [4].

DIGEST-MD5
This mechanism improves upon the CRAM-MD5 mechanism by avoiding the need for the server to store plaintext passwords. With digest authentication the server needs to store the MD5 digest of the users password which helps to make the system more secure. As in CRAM-MD5 the password is hashed with a server nonce and other data before being transmitted across the network. Specified in [3].

NTLM
This is a proprietary protocol developed by Microsoft [5] and is in common use for authenticating users in a Windows network environment. NTLM uses DES encryption and MD4 digests of the users password to authenticate a connection. Certain weaknesses have been found in NTLM and thus there are a number of versions of the protocol.

EXAMPLES

See the examples subdirectory for more complete samples using SASL with network protocols. The following should give an idea how the SASL commands are to be used. In reality this should be event driven. Each time the step command is called, the last server response should be provided as the command argument so that the SASL mechanism can take approriate action.

 
proc Callback {context command args} {
    switch -exact -- $command {
        login    { return "" }
        username { return $::tcl_platform(user) }
        password { return "SecRet" }
        realm    { return "" }
        hostname { return [info host] }
        default  { return -code error unxpected }
    }
}

proc Demo {{mech PLAIN}} {
    set ctx [SASL::new -mechanism $mech -callback Callback]
    set challenge ""
    while {1} {
        set more_steps [SASL::step $ctx challenge]
        puts "Send '[SASL::response $ctx]'"
        puts "Read server response into challenge var"
        if {!$more_steps} {break}
    }
    SASL::cleanup $ctx
}

REFERENCES

  1. Myers, J. "Simple Authentication and Security Layer (SASL)", RFC 2222, October 1997. (http://www.ietf.org/rfc/rfc2222.txt)

  2. Newman, C. "Anonymous SASL Mechanism", RFC 2245, November 1997. (http://www.ietf.org/rfc/rfc2245.txt)

  3. Leach, P., Newman, C. "Using Digest Authentication as a SASL Mechanism", RFC 2831, May 2000, (http://www.ietf.org/rfc/rfc2831.txt)

  4. Klensin, J., Catoe, R. and Krumviede, P., "IMAP/POP AUTHorize Extension for Simple Challenge/Response" RFC 2195, September 1997. (http://www.ietf.org/rfc/rfc2195.txt)

  5. No official specification is available. However, http://davenport.sourceforge.net/ntlm.html provides a good description.

AUTHORS

Pat Thoyts

KEYWORDS

SASL, authentication

COPYRIGHT

Copyright © 2005, Pat Thoyts <patthoyts@users.sourceforge.net>