NAME¶
SASL - Implementation of SASL mechanisms for Tcl
SYNOPSIS¶
package require
Tcl 8.2
package require
SASL ?1.3.3?
::SASL::new option value ?...?
::SASL::configure option value ?
...?
::SASL::step context challenge ?
...?
::SASL::response context
::SASL::reset context
::SASL::cleanup context
::SASL::mechanisms ?
type? ?
minimum?
::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 using 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. All mechanisms should accept an initial empty
challenge.
- ::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 ?type? ?minimum?
- 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. Any mechanism with a preference value
less than the minimum (which defaults to 0) is removed from the
returned list. This permits a security threshold to be set. Mechanisms
with a preference less that 25 transmit authentication are particularly
susceptible to eavesdropping and should not be provided unless a secure
channel is in use (eg: tls).
The type parameter may be one of client or server and
defaults to client. Only mechanisms that have an implementation
matching the type are returned (this permits servers to correctly
declare support only for mechanisms that actually provide a server
implementation).
- ::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. If the mechanism is already registered then the recorded values
are updated.
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.
For server mechanisms the password callback should always be called with the
authentication identity and the realm as the first two parameters.
- 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 the case of the NTLM mechanism it is the Windows NT 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].
- OTP
- OTP is the One-Time Password system described in RFC 2289 [6]. This
mechanism is secure against replay attacks and also avoids storing
password or password equivalents on the server. Only a digest of a seed
and a passphrase is ever transmitted across the network. Requires the
otp package from tcllib and one or more of the cryptographic digest
packages (md5 or sha-1 are the most commonly used).
- 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. As this mechanism has additional
dependencies it is made available as a separate sub-package. To enable
this mechanism your application must load the SASL::NTLM
package.
- X-GOOGLE-TOKEN
- This is a proprietary protocol developed by Google and used for
authenticating users for the Google Talk service. This mechanism makes a
pair of HTTP requests over an SSL channel and so this mechanism depends
upon the availability of the tls and http packages. To enable this
mechanism your application must load the SASL::XGoogleToken
package. In addition you are recommended to make use of the autoproxy
package to handle HTTP proxies reasonably transparently.
- SCRAM
- This is a protocol specified in RFC 5802 [7]. To enable this mechanism
your application must load the SASL::SCRAM package.
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 appropriate action.
proc ClientCallback {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 ClientCallback]
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.
- [6]
- Haller, N. et al., "A One-Time Password System", RFC 2289,
February 1998, ( http://www.ieft.org/rfc/rfc2289.txt)
- [7]
- Newman, C. et al., "Salted Challenge Response Authentication
Mechanism (SCRAM) SASL and GSS-API Mechanisms", RFC 5802, July 2010,
( http://www.ieft.org/rfc/rfc5802.txt)
AUTHORS¶
Pat Thoyts
BUGS, IDEAS, FEEDBACK¶
This document, and the package it describes, will undoubtedly contain bugs and
other problems. Please report such in the category
sasl of the
Tcllib Trackers [
http://core.tcl.tk/tcllib/reportlist]. Please also
report any ideas for enhancements you may have for either package and/or
documentation.
KEYWORDS¶
SASL, authentication
CATEGORY¶
Networking
COPYRIGHT¶
Copyright (c) 2005-2006, Pat Thoyts <patthoyts@users.sourceforge.net>