TLS (SSL) Tcl Commands

TLS (SSL)

tls - binding to OpenSSL toolkit.

SYNOPSIS
package require Tcl ?8.2?
package require tls ?1.5?
 
tls::init ?options?
tls::socket ?options? host port
tls::socket ?-server command? ?options? port
tls::handshake channel
tls::status ?-local? channel
tls::import channel ?options?
tls::unimport channel
tls::ciphers protocol ?verbose?
tls::version
COMMANDS
CALLBACK OPTIONS
HTTPS EXAMPLE
SPECIAL CONSIDERATIONS
SEE ALSO

NAME

tls - binding to OpenSSL toolkit.

SYNOPSIS

package require Tcl 8.2
package require tls 1.6

tls::init ?options?
tls::socket ?options? host port
tls::socket ?-server command? ?options? port
tls::status ?-local? channel
tls::handshake channel

tls::import channel ?options?
tls::unimport channel
tls::ciphers protocol ?verbose?
tls::version

DESCRIPTION

This extension provides a generic binding to OpenSSL, utilizing the Tcl_StackChannel API for Tcl 8.2 and higher. The sockets behave exactly the same as channels created using Tcl's built-in socket command with additional options for controlling the SSL session. To use TLS with an earlier version of Tcl than 8.2, please obtain TLS 1.3. Please note that there are known limitations with the stacked channel implementation prior to 8.3.2, so it is recommended that TLS is used with an 8.3.2+ interpreter. The current version of TLS will work with Tcl 8.2+, it is just more stable with 8.3.2+.

COMMANDS

Typically one would use the tls::socket command which provides compatibility with the native Tcl socket command. In such cases tls::import should not be used directly.

tls::init ?options?
This routine sets the default options used by tls::socket and is optional. If you call tls::import directly this routine has no effect. Any of the options that tls::socket accepts can be set using this command, though you should limit your options to only TLS related ones.
 
tls::socket ?options? host port
tls::socket ?-server command? ?options? port
This is a helper function that utilizes the underlying commands (tls::import). It behaves exactly the same as the native Tcl socket command except that the options can include any of the applicable tls:import options.
 
tls::handshake channel
Forces handshake to take place, and returns 0 if handshake is still in progress (non-blocking), or 1 if the handshake was successful. If the handshake failed this routine will throw an error.
 
tls::status ?-local? channel
Returns the current security status of an SSL channel. The result is a list of key-value pairs describing the connected peer. If the result is an empty list then the SSL handshake has not yet completed. If -local is given, then the certificate information is the one used locally.
issuer dn
The distinguished name (DN) of the certificate issuer.
subject dn
The distinguished name (DN) of the certificate subject.
notBefore date
The begin date for the validity of the certificate.
notAfter date
The expiry date for the certificate.
serial n
The serial number of the certificate.
cipher cipher
The current cipher in use between the client and server channels.
sbits n
The number of bits used for the session key.
tls::import channel ?options?
SSL-enable a regular Tcl channel - it need not be a socket, but must provide bi-directional flow. Also setting session parameters for SSL handshake.
-cadir dir
Provide the directory containing the CA certificates.
-cafile filename
Provide the CA file.
-certfile filename
Provide the certificate to use.
-cipher string
Provide the cipher suites to use. Syntax is as per OpenSSL.
-command callback
If specified, this callback will be invoked at several points during the OpenSSL handshake. It can pass errors and tracing information, and it can allow Tcl scripts to perform their own validation of the certificate in place of the default validation provided by OpenSSL.
See CALLBACK OPTIONS for further discussion.
-keyfile filename
Provide the private key file. (default: value of -certfile)
-model channel
This will force this channel to share the same SSL_CTX structure as the specified channel, and therefore share callbacks etc.
-password callback
If supplied, this callback will be invoked when OpenSSL needs to obtain a password, typically to unlock the private key of a certificate. The callback should return a string which represents the password to be used.
See CALLBACK OPTIONS for further discussion.
-request bool
Request a certificate from peer during SSL handshake. (default: true)
-require bool
Require a valid certificate from peer during SSL handshake. If this is set to true then -request must also be set to true. (default: false)
-server bool
Handshake as server if true, else handshake as client.(default: false)
-servername host
Only available if the OpenSSL library the package is linked against supports the TLS hostname extension for 'Server Name Indication' (SNI). Use to name the logical host we are talking to and expecting a certificate for
-ssl2 bool
Enable use of SSL v2. (default: true unless -DNO_PATENTS was specified in build)
-ssl3 bool
Enable use of SSL v3. (default: true)
-tls1 bool
Enable use of TLS v1. (default: false)
tls::unimport channel
Provided for symmetry to tls::import, this unstacks the SSL-enabling of a regular Tcl channel. An error is thrown if TLS is not the top stacked channel type.
tls::ciphers protocol ?verbose?
Returns list of supported ciphers based on the protocol you supply, which must be one of ssl2, ssl3, or tls1. If verbose is specified as true then a verbose, semi-human readable list is returned providing additional information on the nature of the cipher support. In each case the result is a Tcl list.
tls::version
Returns the version string defined by OpenSSL.

CALLBACK OPTIONS

As indicated above, individual channels can be given their own callbacks to handle intermediate processing by the OpenSSL library, using the -command and -password options passed to either of tls::socket or tls::import.

-command callback
Invokes the specified callback script at several points during the OpenSSL handshake. Except as indicated below, values returned from the callback are ignored. Arguments appended to the script upon callback take one of the following forms:

info channel major minor message
This form of callback is invoked by the OpenSSL function SSL_CTX_set_info_callback().
The major and minor arguments are used to represent the state information bitmask.
Possible values for major are:
handshake, alert, connect, accept.
Possible values for minor are:
start, done, read, write, loop, exit.
The message argument is a descriptive string which may be generated either by SSL_state_string_long() or by SSL_alert_desc_string_long(), depending on context.

verify channel depth cert status error
This form of callback is invoked by the OpenSSL function SSL_set_verify().
The depth argument is an integer representing the current depth on the certificate chain, with 0 as the subject certificate and higher values denoting progressively more indirect issuer certificates.
The cert argument is a list of key-value pairs similar to those returned by tls::status.
The status argument is an integer representing the current validity of the certificate. A value of 0 means the certificate is deemed invalid. A value of 1 means the certificate is deemed valid.
The error argument supplies the message, if any, generated by X509_STORE_CTX_get_error().

The callback may override normal validation processing by explicitly returning one of the above status values.

-password callback
Invokes the specified callback script when OpenSSL needs to obtain a password. The callback should return a string which represents the password to be used. No arguments are appended to the script upon callback.

Reference implementations of these callbacks are provided in the distribution as tls::callback and tls::password respectively. Note that these are sample implementations only. In a more realistic deployment you would specify your own callback scripts on each TLS channel using the -command and -password options.

The default behavior when the -command option is not specified is for TLS to process the associated library callbacks internally. The default behavior when the -password option is not specified is for TLS to process the associated library callbacks by attempting to call tls::password. The difference between these two behaviors is a consequence of maintaining compatibility with earlier implementations.

The tls::debug variable provides some additional control over these reference callbacks. Its value is zero by default. Higher values produce more diagnostic output, and will also force the verify method in tls::callback to accept the certificate, even when it is invalid.

The use of the reference callbacks tls::callback and tls::password is not recommended. They may be removed from future releases.

The use of the variable tls::debug is not recommended. It may be removed from future releases.

HTTPS EXAMPLE

This example requires a patch to the http module that ships with Tcl - this patch has been submitted for inclusion in Tcl 8.2.1, but is also provided in the tls directory if needed. A sample server.pem is provided with the TLS release, courtesy of the OpenSSL project.

package require http
package require tls

http::register https 443 [list ::tls::socket -require 1 -cafile ./server.pem]

set tok [http::geturl https://developer.netscape.com/]

SPECIAL CONSIDERATIONS

The capabilities of this package can vary enormously based upon how your OpenSSL library was configured and built. At the most macro-level OpenSSL supports a "no patents" build, which disables RSA, IDEA, RC(2,4,5) and SSL2 - if your OpenSSL is configured this way then you will need to build TLS with the -DNO_PATENTS option - and the resultant module will function correctly and also support ADH certificate-less encryption, however you will be unable to utilize this to speak to normal Web Servers, which typically require RSA support. Please see http://www.openssl.org/ for more information on the whole issue of patents and US export restrictions.

SEE ALSO

socket, fileevent, OpenSSL


Copyright © 1999 Matt Newman.
Copyright © 2004 Starfish Systems.