order to use the <b class="option">-require</b> option, one of the following must be true:</p>
<ul class="doctools_itemized">
<li><p>On Linux and Unix systems with OpenSSL already installed, if the CA
certificates are stored in the standard locations, or if the <b class="variable">SSL_CERT_DIR</b>
or <b class="variable">SSL_CERT_FILE</b> environment variables are set, then <b class="option">-cadir</b>,
<b class="option">-cadir</b>, and <b class="option">-castore</b> aren't needed.</p></li>
<li><p>If OpenSSL is not installed in the default location, or when using Mac OS
or Windows and OpenSSL is installed, the <b class="variable">SSL_CERT_DIR</b> and/or
<b class="variable">SSL_CERT_FILE</b> environment variables or the one of the <b class="option">-cadir</b>,
<b class="option">-cadir</b>, or <b class="option">-castore</b> options must be defined.</p></li>
<li><p>On Windows, starting in OpenSSL 3.2, it is now possible to access the
built-in Windows Certificate Store from OpenSSL. This can be achieved by
setting the <b class="option">-castore</b> option to &quot;<b class="const">org.openssl.winstore://</b>&quot;.</p></li>
<li><p>If OpenSSL is not installed, the CA certificates must be downloaded and
installed with the user software. The CURL team makes them available at

[opt_def -session_id [arg binary_string]]
Specifies the session id to resume a session. Not supported yet.

[opt_def -ssl2 [arg bool]]
Enable use of SSL v2. The default is [const false]. Note: Recent versions of
OpenSSL no longer support SSLv2, so this may not have any effect. See the
[cmd tls::protocols] command for supported protocols.

[opt_def -ssl3 [arg bool]]
Enable use of SSL v3. The default is [const false]. Note: Recent versions
of OpenSSL may have this disabled at compile time, so this may not have any
effect. See the [cmd tls::protocols] command for supported protocols.

[opt_def -tls1 [arg bool]]
Enable use of TLS v1. The default is [const true]. Note: TLS 1.0 needs

On Linux and Unix systems with OpenSSL already installed, if the CA
certificates are stored in the standard locations, or if the [var SSL_CERT_DIR]
or [var SSL_CERT_FILE] environment variables are set, then [option -cadir],
[option -cadir], and [option -castore] aren't needed.

[item]
If OpenSSL is not installed in the default location, or when using Mac OS
or Windows and OpenSSL is installed, the [var SSL_CERT_DIR] and/or
[var SSL_CERT_FILE] environment variables or the one of the [option -cadir],
[option -cadir], or [option -castore] options must be defined.

[item]
On Windows, starting in OpenSSL 3.2, it is now possible to access the
built-in Windows Certificate Store from OpenSSL. This can be achieved by
setting the [option -castore] option to "[const org.openssl.winstore://]".

[subsection "Values for Command Callback"]

The callback for the [option -command] option is invoked at several points during the
OpenSSL handshake and during routine operations. See below for the possible
arguments passed to the callback script. Values returned from the callback are
ignored.

[list_begin options]

[opt_def error [arg "channelId message"]]
This form of callback is invoked whenever an error occurs during the initial
connection, handshake, or I/O operations. The [arg message] argument can be
from the Tcl_ErrnoMsg, OpenSSL function [fun ERR_reason_error_string()],
or a custom message. This callback is new for TclTLS 1.8.

Descriptive message string which may be generated either by
[fun SSL_state_string_long()] or [fun SSL_alert_desc_string_long()],
depending on the context.

[def [arg type]]
For alerts, the possible values are: [const warning],
[const fatal], and [const unknown]. For others, [const info] is used.
This argument is new for TclTLS 1.8.

[list_end]

[opt_def message [arg "channelId direction version content_type message"]]
This form of callback is invoked by the OpenSSL function
[fun SSL_set_msg_callback()] whenever a message is sent or received during the
initial connection, handshake, or I/O operations. It is only available when

[subsection "Values for Password Callback"]

The callback for the [option -password] option is invoked by TclTLS whenever OpenSSL needs
to obtain a password. See below for the possible arguments passed to the
callback script. The user provided password is expected to be returned by the
callback.

[list_begin options]

[opt_def password [arg "rwflag size"]]
Invoked when loading or storing an encrypted PEM certificate. The arguments are:

[list_begin definitions]

The callback for the [option -validatecommand] option is invoked during the handshake
process in order for the application to validate the provided value(s). See
below for the possible arguments passed to the callback script. If not
specified, OpenSSL will accept all valid certificates and extensions. To reject
the value and abort the connection, the callback should return 0. To accept the
value and continue the connection, it should return 1. To reject the value, but
continue the connection, it should return 2. This callback is new for TclTLS 1.8.

[list_begin options]

[opt_def alpn [arg "channelId protocol match"]]
For servers, this form of callback is invoked when the client ALPN extension is
received. If [arg match] is true, then [arg protocol] is the first
[option -alpn] protocol option in common to both the client and server.
If not, the first client specified protocol is used. This callback is called

    statePtr->flags	= flags;
    statePtr->interp	= interp;
    statePtr->want	= 0;
    statePtr->vflags	= verify;
    statePtr->err	= "";

    /* Allocate callback script */
    if (script) {
	(void) Tcl_GetStringFromObj(script, &len);
	if (len) {
	    statePtr->callback = script;
	    Tcl_IncrRefCount(statePtr->callback);
	}
    }

    /* Allocate password callback */
    if (password) {
	(void) Tcl_GetStringFromObj(password, &len);
	if (len) {
	    statePtr->password = password;
	    Tcl_IncrRefCount(statePtr->password);
	}
    }

    /* Allocate validate callback */
    if (vcmd) {
	(void) Tcl_GetStringFromObj(vcmd, &len);
	if (len) {
	    statePtr->vcmd = vcmd;
	    Tcl_IncrRefCount(statePtr->vcmd);
	}
    }

    /* Set default CA store on Windows */
#if OPENSSL_VERSION_NUMBER >= 0x30200000L && (defined(_WIN32))
    if (CAstore == NULL && CAfile == NULL && CApath == NULL) {
	CAstore = "org.openssl.winstore://";
    }
#endif

    if (model != NULL) {
	int mode;
	/* Get the "model" context */
	chan = Tcl_GetChannel(interp, model, &mode);
	if (chan == (Tcl_Channel) NULL) {
	    Tls_Free((tls_free_type *) statePtr);