=pod

=head1 NAME

SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth - set peer certificate verification parameters

=head1 SYNOPSIS

 #include <openssl/ssl.h>

 void SSL_CTX_set_verify(SSL_CTX *ctx, int mode,
                         int (*verify_callback)(int, X509_STORE_CTX *));
 void SSL_set_verify(SSL *s, int mode,
                     int (*verify_callback)(int, X509_STORE_CTX *));
 void SSL_CTX_set_verify_depth(SSL_CTX *ctx,int depth);
 void SSL_set_verify_depth(SSL *s, int depth);

 int verify_callback(int preverify_ok, X509_STORE_CTX *x509_ctx);

=head1 DESCRIPTION

SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and
specifies the B<verify_callback> function to be used. If no callback function
shall be specified, the NULL pointer can be used for B<verify_callback>.

SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and
specifies the B<verify_callback> function to be used. If no callback function
shall be specified, the NULL pointer can be used for B<verify_callback>. In
this case last B<verify_callback> set specifically for this B<ssl> remains. If
no special B<callback> was set before, the default callback for the underlying
B<ctx> is used, that was valid at the the time B<ssl> was created with
L<SSL_new(3)|SSL_new(3)>.

SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
verification that shall be allowed for B<ctx>. (See the BUGS section.)

SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
verification that shall be allowed for B<ssl>. (See the BUGS section.)

=head1 NOTES

The verification of certificates can be controlled by a set of logically
or'ed B<mode> flags:

=over 4

=item SSL_VERIFY_NONE

B<Server mode:> the server will not send a client certificate request to the
client, so the client will not send a certificate.

B<Client mode:> if not using an anonymous cipher (by default disabled), the
server will send a certificate which will be checked. The result of the
certificate verification process can be checked after the TLS/SSL handshake
using the L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> function.
The handshake will be continued regardless of the verification result.

=item SSL_VERIFY_PEER

B<Server mode:> the server sends a client certificate request to the client.
The certificate returned (if any) is checked. If the verification process
fails as indicated by B<verify_callback>, the TLS/SSL handshake is
immediately terminated with an alert message containing the reason for
the verification failure.
The behaviour can be controlled by the additional
SSL_VERIFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE flags.

B<Client mode:> the server certificate is verified. If the verification process
fails as indicated by B<verify_callback>, the TLS/SSL handshake is
immediately terminated with an alert message containing the reason for
the verification failure. If no server certificate is sent, because an
anonymous cipher is used, SSL_VERIFY_PEER is ignored.

=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT

B<Server mode:> if the client did not return a certificate, the TLS/SSL
handshake is immediately terminated with a "handshake failure" alert.
This flag must be used together with SSL_VERIFY_PEER.

B<Client mode:> ignored

=item SSL_VERIFY_CLIENT_ONCE

B<Server mode:> only request a client certificate on the initial TLS/SSL
handshake. Do not ask for a client certificate again in case of a
renegotiation. This flag must be used together with SSL_VERIFY_PEER.

B<Client mode:> ignored

=back

Exactly one of the B<mode> flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be
set at any time.

SSL_CTX_set_verify_depth() and SSL_set_verify_depth() set the limit up
to which depth certificates in a chain are used during the verification
procedure. If the certificate chain is longer than allowed, the certificates
above the limit are ignored. Error messages are generated as if these
certificates would not be present, most likely a
X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY will be issued.
The depth count is "level 0:peer certificate", "level 1: CA certificate",
"level 2: higher level CA certificate", and so on. Setting the maximum
depth to 2 allows the levels 0, 1, and 2. The default depth limit is 9,
allowing for the peer certificate and additional 9 CA certificates.

The B<verify_callback> function is used to control the behaviour when the
SSL_VERIFY_PEER flag is set. It must be supplied by the application and
receives two arguments: B<preverify_ok> indicates, whether the verification of
the certificate in question was passed (preverify_ok=1) or not
(preverify_ok=0). B<x509_ctx> is a pointer to the complete context used
for the certificate chain verification.

The certificate chain is checked starting with the deepest nesting level
(the root CA certificate) and worked upward to the peer's certificate.
At each level signatures and issuer attributes are checked. Whenever
a verification error is found, the error number is stored in B<x509_ctx>
and B<verify_callback> is called with B<preverify_ok>=0. By applying
X509_CTX_store_* functions B<verify_callback> can locate the certificate
in question and perform additional steps (see EXAMPLES). If no error is
found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
before advancing to the next level.

The return value of B<verify_callback> controls the strategy of the further
verification process. If B<verify_callback> returns 0, the verification
process is immediately stopped with "verification failed" state. If
SSL_VERIFY_PEER is set, a verification failure alert is sent to the peer and
the TLS/SSL handshake is terminated. If B<verify_callback> returns 1,
the verification process is continued. If B<verify_callback> always returns
1, the TLS/SSL handshake will never be terminated because of this application
experiencing a verification failure. The calling process can however
retrieve the error code of the last verification error using
L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> or by maintaining its
own error storage managed by B<verify_callback>.

If no B<verify_callback> is specified, the default callback will be used.
Its return value is identical to B<preverify_ok>, so that any verification
failure will lead to a termination of the TLS/SSL handshake with an
alert message, if SSL_VERIFY_PEER is set.

=head1 BUGS

In client mode, it is not checked whether the SSL_VERIFY_PEER flag
is set, but whether SSL_VERIFY_NONE is not set. This can lead to
unexpected behaviour, if the SSL_VERIFY_PEER and SSL_VERIFY_NONE are not
used as required (exactly one must be set at any time).

The certificate verification depth set with SSL[_CTX]_verify_depth()
stops the verification at a certain depth. The error message produced
will be that of an incomplete certificate chain and not
X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.

=head1 RETURN VALUES

The SSL*_set_verify*() functions do not provide diagnostic information.

=head1 EXAMPLES

The following code sequence realizes an example B<verify_callback> function
that will always continue the TLS/SSL handshake regardless of verification
failure, if wished. The callback realizes a verification depth limit with
more informational output.

All verification errors are printed, informations about the certificate chain
are printed on request.
The example is realized for a server that does allow but not require client
certificates.

The example makes use of the ex_data technique to store application data
into/retrieve application data from the SSL structure
(see L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>,
L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>).

 ...
 typedef struct {
   int verbose_mode;
   int verify_depth;
   int always_continue;
 } mydata_t;
 int mydata_index;
 ...
 static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
 {
    char    buf[256];
    X509   *err_cert;
    int     err, depth;
    SSL    *ssl;
    mydata_t *mydata;

    err_cert = X509_STORE_CTX_get_current_cert(ctx);
    err = X509_STORE_CTX_get_error(ctx);
    depth = X509_STORE_CTX_get_error_depth(ctx);

    /*
     * Retrieve the pointer to the SSL of the connection currently treated
     * and the application specific data stored into the SSL object.
     */
    ssl = X509_STORE_CTX_get_ex_data(ctx, SSL_get_ex_data_X509_STORE_CTX_idx());
    mydata = SSL_get_ex_data(ssl, mydata_index);

    X509_NAME_oneline(X509_get_subject_name(err_cert), buf, 256);

    /*
     * Catch a too long certificate chain. The depth limit set using
     * SSL_CTX_set_verify_depth() is by purpose set to "limit+1" so
     * that whenever the "depth>verify_depth" condition is met, we
     * have violated the limit and want to log this error condition.
     * We must do it here, because the CHAIN_TOO_LONG error would not
     * be found explicitly; only errors introduced by cutting off the
     * additional certificates would be logged.
     */
    if (depth > mydata->verify_depth) {
        preverify_ok = 0;
        err = X509_V_ERR_CERT_CHAIN_TOO_LONG;
        X509_STORE_CTX_set_error(ctx, err);
    } 
    if (!preverify_ok) {
        printf("verify error:num=%d:%s:depth=%d:%s\n", err,
                 X509_verify_cert_error_string(err), depth, buf);
    }
    else if (mydata->verbose_mode)
    {
        printf("depth=%d:%s\n", depth, buf);
    }

    /*
     * At this point, err contains the last verification error. We can use
     * it for something special
     */
    if (!preverify_ok && (err == X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT)
    {
      X509_NAME_oneline(X509_get_issuer_name(ctx->current_cert), buf, 256);
      printf("issuer= %s\n", buf);
    }

    if (mydata->always_continue)
      return 1;
    else
      return preverify_ok;
 }
 ...

 mydata_t mydata;

 ...
 mydata_index = SSL_get_ex_new_index(0, "mydata index", NULL, NULL, NULL);

 ...
 SSL_CTX_set_verify(ctx, SSL_VERIFY_PEER|SSL_VERIFY_CLIENT_ONCE,
                    verify_callback);

 /*
  * Let the verify_callback catch the verify_depth error so that we get
  * an appropriate error in the logfile.
  */
 SSL_CTX_set_verify_depth(verify_depth + 1);

 /*
  * Set up the SSL specific data into "mydata" and store it into th SSL
  * structure.
  */
 mydata.verify_depth = verify_depth; ...
 SSL_set_ex_data(ssl, mydata_index, &mydata);
					     
 ...
 SSL_accept(ssl);	/* check of success left out for clarity */
 if (peer = SSL_get_peer_certificate(ssl))
 {
   if (SSL_get_verify_result(ssl) == X509_V_OK)
   {
     /* The client sent a certificate which verified OK */
   }
 }

=head1 SEE ALSO

L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>,
L<SSL_CTX_get_verify_mode(3)|SSL_CTX_get_verify_mode(3)>,
L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>,
L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>,
L<SSL_get_peer_certificate(3)|SSL_get_peer_certificate(3)>,
L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>,
L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>

=cut