~ubuntu-branches/ubuntu/oneiric/openssl/oneiric-security

=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 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, 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, 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.

The actual verification procedure is performed either using the built-in

verification procedure or using another application provided verification

function set with

L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>.

The following descriptions apply in the case of the built-in procedure. An

application provided procedure also has access to the verify depth information

and the verify_callback() function, but the way this information is used

may be different.

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 not be terminated with respect to verification

failures and the connection will be established. 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; information about the certificate chain

is 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_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(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