SSL_CTX_set_verify(3)       OpenSSL       SSL_CTX_set_verify(3)





NAME
       SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_ver-
       ify_depth, SSL_set_verify_depth - set peer certificate
       verification parameters

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);

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

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

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

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

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

       SSL_VERIFY_NONE
           Server mode: the server will not send a client cer-
           tificate request to the client, so the client will
           not send a certificate.

           Client mode: if not using an anonymous cipher (by
           default disabled), the server will send a certifi-
           cate which will be checked. The result of the cer-
           tificate verification process can be checked after
           the TLS/SSL handshake using the SSL_get_ver-
           ify_result(3) function.  The handshake will be con-
           tinued regardless of the verification result.

       SSL_VERIFY_PEER
           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 con-
           trolled by the additional SSL_VER-
           IFY_FAIL_IF_NO_PEER_CERT and SSL_VERIFY_CLIENT_ONCE
           flags.

           Client mode: the server certificate is verified. If
           the verification process fails, the TLS/SSL hand-
           shake is immediately terminated with an alert mes-
           sage containing the reason for the verification
           failure. If no server certificate is sent, because
           an anonymous cipher is used, SSL_VERIFY_PEER is
           ignored.

       SSL_VERIFY_FAIL_IF_NO_PEER_CERT
           Server mode: if the client did not return a certifi-
           cate, the TLS/SSL handshake is immediately termi-
           nated with a "handshake failure" alert.  This flag
           must be used together with SSL_VERIFY_PEER.

           Client mode: ignored

       SSL_VERIFY_CLIENT_ONCE
           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_VER-
           IFY_PEER.

           Client mode: ignored

       Exactly one of the 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 SSL_CTX_set_cert_verify_callback(3).  The following
       descriptions apply in the case of the built-in proce-
       dure. An application provided procedure also has access
       to the verify depth information and the verify_call-
       back() 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 cer-
       tificate chain is longer than allowed, the certificates
       above the limit are ignored. Error messages are gener-
       ated 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 cer-
       tificate", "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 verify_callback function is used to control the be-
       haviour when the SSL_VERIFY_PEER flag is set. It must be
       supplied by the application and receives two arguments:
       preverify_ok indicates, whether the verification of the
       certificate in question was passed (preverify_ok=1) or
       not (preverify_ok=0). x509_ctx is a pointer to the com-
       plete context used for the certificate chain verifica-
       tion.

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

       The return value of verify_callback controls the strat-
       egy of the further verification process. If verify_call-
       back returns 0, the verification process is immediately
       stopped with "verification failed" state. If SSL_VER-
       IFY_PEER is set, a verification failure alert is sent to
       the peer and the TLS/SSL handshake is terminated. If
       verify_callback returns 1, the verification process is
       continued. If verify_callback always returns 1, the
       TLS/SSL handshake will not be terminated with respect to
       verification failures and the connection will be estab-
       lished. The calling process can however retrieve the
       error code of the last verification error using
       SSL_get_verify_result(3) or by maintaining its own error
       storage managed by verify_callback.

       If no verify_callback is specified, the default callback
       will be used.  Its return value is identical to prever-
       ify_ok, so that any verification failure will lead to a
       termination of the TLS/SSL handshake with an alert mes-
       sage, if SSL_VERIFY_PEER is set.

BUGS
       In client mode, it is not checked whether the SSL_VER-
       IFY_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.

RETURN VALUES
       The SSL*_set_verify*() functions do not provide diagnos-
       tic information.

EXAMPLES
       The following code sequence realizes an example ver-
       ify_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 exam-
       ple 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 SSL_get_ex_new_index(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 */
          }
        }

SEE ALSO
       ssl(3), SSL_new(3), SSL_CTX_get_verify_mode(3),
       SSL_get_verify_result(3), SSL_CTX_load_verify_loca-
       tions(3), SSL_get_peer_certificate(3),
       SSL_CTX_set_cert_verify_callback(3),
       SSL_get_ex_data_X509_STORE_CTX_idx(3),
       SSL_get_ex_new_index(3)



0.9.7c                     2003-06-26     SSL_CTX_set_verify(3)
