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

 #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);

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.)

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 certificate 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 certificate which will be checked. The result of the certificate verification process can be checked after the TLS/SSL handshake using the SSL_get_verify_result(3) function. The handshake will be continued 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 controlled by the additional SSL_VERIFY_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 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.

SSL_VERIFY_FAIL_IF_NO_PEER_CERT

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. 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_VERIFY_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 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 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: 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 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 x509_ctx and verify_callback is called with preverify_ok=0. By applying X509_CTX_store_* functions verify_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 strategy of the further verification process. If 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 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 established. 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 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.

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.

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

The following code sequence realizes an example 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 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 */
   }
 }

ssl(3), SSL_new(3), SSL_CTX_get_verify_mode(3), SSL_get_verify_result(3), SSL_CTX_load_verify_locations(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)