- NAME
- SYNOPSIS
- DESCRIPTION
- NOTES
- BUGS
- RETURN VALUES
- EXAMPLES
- SEE ALSO
- COPYRIGHT

# NAME

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

# SYNOPSIS

     #include <openssl/ssl.h>

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

     typedef int (*SSL_verify_cb)(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 time ssl was created with 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 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_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

If the mode is SSL_VERIFY_NONE none of the other flags may be set.

The actual verification procedure is performed either using the built-in verification procedure or using another application provided verification function set with 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.

# BUGS

In client mode, it is not checked whether the SSL_VERIFY_PEER flag is set, but whether any flags are set. This can lead to unexpected behaviour if SSL_VERIFY_PEER and other flags are not used as required.

# RETURN VALUES

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

# EXAMPLES

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; 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 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(err_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_new(3), SSL_get_verify_result(3), SSL_get_peer_certificate(3), SSL_get_ex_data_X509_STORE_CTX_idx(3), COPYRIGHT

Copyright 2000-2016 The OpenSSL Project Authors. All Rights Reserved.

Licensed under the OpenSSL license (the "License"). You may not use this file except in compliance with the License. You can obtain a copy in the file LICENSE in the source distribution or at https://www.openssl.org/source/license.html.