MirBSD manpage: SSL_CTX_set_verify(3), SSL_CTX_set_verify_depth(3), SSL_set_verify(3), SSL_set_verify_depth(3)



     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 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 certi-
     ficate 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:

         Server mode: the server will not send a client certifi-
         cate 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

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         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 verif-
         ication result.

         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 addi-
         tional 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.

         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

         Server mode: only request a client certificate on the
         initial TLS/SSL handshake. Do not ask for a client cer-
         tificate 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 appli-
     cation provided verification function set with
     SSL_CTX_set_cert_verify_callback(3). The following descrip-
     tions apply in the case of the built-in procedure. An appli-
     cation 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

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     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 certifi-
     cates would not be present, most likely a
     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 cer-
     tificate in question was passed (preverify_ok=1) or not
     (preverify_ok=0). x509_ctx is a pointer to the complete con-
     text 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 advanc-
     ing 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 ter-
     minated with respect to verification failures and the con-
     nection 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.

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     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 incom-
     plete certificate chain and not
     X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.


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


     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

     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),

      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;

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         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;
           return preverify_ok;

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      mydata_t mydata;

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


       * 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),

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