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SSL_CTX_set_verify(3)               OpenSSL              SSL_CTX_set_verify(3)



NAME
       SSL_get_ex_data_X509_STORE_CTX_idx, SSL_CTX_set_verify, SSL_set_verify,
       SSL_CTX_set_verify_depth, SSL_set_verify_depth, SSL_verify_cb,
       SSL_verify_client_post_handshake, SSL_set_post_handshake_auth,
       SSL_CTX_set_post_handshake_auth - set peer certificate verification
       parameters

LIBRARY
       libcrypto, -lcrypto

SYNOPSIS
        #include <openssl/ssl.h>

        typedef int (*SSL_verify_cb)(int preverify_ok, X509_STORE_CTX *x509_ctx);

        void SSL_CTX_set_verify(SSL_CTX *ctx, int mode, SSL_verify_cb verify_callback);
        void SSL_set_verify(SSL *ssl, int mode, SSL_verify_cb verify_callback);
        SSL_get_ex_data_X509_STORE_CTX_idx(void);

        void SSL_CTX_set_verify_depth(SSL_CTX *ctx, int depth);
        void SSL_set_verify_depth(SSL *ssl, int depth);

        int SSL_verify_client_post_handshake(SSL *ssl);
        void SSL_CTX_set_post_handshake_auth(SSL_CTX *ctx, int val);
        void SSL_set_post_handshake_auth(SSL *ssl, int val);

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 SSL_new(3). Within the callback function,
       SSL_get_ex_data_X509_STORE_CTX_idx can be called to get the data index
       of the current SSL object that is doing the verification.

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

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

       SSL_CTX_set_post_handshake_auth() and SSL_set_post_handshake_auth()
       enable the Post-Handshake Authentication extension to be added to the
       ClientHello such that post-handshake authentication can be requested by
       the server. If val is 0 then the extension is not sent, otherwise it
       is. By default the extension is not sent. A certificate callback will
       need to be set via SSL_CTX_set_client_cert_cb() if no certificate is
       provided at initialization.

       SSL_verify_client_post_handshake() causes a CertificateRequest message
       to be sent by a server on the given ssl connection. The SSL_VERIFY_PEER
       flag must be set; the SSL_VERIFY_POST_HANDSHAKE flag is optional.

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_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, SSL_VERIFY_CLIENT_ONCE
           and SSL_VERIFY_POST_HANDSHAKE 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 once during the
           connection. Do not ask for a client certificate again during
           renegotiation or post-authentication if a certificate was requested
           during the initial handshake. This flag must be used together with
           SSL_VERIFY_PEER.

           Client mode: ignored

       SSL_VERIFY_POST_HANDSHAKE
           Server mode: the server will not send a client certificate request
           during the initial handshake, but will send the request via
           SSL_verify_client_post_handshake(). This allows the SSL_CTX or SSL
           to be configured for post-handshake peer verification before the
           handshake occurs. This flag must be used together with
           SSL_VERIFY_PEER. TLSv1.3 only; no effect on pre-TLSv1.3
           connections.

           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_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 a limit on
       the number of certificates between the end-entity and trust-anchor
       certificates.  Neither the end-entity nor the trust-anchor certificates
       count against depth. If the certificate chain needed to reach a trusted
       issuer is longer than depth+2, X509_V_ERR_CERT_CHAIN_TOO_LONG 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, 2 and 3 (0 being
       the end-entity and 3 the trust-anchor).  The default depth limit is
       100, allowing for the peer certificate, at most 100 intermediate CA
       certificates and a final trust anchor certificate.

       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.

       After calling SSL_set_post_handshake_auth(), the client will need to
       add a certificate or certificate callback to its configuration before
       it can successfully authenticate. This must be called before
       SSL_connect().

       SSL_verify_client_post_handshake() requires that verify flags have been
       previously set, and that a client sent the post-handshake
       authentication extension. When the client returns a certificate the
       verify callback will be invoked. A write operation must take place for
       the Certificate Request to be sent to the client, this can be done with
       SSL_do_handshake() or SSL_write_ex().  Only one certificate request may
       be outstanding at any time.

       When post-handshake authentication occurs, a refreshed NewSessionTicket
       message is sent to the client.

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.

       The SSL_verify_client_post_handshake() function returns 1 if the
       request succeeded, and 0 if the request failed. The error stack can be
       examined to determine the failure reason.

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
       CRYPTO_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(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(7), 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_CTX_set_client_cert_cb(3),
       CRYPTO_get_ex_new_index(3)

HISTORY
       The SSL_VERIFY_POST_HANDSHAKE option, and the
       SSL_verify_client_post_handshake() and SSL_set_post_handshake_auth()
       functions were added in OpenSSL 1.1.1.

COPYRIGHT
       Copyright 2000-2018 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>.



1.1.1                             2018-09-17             SSL_CTX_set_verify(3)