Provided by: libssl-doc_3.0.13-0ubuntu3.5_all bug

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 various SSL/TLS parameters for peer certificate verification

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.

       In client mode verify_callback may also call the SSL_set_retry_verify(3) function on the SSL object set
       in the x509_store_ctx ex data (see SSL_get_ex_data_X509_STORE_CTX_idx(3)) and return 1.  This would be
       typically done in case the certificate verification was not yet able to succeed.  This makes the
       handshake suspend and return control to the calling application with SSL_ERROR_WANT_RETRY_VERIFY.  The
       application can for instance fetch further certificates or cert status information needed for the
       verification.  Calling SSL_connect(3) again resumes the connection attempt by retrying the server
       certificate verification step.  This process may even be repeated if need be.  Note that the handshake
       may still be aborted if a subsequent invocation of the callback (e.g., at a lower depth, or for a
       separate error condition) returns 0.

       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 (see BUGS)

       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 (see BUGS)

       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 (see BUGS)

       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 other
       than SSL_VERIFY_NONE 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 2000-2022 The OpenSSL Project Authors. All Rights Reserved.

       Licensed  under  the  Apache License 2.0 (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>.

3.0.13                                             2025-02-05                           SSL_CTX_SET_VERIFY(3SSL)