As discussed recently on openssl-users.

doc/ssl/SSL_CTX_set_cert_verify_callback.pod

+=pod
+
+=head1 NAME
+
+SSL_CTX_set_cert_verify_callback - set peer certificate verification procedure
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ void SSL_CTX_set_cert_verify_callback(SSL_CTX *ctx, int (*callback)(),
+                                       char *arg);
+ int (*callback)();
+
+=head1 DESCRIPTION
+
+SSL_CTX_set_cert_verify_callback() sets the verification callback function for
+B<ctx>. SSL objects, that are created from B<ctx> inherit the setting valid at
+the time, L<SSL_new(3)|SSL_new(3)> is called. B<arg> is currently ignored.
+
+=head1 NOTES
+
+Whenever a certificate is verified during a SSL/TLS handshake, a verification
+function is called. If the application does not explicitly specify a
+verification callback function, the built-in verification function is used.
+If a verification callback B<callback> is specified via
+SSL_CTX_set_cert_verify_callback(), the supplied callback function is called
+instead. By setting B<callback> to NULL, the default behaviour is restored.
+
+When the verification must be performed, B<callback> will be called with
+the argument callback(X509_STORE_CTX *x509_store_ctx). The arguments B<arg>
+that can be specified when setting B<callback> are currently ignored.
+
+B<callback> should return 1 to indicate verification success and 0 to
+indicate verification failure. If SSL_VERIFY_PEER is set and B<callback>
+returns 0, the handshake will fail. As the verification procedure may
+allow to continue the connection in case of failure (by always returning 1)
+the verification result must be set in any case using the B<error>
+member of B<x509_store_ctx>, so that the calling application will be informed
+about the detailed result of the verification procedure! 
+
+Within B<x509_store_ctx>, B<callback> has access to the B<verify_callback>
+function set using L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>.
+
+=head1 WARNINGS
+
+Do not mix the verification callback described in this function with the
+B<verify_callback> function called during the verification process. The
+latter is set using the L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>
+family of functions.
+
+Providing a complete verification procedure including certificate purpose
+settings etc is a complex task. The built-in procedure is quite powerful
+and in most cases it should be sufficient to modify its behaviour using
+the B<verify_callback> function.
+
+=head1 BUGS
+
+It is possible to specify arguments to be passed to the verification callback.
+Currently they are however not passed but ignored.
+
+The B<callback> function is not specified via a prototype, so that no
+type checking takes place.
+
+=head1 RETURN VALUES
+
+SSL_CTX_set_cert_verify_callback() does not provide diagnostic information.
+
+=head1 SEE ALSO
+
+L<ssl(3)|ssl(3)>, L<SSL_CTX_set_verify(3)|SSL_CTX_set_verify(3)>,
+L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>,
+L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>
+
+=cut

doc/ssl/SSL_CTX_set_verify.pod

@@ -59,14 +59,14 @@ The handshake will be continued regardless of the verification result.
 
 B<Server mode:> the server sends a client certificate request to the client.
 The certificate returned (if any) is checked. If the verification process
-fails as indicated by B<verify_callback>, the TLS/SSL handshake is
+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.
 
 B<Client mode:> the server certificate is verified. If the verification process
-fails as indicated by B<verify_callback>, the TLS/SSL handshake is
+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.
@@ -92,6 +92,15 @@ B<Client mode:> ignored
 Exactly one of the B<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
+L<SSL_CTX_set_cert_verify_callback(3)|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
@@ -278,6 +287,7 @@ L<SSL_CTX_get_verify_mode(3)|SSL_CTX_get_verify_mode(3)>,
 L<SSL_get_verify_result(3)|SSL_get_verify_result(3)>,
 L<SSL_CTX_load_verify_locations(3)|SSL_CTX_load_verify_locations(3)>,
 L<SSL_get_peer_certificate(3)|SSL_get_peer_certificate(3)>,
+L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>,
 L<SSL_get_ex_data_X509_STORE_CTX_idx(3)|SSL_get_ex_data_X509_STORE_CTX_idx(3)>,
 L<SSL_get_ex_new_index(3)|SSL_get_ex_new_index(3)>
 

doc/ssl/ssl.pod

@@ -299,7 +299,7 @@ protocol context defined in the B<SSL_CTX> structure.
 
 =item void B<SSL_CTX_set_cert_store>(SSL_CTX *ctx, X509_STORE *cs);
 
-=item void B<SSL_CTX_set_cert_verify_cb>(SSL_CTX *ctx, int (*cb)(SSL_CTX *), char *arg)
+=item void B<SSL_CTX_set_cert_verify_cb>(SSL_CTX *ctx, int (*cb)(), char *arg)
 
 =item int B<SSL_CTX_set_cipher_list>(SSL_CTX *ctx, char *str);
 
@@ -663,6 +663,7 @@ L<SSL_CTX_sess_set_cache_size(3)|SSL_CTX_sess_set_cache_size(3)>,
 L<SSL_CTX_sess_set_get_cb(3)|SSL_CTX_sess_set_get_cb(3)>,
 L<SSL_CTX_sessions(3)|SSL_CTX_sessions(3)>,
 L<SSL_CTX_set_cert_store(3)|SSL_CTX_set_cert_store(3)>,
+L<SSL_CTX_set_cert_verify_callback(3)|SSL_CTX_set_cert_verify_callback(3)>,
 L<SSL_CTX_set_cipher_list(3)|SSL_CTX_set_cipher_list(3)>,
 L<SSL_CTX_set_client_CA_list(3)|SSL_CTX_set_client_CA_list(3)>,
 L<SSL_CTX_set_default_passwd_cb(3)|SSL_CTX_set_default_passwd_cb(3)>,