--- /dev/null
+=pod
+
+=head1 NAME
+
+SSL_CTX_set_verify, SSL_set_verify, SSL_CTX_set_verify_depth, SSL_set_verify_depth - set peer certificate verification parameters
+
+=head1 SYNOPSIS
+
+ #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);
+
+=head1 DESCRIPTION
+
+SSL_CTX_set_verify() sets the verification flags for B<ctx> to be B<mode> and
+specifies the B<verify_callback> function to be used. If no callback function
+shall be specified, the NULL pointer can be used for B<verify_callback>.
+
+SSL_set_verify() sets the verification flags for B<ssl> to be B<mode> and
+specifies the B<verify_callback> function to be used. If no callback function
+shall be specified, the NULL pointer can be used for B<verify_callback>. In
+this case last B<verify_callback> set specifically for this B<ssl> remains. If
+no special B<callback> was set before, the default callback for the underlying
+B<ctx> is used, that was valid at the the time B<ssl> was created with
+L<SSL_new(3)|SSL_new(3)>.
+
+SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
+verification that shall be allowed for B<ctx>. (See the BUGS section.)
+
+SSL_set_verify_depth() sets the maximum B<depth> for the certificate chain
+verification that shall be allowed for B<ssl>. (See the BUGS section.)
+
+=head1 NOTES
+
+The verification of certificates can be controlled by a set of logically
+or'ed B<mode> flags:
+
+=over 4
+
+=item SSL_VERIFY_NONE
+
+B<Server mode:> the server will not send a client certificate request to the
+client, so the client will not send a certificate.
+
+B<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 L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> function.
+The handshake will be continued regardless of the verification result.
+
+=item SSL_VERIFY_PEER
+
+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
+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
+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.
+
+=item SSL_VERIFY_FAIL_IF_NO_PEER_CERT
+
+B<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.
+
+B<Client mode:> ignored
+
+=item SSL_VERIFY_CLIENT_ONCE
+
+B<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.
+
+B<Client mode:> ignored
+
+=back
+
+Exactly one of the B<mode> flags SSL_VERIFY_NONE and SSL_VERIFY_PEER must be
+set at any time.
+
+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
+above the limit are ignored. Error messages are generated as if these
+certificates would not be present, most likely a
+X509_V_ERR_UNABLE_TO_GET_ISSUER_CERT_LOCALLY 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, and 2. The default depth limit is 9,
+allowing for the peer certificate and additional 9 CA certificates.
+
+The B<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: B<preverify_ok> indicates, whether the verification of
+the certificate in question was passed (preverify_ok=1) or not
+(preverify_ok=0). B<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 B<x509_ctx>
+and B<verify_callback> is called with B<preverify_ok>=0. By applying
+X509_CTX_store_* functions B<verify_callback> can locate the certificate
+in question and perform additional steps (see EXAMPLES). If no error is
+found for a certificate, B<verify_callback> is called with B<preverify_ok>=1
+before advancing to the next level.
+
+The return value of B<verify_callback> controls the strategy of the further
+verification process. If B<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 B<verify_callback> returns 1,
+the verification process is continued. If B<verify_callback> always returns
+1, the TLS/SSL handshake will never be terminated because of this application
+experiencing a verification failure. The calling process can however
+retrieve the error code of the last verification error using
+L<SSL_get_verify_result(3)|SSL_get_verify_result(3)> or by maintaining its
+own error storage managed by B<verify_callback>.
+
+If no B<verify_callback> is specified, the default callback will be used.
+Its return value is identical to B<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.
+
+=head1 BUGS
+
+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 incomplete certificate chain and not
+X509_V_ERR_CERT_CHAIN_TOO_LONG as may be expected.
+
+=head1 RETURN VALUES
+
+The SSL*_set_verify*() functions do not provide diagnostic information.
+
+=head1 EXAMPLES
+
+The following code sequence realizes an example B<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
+certificates.
+
+ ...
+ int verbose_mode;
+ int verify_depth;
+ int always_continue;
+ ...
+ static int verify_callback(int preverify_ok, X509_STORE_CTX *ctx)
+ {
+ char buf[256];
+ X509 *err_cert;
+ int err, depth;
+
+ err_cert = X509_STORE_CTX_get_current_cert(ctx);
+ err = X509_STORE_CTX_get_error(ctx);
+ depth = X509_STORE_CTX_get_error_depth(ctx);
+
+ 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_TO_LONG error would not
+ * be found explicitly; only errors introduced by cutting off the
+ * additional certificates would be logged.
+ */
+ if (depth > 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 (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 (always_continue)
+ return 1;
+ else
+ return preverify_ok;
+ }
+ ...
+ 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);
+
+ ...
+ 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 */
+ }
+ }
+
+=head1 SEE ALSO
+
+L<ssl(3)|ssl(3)>, L<SSL_new(3)|SSL_new(3)>,
+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)>
+
+=cut
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