=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.
+When a peer certificate has been received during a SSL/TLS handshake,
+a verification function is called regardless of the verification mode.
+If the application does not explicitly specify a verification callback function,
+the built-in verification function is used.
If a verification callback I<callback> is specified via
SSL_CTX_set_cert_verify_callback(), the supplied callback function is called
-instead. By setting I<callback> to NULL, the default behaviour is restored.
-
-When the verification must be performed, I<callback> will be called with
-the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg). The
-argument I<arg> is specified by the application when setting I<callback>.
-
-I<callback> should return 1 to indicate verification success and 0 to
-indicate verification failure. If SSL_VERIFY_PEER is set and I<callback>
-returns 0, the handshake will fail.
-
-In client mode I<callback> may also return -1,
-typically on failure verifying the server certificate.
-This makes the handshake suspend and return control to the calling application
-with B<SSL_ERROR_WANT_RETRY_VERIFY>.
-The app can for instance fetch further certificates or cert status information
-needed for the verification.
-Calling L<SSL_connect(3)> again resumes the connection attempt
-by retrying the server certificate verification step.
+instead with the arguments callback(X509_STORE_CTX *x509_store_ctx, void *arg).
+The argument I<arg> is specified by the application when setting I<callback>.
+By setting I<callback> to NULL, the default behaviour is restored.
+
+I<callback> should return 1 to indicate verification success
+and 0 to indicate verification failure.
+In server mode, a return value of 0 leads to handshake failure.
+In client mode, the behaviour is as follows.
+All values, including 0, are ignored
+if the verification mode is B<SSL_VERIFY_NONE>.
+Otherwise, when the return value is less than or equal to 0, the handshake will
+fail.
+
+In client mode I<callback> may also call the L<SSL_set_retry_verify(3)>
+function on the B<SSL> object set in the I<x509_store_ctx> ex data (see
+L<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 B<SSL_ERROR_WANT_RETRY_VERIFY>. The app can for
+instance fetch further certificates or cert status information needed for
+the verification. Calling L<SSL_connect(3)> again resumes the connection
+attempt by retrying the server certificate verification step.
This process may even be repeated if need be.
-As the verification procedure may
-allow the connection to continue in the case of failure (by always
-returning 1) the verification result must be set in any case using the
-B<error> member of I<x509_store_ctx> so that the calling application
-will be informed about the detailed result of the verification procedure!
+In any case a viable verification result value must be reflected
+in the B<error> member of I<x509_store_ctx>,
+which can be done using L<X509_STORE_CTX_set_error(3)>.
+This is particularly important in case
+the I<callback> allows the connection to continue (by returning 1).
+Note that the verification status in the store context is a possibly durable
+indication of the chain's validity!
+This gets recorded in the SSL session (and thus also in session tickets)
+and the validity of the originally presented chain is then visible
+on resumption, even though no chain is presented int that case.
+Moreover, the calling application will be informed about the detailed result of
+the verification procedure and may elect to base further decisions on it.
Within I<x509_store_ctx>, I<callback> has access to the I<verify_callback>
function set using L<SSL_CTX_set_verify(3)>.
=head1 SEE ALSO
L<ssl(7)>, L<SSL_CTX_set_verify(3)>,
+L<X509_STORE_CTX_set_error(3)>,
L<SSL_get_verify_result(3)>,
+L<SSL_set_retry_verify(3)>,
L<SSL_CTX_load_verify_locations(3)>
=head1 COPYRIGHT
-Copyright 2001-2018 The OpenSSL Project Authors. All Rights Reserved.
+Copyright 2001-2022 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
projects / openssl.git / blobdiff