The client-side cert verification callback function may not only return
as usual for success or 0 for failure, but also -1,
typically on failure verifying the server certificate.
This makes the handshake suspend and return control to the calling application
with SSL_ERROR_WANT_RETRY_VERIFY.
The app can for instance fetch further certificates or cert status information
needed for the verification.
Calling SSL_connect() again resumes the connection attempt
by retrying the server certificate verification step.
This process may even be repeated if need be.
The core implementation of the feature is in ssl/statem/statem_clnt.c,
splitting tls_process_server_certificate() into a preparation step
that just copies the certificates received from the server to s->session->peer_chain
(rather than having them in a local variable at first) and returns to the state machine,
and a post-processing step in tls_post_process_server_certificate() that can be repeated:
Try verifying the current contents of s->session->peer_chain basically as before,
but give the verification callback function the chance to pause connecting and
make the TLS state machine later call tls_post_process_server_certificate() again.
Otherwise processing continues as usual.
The documentation of the new feature is added to SSL_CTX_set_cert_verify_callback.pod
and SSL_want.pod.
This adds two tests:
* A generic test in test/helpers/handshake.c
on the usability of the new server cert verification retry feature.
It is triggered via test/ssl-tests/03-custom_verify.cnf.in (while the bulky auto-
generated changes to test/ssl-tests/03-custom_verify.cnf can be basically ignored).
* A test in test/sslapitest.c that demonstrates the effectiveness of the approach
for augmenting the cert chain provided by the server in between SSL_connect() calls.
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13906)
The default is openssl.cnf The project seems to prefer xxx.conf these
days, but we should use the default convention.
Rename all foo.conf (except for Configurations) to foo.cnf
Fixes#11174
Reviewed-by: Paul Yang <kaishen.yy@antfin.com>
Reviewed-by: Tomas Mraz <tmraz@fedoraproject.org>
(Merged from https://github.com/openssl/openssl/pull/11176)
