title: OpenSSL 1.1.1d - Async use of SSL_CTX_set_cert_verify_callback?
body: <p>Certificate validation in general requires asynchronous operations such as: OCSP/CRL fetch.</p>
  
  <p>On the other hand, the callback from <code>SSL_CTX_set_cert_verify_callback</code> expects a synchronous result:
  1 (success) / 0 (failure). (<a href="https://www.openssl.org/docs/man1.1.1/man3/SSL_CTX_set_cert_verify_callback.html" rel="nofollow noreferrer">reference</a>).</p>
  
  <p>I find it kind of odd that any validation of this type is expected to be synchronous.</p>
  
  <p>Is there an alternative API for certificate chain validation that supports asynchronous operations?</p>