Question

When using OpenSSL with non blocking sockets its convenient to use the transparent negotiation mode where the negotiation is initiated by calling SSL_write on a not-yet-securely-connected SSL context, and then handling the resulting error (usually SSL_WANT_READ) to read a handshake packet from the output BIO and send it over the transport.

However, without the explicit (blocking) call to SSL_do_handshake its not clear to me when to try to validate the certificate.

Do I just have to do a certificate check as soon as SSL_write returns success for the first time? Or is there a better signal?

Answer 1

Before you initiate the negotiation, you should use SSL_set_verify() to set the verification mode and optionally a verification callback. Handle any additional application-specific validation you need in the callback.

Then, if SSL_write() and/or SSL_read() return successfully, you know that the certificate has been validated.