Custom verification of peer certificates in C programs

Every TLS connection is established between two peers: the client and the server. Each peer can request and verify the other peer’s certificate. In real life, the server certificate is almost always verified during the TLS handshake. Before TLS 1.3, the TLS protocol supported anonymous ciphers, which allowed the server to operate without a certificate. In practice, those anonymous ciphers were rarely used and were forbidden by default. So, in practice, a certificate for TLS has always been required. On the contrary, TLS client certificates are seldom used. However, verifying a client certificate in an application using OpenSSL is very similar to verifying a server certificate. Therefore, it makes sense to talk about peer certificate verification instead of limiting ourselves to server certificate verification. We will verify the server certificate in most of our code examples. More information about verifying client certificates...