HTTPS: the TLS 1.3 handshake

How a browser and server agree on secret keys over a public network, and how the certificate stops impostors.

The idea

Both sides contribute a key share. From the two shares each side computes the same secret without ever sending it. The certificate, signed by a trusted certificate authority, proves the server owns the domain, so a man in the middle cannot pretend to be it.

Code

// What the client checks, in order (simplified)
void Handshake()
{
    SendClientHello(supportedCiphers, keyShare);
    var (serverShare, cert) = ReceiveServerHello();
    if (!cert.IsSignedByTrustedCA() || cert.Host != host)
        throw new AuthenticationException();
    var keys = DeriveKeys(myPrivate, serverShare);
    SendEncrypted("GET / HTTP/1.1");
}

Tips

• TLS gives confidentiality (encrypted), integrity (tamper-proof) and server authentication.
• HTTPS does not make a site safe from XSS or SQL injection; it protects data in transit only.
• HSTS tells browsers to always use HTTPS for a site, closing the downgrade gap.

In the .NET library

In ASP.NET Core, UseHttpsRedirection and UseHsts enforce HTTPS; certificates are usually terminated at a reverse proxy or load balancer.