TLS

TLS is the encryption layer that puts the “S” in HTTPS: it makes sure nobody between you and the server can read or tamper with what you send, and that the server really is who it claims to be. At the start of a connection, the two sides do a key exchange (usually Diffie–Hellman) that lets them agree on a shared secret even though anyone can watch the conversation. The server also presents a certificate, a signed statement that this key belongs to this domain, which your browser checks back through a chain of signatures to an authority it already trusts. After that handshake, everything is encrypted with the shared key, which is fast.