TLSHandshake

TLSHandshake is the process by which a Transport Layer Security (TLS) connection is established between a client and a server. It negotiates the protocol version and cryptographic algorithms, authenticates endpoints, and creates a shared secret used to encrypt subsequent data.

The handshake typically starts with the client sending a ClientHello that lists the supported TLS versions, cipher suites, a random value, and extensions such as Server Name Indication (SNI). The server replies with a ServerHello selecting the version and cipher suite, sends its certificate chain to prove its identity, and may request a client certificate for mutual authentication. Depending on the chosen cipher suite, additional messages such as a ServerKeyExchange may be exchanged to carry key-exchange parameters. The client may then send a ClientKeyExchange, and both parties proceed to establish the shared secret.

Following the key exchange, both sides derive symmetric encryption and integrity keys from the negotiated material and exchange Finished messages to verify the successful authentication and key establishment. After this point, application data is transmitted under the negotiated security parameters. The protocol may also support session resumption via session identifiers or tickets to skip parts of the handshake on subsequent connections.

TLS 1.3 streamlines the handshake by reducing round trips and consolidating messages, often enabling faster connection setup and, with features like 0-RTT in resumed sessions, faster reuse of security credentials. Security considerations include ensuring valid certificate validation, using ephemeral keys for forward secrecy, and selecting strong, modern cipher suites to mitigate downgrade and MITM risks.