ChangeCipherSpec

ChangeCipherSpec is a protocol message used in the TLS and DTLS handshakes to signal that subsequent records will be protected under the newly negotiated cryptographic parameters. It plays a key role in transitioning from the unauthenticated or partially authenticated handshake phase to the encrypted application data phase. In TLS versions up to 1.2, it is an explicit step in the handshake; TLS 1.3 does not use ChangeCipherSpec, as its handshake and key schedule are designed differently and do not rely on a separate changeover message.

Format and sequencing: The ChangeCipherSpec message is carried as a TLS/DTLS record with the content type ChangeCipherSpec (the numeric value 20). The payload is a single byte with the value 1. It is transmitted after the parties have agreed on a cipher suite and generated the new keys. In a typical TLS 1.0–1.2 handshake, the client sends ChangeCipherSpec followed by its Finished message, and the server responds with its own ChangeCipherSpec followed by its Finished message. Once both sides have processed these messages, subsequent records are encrypted and integrity-protected using the negotiated parameters.

Operational considerations: Correct sequencing is essential; if ChangeCipherSpec messages are lost, reordered, or not processed in time, peers may fail to decrypt subsequent records. Implementations rely on strict state management to ensure the transition occurs coherently and to detect any handshake abnormalities or renegotiation attempts.

See also: Transport Layer Security, Datagram Transport Layer Security, Finished message, Record layer, Cipher suite, TLS 1.3.