2 Signal Protocol

Signal Protocol is the encryption protocol that Signal uses to provide end-to-end encryption. It combines three subprotocols: the Double Ratchet Algorithm, the X3DH Key Agreement Protocol, and the Prekey Server System. These subprotocols allow Signal to generate and exchange encryption keys dynamically, prevent replay attacks, and enable asynchronous communication.

3 TLS extensions

Signal uses several TLS extensions to enhance its encryption protocol. For example, it uses the Session Tickets extension to resume previous sessions without performing a full handshake, which reduces latency and bandwidth. It also uses the Supported Groups extension to negotiate the elliptic curve groups for key exchange, which increases security and efficiency. Additionally, it uses the Encrypted Client Hello extension to protect the client's identity and preferences from network observers.

4 TLS features

Signal also leverages some TLS features to improve its encryption protocol. For instance, it uses the 0-RTT feature to send application data in the first message of the handshake, which reduces latency and improves user experience. It also uses the Post-Handshake Authentication feature to authenticate the client after the handshake, which allows for more flexibility and privacy. Furthermore, it uses the TLS False Start feature to send application data before receiving the final confirmation from the server, which reduces latency and increases throughput.

5 TLS 1.3

Signal supports the latest version of TLS, which is TLS 1.3. This version offers several advantages over previous versions, such as faster and simpler handshakes, stronger encryption algorithms, better forward secrecy, and more robust security guarantees. Signal also implements some of the draft specifications of TLS 1.3, such as the Delegated Credentials extension, which allows the server to delegate its authentication to a third party, and the External PSK Importer extension, which allows the client and the server to use pre-shared keys from other protocols.

6 TLS best practices

Signal follows some of the best practices for implementing TLS, such as using strong ciphers, enabling certificate transparency, disabling weak protocols and extensions, and updating the software regularly. Signal also monitors the security and performance of its TLS implementation using various tools and metrics, such as Certificate Transparency logs, Qualys SSL Labs, and Signal TLS Proxy. By doing so, Signal ensures that its encryption protocol is reliable, secure, and efficient.

7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?