Signal Protocol

Signal Protocol. It is an end-to-end encryption protocol designed to provide strong security for instant messaging and voice calls. Developed by Open Whisper Systems, it combines the Double Ratchet Algorithm, prekeys, and a 3-DH handshake to ensure forward secrecy and future secrecy. The protocol forms the cryptographic core of the Signal application and has been widely adopted by other major platforms.

Overview

The protocol was created to address security shortcomings in earlier systems like Off-the-Record Messaging and protocols used by Silent Circle. Its primary innovation is the Double Ratchet Algorithm, which independently updates encryption keys for each message. This design ensures that compromise of a single key does not affect the security of past or future communications. The system utilizes a central server to facilitate the initial exchange of public keys and prekeys between users, but all message content remains encrypted and inaccessible to the server itself.

Technical details

The technical foundation integrates several advanced cryptographic constructs. The initial key agreement is performed using an extended Diffie-Hellman handshake, often referred to as the X3DH protocol. This process combines long-term identity keys, medium-term signed prekeys, and one-time prekeys to establish a shared secret. Following this setup, the Double Ratchet Algorithm takes over, combining a symmetric-key ratchet for per-message encryption and an asymmetric ratchet for updating key material. Encryption of message payloads typically uses AES-256 in Galois/Counter Mode and authentication via HMAC-based key derivation functions.

Security properties

The protocol is renowned for providing strong, practical security guarantees. It achieves forward secrecy by ensuring that session keys are constantly updated, so past messages cannot be decrypted if long-term keys are compromised. It also provides future secrecy, or post-compromise security, meaning the protocol can heal from a key compromise after a few exchanged messages. The design prevents mass decryption by network observers and resists sophisticated attacks, including those from powerful adversaries like the National Security Agency. Its security model has been formally analyzed in academic papers presented at conferences like IEEE Symposium on Security and Privacy.

Implementation and adoption

The reference implementation is open-source, primarily written in Java and C++, and maintained by the Signal Foundation. The most famous implementation is within the Signal application for Android and iOS. Notably, the protocol has been integrated into several major communication platforms, including WhatsApp, Facebook Messenger's Secret Conversations, and Google Messages for RCS. This widespread adoption by Meta and Google has made it one of the most widely deployed encryption systems globally, protecting billions of conversations.

History and development

Early work began with Moxie Marlinspike and Trevor Perrin at Open Whisper Systems, building upon concepts from the Axolotl (protocol) and critiques of Off-the-Record Messaging. The first version was deployed in the TextSecure application in 2013. A significant evolution occurred with the formal specification of the Double Ratchet Algorithm in 2016. Following the creation of the Signal Foundation in 2018 with initial funding from Brian Acton, development and maintenance continued independently. The protocol's design has influenced standards at the Internet Engineering Task Force and remains a benchmark for private communication. Category:Cryptographic protocols Category:Computer security Category:Free software