Double Ratchet Algorithm

The algorithm that gives Signal, WhatsApp, and Matrix end-to-end encryption — where compromising one message key reveals nothing about past or future messages.

Key Takeaways

1. The Double Ratchet algorithm provides forward and backward secrecy for messaging — compromising one message key doesn't reveal past or future messages
2. Two ratchets operate together — a Diffie-Hellman ratchet generates new shared secrets, a symmetric ratchet derives per-message keys
3. Each message uses a unique encryption key — even if an attacker captures one key, only that single message is compromised
4. Signal, WhatsApp, and Matrix all use the Double Ratchet — it's the standard for end-to-end encrypted messaging