Signal Protocol

Cryptography · Axiom Academy

How billions of people send truly private messages every day The Private Messaging Challenge Imagine you want to send a private message to your friend. Simple, right? But here's the catch: This isn't a hypothetical problem. Every day, billions of people send messages through: All of these apps use the same underlying cryptographic protocol: The Signal Protocol , developed by Open Whisper Systems and now maintained by the Signal Foundation. But secure messaging isn't just about encrypting a single message. It needs to solve several hard problems simultaneously... Why Is Secure Messaging Different? Secure messaging has unique challenges that traditional encryption doesn't face: The Signal Protocol solves all of these problems using two main components: X3DH for initial key agreement and the Double Ratchet Algorithm for ongoing message encryption. X3DH: Extended Triple Diffie-Hellman Before Alice and Bob can exchange messages, they need to establish a shared secret key. X3DH accomplishes this with three types of keys: Identity Key (IK) - Long-term key that represents your identity. Changes very rarely. Signed Pre-Key (SPK) - Medium-term key signed by your identity key. Rotated periodically (e.g., weekly). One-Time Pre-Keys (OPK) - Single-use keys for extra security. Deleted after use to provide perfect forward secrecy. Here's how it works when Alice wants to message Bob for the first time:

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