On a high level, the Signal Protocol is a security library on steroids. Despite its novelty and growing importance, there has been few formal analyses of this protocol, whilst it has been a driving force in the world of cybersecurity. So, what makes its End-to-End Encryption (E2EE) so powerful?
The Signal Protocol amalgamates the Extended Triple Diffie-Hellman (X3DH) key agreement protocol, Double Ratchet algorithm, pre-keys, and uses Curve25519, AES-256, and HMAC-SHA256 as cryptographic primitives. These are all well-established, low-level cryptographic algorithms that are frequently used to build computer security systems.
Looking for more details? Check out the research paper here.