"X3DH establishes a shared secret key between two parties who mutually authenticate each other based on public keys. X3DH provides forward secrecy and cryptographic deniability.
X3DH is designed for asynchronous settings where one user ("Bob") is offline but has published some information to a server. Another user ("Alice") wants to use that information to send encrypted data to Bob, and also establish a shared secret key for future communication."
This algorithm is recommended for implementing the Double Ratchet algorithm (E2EE) initialization.
Note
There's a rust library for x3dh, however, the library use the NIST P-256 elliptic curve instead of Curve25519 or Curve448 according to Signal recommendations.
The use of another curve than the one expected lead to different key size, which make the implementation of the Double Ratchet algorithm using the recommended curve impossible. That's why I decided to implemented my own X3DH using the Curve25519 for learning purpose.
| Parameters | |
|---|---|
| curve | Curve25519 |
| hash | SHA-512 |
| info | RedWheelbarrow |
The algorithm is well described on Signal.
However, I have tried to draw a diagram to make it easier to understand the protocol.
