The Double Ratchet is a procedure that uses the shared key generated by the X3DH protocol to create and update new keys used for encrypting and decrypting messages.

Linphone's message encryption (LIME) adopts the X3DH and Double Ratchet protocols, which were published by Signal. X3DH is a procedure for generating a shared key based on ECDH, while the Double Ratchet uses the shared key generated by the X3DH protocol to generate and update new keys for encrypting and decrypting messages. Here, I will first summarize X3DH and ECDH.