double-ratchet E2E encryption using AES-256-GCM cipher with X3DH key agreement using 2 ephemeral Curve448 keys to derive secrets for ratchet initialization. These keys and secrets are separate for each contact, group membership and file transfer

additional encryption of messages delivered from servers to recipients, also using Curve25519 DH exchange and NaCl crypto-box - to avoid shared cipher-text in sent and received traffic