Encryption
Safety numbers, explained
End-to-end encryption guarantees that only the holders of the keys can read a conversation. It does not, on its own, guarantee that the keys belong to the person you think they do. If the service that distributes public keys hands you the wrong one, the encryption works perfectly and protects a conversation with an impostor. This is the machine-in-the-middle problem, and the only complete answer to it is verification outside the system.
How Enclessa uses it
Enclessa computes a safety number for each end-to-end encrypted conversation and shows it to both sides, and it pins the other person’s user key on first contact. A key that changes afterwards blocks the conversation with a warning rather than quietly re-establishing it, which is the behaviour the architecture requires rather than an option an administrator can switch off.