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X3DH: key agreement while offline
Simulate Bob publishing a key bundle and going offline, then Alice computing a shared secret from it anyway — the mechanism behind Signal's first-message problem.
What's happening under the hood →·Read the End-to-end encrypted messaging: the Signal Protocol module for the full explanation →
1. Bob publishes a key bundle, then goes offline
Three real ECDH (P-256) key pairs: an identity key, a signed prekey, and a one-time prekey.
2. Alice comes online and computes a shared secret — Bob still isn't here
3. Bob comes back online