Suppose Alice starts a conversation while Bob is offline. A one-time prekey lets Bob prepare for that moment: it is a public, ephemeral Diffie-Hellman key uploaded in advance and used once. section 10.4.2
The mechanism follows a small lifecycle:
- Bob generates multiple ephemeral key pairs and uploads their public parts to a central server. section 10.4.2
- The selected public key is deleted after use, helping add forward secrecy even when Bob is offline. section 10.4.2
When the supply runs out
One-time prekeys can be depleted. A signed prekey then provides a fallback with forward secrecy limited to the last signed-prekey change, so fresh one-time prekeys must be uploaded. section 10.4.2
