Prekeys

A prekey in cryptography is a public key that is generated in advance and published so that others can initiate a secure session with the key owner even if the owner is offline. The corresponding private key remains with the owner and is used later to complete the key exchange.

In modern messaging protocols, a prekey is part of a bundle that a user uploads to a server. A typical bundle includes the user’s identity key (a long-term public key), a signed prekey (a long-lived but periodically rotated key with a signature from the identity key), and a set of one-time prekeys (short-lived keys used once). The server stores these public keys and serves them to someone who wants to start a conversation.

To initiate a session, the sender downloads the recipient’s prekey bundle and performs a key exchange using its own ephemeral key and the recipient’s prekeys. This yields a shared secret that establishes a secure session with forward secrecy. The one-time prekeys are consumed when used, while the identity key and signed prekey remain for future sessions. This process lets messages be delivered and decrypted asynchronously, even if the recipient is not online when the session is started.

Security and lifecycle considerations include generating prekeys on the device, rotating keys to limit exposure, and ensuring private keys stay on the device. The server generally stores only public keys to reduce risk. Prekeys underpin asynchronous, end-to-end encrypted communication and are central to protocols such as the Signal Protocol.