Recipient's published PreKeyBundle (SPK signature will be re-verified inside initiateX3DH).
Initiator's Ed25519 identity key pair.
Application context salt (e.g. groupId, recordingId). MUST be identical for wrap and unwrap.
Purpose string, e.g. 'me2em/crypto/v1/group-key'.
The 32-byte key to wrap (e.g. a group key or file key).