pub const ACK_STATUS_UNDECRYPTABLE: &str = "undecryptable";Expand description
ACK_STATUS_KEY value: the recipient can never decrypt the frame it
names (a spent ratchet generation, a dead epoch, or the pending-decryption
queue gave up on it). The sender resends the message once; it is
never message_delivered. Still a delivery ACK in shape, so a relay holding
the frame drops it.