This string holds the pre-shared secret used either directly as the encryption key or to authenticate a Diffie-Hellman key exchange and prevent man-in-the-middle attacks. Admins must configure matching values on both ends of the encrypted uplink, since a mismatch prevents the encryption state machine from reaching the active state securely. For example, an admin enters the same pre-shared passphrase on both transponders at either end of the link before enabling encryption.