A method and apparatus for encrypting a Mode-S extended squitter exploiting a provided key. The key includes a control key segment, an address key segment, a first message key segment, and a first parity/identity key segment. Adding modulo two the control segment to the control key segment produces an intermediate control segment. Adding modulo two the address segment to the address key segment produces an intermediate address segment. Adding modulo two the message segment to the first message key segment produces an intermediate message segment. Adding modulo two the parity/identity segment to the first parity/identity key segment produces an first intermediate parity/identity segment. Concatenating the intermediate control segment, the intermediate address segment, the intermediate message segment, and the intermediate parity/identity segment produces an intermediate extended squitter. Shifting the intermediate extended squitter according to a linear function shift register produces a shifted-intermediate parity/identity segment.