Key code correlation security

A transponder type key security system having a plurality of authorized keys efficiently correlates a key code transmitted by a key-based transponder unit with codes stored in an interrogator unit (100) while inhibiting reception interference by multiple signals transmitted by a plurality of key-based transponder units (200). The interrogator unit (100) transmits a general request for key-based transponder units (200) having pre-stored key codes allocated thereto to respond by giving their respective key codes. A transponder unit (200), upon receiving the request, transmits its key code to the interrogator unit (100). The interrogator unit (100) compares a received key code with key codes previously stored to determine if the transponder unit (200) responding is authorized or not. In the event that multiple transponder units (200) respond to the request, the interrogator unit (100) transmits a key-specific request so that only the targeted key-based transponder unit (200) will respond. This prevents multiple transponder requests from interfering with one another. Usually only a single key is present and there is no need to transmit key-specific requests. Thus, the number of key code requests to be transmitted is reduced and a determination of whether a key is authorized or not can be carried out efficiently.