An identification friend or foe (IFF) transponder includes: an electronic receiver to receive encoded challenges from an IFF interrogator; a processing circuit including a decode circuit to decode the received challenges into encrypted challenges using a time of day (TOD) and/or a cryptographic key, a decrypt circuit to decrypt the encrypted challenges into challenges using the TOD and/or the key, a reply circuit to generate replies to the challenges, and an encode circuit to encode the generated replies using the TOD and/or the key; an electronic transmitter to transmit the encoded replies to the IFF interrogator; and an alert circuit to detect the encrypted challenges from the received challenges without using the TOD or the key, use the detected challenges to determine if the TOD is incorrect and/or the key is wrong, and issue an alert in response to determining the TOD is incorrect and/or the key is wrong.
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