Aspects of the disclosure are directed to detecting interactions with signals, such as by an attacker attempting to gain access to a vehicle. Signal waveforms used for authentication are evaluated, for communications between respective circuits. Possible interaction by a third circuit is analyzed by detecting variations in characteristics of a leading portion of a data symbol relative to known characteristics of the leading portion of the data signal. A condition indicative of whether the signal waveform has been interacted with and retransmitted is determined, based on the detected variations. For instance, if the variations are indicative of a known type of variation induced by interaction and retransmission, such interaction and transmission can be detected. Where the determined condition is not deemed an attack, an output signal that provides vehicle access is generated based on the determined condition.