It would be an interesting curiosity, probably not practically useful, to create a locking keypad that does challenge-response. Along with a regular 10-digit keypad, it also has a display for a (say) 5 digit number. We might have the display polarized or hooded so only the person operating the lock can read it. The display displays a nonce, a "challenge". The user uses his private secret to calculate the response to the challenge and types it into the keypad. The door checks the answer using the user's public key.
If the lock is reverse engineered, you still cannot learn the user's private key.
This would be a great idea if only people could do modular exponentiation in their head.
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