In Digital certificate authentication, which key does the supplicant use to encrypt the challenge message before sending it to the verifier?

Supplicant's Private Key

Explanation:

The supplicant uses its PRIVATE KEY to encrypt the challenge message before sending it to the verifier.

This is evident in that in a Digital Certificate Authentication, concerning challenge-response, when the verifier sends a challenge message to the supplicant, the supplicant would encrypt the challenge message with its PRIVATE KEY and send the response message back to the verifier. In return, the verifier would decrypt the response message with the true party’s public key.

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i believe the above answer to be correct;

i think the answer is b.