Function answer_challenge 

pub fn answer_challenge(
    challenge: &Challenge,
    responder_did: &str,
    key: &SigningKey,
) -> ChallengeResponse

Sign a challenge, producing a ChallengeResponse the initiator can verify.

Signs signed_message(challenge.nonce, responder_did) with key and embeds the corresponding public key so the verifier needs nothing but the response and the expected DID.