Function challenge_response 

pub fn challenge_response(
    approvals: &[CommandApprovalPayload],
    identity_provider: &impl IdentityProvider,
    request: &IdentityAuthedRequestPayload,
    quorum_size: usize,
    verified_parties: &[VerifyingKey],
) -> Result<QuorumApprovedChallengeResponsePayload, QuorumApprovedRequestError>

Given a list of command approval payloads, an identity provider, a quorum approved request initialization payload, a quorum size and a list of verifying keys for the other parties, returns an ok result with a quorum approved challenge response payload or an appropriate error result for an invalid request.