Function respond 

pub fn respond(
    challenge: &Challenge,
    credentials: &Credentials,
    method: &str,
    uri: &str,
    nonce_count: u32,
    cnonce: &str,
) -> String

Answer a challenge.

uri is the Request-URI of the request being authorized — not the user’s URI. They differ for REGISTER, which is the first request anyone sends, so getting it wrong fails immediately and confusingly.