Function client_final_message 

pub fn client_final_message(
    password: &str,
    client_nonce: &str,
    server_first_b64: &str,
    username: &str,
) -> Result<(String, Vec<u8>), AuthError>

Client-side: Process server-first-message, produce client-final-message.

username is the same value originally passed to client_first_message. password is the user’s plaintext password. client_nonce is the nonce returned by client_first_message. server_first_b64 is the base64 server-first-message data received from the server.

Returns (client_final_data_base64, expected_server_signature).