code: def calculate_u(big_a, big_b):
      
      u_hex_hash = hex_hash(pad_hex(big_a) + pad_hex(big_b))
      return hex_to_long(u_hex_hash)
docs: Calculate the client's value U which is the hash of A and B
      :param {Long integer} big_a Large A value.
      :param {Long integer} big_b Server B value.
      :return {Long integer} Computed U value.