code: def calculate_a(self):
          
          big_a = pow(self.g, self.small_a_value, self.big_n)
          # safety check
          if (big_a % self.big_n) == 0:
              raise ValueError('Safety check for A failed')
          return big_a
docs: Calculate the client's public value A = g^a%N
          with the generated random number a
          :param {Long integer} a Randomly generated small A.
          :return {Long integer} Computed large A.