const func bigInteger: modPow (in var bigInteger: base,
    in var bigInteger: exponent, in bigInteger: modulus) is func
  result
    var bigInteger: power is 1_;
  begin
    if exponent < 0_ or modulus < 0_ then
      raise RANGE_ERROR;
    else
      while exponent > 0_ do
        if odd(exponent) then
          power := (power * base) mod modulus;
        end if;
        exponent >>:= 1;
        base := base ** 2 mod modulus;
      end while;
    end if;
  end func;