def fixed_length_secure_compare(a, b)
      raise ArgumentError, "string length mismatch." unless a.bytesize == b.bytesize

      l = a.unpack "C#{a.bytesize}"

      res = 0
      b.each_byte { |byte| res |= byte ^ l.shift }
      res == 0
    end
def fixed_length_secure_compare(a, b)
      raise ArgumentError, "string length mismatch." unless a.bytesize == b.bytesize

      l = a.unpack "C#{a.bytesize}"

      res = 0
      b.each_byte { |byte| res |= byte ^ l.shift }
      res == 0
    end
Constant time string comparison, for fixed length strings.

 The values compared should be of fixed length, such as strings
 that have already been processed by HMAC. Raises in case of length mismatch.