AES permits the use of 256 bit keys. A 256 bit key requires not merely twice as long to crack as a 128 bit key, but rather 2128 times as long. If a device could be built that would crack a billion billion (1018) AES keys per second, it would require 3,671,743,063,080,802,746,815,416,825,491,118,336,290,905,145,409,708 years to exhaust the 256 bit key space.