Flexible block ciphers: Modifying LEX

In this paper, we review LEX, a stream cipher that harnesses the strength of the Advanced Encryption Standard (AES) block cipher to produce ciphertext at 2.5 times the speed of the AES. Both AES and LEX have been attacked due to the way that they convert master keys into round keys or state. We consider ways in which to remove the vulnerabilities of LEX without diminishing its throughput. We propose FLEX, a more secure version of LEX that produces keystream at 2.4 times the speed of the underlying block cipher.