TweLEX: A Tweaked Version of the LEX Stream Cipher

LEX is a stream cipher proposed by Alex Biryukov. It was selected to phase 3 of the eSTREAM competition. LEX is based on the Advanced Encryption Standard (AES) block cipher and uses a methodology called Leak Extraction, proposed by Biryukov himself. However Dunkelman and Keller show that a key recovery attack exists against LEX. Their attack requires 2 bytes of keystream produced by the same key and works with a time complexity of 2 operations. In this work we explore LEX further and have shown that under the assumption of a related key model we can obtain 24 secret state bytes with a time complexity of 2 and a data complexity of 2. Subsequently, we introduce a tweaked version of LEX, called TweLEX, which is shown to resist all known attacks against LEX. Though the throughput of TweLEX is half of LEX, it is still 1.25 times faster than AES, the underlying block cipher. This work attempts to revive the principle of leak extraction as a simple and elegant method to design stream ciphers.