Differential Attacks against the Helix Stream Cipher

In this paper, we analyze the security of the stream cipher Helix, recently proposed at FSE’03. Helix is a high-speed asynchronous stream cipher, with a built-in MAC functionality. We analyze the differential properties of its keystream generator and describe two new attacks. The first attack requires 2 basic operations and processes only 2 words of chosen plaintext in order to recover the secret key for any length up to 256 bits. However, it assumes the attacker can force nonces to be used twice. Our second attack relies on weaker assumptions. It is a distinguishing attack that detects internal state collisions after 2 words of chosen plaintext.