Yet Another Attack on Vest

We present a new side-channel attack against VEST, a set of four stream ciphers which reached the second phase of the eSTREAM project (the European stream cipher project). The proposed attacks target the counter part of the ciphers, composed of 16 short-length nonlinear feedback shift registers (NLFSR) independently updated. Our aim is to retrieve the whole initial state of the counter (163 to 173 bits) which is a part of the keyed state. The first attack is directly adapted from previous works on differential side-channel attacks. The second attack is brand new. It involves a unique measurement thus it can be seen as a simple side-channel attack. However, it requires some signal processing so we call it Refined Simple Power Analysis. As we expect full recovery of the initial state with minimal complexity, one should carefully consider implementing any VEST cipher in an embedded device.