International Association
for Cryptologic Research

Scan chains, a design for testability (DFT)

feature, are included in most modern-day ICs. But, it

opens a side channel for attacking cryptographic chips.

We propose a methodology by which we can recover

internal states of any stream cipher using scan chains

without knowledge of its design. We consider conven-

tional scan-chain design which is normally not scram-

bled or protected in any other way. In this scenario

the challenge of the adversary is to obtain the corre-

spondence of output of the scan chain and the internal

state registers of the stream cipher. We present a math-

ematical model of the attack and the correspondence

between the scan chain-outputs and the internal state

bits have been proved under this model. We propose an

algorithm that through o-line and on-line simulation

forms bijection between the above mentioned sets and

thus nds the required correspondence. We also give an

estimate of the number of o-line simulations necessary

for nding the correspondence.

The proposed strategy is successfully applied to eS-

tream hardware based nalists MICKEY-128 2.0, Triv-

ium and Grain-128. To the best of our knowledge, this is

the rst scan based attack against full round Grain-128

and only the fourth reported cryptanalysis. This attack

on Trivium is better than that of the published scan-

attack on Trivium. This scan-based attack is also the

rst reported scan based cryptanalysis against MICKEY-

128 2.0.