  The implementation of cryptographic primitives in integrated circuits (ICs)
continues to increase over the years due to the recent advancement of
semiconductor manufacturing and reduction of cost per transistors. The hardware
implementation makes cryptographic operations faster and more energy-efficient.
However, various hardware attacks have been proposed aiming to extract the
secret key in order to undermine the security of these primitives. In this
paper, we focus on the widely used advanced encryption standard (AES) block
cipher and demonstrate its vulnerability against tampering attack. Our proposed
attack relies on implanting a hardware Trojan in the netlist by an untrusted
foundry, which can design and implement such a Trojan as it has access to the
design layout and mask information. The hardware Trojan's activation modifies a
particular round's input data by preventing the effect of all previous rounds'
key-dependent computation. We propose to use a sequential hardware Trojan to
deliver the payload at the input of an internal round for achieving this
modification of data. All the internal subkeys, and finally, the secret key can
be computed from the observed ciphertext once the Trojan is activated. We
implement our proposed tampering attack with a sequential hardware Trojan
inserted into a 128-bit AES design from OpenCores benchmark suite and report
the area overhead to demonstrate the feasibility of the proposed tampering
attack.
