ABSTRACT
With the increase in the applications and complexity of cryptodevices, reliability of such devices has raised concern. Scan chains are the most popular testing technique due to their high fault coverage and least hardware overhead. However, scan chains open side channels for cryptanalysis as discussed in Chapter 7. Scan chains are used to access intermediate values stored in the flip-flops, thereby, ascertaining the secret information, often known as key. These attack techniques exploit the fact that most of modern day ICs are designed for testability. If scan chains are used for testing then they serve as a double-edged sword. In this test scheme all flip-flops (FFs) are connected in a chain and the states of the FFs can be scanned out through the chain. Scan testing equips a user with two very powerful features namely controllability and observability. Controllability refers to the fact that the user can set the FFs to a desired state, while observability refers to the power to observe the content of the FFs. Both these desirable features of a scan-chain testing methodology can be fatal from cryptographic point of view. In cryptographic algorithms, knowledge of intermediate
FIGURE 11.1: Design of a Scan Chain
values of the cipher can seriously reduce the complexity of breaking it. As seen in Section 7.7 of Chapter 7, the availability of scan chains to an attacker can be used efficiently to determine the key of a block cipher.
