Encryption of test data: which cipher is better?

Testing is a mandatory step in the Integrated Circuit (IC) production because it ensures the required quality of the devices. The most common solution for easing IC testing is the scan chain insertion. This way, a tester can control and observe the internal states of the circuit through dedicated pins. However, a malicious user can exploit this infrastructure in order to extract secret information stored inside the chip. This is the case for cryptographic circuits where partially encrypted results can be observed by shifting out the scan chain content and exploited to retrieve secret keys. Existing countermeasures consist in encrypting the scan content, ensuring the confidentiality of the exchanged messages between the circuit and the tester. The encryption techniques that have been proposed so far rely on the use of two different ciphers: stream ciphers and block ciphers. In this paper, we present pros and cons of both solutions in terms of security and performance. The purpose is to provide an overview of the state-of-the-art in test data encryption and to give elements of comparison between the two ciphers.