Does stream cipher-based scan chains encryption really prevent scan attacks?

Abstract : Scan chains offer facilities to steal secret information embedded in a circuit. For instance on a crypto-processor, collecting data related to the round register leads to reveal the secret key used for encryption. To protect against this threat while maintaining the test and debug features, countermeasures are implemented on the test interface or on the scan chain itself. Among the countermeasures, some of them are based on stream ciphers, consisting in the encryption of both controlled and observed data in the scan chain. We show in this paper that the stream cipher protection presents vulnerabilities if the keystream generated by the stream cipher is identical after a reset.