Collision Attack on XTR and a Countermeasure with a Fixed Pattern

Recently, XTR is considered as one of good candidates for more energy efficient cryptosystems. Among the family of XTR algorithms, the Improved XTR Single Exponentiation (XTR-ISE) is the most efficient one suitable for ubiquitous computer. Even though the security of such devices against side channel attacks is very dangerous, there are few works on side channel attacks against XTR-ISE. In this paper we propose a new collision attack on XTR-ISE. The analysis complexity of the proposed one is about 240 where the key size is 160-bit, which is 55% improvement from the previously best known analysis of Page-Stam. We also propose a novel countermeasure using a fixed pattern which is secure against SPA. In the sense of both efficiency and security the proposed countermeasure is the best one among the previous countermeasures- it is about 30% faster.