Public-key cryptosystem (PKC) is one of inevitable key technologies in order to accomplish fruitful security applications in ubiquitous computing systems. The ubiquitous computer only has scarce computational resources (like Smart cards, RFID, Sensor Network), however, so that the light weight PKC is necessary for those miniaturized low-power devices. Recently, XTR is considered as one of good candidates for more energy efficient cryptosystems. Among XTR exponentiation algorithms, the most efficient one is the Improved XTR Single Exponentiation (XTR-ISE) proposed by Stam-Lenstra. Thus among the family of XTR algorithms, 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, derived from the structural properties of XTR-ISE. The analysis complexity of the proposed one...