We consider a simple conceptual model of random accessing to a receiver deploying multiuser detection techniques. The model is a simple extension of Massey and Mathys' collision channel without feedback in which a certain probability of recovery from collision is introduced. As the main result, we derive the slot-synchronized capacity region of this recovery channel and show that for sufficiently powerful multiuser detectors, maximal performance is achieved without any sophisticated transmission protocols. We also explore the effect of a simple fading model.

In this paper, we introduce a new class of side-channel attacks called partitioning attacks. We have successfully launched a version of the attack on several implementations of COMP128, the popular GSM authentication algorithm that has been deployed by different service providers in several types of SIM cards, to retrieve the 128 bit key using as few as 8 chosen plaintexts. We show how partitioning attacks can be used effectively to attack implementations that have been equipped with ad hoc and inadequate countermeasures against side-channel attacks. Such ad hoc countermeasures are systemic in implementations of cryptographic algorithms, such as COMP128, which require the use of large tables since there has been a mistaken belief that sound countermeasures require more resources than are available. To address this problem, we describe a new resource-efficient countermeasure for protecting table lookups in cryptographic implementations and justify its correctness rigorously.