Randomised Mutual Search

We study the efficiency of randomised solutions to the mutual search problem of finding k agents distributed over n nodes. For a restricted class of so-called linear randomised mutual search algorithms we derive a lower bound of k−1 k+1 (n+1) expected calls in the worst case. A randomised algorithm in the shared-coins model matching this bound is also presented. Finally we show that in general more adaptive randomized mutual algorithms perform better (using k−1+k−1k+1− k−2n(n−k) worst case expected calls in the shared coins model) than the lower bound for the restricted case, even when given only private coins. A lower bound of k − 1 + n−k k+1 for this case is also derived

Practical Schemes For Privacy & Security Enhanced RFID

Proper privacy protection in RFID systems is important. However, many of the schemes known are impractical, either because they use hash functions instead of the more hardware efficient symmetric encryption schemes as a efficient cryptographic primitive, or because they incur a rather costly key search time penalty at the reader. Moreover, they do not allow for dynamic, fine-grained access control to the tag that cater for more complex usage scenarios. In this paper we investigate such scenarios, and propose a model and corresponding privacy friendly protocols for efficient and fine-grained management of access permissions to tags. In particular we propose an efficient mutual authentication protocol between a tag and a reader that achieves a reasonable level of privacy, using only symmetric key cryptography on the tag, while not requiring a costly key-search algorithm at the reader side. Moreover, our protocol is able to recover from stolen readers.Comment: 18 page

Toward self-stabilizing wait-free shared memory objects

Past research on fault tolerant distributed systems has focussed on either processor failures, ranging from benign crash failures to the malicious byzantine failure types, or on transient memory failures, which can suddenly corrupt the state of the system. An interesting question in the theory of distributed computing is whether one can device highly fault tolerant protocols which can tolerate both processor failures as well as transient errors. To answer this question we consider the construction of self-stabilizing wait-free shared memory objects. These objects occur naturally in distributed systems in which both processors and memory may be faulty. Our contribution in this paper is threefold. First, we propose a general definition of a self-stabilizing wait-free shared memory object that expresses safety guarantees even in the face of processor failures. Second, we show that within this framework one cannot construct a self-stabilizing single-reader single-writer regular bit from single-reader single-writer safe bits. This result leads us to postulate a self-stabilizing dual-reader single-writer safe bit with which, as a third contribution, we construct self-stabilizing regular and atomic registers