COWpads: Sharing iPads in a range of secondary school classrooms

This article outlines a mid-point snapshot of the progress of a small teaching-as-inquiry project at Hillcrest High School in 2013. Three teachers (music, mathematics, French) volunteered to focus on using iPads in a COW (computers on wheels, hence the term COWPads) configuration with a junior class during 2013. Each teacher created their own teaching-as-inquiry question focused on specific aspects of their practice. A University of Waikato researcher supported the teachers by observing classes and meeting regularly for feedback, reflection and discussion. Halfway through the year the following themes have emerged: the technical challenges to using a device designed for personal use as a shared device; a positive impact on students’ concentration levels and spans when using iPads, and shifts in teachers’ pedagogical design and practice. The teachers individually contribute their voices to this article, describing their initial experiences of using iPads on a regular basis and what they concentrated on most during the first few months of the project

On the insecurity of arithmetic coding

Arithmetic coding is a technique which converts a given probability distribution into an optimal code and is commonly used in compression schemes. The use of arithmetic coding as an encryption scheme is considered. The simple case of a single binary probability distribution with a fixed (but unknown) probability is considered. We show that for a chosen plaintext attack w+ 2 characters is sufficient to uniquely determine a w-bit probability. For many known plaintexts w+ m+ O(log m) symbols where mis the length of an initial sequence containing just one of (the two possible) symbols is sufficient. It is noted that many extensions to this basic scheme are vulnerable to the same attack provided the arithmetic coder can be repeatedly reset to its initial state. If it cannot be reset then their vulnerability remains an open question

The subset sum problem and arithmetic coding

The security offered by symmetric cryptosystems based on the arithmetic coding algorithm is examined. It is shown that this can be reduced naturally to the subset sum problem. The subset sum problem is NP-complete, however, the cases which arise in practical cryptosystems based on this problem tend to be solvable in polynomial time because the sums formed are either superincreasing or of low density. Our attack is therefore similar to attacks on public-key cryptosystems based on the subset sum problem (knapsack systems)