SPARKSkein: A Formal and Fast Reference Implementation of Skein

This paper describes SPARKSkein – a new reference implementation of the Skein algorithm, written and verified using the SPARK language and toolset. This paper is aimed at readers familiar with the Skein algorithm and its existing reference implementation, but who might not be familiar with SPARK. The new implementation is readable, completely portable to a wide-variety of machines of differing word-sizes and endian-ness, and “formal” in that it is subject to a proof of type safety. This proof also identified a subtle bug in the implementation which persists in the C version of the code. The new code offers similar performance to the existing reference implementation. As a further result of this work, we have identified several opportunities to improve both the SPARK tools and GCC.