SPARKSkein: A formal and fast reference implementation of Skein This paper describes SPARKSkein – a new reference implementation of the Skein cryptographic hash algorithm, written and verified using the SPARK language and toolset. 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 original reference implementation which persists in the C version of the code. Performance testing has been carried out using three generations of the GCC compiler. With the latest compiler, the SPARK code offers identical performance to the existing C reference implementation. As a further result of this work, we have identified several opportunities to improve both the SPARK tools and GCC