Does anyone know the algorithm used by [Turing's Colossus](https://en.wikipedia.org/wiki/Colossus_computer) inference engine, so highly classified that the Brits kept it secret for decades after WW II?

Indeed, it may still be classified. Several years ago a colleague of mine, [the late Robert Johnson](http://www.sltrib.com/news/3747348-155/u-professor-helped-shape-computer-science), sent me an email response to my announcement about the [Hutter Prize for Lossless Compression of Human Knowledge](http://prize.hutter1.net/). Bob was the guy who designed the Burroughs zero address architecture used for so many years in banking (and incidentally invented magnetic ink used on checks). He certainly had a lot of background he didn't advertise, including classified work.

> 
> Computerdom does not have a lot of art in inference engines (making
>  predictions). The most effective inference engine that I know of is
>  the software done for Colossus, Turing's code breaking "computer" of
>  WWII. The Brits still treat that software as classified even though
>  the hardware has been declassified for years. So far as I know,
>  nobody outside of UK knows the details of that software. My point
>  here is that drawing understanding from natural languages is a
>  relatively small art practiced mostly by cryptoanalysts. And my
>  further point is that the natural language of interest (be it English,
>  Chinese, Mayan or ...) has a major influence on how one (person or
>  program) goes about doing analyses and making inferences. From a
>  practical perspective, the Hutter challenge would be much more
>  tractable for at least me if I could do it in Chinese. My first PhD
>  student was Jun Gu who is currently Chief Information Scientist for
>  PRC. His thesis was on efficient compression technologies. If you
>  wish, you can share these thoughts with whomever you please.
> 
> 
> Bob Johnson Prof. Emeritus Computer Science Univ. of Utah
> 
> 
>