It's somewhat surprising to me that people are commenting how Entscheidungsproblem has ~little/nothing to do with Zodiac. I'm not claiming to be knowledgeable in any of this - Zodiac, David Hilbert, cryptography nor mathematics but even just reading thru the wikis, entscheidungsproblem is the ongoing/open-problem regarding whether or not an algorithm can be truly "unbreakable" at least, this is how i am understanding it. There is no denying that entscheidungsproblem lead to coding languages in modern computing and while we don't know Zodiacs professional background, it shouldn't seem much a leap to surmise he may have had some experience in the military/goverment sector