Stross fans will know what I mean when I say this nomenclature sounds like something out of "A Colder War" [1]. Those who don't know what I mean are strongly encouraged to follow that link, at the far end of which is a rather good piece of short fiction that answers the question: "What if Lovecraft was right - and Kubrick, in Dr. Strangelove, was right, too?"
Sure. But since neither XK nor ZK is known to have been used by CIA, and XK is known to be used in the novella, and the snark/boojum contrast exists and would fit well with the story, that was what I referenced.
Better a ZK-SNARK than an XK-BOOJUM, I suppose...
[1] http://www.infinityplus.co.uk/stories/colderwar.htm