I think thats just a particular naming style used by people like the CIA:
https://en.wikipedia.org/wiki/CIA_cryptonym
e.g. MKUltra - the MK tells you it was sponsored by the Technical Services Division
The UK had Rainbow Codes:
https://en.wikipedia.org/wiki/List_of_Rainbow_Codes
e.g. Blue Bunny - a 10kt nuclear mine
Edit: I agree that A Colder War is superb - wonderfully bleak ending (if you like that kind of thing).
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