[arethuza]

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).

[throwanem]

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.