I read a novel by a former SAS soldier about a former SAS soldier who's sent on deniable operations by the British government, and who uses what is clearly a on

I read a novel by a former SAS soldier about a former SAS soldier who's sent on deniable operations by the British government, and who uses what is clearly a one-time pad to communicate with his handlers. The detailed description of the precise method used, the fact that the term "one-time pad" is never used, and the narrator's apparent lack of understanding of how exactly it works (encryption/decryption is done via modulo-10 addition/subtraction, which the narrator derisively refers to as "spook maths") suggest that the author himself, or a contact of his in the same circles, has used a very similar system for real.