Decrypts, and SIGINT in general, are boring


With all due honour to the exploits of those clever-clogs at Bletchley Park and the intrigue that surrounded the use of their ULTRA, decryption (and SIGINT in general) don’t easily make for a variety of good RPG adventures. You can devise some adventures that make good use of remote cracking and the security discipline that opposes it, but there is a persistent danger that if you make it realistic it is slow and puts the key actors in secret rooms remote from interaction with NPCs¹, while if you make it cinematic without letting institutions respond realistically to it it becomes a setting-wrecking superpower. So in Flat Black I want decryption of intercepted broadcasts not to be a common thing.

It seems to me that I have two options. Either

  1. Nobody much broadcasts a lot of encrypted secrets or transmits them along cables that might be tapped, because they know that anything they encrypt might be decrypted. There are therefore adventures to be had smuggling and intercepting physical deliveries of secure material.
  2. Most everyone routinely broadcasts encrypted secrets or transmits them through networks, but you can’t crack their encryption so you have to subvert their people or infiltrate their premises to get a look at their source texts or decrypts.

The two possibilities suggest two different sorts of adventure, one involving either carrying a message or intercepting a courier, the other involving infiltration by stealth or social engineering, to steal records or codebooks.

What if I say that everything is crackable except for a one-time pad? Do I then get both kinds of adventure because of couriers trying to deliver stacks of pads?

¹ Once upon a time I ran a Flat Black adventure for a very small group of PCs who were working for some sort of Imperial intelligence outfit, and that involved them monitoring the mysterious activities of some shadowy figures on a world that had such development that iron-hulled ocean-steamers were a marvel. The shadowy figures had a ship that I based on the idea of Isambard Kingdom Brunel’s SS Great Eastern, which they were using to operate a bathysphere at great depth and recover something from the bottom of the ocean. The PCs thought that they were being very clever to hide below the horizon and use an aerial remote probe to set the ship’s main deck and principal cabins with bugs and surveillance devices, but (a) the oppo did not conduct any “As you know, Jim” conversations, and (b) when the shit hit the fan the PCs were fifteen minutes away and unable to intervene. Fortunately the shady characters weren’t interested in the Eichberger drive on the sunken pirate ship, but the priceless AI running on that ship’s computers was killed by the secret police.

Think once. Think twice. Get on the ship and interact with NPCs!


When e.g. Bletchley Park appears in fiction, the usual approach seems to be to use it as the unexamined backdrop for a conventionally human story. (Then again, this is most writers’ approach to anything complicated that the audience won’t be expected to understand.)

In my Reign of Steel games I assumed that a GURPS TL9 computer could readily decrypt anything encrypted with a TL8 algorithm, and so on. This meant in practice that the human resistance had to use codes rather than ciphers, and one-time pads when they did have to use ciphers, though that wasn’t something the PCs got directly involved with most of the time. On the other hand, when an AI wanted to eavesdrop on another AI, it would have to get sneaky.

People still make mistakes. They still think “oh, nobody will ever compromise the armoured cable inside the secure building, I can let data go across there in plain text”.

One-time pads are bulky and inconvenient, whatever the tech level (unless you have teleportation). Chances are someone will say “eh, this isn’t so important, we can re-use the old pad”. They really do, the fools.

And of course while you should have a different pad for each pair of communicating entities, you often don’t. So the adventure about getting data flowing between A and B may include both the cable-tap between them and the subversion of C to get a copy of their Official Pad for October.


A friend of mine was on an inspection tour of ASIO’s purpose-built secure building, being led around inside the supposed Faraday cage by a representative of the construction contractors, when his guide’s mobile phone rang.

One-time pads are bulky and inconvenient, whatever the tech level (unless you have teleportation). Chances are someone will say “eh, this isn’t so important, we can re-use the old pad”. They really do, the fools.

A quondam girlfriend of mine once maintained that her grandfather had been awarded his BEM for the brilliant idea of using the last pad in a set to send a general order to South East Asia Command to re-use the month’s used pads, when they ran out one time. There’s something wrong with that story, but I never thought it worthwhile or tactful to find out what.


Bletchley Park, by Jane Austen. . . .


“It is a truth universally acknowledged, that a single decryptor in possession of a good ciphertext, must be in want of a crib.”


There is a memoir worth reading by Leo Marks, who was the cryptographer sent from Bletchley Park to work at SOE: Between Silk and Cyanide. It is mostly about how terrible SOE was. In particular the title refers to Marks’ inability to persuade them to issue one-time pads printed on silk and concealed in the linings of clothes.


It’s on my to-read list and has been for some time. :slight_smile:


Have you read R.V. Jones’ Most Secret War? Jones was in the SIS (IId), so it’s a lot less hopeless and futile than the Marks.


Some time ago, but yes. And I have a fair bit of the general background of course. (I find it quite worrying that these days, when I hear non-historians saying something about WWII at least in the European theatre, I generally know what they’re talking about in more detail than they do. I didn’t set out to do this! I just wanted to take advantage of Warehouse 23 selling off GURPS WWII cheaply when GURPS 4e had come out!)


Damn those Satanic tabletop RPGs! First, they lure us into devil-worship, and then… They teach us trivia!