You’d be be amazed how bad tradecraft is in Elizabethan England., Seriously, some of the techniques they used are now games for children. Let’s think about what they were doing, and what your character can do about it.
The basic technique of sigint that Walsingham uses is interception of mail. There’s no postal system as such, instead people have mail passed by trusted couriers. These human elements are easy to suborn. For example, Mary Queen of Scots was fund guilty of treason after her letters were presented to court. Her way of getting letters out, smuggled in barrels, was completely compromised by Walsingham, whose proxies helped set it up. They let it run for ages, until they had what they considered a smoking gun. “Let the great plot commence. Mary” Walsingham also did similar things for the foreign services working in England: a heap of mail was intercepted in the Spanish Netherlands, copied, and then let flow on.
“Why not encode your letters?”, you may ask. People did, but they were horrible at it. Thomas Phellepes, Walsingham’s codebreaker, did some work with null characters in substitutions, and each of his agents had a separate cypher key. Walsingham has The Book of Keys, the loss of which would be a critical strike against his organisation. He imagines that a similar book exists for all of his rival services, and may dispatch agents to destroy or copy them, but this may not be accurate.
Cardinal William Allen, who is one of the leaders of the Catholic networks, gives one of his agents a key of “Deus”. That is, his cypher is a straight substitution where instead of A, B, C, and D the letters are A, B, C and D, then all of the letters just fill out each line. This is childishly simple to crack: frequency analysis has been invented by now, after all. If the plaintext is in English, the DEUS cypher has the basic problem that single letters are likely A or I, and that the likeliest characters are E or T. You crack a couple of words and the whole thing falls apart. Phelleppes notes in one of his letters that the reason he was having trouble cracking a particular message was because the person writing it didn’t know as much Latin as they thought they did, and their spelling was terrible. That is, the plaintext was so bad it made the cyphertext hideous.
What if we don’t use a cypher? What if we use a code? The best known ones in period are book codes. In that case, the key is a particular edition of a book, and the code is a series of non-repeating locations in the book. The problem here is that the most common books in England are the approved Bible and Foxe’s Book of Martyrs, which you don’t want an agent in Europe to be lugging about. Similarly the big seller on the continent is the Catholic version of the Bible, and you don’t want the English to catch you with one of those. Another problem with Biblical codes is that you can’t directly mention many proper nouns, like place names or surnames, because they aren’t in the book. You can spell them out, but it makes your message a lot longer. Swapping out the book is hard, but I was thinking the other day that if you were a Cellini fan and someone sent you the tail and claws a scorpion, you’d know it was a signal to switch to his biography.
Some of Walsingham’s agents use single signal or idiot code triggers. Basically in the first case, the signal is a context message. You show a red light in a window and that means “Yes” or “Now!” or whatever the agent has been primed with in advance. This is pretty common in ships’ actions. An idiot code is an activation phrase that can be hidden in another conversation. So, as an agent, your code might be “Your aunt, Lady Catherine…” and so whenever you see that, you know the next bit is an instruction. “Your aunt, Lady Catherine, has gone to Paris.” for example. It has virtually no bandwidth, but I once read a book that used idiot code the whole way through and didn’t spot it. At the end there was a flashback where the characters set it up.
In the game period, suddenly the French do get an “unbreakable” cypher. I’m not sure how widely the Vigenere cypher table is known, but here’s how it works. You make a 26 row and column grid. The top line is the standard A-Z, the next line is B-A, the next C-B and so on. There’s a graphic of it in the episode art on the blog.
You pick a keyword, then extend it to be the length of the message, then chunk the message into 5 letter batches. The first letter in the keyword gives you the row and the plaintext gives you the column, and you work the whole way through. The receiver needs to know your code, of course. This simply works, and isn’t broken until the 19th Century, but its a bit finicky for some field agents. This is why even though they have this, Cardinal Richelieu uses a grille instead.
A grille, by the way, is a template that obscures all of the words on a page except the ones to be read. They don’t work well in modern writing, because we believe terse sentences are glorious. Back then, language was so flowery, and even simple letters so long, that you could work in messages less obviously. This brings us to steganography – that is, hiding the message. Tacitus, and early adopter, mentions a couple of ideas in his book on sieges. His preferred idea was to make pinpricks under letters in a book, I believe. More generally, there’s so much smuggling to and from the Continent that ships decked out precisely to hide stuff in are found along much of the south coast of England.
Invisible inks do exist. The Arabs invented lemon juice ink a while back. In the Ars Magica period, badly made ink _is_ invisible ink. Oak gall ink with too little iron in it is clearish, and over time etches into the parchment, becoming visible. This is why palimpsests exist. That’s where parchment that’s been scraped clean for reuse sometimes allows its first text to remerge. In the Magonomia period, the recipe for invisible ink that a London alchemist would know is vinegar and alum. That seems like a low-level spell.
As we close out I just want to mention one book, which is so wonderful if I put it in the setting people would think I’d made it up: Steganographia by Johannes Trithemius. It was was written in the 1490s, but was only widely published in the early 1600s. It was banned by the Church as it is, superficially, about how to send messages great distances using conjured spirits. The thing is, though, that this is all just covertext – the book is really about how cryptography, its techniques hidden under a layer of metaphysics. A separate key for the third volume, which for a long while after the first two volumes were keyed was thought to actually be about magical communication, has since been cracked, and it’s even more information about cyphers. Robert Hooke, who was eventually the master of experiments for the Royal Society, reckoned he could decode Trithemian codes in John Dee’s writing to Queen Elizabeth. It’s a stretch, but it suits us.
Games From Folktales 