Joanna Bourne cracks the code on spy communications

Joanna Bourne, author of the new Rogue Spy, joins HEA to share how spies communicated back in the day.

Joanna: Do I love cryptography and codes because I write about Regency spies? Or do I write about spies because I love all that devious secret stuff?

One of those unanswerable philosophical questions.

We pick up our cellphones and chat or text in the blithe certainty (probably ill-founded) that no one overhears. Drop back two millennia and communications are somewhat more vulnerable.

There you are, sitting in your grain storage in Some Ancient City and you want to warn your boneheaded son-in-law in the provinces that the tax man is headed his way. (This is assuming there were Ancient Tax Audits, which is one of those historical likelihoods.) Instead of tapping your message into a damp clay tablet or scratching it out on lambskin in some straightforward way that will be read by your messenger and gossiped over at the first pub, maybe you write in code.

And the great game of hidden messages and finding hidden messages begins.

Probably, in 600 BCE and for the next two millennia, give or take, you use a substitution code.

In a substitution code, each letter stands for a different letter. This can be a straight swap out of one for the other. The Secret Decoder Ring you got in your breakfast cereal when you were 6 was this sort of cypher. Spin it and you learned that B=W, N=T, and so on.

Thus:

"The Taxman Cometh!"

Might be rendered as

"Yuv Ypchpt Alhvyu!"

Which sounds rather emphatic, doesn't it? Your code would puzzle everybody who got his hands on your message but probably not keep your son-in-law out of the hoosegow if they do a careful accounting of his jars of honey and vats of oil.

Substitution codes worked so well folks with something to hide kept using them till the 20th century, adding bells and whistles as they went along. Substitution codes became less and less penetrable. My Regency spies were probably the last generation of decoders who could work their way through the tricks and traps and variations of the better substitution codes. Cami, the heroine of my book Rogue Spy, breaks and makes such codes in her quiet little cottage in the country. It's likely she had instruments to help her out, metal disks and cylinders with the letters on them.

My Cami would also be wearily familiar with the book cipher. In this kind of code, both sender and receiver refer to a particular book. It has to be the same edition, absolutely identical in every way. The book cipher gives page number, line number, and place in the line of type.

So let's say you want to shout "Run for it " to a particularly endangered friend, and you're both in possession of Rogue Spy. You tweet a string of numbers: 1-2-4, 2-13-6, 2-18-3. Your buddy pulls out her copy of my book, decodes in two minutes, and scarpers.

A book cipher is unbreakable — so long as the book remains unknown.

In the new Sherlock series' episode The Blind Banker, Sherlock has a book cipher to solve. He searches the shelves of the men involved, looking through book after book ... till he works out the one book they have in common.

I love Sherlock.

What other cool ways can my spies communicate?

So many ways an ordinary letter can carry secrets without resorting to invisible ink. For instance, if you read every fifth word:

Bring Jenny to help. She's more use here when the chips are down, you know, and she's less likely to dip fingers in the till. We're short handed, so look into hiring Jerry Smith from over in New Carrolton this time.

It becomes:

Bring more chips and dip. We're into overtime.

And no spy's ensemble is complete without his handy secret code. In this recherché little volume or list, "Aunt Fanny" translates into "the Head of Intelligence." "Drinking hot chocolate" means "Found a weapons cache." "Taking outdoor exercise" is "Fly at once. All Is Discovered."

The problem with a written-down code is the spy needs to keep a betraying physical object near at hand. It marks him as a spy. If he's captured by the enemy ... Well, "All Is Discovered."

The next book in my Spymaster's series, Rogue Spy, is on the stands now and available online in paperback, e-book and audio.

Rogue Spy is Pax's story. Ten years ago he was a boy, given the name Thomas Paxton and sent by Revolutionary France to infiltrate the British Intelligence Service. Now he's back in London, alone and unarmed, to confess. He's given one last impossible assignment to prove his loyalty.

Lovely, lying, former French spy Camille Leyland is dragged from her safe rural obscurity by threats and blackmail. Dusting off her spy skills, she sets out to track down a ruthless French fanatic and rescue the innocent victim he's holding — only to find an old colleague already on the case. Pax.

Old friendship turns to new love. Pax's and Camille's dark secrets loom up from the past. And Pax is left with a choice — go rogue from the Service or lose Camille forever.

I will be so happy to send a copy to one of the commenters here.

Find out more at www.joannabourne.com.