Cryptography has had a long affiliation with magic and the occult, as in Tetraktys. But there’s a more natural affinity between magic and what’s called steganography. Cryptography is the science of hiding the meaning of a message; steganography is the science of hiding the existence of a message within a document.

Steganography’s operating principle is diversion of attention. Many magic tricks work the same way.  Here’s an example. Below are pictured six cards. Now pick one—it doesn’t matter which—and focus carefully on it. You’ll need to remember it when you reach the end of this blog entry.

Sir Robert Baden-Powell, the founder of the Boy Scouts, offers an excellent illustration of steganography at work. His memoir My Adventures as a Spy recounts his use of steganography while spying on a military installation in Dalmatia. Posing as a butterfly collector, he sketched butterflies in a notebook. Within the marks of the wings of one specimen, he encoded the positions of the guns in a fortress. Here’s the butterfly:

And here’s the fortress, with its guns:

The spots between the lines on the butterfly are decoys. Those straddling the lines denote gun positions, with a different type of spot used for each of the three sizes of guns. I’ve marked one example of each gun type here (leaving the rest for you to figure out).

“It is astounding how little the ordinary person notices butterflies,” observed the writer and lepidopterist Vladimir Nabokov. So it’s not astounding that even in the neighborhood of the fortress, people inspecting Baden-Powell’s notebook failed to notice that one sketch pictured not a real butterfly, but a map of gun placements. People tend to focus their attention only on what is contextually salient–or on what they have pointed out to them.

To illustrate what I mean, let me complete my magic trick. I’m going to guess the card you selected, and remove it from the lineup. Here we go:

I did remove the right card, didn’t I? Yes? But how did I guess it? Well, I didn’t, of course. If you compare the cards here with the original set above, you’ll see that I removed all of the originals, and supplanted them with five impostors. You were focusing only on your card, so you probably didn’t notice the switcheroo.

The motto of the Boy Scouts is “Be Prepared.” Did Baden-Powell conceal a subtext in this motto? He himself was evasive on this point. But it’s not a stretch to suppose that the Boy Scout motto was meant as an injunction to cultivate young boys with military preparedness—not to mention the multifarious skills of spycraft.

(In fact, here’s a recent mention of Boy Scouts being trained as border-control agents.)

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Notes:

(1) The card trick in this blog is drawn from here.

(2) The term “steganography” derives from the 15th century work Steganographia by the abbot, humanist, and (pseudo-)occultist Trithemius.  The Steganographia was influential during the Renaissance as a work on black magic. It purports to teach the use of angels and demons to transmit messages. Recent decipherment of Book III, however, has shown that Trithemius’ Steganographia is not a work on black magic. It is, to give an aptly convoluted characterization, a steganographic cover for a work on cryptology and steganography.

(3) Modern steganography looks to hide messages in high-information-density documents, primarily digital images. The basic principle is unchanged: The image serves as cover for the message. But a potent combination of image-encoding techniques and cryptography now places steganographic messages beyond the realm of human observation. Instead, detection of steganographic messages—which can be challenging—requires the use of modern statistical techniques.