Communication is an essential part of humanity, and, presumably, we need to be honest in most of our communicative acts in order for society to function. Yet, dishonest communication has often been a driver of history, and systemic communicative dishonesty — like encryption and cryptography — has been around for millennia. In fact, the development of this practice in the ancient world and humanity’s continuous engagement in it can be seen as a testament to the ingenuity and intellectual capacity of human thinking.
Discussions of ancient cryptology consistently reference the “Caesar shift,” but cryptographic symbol-making and code breaking happened long before Caesar. These were done to protect the commercial secrets of craftsmen and merchants, rather than exclusively for warfare. For example, a clay tablet in Mesopotamia from around 1500 BC encrypted a secret recipe for pottery glaze. The Kama Sutra includes information for secret cryptography between lovers, something that contemporary politicians may want to make use of.
Still, military use seems to be the main purpose for the growth of cryptography in the ancient world. Take the Caesar shift, a monoalphabetic substitution code. It simply called for a shift of three letters; in English, A becomes D, B becomes E, Z becomes C, and so on. Although it seems obvious now, “the shift” — or Caesar cipher — served its purpose for hundreds of years, likely because many enemy troops were illiterate and even the most learned officers and analysts may not have had sophisticated enough command of enemy language to know what characters made up a foreign alphabet.
Around 800 A.D., Arab mathematician and philosopher Al-Kindi developed the technique of frequency analysis, forever cracking Caesar cyphers. Still, shifting letters could work, it seemed, if the shifts were not of a simple-minded consistency. Enter the polyalphabetic encryption method: the first letter of a message could use one shift. The second could use another shift, and so on. Frequency analysis would be capable of breaking these codes too, but doing so would take much longer. This cat-and-mouse game between frequency analysis and symbol-shifting would become the trans-historical theme of symbol-based cryptography.
Another step in the evolution of cryptography was the homophonic substitution cipher, which replaced alphabetic systems with non-alphabetic symbols. This technique is thought to have originated in the fourteenth century. Like the polyalphabetic variant of substitution, homophonic systems could also vary; this time, however, the variations themselves would be varied, with high-frequency letters having greater variations than low-frequency. For example, if the letter S is a commonly used letter, the code would create different substitutions for S in the first instance, the second instance, etc. This created yet another challenge for frequency analysis, and the cat-and-mouse game of encryption and decryption continued.
By WWII, complex and temporally variable substitution cyphers (codes changed each day) were testing the limits of computers. The machines would have to slowly calculate tens of thousands of combinations, in the hopes of cracking codes within 24-hour cycles in order to gather bits of information. Meanwhile, revolutionary digital and sound-based encoding were changing the entire nature of secret communication through pulse-code modulation.
Despite the frequency of dishonest communication and the relevance of such communication to the functioning and dynamism of human society, only a tiny percentage of humans actually understand the processes of encryption. It’s fun, regardless, to see the thread running from rudimentary symbolic manipulation in Mesopotamia or the Roman Empire to pulse-code modulation for verbal and audio encryption to the locked (and sometimes unlocked) encrypted currencies of today. The common denominator is substitution — the (dishonest) gesture of making a symbol mean something that’s not what it says.