Today is the first day of Illinois’ shelter-in-place order. The coronavirus pandemic is forcing everyone to hunker down and stay at home in order to flatten the curve and give the health care system a fighting chance at caring for those infected with Covid-19. My kids are home for what is likely to be the rest of the school year and I’m frantically trying to transition from a small classroom, face-to-face teaching paradigm to on-line, remote teaching. It’s been a real hoot so far.

If you’re like me, you’ve probably noticed on social media an influx of live and recorded musical performances, zoo tours, art lessons, and enriching things to do from home. I thought I’d get in on the fun, tap into my love of old-school, pre-computer era cryptography, and start a regular series of cryptanlysis puzzles I’ll be calling Coronacrypto.

## The Problem with Cryptogram Puzzles

If you’ve ever done a classic cryptogram puzzle then you have a pretty good idea of what to expect, at least at first. Your typical cryptogram is what’s known as a monoalphabetic, unilateral substitution cipher. It’s unilateral because the code works in terms of single letters. It’s monoalphabetic because each letter in the message is replaced with one and only one letter. This results in a cipher where you replace one letter with another in order to transform the original message (plaintext) into the the enciphered message (ciphertext). The correspondence between letters in the ciphertext and letters in the plaintext is called the key. When you solve these puzzles you’re attempting to break the cipher by revealing the plaintext and recovering the key.

Cryptogram puzzles typical retain word-length, punctuation, and capitalization. They only hide the true message by substituting letters. When these systems were used in practice, all of word and sentence level structure was scrubbed from the message and only the letters were retained. All of that structure offered clues to the code breaker, the cryptanalyst. It’s easy to spot three letter words, proper nouns, and all the other patterns that arise in our use of language when you retain the essential structure of the message.

More often than not the substitution pattern, the key, used for cryptogram puzzles is random. From a security standpoint this is great. Random keys have no predictable pattern to them. This also makes them harder to remember and therefore harder to use as the basis for secure communication. In practice, the code maker, the cryptographer, might instead devise a system for generating keys that was easy to remember. This might include reusing keys from day to day or having a way of rotating keys. These key management systems are not random because people need to know and remember how to use them. They necessarily have some kind of pattern. A cryptanalyst’s true goal is to not only decipher individual messages but to discover the pattern in the system behind the keys. By knowing the system’s pattern they can decipher future messages with less effort and less time. If you already know what the key for tomorrow’s messages will be, then deciphering the message will be quick and easy.

When working on cryptogram puzzles you sometimes are given a clue about the source of the message. Rarely is there a relationship between subsequent messages. In practice, enciphered messages are not random. They are written records of conversations or reports about a topic of interest. These social and situational patterns are also important to the cryptanalyst. They too can aid in breaking a whole cryptosystem: the practices for generating keys, enciphering and decipering messages, and the organizational practice of using the cipher, in which the cipher is deployed.

Cryptogram puzzles are fun but all of the puzzles and puzzle sets I’ve encountered are simply sets of unrelated puzzles. You get the thrill of deciphering the message, but they do not provide you with the opportunity to hunt down and break a whole cryptosystem.

I’ve always wanted a cryptogram series that is more like real-world, historical cryptography: word and sentence structure is not maintained, keys are generated by a underlying system, and messages come from a related source or are about a related topic. Such a series has the usual reward of deciphering individual messages but the bigger prize of breaking the system and realizing that you know how decipher future messages without even needing to analyze them.

Coronacrypto is not a series of individual message puzzles but a series of cryptosystems. I’ll release the puzzles in numbered series. Each series is enciphered with a specific cryptosystem. Your job is to decipher each puzzle and try to recover the underlying system. At the end of each series I’ll spill the beans, give you the original messages, and explain the system.

## So You Want to be a Cryptanalyst

If you’re not interested in breaking whole cryptosystems, then you can still play Coronacrypto. I’ll give you some hints along the way and will mostly stick to simple, cryptogram-like systems for most of the series. From this perspective you can think of them as cryptogram puzzles with the added challenge of having all the helpful word and sentence structure removed.

If, however, you want to really dive into the world of cryptology and go for the real prize, breaking my cryptosystems, then I highly recommend you start with FM 34-40-2 Basic Cryptanalysis. This old US Army field manual is a great, to the point introduction to the cryptosystems I’ll be using in Coronacrypto and equips you with all the tools you need to break them.

If you want some motivation from the annals of history, then look no further than Jason Fagone’s The Woman Who Smashed Codes. I was recently gifted this book and could not put it down. It chronicles the life and times of Elizabeth Friedman. By reading about Elizabeth’s cryptanalytic exploits you’ll gain a real appreciation for the thrill of breaking whole cryptosystems. Elizabeth and her husband, William Friedman, were integral parts of the US code breaking efforts during WWII and helped shaped modern cryptology (cryptography and cryptanalysis combined) as we know it. Much of their insights and cryptologic discoveries underly what you’ll learn from FM 34-40-2. They quite literally wrote the book on early cryptanalysis.

Finally, I’d be remiss if I didn’t point you towards David Khan’s The Codebreakers and Simon Singh’ The Code Book. Both books tell the history, ancient and modern, of cryptology. Khan’s book is expansive and detailed. Singh’s book is lighter, literally, and where I recommend you start if you want a quick but still accurate look at the art and science of secrets.

## Enough Chit-Chat. Let’s Break Ciphers!

Ready to get to work? Good. Series 1 starts right now!. You’ve already been given a major hint: this system uses a monoalphabetic unilateral substitution cipher. You need look no further than FM 34-40-2 Basic Cryptanalysis for techniques for breaking this cipher and the underlying system. If, however, you’d rather tackle this puzzle using your wits alone, then remember, there are patterns in the way we use letters and changing the letter just shifts the pattern to a different letter.