Let us talk, dearly beloved, about a reasonable pencil-and-paper crypto system, for those times when you don’t want to use your computer to encrypt stuff that you need to send to some other pal.
Nothing is totally secure. Aside from strong-arm mathematical cryptanalysis, any crypto system has weaknesses. The first of them is this: All pipes leak at both ends.
Someone knows what the message was before it was encrypted. Someone else knows what it is after it’s decrypted. Both of those people are vulnerable to black bag cryptanalysis, to checkbook cryptanalysis, and to rubber hose cryptanalysis. They are even more vulnerable to dumb-shit cryptanalysis. The more people who know the contents of the message the greater the vulnerabilities up.
But leave that aside. Here’re the workshop instructions.
Start with the Straddling Checkerboard. This is a substitution cypher devised by those clever buggers the Jesuits back during the Enlightenment.
Neat little table there. Notice how it’s different from those little 5x5 checker boards that everyone knows. The first thing you’ll notice is that it uses all the numbers, and all the letters are available. The second thing you’ll notice is that one of the rows has no number at all. I’ve stashed the most common English letters up there.
One of the disadvantages of a standard checkerboard is that it doubles the message length. With a straddling checkerboard, that isn’t the case.
Here’s how it works:
Let’s take our message: “Don’t let them scare you.”
D is at the intersection of 1 and 5, so it becomes 15. O is on the first row, so it just becomes 6. N is on the top row too, so it becomes 7. T, similarly, becomes 4.
Proceed in similar manner until you have the entire message:
15674 11344 18312 01458 32962 5
There it is, broken into five-number groups (for ease in transmission). Fill up the last group with random numbers until it, too, is five numbers long.
In order to unambiguously decrypt the message, go through it. Every time you see a 1 or a 2, circle it and the next number. All the remaining numbers will be single letters.
Right. Very good. But still not secure. Let’s do something else with it. This is a wrinkle added to the old Jesuit Straddling Checkerboard by the Red Orchestra, the Soviet spy ring that operated in Berlin during WWII.
We’re going to add pseudo-random numbers to the mix. We’ll get them from an almanac: in this case the World Almanace and Book of Facts, 2008 edition.
Find a page within that contains a Whole Lot of Numbers. For example, take page 160, Non-marital Childbearing in the US, 1970-2004.
We’ll also need to know how to do non-carrying addition. 1+1=2. 6+6 also = 2.
In that World Factbook, on page 160, I’m going to start on line 07. That’s the line that starts “All races.” We’ll just follow the numbers across the page, then down the page, then to the next page, and as far as we need until we have enough numbers to write under our little encyphered text.
15674 11344 18312 01458 32962 51234 (note the padding)
10714 31842 20280 32232 83303 32335 (and so on)
Add the numbers, vertically, with non-carrying addition and you get your final ciphertext:
25388 42186 38592 33680 15265 83569
To reverse this, just subtract the keynumbers from the ciphertext, borrowing as necessary.
One more step. You’ll need a couple of other five-digit numbers, now, which you’ll have memorized: Say the first one is 12121 and the second is 98765. You and your buddy know these numbers; no one else does. This is how you’re going to tell your chum which page and line to start on.
Page 160 line 07 becomes 16007. When added to 12121 that becomes 28128. When added to 98765 that becomes 04762. Now add in the date of transmission: 090908 (drop the last 8 because you only need five numbers) and you get: 27118 and 03752.
Put those numbers in pre-arranged places in your ciphertext (say second from the start and third from the end), and you’re done.
25388 27118 42186 38592 33680 03752 15265 83569
The only punctuation you use is the period (.) which is also encrypted. The slash (/) is the numeral shift sign. In order to put numbers into your text, you put in the shift sign, then repeat each digit three times, then the shift sign again. Thus “Meet me at 247 Main Street” becomes “Meet me at /222444777/ Main Street.”
Some other notes on craft: Always construct your checkerboard from memory each time you use it, and destroy it immediately afterward. Do not do any work on an electronic device. Stay away from windows. The checkerboard and the plaintext should never be on the same physical sheet of paper. The plaintext and the ciphertext should never be on the same physical sheet of paper. Work on a hard surface that will not take impressions.
Immediately after you’ve created the ciphertext, destroy all the intermediate sheets of paper.
Since the beginnings and ends of messages are often standard, split your message in two and reverse the parts so the beginning and end are together, somewhere near the middle.
You and your pal will need to agree on a book to use. Have several different books of tables in your house (so which one you used isn’t obvious).
All that a crypto system buys you is time. Don’t send anything where the information has a longer shelf life than your cipher.
Assume that the bad guys know everything about your crypto system except the specific keys you used today.
Make all arrangements (keys, etc.) face to face. Nothing by any electronic means of communication. If anything at all makes you feel hinky, change the keys right then.
You can change the arrangement of letters inside the checkerboard, and which numbers are the straddle, regularly. Like, daily. (One way to change the arrangement of letters is to use a keyphrase, writing it into the checkerboard and using each letter only once. One advantage of using keyphrases is that they’ll near-automatically put the most common letters in the top row.) For example, Now is the winter of our discontent yields: