Codes, Ciphers and Secret Writing

Not long ago someone asked the famous Chinese physicist Chen Ning Yang what we should do if we ever received an unmistakable radio message from outer space. His reply was:
“Don’t answer.”

JEFFERSON’S WHEEL CIPHER
In the picture, the wheels have been set so that the horizontal line of letters, above the cross-bar, reads: “Have just reached eastern edg.” These are the first 25 letters of a message. The encoder copies down any other horizontal line of letters. The decoder sets the wheels to align the cipher text, then he looks around the cylinder for a line which makes sense. All the other lines, Jefferson wrote, “will be jumbled and have no meaning, so that he cannot mistake the true one intended.”

Porta’s Digraphic Cipher
A digraphic cipher is one in which pairs of letters, instead of individual letters, provide the basis of the cipher text. In the Porta Cipher, a single symbol is substituted for every pair of letters in the message. The method was invented by Giovanni Battista Porta, an Italian writer, scientist and magician. At the age of 28 he published (in 1563) a delightful book on codes, which included this one. It is the first known digraphic cipher.

Lewis Carroll’s Vigenère Cipher
This ingenious cipher is named for Blaise de Vigenère, a sixteenth-century Frenchman who invented many ciphers and wrote about them. Variations of the basic idea were later rediscovered by others. The version given here was invented by Lewis Carroll, the Oxford University mathematician who wrote Alice’s Adventures in Wonderland and Through the Looking-Glass.

THAT has the same letter at the beginning and at the end. When one or more letters appear more than once in a word, it is called a “pattern word.” In solving a cryptogram, pattern words provide invaluable clues.
For example, suppose you saw the word XPP in a cryptogram. It is most likely to be a common word such as ALL, SEE or TOO, although it could be a less common word such as ODD, ADD, BOO, INN, EGG, ZOO and scores of others. As we have learned, XYZX is most likely to be THAT.

First, some important facts about the English language:
1. The most often used letter is E, followed (in order of frequency) by T, A, O, N. (E is also the most common letter in German, French, Italian and Spanish, but in many other languages it is not. In Russian, for instance, O is the most used letter.)
2. The most common letter at the end of a word is E.
3. The most common beginning letter of a word is T.
4. A single-letter word is A or I, and on rare occasions, O.
5. The most frequent two-letter word is OF, followed by TO and IN.
6. The most used three-letter word is THE. The next most common is AND.
7. Q is always followed by U.
8. The consonant that most often follows a vowel is N.
9. The most common double letters are, in order of frequency, LL, EE, SS, OO, TT, FF, RR, NN, PP and CC.
10. The most frequently occurring four-letter word is THAT.