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.