BIS 380 Intro to Cryptography, Caesar Cipher The Caesar Cipher is so named because, according to Suetonius, Julius Caesar used it to encode anything of military significance: "If he had anything confidential to say, he wrote it in cipher, that is, by so changing the order of the letters of the alphabet, that not a word could be made out. If anyone wishes to decipher these, and get at their meaning, he must substitute the fourth letter of the alphabet, namely D, for A, and so with the others." - Seutonius, Life of Caesar This cipher is a simple substitution cipher, that is, we shift each letter a given amount in the alphabet. As long as the person to whom you are sending the encrypted message knows how many to shift, they can decode your message. Here is the alphabet shifted as Caesar would have done: abcdefghijklmnopqrstuvwxyz defghijkImnopqrstuvwxyzabc So if you had an ' a ' in your message you would replace it with a ' d ' Use your cipher wheel to encrypt the following (the amount to shift is in parens) trade (3) encode (18) Decrypt the following: PLORE EBPXF (13) Part of the problem with a cipher like this is how week it is. Without knowing the 'key' see if you can figure out this one: qbsuz po We can use our knowledge of English (assuming the message is in English) to help decrypt this. There are only a few two-letter words in English. Since we know there is a two-letter word in the encrypted text, that really narrows down the choices we have to make to try to decrypt. The ' p ' could be a vowel or one of a few consonants. We don't need to try Z,X or quite a few others. Doing this by hand won't take too long, but we could also write a computer program to very quickly iterate through all possibilities. This is called a "brute force" attempt. We are going to use processing power to just try all possible shifts until we see one that makes sense. It points out problems with ciphers like this, namely we don't want a single character in, to give us a single character out. This is what led Alan Turing (and others) to be able to crack the German Enigma code used in WWII. For better security, we need a more complex way to manipulate our data. Exclusive OR (XOR) is one method we can use, as long as we are doing this on a computer, because of ASCll code and binary. XOR is not a secure method on its own, but is a start and is used in many different ciphers like AES. We'll cover this in a bit more detail in class