Caesar Shift / ROT13 Cipher

Caesar shift is a very common cipher, that is incredibly easy to use. This being said, it is also incredibly easy to break. In this cipher, the alphabet used to encode the text is simply the standard alphabet, rotated round by an agreed number of positions. This cipher is said to have been used by Julius Caesar to communicate with his army. This is quite likely the first form of encryption that was ever used to secure communications.

ROT13 is a common example of this cipher that was used quite frequently on the Usenet. It wasn’t ever really used (to my knowledge) to prevent people from reading a message, but rather just to hide text from the casual eye.

Most NNTP readers now come with a function to ROT13 a message. Because with ROT13, you are shifting the alphabet 13 places, the same shift will return the text to it’s original position.

The following is an example of Caesar Shift, using a shift of 21

Breaking The Caesar Shift

This cipher is incredibly easy to break, as there is only 25 different answers (not 26, a rotation of 26 would be the same as the plain text alphabet!).

Trends in the English language can also give you a vital clue as to the value of the shift. There are only 2 letters in the English language that appear on thier own for example, ‘I’ and ‘A’. This means that if there is a single letter in the cipher text, it limits you to 2 possible shifts. This is of course, making the assumption that the message is in English, and that the spaces havent’ been stripped out.

There is a technique called frequency analysis, but I will cover this in the Monoalphabetic Substitutions section.

My encoder / decoder tool can be used to encode/decode this type of encryption.