Classic Cryptography: Caesar Substitution

Caesar Substitution
The Caesar Cipher is one of the simplest monoalphabetic substitutions one may use, and it is also one of the easiest to break.

It is said that Julius Caesar wrote to his friends using a simple substitution cipher, where the plaintext letter was replaced by the ciphertext three places down the alphabet, so that the letter M is replaced by P and so on.

The Caesar cipher may be summarised in a table, shown below. It is now the case that any cipher whose cipher alphabet consists of the letters in their normal order is called a Caesar cipher.

PLAINTEXT   abcdefghijklmnopqrstuvwxyz
CIPHERTEXT  DEFGHIJKLMNOPQRSTUVWXYZABC

As there are only 26 Caesar alphabets it is trivial to solve a caesar cipher by exhaustive search. This is can be done by listing the alphabet under each letter of a section of the ciphertext. The line which contains legible plaintext is the correct one.

plain text this is a simple cipher
cipher text vjku ku c ukorng ekrjgt

v j k u k u c u k o r n g e k r j g t
-------------------------------------
v j k u k u c u k o r n g e k r j g t
u i j t j t b t j n q m p d j q i f s
t h i s i s a s i m p l e c i p h e r
. g h r h r . r h l o . d b h o . d q
. f g . g . . q g k . . . . . n . c p
. e . . f . . . f . . . . . . . . b .
. . . . .   . e . .   . . . .   a .
. .   .     . . .                . .
.     .       .                  .
                .                  .

To save time a series of strips could be prepared. Each with 52 letters, the letters would be in alphabetical order twice over.The strips would be aligned so that the ciphertext read in one position. They would then be scanned to reveal the plaintext.

If you use usenet you may very well may see a caesar cipher used as "spoiler protection". That cipher is called, so called because it rotates the alphabet by 13 places. If ROT13 is applied twice then the plaintext is obtained again. Your newsreader may well have a ROT13 function. The next section has some text coded with ROT13, try to decode it before you move on.

Ciphertext: v pna pvcwre

Having cleared that simplest of ciphers you may now move on to more complex substitutions.