Caesar's Cipher

Caesar's Cipher

A substitution cipher is an extremely simple example of conventional cryptography. A substitution cipher substitutes one piece of information for another. This is most frequently done by offsetting letters of the alphabet. In Julius Caesar's cipher, the algorithm is to offset the alphabet and the key is the number of characters to offset it.

For example, if we encode the word "SECRET" using Caesar's key value of 3, we offset the alphabet so that the 3rd letter down (D) begins the alphabet.

Plaintext	ABCDEFGHIJKLMNOPQRSTUVWXYZ
Ciphertext	DEFGHIJKLMNOPQRSTUVWXYZABC

where A=D (A encrypts as D) , B=E, C=F, and so on.

Using this scheme, the plaintext, "SECRET" encrypts as "VHFUHW." To allow someone else to read the ciphertext, you tell them that the key is 3.

Breaking of Ceaser's Cipher

Obviously, it had two weaknesses. The first was that the algorithm was not particularly strong. If trial and error couldn't crack the algorithm, then some simple analysis would. If English text was being encrypted, then it would be relatively simple to compare the frequency of letters in the cipher text against the frequency of letters in standard English. Statistics would soon reveal patterns that pointed out the probable plain text letter associated with each cipher text letter. Once a single association was found the entire algorithm could be cracked. No message would be secure.

But, it worked for Caesar, and it illustrates how conventional cryptography works.

Frequency analysis

Caesar's Cipher is so vulnerable to frequency analysis. It is because there is a one-to-one relationship between each letter. If a sufficiently large ciphertext is given, the plaintext can be found out by frequency analysis.

The frequency table of the letter appear in English

If there is a sufficiently large ciphertext, it would be solved by comparing the frequency of letters in the cipher text against the frequency of letters in standard English. If the frequency of the letter in the cipher text is almost the same as the frequency of letters in standard English, we can find out which letter is substituted for the letter in ciphertext. Then the message would be decrypted.