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