Convention Cryptosystem

Illustrates the convention a encryption process. The original “plaintext” is converted into apparently random nonsense, called “ciphertext”. The encryption process consists of an algorithm and a key. The key is a value independent of the plaintext. The algorithm will produce a different output depending on the specific key being used at the time.Changing the key changes the output of the algorithm, i.e., the ciphertext. Once the ciphertext is produced, it may be transmitted. Upon reception, the ciphertext can be transformed back to the original plaintext by using a decryption algorithm and the same key that was used for encryption. In conventional cryptography, also called secret-key or symmetric-key encryption, one key is used both for encryption and decryption.

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.

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.