Simple ciphers are both easy to encrypt and decrypt, the only problem with this is that they are also easy so crack. The two main simple ciphers are explained below:.
A Transposition Cipher is a cipher where no letters in the message are changed around but simply positioned in a different order, for example THIS IS AN EXAMPLE could be encrypted as SIHT SI NA ELPMAXE. This cipher is simple to recognize and decode as the letters in each word are simply reversed.
A Substitution Cipher is a process where the orders of the letters are not changed but simply their identity. For example, using the table below the word DOG can be encoded as JUM. We do this by finding the first letter in the original message (plaintext) in the top row of the table and then replacing it with the letter directly below it in the table (ciphertext).
The main problem with these two types of cipher is that they can be easily cracked. This is because the ciphertext they produce contains the same frequency of letters as the plaintext. Through a process called frequency analysis, given a sufficiently large ciphertext it can easily be cracked by mapping the frequency of its letters to the known frequency of, for example, English text.
Because of the problems caused by the above ciphers a new type of cipher was needed. A Polyalphabetic cipher involves the use of more than one cipher alphabet. Therefore each letter would have a one to many relationship with its substitute as oppose to a one to one.
The Vigenere Cipher was devised by Blaise de Vigenere from the court of Henry III of France in the sixteenth century. This polyalphabetic cipher is based on the following tableau. As you can see on the table the first row ia a shift of 0, the second is a shift of 1, the last is a shift of 25.
Method: If we wanted to encipher the plaintext ANNA LIKES BANANAS using the Vigenere cipher we would first have to choose a key word, we will use MEDIA.