Security Value

It is also a substitution cipher and is difficult to break compared to the simple substitution cipher. As in case of substitution cipher, cryptanalysis is possible on the Playfair cipher as well, however it would be against 625 possible pairs of letters (25×25 alphabets) instead of 26 different possible alphabets.

The Playfair cipher was used mainly to protect important, yet non-critical secrets, as it is quick to use and requires no special equipment.

Vigenere Cipher

This scheme of cipher uses a text string (say, a word) as a key, which is then used for doing a number of shifts on the plaintext.

For example, let’s assume the key is ‘point’. Each alphabet of the key is converted to its respective numeric value: In this case,

p → 16, o → 15, i → 9, n → 14, and t → 20.

Thus, the key is: 16 15 9 14 20.

Process of Vigenere Cipher

• The sender and the receiver decide on a key. Say ‘point’ is the key. Numeric representation of this key is ‘16 15 9 14 20’.
• The sender wants to encrypt the message, say ‘attack from south east’. He will arrange plaintext and numeric key as follows −

• He now shifts each plaintext alphabet by the number written below it to create ciphertext as shown below −

• Here, each plaintext character has been shifted by a different amount – and that amount is determined by the key. The key must be less than or equal to the size of the message.
• For decryption, the receiver uses the same key and shifts received ciphertext in reverse order to obtain the plaintext.