Playfair Cipher - Modern Comparisons

Modern Comparisons

Computer-run block ciphers work in a manner similar to Playfair’s: they break the original message into blocks of characters and apply a complex mathematical transformation, based upon the key, to each of those blocks.

Naturally, modern ciphers are not restricted to upper-case, no-punctuation, J-less messages. Any form of data that can be stored on a computer can be encrypted with a modern cipher.

A modern block cipher can be run in a mode similar to that of Playfair, where the same block (in Playfair, a pair of letters) always encrypts to the same bit of ciphertext: in our example, CO will always come out as OW. Indeed, many poorly written encryption programs use just this technique, called Electronic Codebook, or ECB.

More sophisticated implementation of a cipher will use one of many other modes. The most common is called Cipher Feedback Mode, or CFB.

CFB starts by encrypting something other than the message. This bit at the front of things is called an initialization vector, or IV. The IV need not be secret, but the same IV should never be re-used with the same encryption key.

First, encrypt the IV. Take the IV and combine it with the first block of the plaintext. With computers, this is done with a mathematical function called a binary XOR; a similar effect could be accomplished with Playfair by "adding" the two together: C + H = K; W + F = B. It is this value which is written to the ciphertext.

Next, take the result from the last step, encrypt it as normally, and add it to the next block from the plaintext. In this way, the encryption of each block depends upon the encryption of each preceding block.

Encrypt IV -> XOR (add) result with first block of plaintext -> write as ciphertext -> encrypt from previous -> XOR with next block of plaintext -> write as ciphertext -> repeat

The example encoded with Playfair modified in this way, using an IV of "AB" might look thus:

OKHKBGVF…

This process greatly increases the security of the encryption system. When done with computers, the speed of the processing of the encryption is not significantly hindered.