# Block Ciphers

This blog will introduce the second type of the symmetric cipher, which is known as “Block Ciphers“.

Claude Shannon is an American mathematician and cryptographer who defined Confusion and Diffusion as the following:

• Confusion: A small change in the key makes a big change in the ciphertext. A common element for achieving Confusion is the substitution operation, which can be found in both AES and DES.
• Diffusion: A small change in the plaintext makes a big change in the ciphertext. Permutation, for example, is the operation that achieve Diffusion such as, DES.

Note: Each operation by itself cannot provide security. However, the idea is to concatenate Confusion and Diffusion elements to build something called product ciphers.

In conclusion, Confusion and Diffusion are two primitive operations with which strong encryption algorithms can be built. Remember: Confusion has to deal with the key, while the Diffusion has to deal with the plaintext. Most of today‘s Block ciphers, in fact, are product ciphers as they consist of rounds which are applied repeatedly to the data. The idea is to use both Confusion and Diffusion repeatedly (again and again and again). The figure below shows the process of the product cipher.

The Diffusion Block cipher below shows how a single bit flip in the plaintext would make many bit flips in the ciphertext.