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.