Types of P-boxes

The chart below shows the three types of p-boxes available in modern block cipher: 

1. Straight P-box

In this type of p-box, the number of inputs and output is the same. If inputs are n and outputs are m, then m=n.  The positions of the arrangement are shown in the figure, where an equal number of bits are transposed to different places.

2. Expansion P-box

An expansion p-box is a p-box with n inputs and m outputs where m>n, i.e., the number of outputs is more than the number of inputs. In this type of p-box, the values get repeated as for one input there is a possibility for more than one output.

3. Compression P-box

A compression p-box is a p-box with n inputs and m outputs where m<n, i.e., the number of outputs is less than the number of inputs. In this type of p-box, few bits are dropped as not all input bits are considered for output. 

P-boxes are permutation boxes which are usually one of the main components of a modern block cipher. They are also known as D-boxes or diffusion boxes. P-boxes are used in the block cipher called the data encryption standard commonly known as DES in cryptography. In this article, we will look into what are p-boxes, the types of p-boxes, and how to identify their types in detail.