Shift rows in DES, is one form of diffusion

In cryptography, we are interested in fields with a finite number of elements

Caeser Cipher is robust against to brute force attack

AES and DES both follow Feistel Cipher Structure and hash algorithms

S-box in AES is used to provide diffusion

The additive inverse for a=12 ,modulas m = 7 is  4  

The key size in DES is 64 bits , and only 56 bits are used for the encryption procedure

A very common field in this category is GF(2) with the set {1, 2} and two operations, addition and multiplication. 

The size of the key in OTP is half the message length

Multiplicative inverse for 2(mod 5)  is 7