S-box in AES is used to provide diffusion

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

AES and DES both follow Feistel Cipher Structure and hash algorithms

Caeser Cipher is robust against to brute force attack

Shift rows in DES, is one form of diffusion

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

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

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

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

Multiplicative inverse for 2(mod 5)  is 7