Understanding basic symmetric ciphers such as SPN and DES is very essential before learning advanced hash functions. This repository contains the implementation of encryption-decryption algorithms of SPN and DES (from Scratch!!!) and linear and differential cryptanalysis of the same. It is an attempt to practically implement the algorithms to get a better insight !! Along with symmetric crptosystems, I also studied various public-key cryptosystems and have implemented RSA encryption and decryption algorithm in python without using any library from scratch.