| Explained that a cryptosystem consists of five components: 1) an alphabet, 2) an enciphering function, 3) a deciphering function, 4) an enciphering key, 5) a deciphering key. By Kerckhoff's Principle we assume that the adversary knows everything about (1), (2) and (3). In symmetric cryptosystems a knowledge of the enciphering key is, by definition,
essentially equivalent to a knowledge of the deciphering key and so in such systems we assume that (4) and (5) are known only to the intended users. In public key cryptography a knowledge of the enciphering key does not yield a knowledge of the deciphering key; in such systems (4) is public knowledge and only (5) is kep secret. Talked more about the Caeser Cipher, HSBC secure key and Enigma Cipher. Introduced the affine cipher. Explained how to use the affine cipher in the Python file cs402.py .