Welcome to MTH6115 Cryptography. In this module you will learn about basic ideas in Cryptography. We will begin with classical cryptography, in which both parties use the same encryption/decryption key and keeping the key secret is essential. Example include Caesar, Affine, Vigenère, or more general stream ciphers. Up until 1970's (eg, during WW1 and WW2) this was the main method used for secret communications. For instance, the Enigma machine was based on a variant of a stream cipher. In the second part of the module, we move on to public-key cryptography. In public-key cryptography, each party has their own key and the encryption key is no longer secret; the security of the communication method now relies on the complexity of the encryption/encryption algorithms. Examples include, RSA, Diffie-Hellman key exchange, El-Gamal, Knapsack cipher, and so on.