Cryptography involves transforming data using an algorithm and key to make it unreadable. Common techniques include encryption algorithms like the Caesar cipher which shifts letters by a set number, and the Vigenère cipher which uses a keyword for polyalphabetic substitution. Modern standards like the Data Encryption Standard and Advanced Encryption Standard use confusion and diffusion principles with substitution boxes and key lengths of 128+ bits for security. Key exchange methods like Diffie-Hellman allow secure key agreement without prior secrets.