The document discusses the Advanced Encryption Standard (AES) algorithm. AES is a symmetric block cipher that encrypts data in blocks of 128 bits using cipher keys of 128, 192, or 256 bits. It operates on a 4x4 column-major order state matrix through 10-14 rounds depending on the key size. Each round consists of four steps: byte substitution, shifting rows, mixing columns, and adding the round key. The key is expanded into a key schedule to derive a different round key for each round.