Rijndael

Rijndael is a family of block ciphers designed by Belgian cryptographers Joan Daemen and Vincent Rijmen. It was submitted in 1997 as a candidate for the U.S. National Institute of Standards and Technology (NIST) Advanced Encryption Standard (AES) competition and was selected as the AES in 2001. The Rijndael family supports block sizes of 128, 192, or 256 bits and key sizes of 128, 192, or 256 bits, though AES uses a fixed 128-bit block with 128/192/256-bit keys.

The algorithm is a substitution-permutation network. It operates on a state arranged as a 4xNb byte matrix, with Nb equal to the block size in 32-bit words. Each round consists of SubBytes (byte substitution using a fixed S-box), ShiftRows (cyclic shifts of rows), MixColumns (linear mixing of bytes within columns, using finite field arithmetic over GF(2^8)), and AddRoundKey (XOR with a round key derived from the key). The number of rounds is 10, 12, or 14, corresponding to 128-, 192-, or 256-bit keys. A key schedule expands the cipher key into all round keys.

Rijndael was designed for simplicity and efficiency in both software and hardware, and the AES standardizes a specific configuration of Rijndael for 128-bit blocks. It remains widely used to secure communications and data, and no practical cryptanalytic break is known against properly implemented AES parameters. Implementations must consider side-channel resistance, as timing and cache leakage can affect real-world security.