Rijndael is a symmetric key encryption algorithm that was selected as the Advanced Encryption Standard (AES) by the U.S. National Institute of Standards and Technology (NIST) in 2001. It was designed by Belgian cryptographers Vincent Rijmen and Joan Daemen. Rijndael operates on blocks of data and supports key sizes of 128, 192, or 256 bits, making it versatile for various security needs. The algorithm uses a series of transformations, including substitution, permutation, and mixing, to encrypt and decrypt data. Its structure allows for efficient implementation in both hardware and software, contributing to its widespread adoption in securing sensitive information across different applications and systems.