The Rijndael algorithm is a symmetric iterated block cipher. The block and key lengths can be 128, 192, or 256 bits. The NIST (National Institute of Standards and Technology) requested that the AES (Advanced Encryption Standard) must implement a symmetric block cipher with a block size of 128 bits. Due to this requirement, variations of Rijndael that can operate on larger block sizes will not be included in the actual standard. Rijndael also has a variable number of iterations or rounds: 10, 12, and 14 when the key lengths are 128, 192, and 256 respectively. The transformations in Rijndael consider the data block as a four-column rectangular array of 4-byte vectors. The key is also considered to be a rectangular array of 4-byte vectors—the number of columns is dependent on key length. Rijndael decryption comprises the inverse of the transformations that encryption uses, performed in reverse order. Decryption commences with the inverse of the final round, followed by the inverses of the rounds, and finishes with the initial data/key addition, which is its own inverse.