Introduction
Governments, companies, and even individuals worldwide rely on encryption to protect confidential data for transmission, processing, and storage.
The ideal encryption algorithm would provide unbreakable security with no significant impact on the normal flow of data. For twenty years the Data Encryption Standard（DES）was the state of the art, but it became unsafe once it was determined how to identify its secret key and decipher the encrypted data. A variant, Triple DES, is more secure, but requires three times the processing time to encrypt or decrypt data.
In September 1997, the National Institute of Standards and Technology（NIST）issued a request for a new Advanced Encryption Standard（AES）to replace DES. Several algorithms were proposed and considered, and in October 2000 the Rijndael block cipher algorithm was chosen as the new AES.
NIST mandated AES for civilian agency use in November 2001 as Federal Information Processing Standard Publication 197（FIPS 197）. In 2003 AES was approved for use with classified information by the US National Security Agency（NSA）. Today AES is the encryption standard used around the world.
This paper gives an introduction to the AES algorithm, explaining its encryption and decryption processes and describing its various options. The paper then discusses how AES is implemented in CAST’s IP cores. These register transfer level（RTL）cores can be used to physically implement the AES algorithm for fast hardware encryption and decryption in ASIC or FPGA technologies.
