The Advanced Encryption Standard (AES), also known as Rijndael, is a block cipher adopted as an encryption standard by the US government. AES was announced by National Institute of Standards and Technology (NIST) as US FIPS PUB 197 November 26 2001 after a 5-year standardization process. It became effective as a standard May 26, 2002.

The cipher was developed by two Belgian cryptographers, Joan Daemen and Vincent Rijmen, and submitted to the AES selection process under the name “Rijndael”, a portmanteau comprising the names of the inventors.

Vincent Rijmen and Joan Daemen.
Photo from the PowerBasic Crypto Archive.

Unlike its predecessor DES, Rijndael is a substitution-permutation network, not a Feistel network. AES is fast in both software and hardware, is relatively easy to implement, and requires little memory. As of 2006, AES is one of the most popular algorithms used in symmetric key cryptography.

AES has a fixed block size of 128 bits and a key size of 128, 192 or 256 bits. The key is expanded into a number of separate round keys using special key schedule known as the Rijndael key schedule. Most of AES calculations are done in a special finite field. AES operates on a 4×4 array of bytes, termed the state. For encryption, each round of AES (except the last round) consists of four stages:

1. AddRoundKey — each byte of the state is combined with the round key; each round key is derived from the cipher key using a key schedule.
2. SubBytes — a non-linear substitution step where each byte is replaced with another according to a lookup table.
3. ShiftRows — a transposition step where each row of the state is shifted cyclically a certain number of steps.
4. MixColumns — a mixing operation which operates on the columns of the state, combining the four bytes in each column using a linear transformation.

The final round replaces the MixColumns stage with another instance of AddRoundKey.

Diagram from A Cryptographic Compendium by John Savard.

As of 2006, the only successful attacks against AES have been side channel attacks, not attacking the underlying cipher, but the implementations of the cipher on systems which inadvertently leak data. In June 2003, the US Government announced that AES may be used for classified information:

“The design and strength of all key lengths of the AES algorithm (i.e., 128, 192 and 256) are sufficient to protect classified information up to the SECRET level. TOP SECRET information will require use of either the 192 or 256 key lengths. The implementation of AES in products intended to protect national security systems and/or information must be reviewed and certified by NSA prior to their acquisition and use.”

Notes on Clipperz’s implementation

• Key size: 256 bits
• Cipher mode: CTR