Advance Encryption Standard Ziza Alexandru 21c [632272]

The Advance Encryption Standard

In 1997, the U.S. National Institute for Standards and Technology (NIST) put out a
public call for a replacement of the symmetric encryption algorithm DES (Data Encryption
Standard). It narrowed down the list of submissions to five finalists, and ultimatel y chose an
algorithm that was then known only as Rijndael, designed by cryptographers Joan Daemen and
Vincent Rijmen, as the one to become the new standard, the Advanced Encryption Standard.
AES is a block cipher that operates on 128 -bit blocks. It is des igned to be used with keys
that are 128, 192 or 256 bits long, yielding ciphers knows as AES -128, AES -192 and AES -256.
A schematic input -output diagram of AES is shown in the figure below.

As of early 2010, AES -256 is widely regarded as the best choic e for a general -purpose
symmetric cryptosystem. It is supported by all mainstream operating systems, including
Windows, Mac OS and Linux.
Typical software implementations of AES are optimized for speed of execution and use
several lookup tables to impleme nt the basic steps of each round. A lookup table stores all the
possible values of a function into an array that is indexed by the input of the function. It can be
shown that the 128 -bit version of the AES algorithm can be implemented using exactly eight
lookup tables, each mapping an input byte (an 8 -bit word) to an output int (a 32 -bit word). Thus,
each of the eight lookup tables stores 256, 32 -bit ints. The lookup tables are precomputed and
accessed during encryption and decryption.
Since the beginning of 2010, AES is considered a highly secure symmetric
cryptosystem. Indeed, the only known practical attacks on AES are side channel attacks.
Variations of a timing attack on high -performance software implementations of AES were
independ ently discovered in 2005 by Bernstein and by Osvik, Shamir and Tromer. Recall that
to speed up the running time of AES, the algorithm is implemented using lookup tables. The
timing attack is based on the fact that cache of the processor where the AES algor ithm is
executed will store portions of the lookup tables use in the implementation of AES. Accessing
table entries stored in the cache is much faster than accessing entries in main memory. By timing
multiple executions of the algorithm using the same key on a series of known plaintexts of
known ciphertexts, the attacker can eventually learn the key.
In conclusion, we can say that encryption is fundamental to co ntemporary digital
security. AES encryption has become the industry standard for data security due to the
increased security it presents . INPUT
(128 bits)AESOUTPUT
(128 bits)
Key
(128, 192 or
256 bits)