What is AES Encryption?
23-11-2006
With DATASAFE™ AES Encryption, your data is encrypted using
the 256-bit AES encryption standard, before you even transmit it over
the internet.
At the click of a button, the DATASAFE™ application generates
a unique mathematical key on your own PC. This key is used to encrypt
and decrypt your files.
Without this key, it is impossible to decipher your files in
the way they are stored on the DATASAFE™ servers, so a copy of the
key should be kept on CD in a safe place other than your office. If you
lose anything from one file to all your data, your data can be retrieved
from the DATASAFE™ servers, following which a decryption step is
required using your unique key.
In the case of data loss, you may request Prisma to assist you
to retrieve and decrypt your data using your unique keys.
But how are your files encrypted?
In cryptography, the Advanced Encryption Standard (AES),
also known as Rijndael, is a block cipher adopted as
an encryption standard by the U.S. government. It is expected to be used
worldwide and analysed extensively, as was the case with its predecessor,
the Data Encryption Standard (DES). AES was announced by National Institute
of Standards and Technology (NIST) as U.S. FIPS PUB 197 (FIPS 197) in
November 26, 2001 after a 5-year standardization process (see Advanced
Encryption Standard process for more details). It became effective as
a standard May 26, 2002. As of 2006, AES is one of the most popular algorithms
used in symmetric key cryptography.
The cipher was developed by two Belgian cryptographers, Joan Daemen and
Vincent Rijmen, both alumni of the Katholieke Universiteit Leuven, and
submitted to the AES selection process under the name "Rijndael",
a combination of the names of the inventors. Rijndael is pronounced [rɛindaːl]
(IPA), a bit like "Rhine dahl", with a long "i" and
a silent "e". In the External links section
is a sound file demonstrating the pronunciation. Now companies can eliminate
the need to manage a daily tape backup process, eliminate the need to
physically move tapes offsite and eliminate the business risk from unreliable
tape backups.
Development
Rijndael was a refinement of an earlier design by Daemen and Rijmen, Square; Square was a development from Shark.
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 a new encryption standard, it is currently being deployed on a large scale.
Description of the cipher
Strictly speaking, AES is not precisely Rijndael (although in practice they are used interchangeably) as Rijndael supports a larger range of block and key sizes; AES has a fixed block size of 128 bits and a key size of 128, 192 or 256 bits, whereas Rijndael can be specified with key and block sizes in any multiple of 32 bits, with a minimum of 128 bits and a maximum of 256 bits.
The key is expanded using Rijndael's 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 (versions of Rijndael with a larger block size have additional columns in the state). For encryption, each round of AES (except the last round) consists of four stages:
- 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.
- SubBytes — a non-linear substitution step where each byte is replaced with another according to a lookup table.
- ShiftRows — a transposition step where each row of the state is shifted cyclically a certain number of steps.
- 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.
Security
As of 2006, the only successful attacks against AES have been side channel attacks. The National Security Agency (NSA) reviewed all the AES finalists, including Rijndael, and stated that all of them were secure enough for US Government non-classified 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."
This marks the first time that the public has had access to a cipher approved by NSA for TOP SECRET information. Many public products use 128-bit secret keys by default; it is possible that NSA suspects a fundamental weakness in keys this short, or they may simply prefer a safety margin for top secret documents (which may require security decades into the future).
The most common way to attack block ciphers is to try various attacks on versions of the cipher with a reduced number of rounds. AES has 10 rounds for 128-bit keys, 12 rounds for 192-bit keys, and 14 rounds for 256-bit keys. As of 2006, the best known attacks are on 7 rounds for 128-bit keys, 8 rounds for 192-bit keys, and 9 rounds for 256-bit keys.
Some cryptographers worry about the security of AES. They feel that the margin between the number of rounds specified in the cipher and the best known attacks is too small for comfort. The risk is that some way to improve these attacks might be found and that, if so, the cipher could be broken. In this meaning, a cryptographic "break" is anything faster than an exhaustive search, so an attack against 128-bit key AES requiring 'only' 2120 operations would be considered a break even though it would be, now, quite unfeasible. In practical application, any break of AES which is only this 'good' would be irrelevant. For the moment, such concerns can be ignored. The largest publicly-known brute-force attack has been against a 64 bit RC5 key by distributed.net (finishing in 2002; Moore's Law implies that this is roughly equivalent to an attack on a 66-bit key as of December 2005).
Other debate centers around the mathematical structure of AES. Unlike most other block ciphers, AES has a very neat algebraic description. This has not yet led to any attacks, but some researchers feel that basing a cipher on a new hardness assumption is risky. This has led Ferguson, Schroeppel, and Whiting to write, "...we are concerned about the use of Rijndael [AES] in security-critical applications."
In 2002, a theoretical attack, termed the "XSL attack", was announced by Nicolas Courtois and Josef Pieprzyk, showing a potential weakness in the AES algorithm. Several cryptography experts have found problems in the underlying mathematics of the proposed attack, suggesting that the authors may have made a mistake in their estimates. Whether this line of attack can be made to work against AES remains an open question. For the moment, the XSL attack against AES appears speculative; it is unlikely that anyone could carry out the current attack in practice.
Side channel attacks
Side channel attacks do not attack the underlying cipher, but attack implementations of the cipher on systems which inadvertently leak data.
In April 2005, D.J. Bernstein announced a cache timing attack that he used to break a custom server that used OpenSSL's AES encryption. The custom server was designed to give out as much timing information as possible, and the attack required over 200 million chosen plaintexts. Some say the attack is not practical over the internet with a distance of one or more hops; Bruce Schneier called the research a "nice timing attack."
Referenced from http://en.wikipedia.org/wiki/Advanced_Encryption_Standard
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