Monday, November 10, 2008

AES (Advanced Encription Algorithm )

The Advanced Encryption Standard (AES Algorithm) is a computer security standard that became effective on May 26, 2002 by NIST to replace DES. The cryptography scheme is a symmetric block cipher that encrypts and decrypts 128-bit blocks of data. Lengths of 128, 192, and 256 bits are standard key lengths used by AES Algorithm.

The algorithm consists of four stages that make up a round which is iterated 10 times for a 128-bit length key, 12 times for a 192-bit key, and 14 times for a 256-bit key. The first stage "SubBytes" transformation is a non-linear byte substitution for each byte of the block. The second stage "ShiftRows" transformation cyclically shifts (permutes) the bytes within the block. The third stage "MixColumns" transformation groups 4-bytes together forming 4-term polynomials and multiplies the polynomials with a fixed polynomial mod (x^4+1). The fourth stage "AddRoundKey" transformation adds the round key with the block of data.

In most ciphers, the iterated transform (or round) usually has a Feistel Structure. Typically in this structure, some of the bits of the intermediate state are transposed unchanged to another position (permutation). AES Algorithm does not have a Feistel structure but is composed of three distinct invertible transforms based on the Wide Trial Strategy design method.


Amber Salm said...

You have posted a brief introduction and history about this popular security algorithm. I read a lot about it but never read about its application or tools which make use of this scheme.
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