Burak Ozdemir ’s Blog » Blog Archive » AES Implementation (Visual C# .NET)

What is AES?
-Advanced Encryption Standard
This is a well-known cryptographic algoritm, which is also known as Rijndael, and it is a block cipher adopted as an encryption standard by the U.S. government. AES was announced by National Institute of Standards and Technology.

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:

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. (You can also search Wikipedia for detailed information.)

I am involving of implementation of this algorithm in .NET platform as a practical tool for testing and learning purposes, and you will be able to download and test this tool soon.!

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