Daily Archives: November 23, 2017

AES Block Cipher

One of the things I meant to do when I started the WaterJuice blog 4 years ago was to build up library of Cryptographic functions. I got as far as putting in some of the common hashes of MD5, SHA1, and SHA2. And I also had RC4 encryption in there. But then got no further. I always meant to put in block ciphers. So today finally I add AES.

AES is a block cipher that operates on 128 bit blocks. A block cipher is a keyed permutation on a block. Every possible value of a 128 block is mapped with a 1-1 correspondence to another block for any given key. AES takes 3 sizes of keys: 128 bit, 192 bit, and 256 bit.

AES was announced by NIST at the end of 2001, this was after a five year process in selecting the new cipher to replace DES which was woefully inadequate by this time. AES is a subset of the Rijndael cipher which was of several algorithms that competed to become the new standard. NIST did not make any changes to Rijndael except to restrict the possible set of key sizes and block sizes. Rijndael works on block sizes and key sizes between 128 and 256 bit that are multiples of 32bit. AES formalises a single block size and 3 key sizes.

In this article I will introduce public domain C code for encrypting and decrypting a block using the AES cipher. This is a basic build block for many other operations. In general AES on its own is not terribly useful as the chances are you’ll want to encrypt something far larger than a single block, however I will talk about modes of operation in a subsequent article. For now I will just present the block cipher primitive.

Before proceeding I’d like to answer the question “why use AES?”. The best answer for that is because it is a safe bet. As the saying went “no one got fired for choosing IBM”, the same can be said about choosing AES. Because it is the standard, it is also the most scrutinised algorithm, and it is is still standing strong 16 years later.

Public domain C source code for AES

The following is a sample program that uses that code to encrypt or decrypt a single block specified by hex on a command line.

AesBlock.c

This has a simple syntax of:

AesBlock [-D]

HexKey must be 128, 192, or 256 bits long, and HexBlock must be 128 bits long. If the -D flag is specified then decryption is peformed

Example:

waterjuice$ AesBlock.exe 2b7e151628aed2a6abf7158809cf4f3c 6bc1bee22e409f96e93d7e117393172a

3ad77bb40d7a3660a89ecaf32466ef97

 

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