From: Brian Candler <b.candler@...>
Date: 2009-09-01T21:00:15+09:00
Subject: Re: AES encryption between c# and ruby

Anth wrote:
> Having some issues getting this to work, dont know much about
> encryption and I think im just doing a silly misstake somewhere. Would
> appreciate a pointer or two in the correct direction.
> 
> Encrypting "a" in c# using AES 256 bit cbc yields
> 
> RZ8NxW9+S1qfzd9VRtzV4sFuHFuXJ3mzBpjwCTzbxJQ=
> 
> Whilst in ruby I get: uNL1Ogvi+mQyWGniJRY88g==
> 
> Ruby code: http://pastie.org/601527
> c# code: http://pastie.org/601526

Observe that the first gives 32 bytes (256 bits), while the second gives 
16 bytes (128 bits).

>> "RZ8NxW9+S1qfzd9VRtzV4sFuHFuXJ3mzBpjwCTzbxJQ=".unpack("m").first.size
=> 32
>> "uNL1Ogvi+mQyWGniJRY88g==".unpack("m").first.size
=> 16

You should see http://en.wikipedia.org/wiki/Aes256

"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 block and key sizes in 
any multiple of 32 bits, with a minimum of 128 bits and a maximum of 256 
bits."

Your C# code appears to be using Rijndael, presumably with a 256 bit 
block size, whereas Openssl is using AES.

Anyway, I can get the same results from both Ruby and the openssl 
command line tool, if I modify your Ruby program as follows:

    cipher_enc.iv = "\x00" * 16
    cipher_enc.key = "\x00" * 32
    ...
    puts output_str.unpack("H*").first

$ echo -n "a" | openssl aes-256-cbc -K 0 -iv 0 | hexdump -C
00000000  b4 a5 e9 8a f6 81 0a 83  bd f5 2b 14 ae 82 2c 37 
|..........+...,7|
00000010

This demonstrates you are doing AES encryption properly. But AES with a 
256 bit key and a 128 bit block size is not the same as Rijndael with a 
256 bit block size.

You may find the AES test vectors on the Wikipedia page helpful too.

HTH,

Brian.
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