From: Waldemar Dick <ruby@...>
Date: 2010-07-06T19:04:58+09:00
Subject: Re: AES block cipher artifacts

Hi,

Am 06.07.2010 11:21, schrieb Jonathan Groll:
 > I've got an oddity with the AES block cipher. There are (seemingly
 > random) artifact bytes that block fill a block passed through the
 > encryption algorith. For instance, starting with "23" this gets
 > transformed to
 > "23\016\016\016\016\016\016\016\016\016\016\016\016\016\016"
 > The question is, how to reversibly perform the cipher without the
 > artifacts?

like you said, you use a BLOCK cipher. So the cipher will pad any data 
to match its block size. In your case the block size is 32 byte big ( 
256 bit key size / 8 ).


> Here is a test program that shows the problem:
>
 >[snip]
> def aes_encrypt(value)
> return Aes.encrypt_buffer(KEY_LENGTH, MODE, @block_secret,
> @iv_secret, value)
> end
>
> def aes_decrypt(v)
> Aes.decrypt_block(KEY_LENGTH, MODE, @block_secret, @iv_secret, v)
> end
> end
 > [snip]

You use encrypt_BUFFER to encrypt, but decrypt_BLOCK to decrypt. They 
are not symmetric. Aes.decrypt_block is a lower level method than 
decrypt_buffer. I don't know the API, but I guess there should be a 
decrypt_buffer, which removes the padding.


Waldemar Dick