Double Encryption Q

[Jack Lloyd]

On Fri, Apr 11, 2008 at 04:30:47PM +0200, COMINT wrote:
> Quick system scenario:
> 
> You have packet [A].
> 
> It gets encrypted using an AES algo in a particular mode and we are
> left with [zA].
> 
> More data [B] is added to that encrypted packet.
> 
> Now I have [zA]+[B] in one packet and I re-encrypt it with the same
> algo/key/mode.
> 
> Have I just compromised the security somehow? I wasn't aware of
> anything but something about this double encryption made something
> ring in my mind so I wanted to double check...

This would certainly cause problems in if "particular mode" == OFB or
counter, since (if you also reuse the IVs), you could have E(zA) == A.

If you use a different (independent, unrelated) key/IV, then the
existence of a weakness in this scheme would seem to provide evidence
of an attack on AES, regardless of mode choice.

-Jack

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