Double Encryption Q

[Pehr Söderman]

There are some situations when this can be dangerous. It's a matter of
implementation. I can directly come up with one trivial scenario that
will end you up in trouble:

Assume that you are using AES-CTR (AES in Counter mode) and do not
change the IV between the two encryptions. In this case you will
correctly encrypt the data, but the second encryption will leave A
unprotected.

/Pehr Söderman

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...
>
> Many thanks,
>
> Mr Pink
>
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