[121224] in cryptography@c2.net mail archive
Re: Double Encryption Q
daemon@ATHENA.MIT.EDU (Martin James Cochran)
Mon Apr 21 12:25:23 2008
Cc: cryptography@metzdowd.com
From: Martin James Cochran <Martin.Cochran@Colorado.EDU>
To: COMINT <comint@gmail.com>
In-Reply-To: <afe51f3f0804110730n7542c555ieceaaf8643d6ab89@mail.gmail.com>
Date: Fri, 18 Apr 2008 17:09:27 -0600
If your original mode of operation is secure, then this should be
secure.
The reduction:
Consider algorithm A that tries to break the double encryption mode of
operation (DM) in the IND-CPA setting. We can construct an algorithm
B that tries to break the original mode of operation (OM) in the IND-
CPA setting. B simply runs A and responds to A's queries by querying
B's oracle twice to simulate A's oracle, and returning the result. B
returns the output of A.
If A breaks the encryption, so does B. So if the original mode is IND-
CPA secure, this double encryption should be okay.
Note that the examples given, OCB and CTR with repeated counters, are
not IND-CPA secure.
Martin Cochran
On Apr 11, 2008, at 8:30 AM, 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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