CDR: RE: Musings on AES and DES

Ray Dillinger bear at sonic.net
Tue Oct 10 08:25:52 PDT 2000



On Tue, 10 Oct 2000, Trei, Peter wrote:

>If you read the ostensible charter of the NSA, its duties include assisting
>in 
>the securing of US civilian communications. While I expect this mainly means
>making sure that Boris & Natasha aren't tapping US internal comm links 
>without permission, it can also be interpreted to make sure we aren't
>using snakeoil ciphers. Making DES not suck seems well within the NSA
>charter. 

True enough.  I have little trust for them though; they have been 
very irritating to american companies who want to make stuff with 
strong ciphers, at least for export. 

>In 1986, when the second recertification came up, I remember considerable
>consternation over the key-length reduction to 56 bits, and the unexplained
>tweaking of the S-boxes. There was serious discussion at the time that one
>or both of these changes were done to introduce backdoors. You'd probably 
>have to find a usenet archive from the period to confirm this. 

No, I wouldn't.  I remember it too, and in fact I was one of the 
conspiracy theorists at that time.  As time went on, though, and 
nobody *outside* the NSA worked out the supposed backdoor, I 
became more and more convinced that the inadequate key length 
was really the only problem.  Switch to independently keyed 
3DES, preferably with a half-block shift between encryptions 
for more diffusion, and that problem goes away. 

>In the end, we now know that the tweaking prevented differential
>cryptanalysis,
>but not linear cryptanalysis. DCA had apparently been discovered internally
>at IBM (and presumably at NSA). LCA was not then known within IBM 
>(whether it was known inside NSA is an interesting question :-)

Hardly matters.  The NSA couldn't realistically have expected to 
exploit linear cryptanalysis on the DES, because it requires them 
to capture something like (IIRC) 2^48 unique plaintext/ciphertext 
pairs.  While that could happen on a high-speed link if they 
monitored it (and the target didn't change keys) for a long time, 
it doesn't seem too likely for the relatively small bandwidths 
employed by terrorist, rebel, and other "fringe" organizations.
If that's a backdoor, it's a backdoor that takes a bulldozer 
to open.  I'm thinking now that they just didn't know about it.

>I would not be suprised if 30 or 50 years down the road, we find out that
>NSA
>did its level best to ensure that the AES selection process picked the best
>candidate. Equally, I would not be suprised to find that they already have 
>some black cryptanalytic technique which can defeat it.  

The NSA was very badly burned in public opinion and by conspiracy 
theorists for their involvement with the DES selection; having 
learned their lesson, I note that they have definitely taken a much 
more hands-off role with AES. Of course, this is also consistent 
with civilian cryptographic know-how having gotten sufficiently 
better that they no longer have to tell us what a secure cipher 
is.

>On the balance
>I favor the former: the NSA is as aware as the rest of us of the huge cost
>(both financial and security) of embedding a broken cipher in the
>infrastructure of the nation.

Hm.  Our opinions differ.  The NSA has a stated agenda to embed 
broken ciphers in the infrastructure of the world, in order to 
preserve their sources of sigint.  In the past, they have been 
perfectly willing to embed broken ciphers in the infrastructure 
of the United states (especially US-produced software) in order 
to further this agenda.  

I don't think I'm a full-out NSA-conspiracy theorist any more, but 
judging strictly from their record,  that is evidently where they 
are coming from.  They have been acting according to the idea that 
the sigint from broken ciphers furthers US interests more 
effectively than having strong ciphers in place.  In the light of 
Echelon and the Crypto AG fiasco, they may even be right about that.

But I don't think it's reasonable for the entire world to suffer 
the pain that broken ciphers in the infrastructure costs, for a 
transitory advantage to one nation.  The best thing the US could 
do, for its allies, for the world at large, and for global trust 
right now would be to just plain quit trying to put broken ciphers 
into the infrastructure of this planet.  This is a direct attack 
on infrastructure, and ought to be treated in world courts just 
as seriously as mining shipping lanes or poisoning water supplies.

I hope somebody has realized this.  It would be nice to think that 
the AES process represents a step in that direction. 

				Bear










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