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Re: The NSA Visits Compendium


In article <[email protected]>,
Andy Brown <[email protected]> wrote:
> On Mon, 16 Oct 1995, Peter Wayner wrote:
> > [...]
> > The most interesting thing that he mentioned was thatthe company had to 
> > guarantee that the data would never be encrypted sequentially by two 
> > _different_ algorithms. Apparently double encryption by 40-bit RC-4 was 
> > okay, but using different algorithms was verboten.
> Very interesting indeed.  With RC4 the bulk of the time is in key setup, 
> so if they could do two setups in parallel then the total time to search 
> a double-encrypted 40 bit keyspace would not be that great.

Hold on -- was the NSA rep talking about double encryption
*with two different independent keys*, or talking about double
encryption with the same key?

Somehow I doubt the latter: for starters, double encryption
with the same key with a stream cipher is generally a Bad Idea.
(Remember Robert Morris's suggestion to ``always look for
plaintext?'') <grin>

In any event, double encryption with the same key is never
gonna be much more secure than single encryption, because it
doesn't increase the key space.

But if the NSA ref was allowing RC4 double encryption *with
two different independent secret keys*, then this *is* interesting!
There are well-known meet-in-the-middle attacks on double
encryption (with independent keys); but the standard one
requires lots of storage (2^40 storage -- this can't be
precomputed if you use 88 extra non-secret salt bits in the
key like SSL); a less well-known more recent attack doesn't
need the storage, but takes a bit longer (probably a few
hundred times longer) than brute force search of single
encryption.  van Oorschot & Wiener have a paper on this subject.

So did you use a SSL-like construction with lots of non-secret
salt bits in the key?

If not, then the 2^40 bytes of storage could be precomputed,
and I'd guess that this NSA position might mean that the NSA
has some Exabytes full of precomputed RC4 output for all possible
40 bit keys. :-)

Dave Wagner
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