Re: Provably "Secure" Crypto (was: IPG Algorithm Broken!)

[Paul Foley <mycroft@actrix.gen.nz>]

On Tue, 26 Nov 1996 03:39:35 +0000 (GMT), The Deviant wrote:

   On Mon, 25 Nov 1996, Dana W. Albrecht wrote:

   > so often, I refer to systems for which rigorous mathematical proof that 
   > "there are no shortcuts" exists.  To my knowledge, no such systems, with 
   > the exception of a real one-time pad, exist today.  However, I also 

   As I have argued many times, that is correct.  OTP, with real random
   numbers, and no-reusage, etc, etc, is the only "perfect" cryptosystem, and
   even it has its problems (like key exchange, for instance).

The only one known at this time, not necessarily the only one
possible.  Are you aware of some proof that no other cryptosystem can
be secure (in the way Dana talks about)?

   > Rigorous proofs of the non-existence of an algorithm are not new.  
   > Neither are rigorous proofs that any algorithm which can solve a given 
   > problem requires a minimal running time.  Or, in an even stronger sense, 

   Hrmmm... I seem to see a problem (namely Moore's first law) in assigning

There's a Moore's second law?

   anything a "minimal running time".  Perhaps "minimal instruction count"
   would be more suited to your example.  Because if you're talking about
   time, it essentially boils down to "the longer something takes the less
   time it takes".

"Minimal running time" doesn't really mean time in hours.  Obviously
hardware gets faster all the time.  It means complexity -- O(n^2)
takes more time than O(log(n)), regardless of how fast your hardware
is.  In other words, if takes f(x) time units, but the units are
arbitrary.

"Minimal instruction count" is pretty meaningless (change the
instruction set to arrive at any figure you like).

   > that a particular known algorithm for a given problem is indeed a 
   > (provably) optimal algorithm for that problem.

   Never happen.  It just won't.  As a rule, there's _always_ a faster way.

But there _are_ such proofs ("reductio ad absurdum".  Assume this is
_not_ the best algorithm.  Then there is some better algorithm.
Figure out some properties this better algorithm must have.  Something
like "1 == 2" comes up, therefore there is no such better algorithm.)

Of course you can do it (whatever "it" is) faster with faster
hardware, or maybe better implementation.  And there are sometimes
special-case shortcuts...(a OTP has innumerable "weak keys" -- all
'0's being the most obvious -- of course it's _possible_ that your
random pad just happens to transform your real message into valid
English text, but I doubt this argument would save you from the firing
squad :-) The chance of generating an all-'0' pad ((1/n)^x, n=range of
values in pad, x=number of blocks (characters) in message) is a lot
better than the chance of getting some unrelated-but-meaningful text
as output (I don't even know where start on that one -- it's the
"monkeys in the British Museum" scenario))

   > Turning once again to cryptography, there is presumably an "optimal" 
   > algorithm for factoring a "general" number in the "worst" case.  Of 

   Ok, now I have to pose a question: If cryptographers actually beleive
   this, why continue to search for a faster one.

Because no-one's found the optimal solution yet (or at least not
proved that it is optimal).

   > "optimal" algorithm.  Worse case bounds on running time for currently 
   > known algorithms can certainly be produced, but no one currently knows 
   > if these are the best algorithms.

   Again I say, there's _always_ a faster way.

But you're arguing "faster" in terms of clock time (which is obviously
true, but not necessarily useful).  Something that takes O(n^2) time
can be done in arbitrarily short clock time (given fast enough
hardware), but is still slower than something that takes O(log(n))
time.  If you make n big enough, the O(n^2) calculation may not be
worth doing, while the O(log(n)) calculation is still fairly fast (in
reality, of course, you'd prefer something that takes much longer than
n^2).

-- 
Paul Foley <mycroft@actrix.gen.nz>       ---         PGPmail preferred

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