As I was reminded last week, you could
achieve any number of results for an
encrypted item.  But you have no way of
knowing whether results end up being
the works of Shakespeare, Chaucer, or Tolkien,
because without the key, you would end
up with all results (actually an
infinite number of results) from which
to choose.

Tracy Johnson
MSI Schaevitz Sensors


-----Original Message-----
From: Roy Brown [mailto:[log in to unmask]]
Sent: Wednesday, January 24, 2001 11:46 AM
In message <[log in to unmask]>, Gavin Scott

But isn't the problem with Quantum Cryptography that it can also *solve*
any encrypted message in finite time?

A quantum machine, given an NP-complete problem to solve, like say the
Travelling Salesman problem for n nodes, should in principle be able to
consider all possible solutions at once (impossible or impracticable
with a conventional linear Von Neumann machine, or even with a barrel of
them strapped together) and arrive at the answer in one-pass, n time,
instead of e^n time.

And as good cryptography is built around NP-completeness, and as it can
be mathematically shown that all NP-complete problems are essentially
identical (a machine to solve one of them can solve them all)... then by
the time you've built a machine to do Quantum Cryptography, you've
practically built the machine to crack the cypher too.

--
Roy Brown        'Have nothing in your houses that you do not know to be
Kelmscott Ltd     useful, or believe to be beautiful'  William Morris