Strengthening quantum cryptography by putting on blinders

[Sean McGrath]

---------- Forwarded message ----------
Date: Thu, 14 Jul 2005 11:11:59 -0400
From: physnews at aip.org
To: sean at MANYBITS.NET
Subject: Physics News Update 737

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 737 July 14, 2005  by Phillip F. Schewe, Ben Stein

[...]

STRENGTHENING QUANTUM CRYPTOGRAPHY BY PUTTING ON BLINDERS.  A
Korea-UK team (contact Myungshik Kim, Queen's University, Belfast,
m.s.kim at qub.ac.uk , or Chilmin Kim, Paichai University) has
introduced a method for preventing several clever attacks against
quantum cryptography, a form of message transmission that uses the
laws of quantum physics to make sure an eavesdropper does not
covertly intercept the transmission.  Making the message sender and
receiver a little blind to each other's actions, the researchers
have shown, can bolster their success against potential
eavesdroppers.
In quantum cryptography, a sender (denoted as Alice) transmits a
message to a receiver (called Bob) in the form of single photons
each representing the 0s and 1s of binary code. If an eavesdropper
(appropriately named Eve) attempts to intercept the message, she
will unavoidably disturb the photon through the Heisenberg
uncertainty principle, which says that even the gentlest observation
of the photon will perturb the particle. This will be instantly
detectable by Alice and Bob, who can stop the message and start
again.  Quantum cryptography is already being used in the real world
and is even available commercially as a way for companies to
transmit sensitive financial data.  But in its real-world
implementation, a weak pulse of light (rather than a perfect stream
of single photons) is sent down a transmission line that is "lossy,"
or absorbs photons.  So feasible attacks on quantum cryptography
include the pulse-splitting attack (in which Eve splits a
transmitted pulse into two pulses and examines one of them for
information), the pulse-cloning attack (in which a transmitted pulse
is copied to relatively high accuracy and then inspected for its
information), and the "man-in-middle" or impersonation attack, in
which Eve could impersonate Alice or Bob by intercepting the
transmission and acting as sender or receiver.
A new paper proposes a solution to these three attacks by proposing
a technique called "blind polarization."  In this technique, Alice
and Bob verify their identities to each other in a rather
paradoxical way, by performing some actions that is their own
private information. Yet these actions make the message completely
indecipherable to a third party. Alice creates a pair of pulses, but
with random polarizations (polarization indicates the direction or
angle in which each pulse's electric field points relative to some
reference, such as a horizontal line)  Alice sends the pulses to
Bob, who does not know the polarizations.  Nonetheless, without
measuring the polarization values, Bob is able to rotate the
polarization of one pulse by one amount and the other pulse by
another amount, but he doesn't tell Alice which pulses got which
treatment.  Alice receives the pulses, and then encodes them with a
message (representing the binary value 0 or 1, which could stand for
"no" or "yes), then blocks one of the pulses, without telling Bob
which one was blocked.  Bob then reverses the various polarizations
by a certain amount to get the desired message.  The various
polarization adjustments are designed in such a way that either
pulse Alice sends will yield the desired information.  According to
researcher Myungshik Kim, Alice has her own private information on
which pulse is blocked, while Bob has his own private information on
which pulse he rotated by a given amount.  Once Alice begins the
transmission, there is no way for Eve to have this private
information which makes their protocol effective against the
man-in-middle and other attacks. (Kye et al., Physical Review
Letters, upcoming article).  This paper is the latest in a wave that
plugs up potential vulnerabilities in quantum cryptography (for an
example of using
"quantum decoys" to thwart attacks, see Lo et al, Physical Review
Letters, 17 June 2005)

***********
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