[CrackMonkey] spooky cryptography

Lukas Eklund leklund at flynn.zork.net
Thu Apr 27 09:43:09 PDT 2000

PHYSICS NEWS UPDATE The American Institute of Physics Bulletin of
Physics News Number 480   April 24, 2000   by Phillip F. Schewe and
Ben Stein

demonstrated for the first time by three independent research groups,
advancing hopes for eventually protecting sensitive data from any kind
of computer attack.  In the latest--and most foolproof--variation yet
of the data-encryption scheme known as quantum cryptography,
researchers employ pairs of "entangled" photons, particles that can be
so intimately interlinked even when far apart that a perplexed
Einstein once derided their behavior as "spooky action at a distance."
Entanglement-based quantum cryptography has unique features for
sending coded data at practical transmission rates and detecting
eavesdroppers.  In short, the entanglement process can generate a
completely random sequence of 0s and 1s distributed exclusively to two
users at remote locations.  Any eavesdropper's attempt to intercept
this sequence will alter the message in a detectable way, enabling the
users to discard the appropriate parts of the data.  This random
sequence of digits, or "key," can then be plugged into a code scheme
known as a "one-time pad cipher,"which converts the message into a
completely random sequence of letters. This code
scheme--mathematically proven to be unbreakable without knowledge of
the key--actually dates back to World War I, but its main flaw had
been that the key could be intercepted by an intermediary.  In the
1990s, Oxford's Artur Ekert (artur.ekert at qubit.org) proposed an
entanglement-based version of this scheme, not realized until now.  In
the most basic version, a specially prepared crystal splits a single
photon into a pair of entangled photons.   Both the message sender
(traditionally called Alice) and the receiver (called Bob) get one of
the photons.  Alice and Bob each have a detector for measuring their
photon's polarization, the direction in which its electric field
vibrates. Different polarizations could represent different digits,
such as the 0 and 1 of binary code.  But according to quantum
mechanics, each photon can be in a combination (or superposition) of
polarization states, and essentially be a 0 and 1 at the same time.
Only when one of them is measured or otherwise disturbed does it
"collapse" to a definite value of 0 and 1, in a random way.  But once
one particle collapses, its entangled partner is also forced to
collapse into a specific digit correlated with the first digit.  With
the right combination of detector settings on each end, Alice and Bob
will get the exact same digit.  After receiving a string of entangled
photons, Alice and Bob discuss which detector settings they used,
rather than the actual readings they obtained, and they discard
readings made with the incorrect settings. At that point, Alice and
Bob have a random string of digits that can serve as a completely
secure key for the mathematically unbreakable one-time pad cipher.  In
their demonstration, Los Alamos researchers (Paul Kwiat,
kwiat at lanl.gov) simulated an eavesdropper (by passing the photons
through a filter on their way to Alice and Bob) and readily detected
disturbances in their transmissions (by employing what may be the
first practical application of the quantum-mechanical test known as
Bell's theorem), enabling them to discard the purloined information.
In a separate demonstration of entangled cryptography for completely
isolated Alice and Bob stations separated by 1 km of fiber optics, an
Austrian research team (Thomas Jennewein, University of Vienna,
thomas.jennewein at univie.ac.at) created a secret key and then securely
transmitted an image of the "Venus" von Willendorf, one of the
earliest known works of art. (See figures at www.quantum.at and
www.aip.org/physnews/graphics.)   Meanwhile, a University of Geneva
group (Nicholas Gisin, Nicolas.Gisin at physics.unige.ch) demonstrates
entangled cryptography over many kilometers of fiber using a photon
frequency closest to what is used on real-life fiber optics lines.  In
these first experiments, the three groups demonstrated relatively slow
data transmission rates. However, entanglement-based cryptography is
potentially faster than non-entangled quantum cryptography, which
requires single-photon sources (and therefore, faint light sources) to
foil eavesdropping. Entangled cryptography also produces relatively
small amounts of excess photons which an eavesdropper could
conceivably skim for information.  (Three upcoming papers in Physical
Review Letters; Select Article.)
lukas	  |  "I don't have to take this abuse from you --
eklund	  |   I've got hundreds of people waiting to abuse me."

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