Noise keeps spooks out of the loop

[R.A. Hettinga]

<http://www.newscientisttech.com/article.ns?id=mg19426055.300&print=true>


- tech - 23 May 2007 - New Scientist Tech
Noise keeps spooks out of the loop

	*	23 May 2007

	*	NewScientist.com news service

	*	D. Jason Palmer


Enlarge image

Noise encryption


SPYING is big business, and avoiding being spied on an even bigger one. So
imagine if someone came up with a simple, cheap way of encrypting messages
that is almost impossible to hack into?

American computer engineer Laszlo Kish at Texas A&M University in College
Station claims to have done just that. He says the thermal properties of a
simple wire can be exploited to create a secure communications channel, one
that outperforms quantum cryptography keys.

His cipher device, which he first proposed in 2005, exploits a property
called thermal noise. Thermal noise is generated by the natural agitation
of electrons within a conductor, which happens regardless of any voltage
passed through it. But it does change depending on the conductor's
resistance.

Kish and his collaborators at the University of Szeged in Hungary say this
can be used to securely pass information, or an encryption key, down any
wire, including a telephone line or network cable. In their device, both
the sender Alice and the receiver Bob have an identical pair of resistors,
one producing high resistance, the other low resistance. The higher the
total resistance on the line, the greater the thermal noise.

Both Alice and Bob randomly choose which resistor to use. A quarter of the
time they will both choose the high resistor, producing a lot of noise on
the line, while a quarter of the time they will both choose the low
resistor, producing little noise. If either detect a high or a low amount
of noise in the line, they ignore any communication.

Half the time, however, they will choose differently, producing an
intermediate level of thermal noise, and it is now that a message can be
sent. If Bob turns on his high resistor, and records an intermediate level
of noise, he instantly knows that Alice has chosen her low resistor, in
essence sending a bit of information such as 1 or 0. Kish's cipher does
this many times, sending a random series of 1s and 0s that can form the
basis of an encryption key, the researchers say
(http://www.arxiv.org/abs/physics/0612153).

That message is also secure. For a start, as Kish notes, it takes an
"educated eavesdropper" to even realise information is being sent when
there seems to be just low-level noise on the line. If they do try to
eavesdrop, they can only tell a message is being sent, not what it is,
because it's impossible to tell whether Alice has a high or low resistor
turned on, and whether the bit of information is a 1 or a 0. What's more,
eavesdropping on the line will naturally alter the level of thermal noise,
so Alice and Bob will know that someone is listening in.

Kish and his team have now successfully built a device that can send a
secure message down a wire 2000 kilometres long, much further than the best
quantum key distribution (QKD) devices tried so far. Tests show a signal
sent via Kish's device is received with 99.98 per cent accuracy, and that a
maximum of just 0.19 per cent of the bits sent are vulnerable to
eavesdropping. The error rate is down to the inherent resistance of the
wire, and choosing a larger wire in future models should help reduce it
further.

However, this level of security already beats QKD. What's more, the system
works with fixed lines, rather than the optical fibres used to carry
photons of light at the heart of quantum encryption devices. It is also
more robust, as QKD devices are vulnerable to corruption by dust, heat and
vibration. It is also much cheaper. "I guess it's around a hundred dollars,
at most," Kish says.

"This is a system that should be taken seriously," says security specialist
Bruce Schneier, who founded network security firm BT Counterpane. He says
he was seduced by the simplicity of the idea when it was first proposed by
Kish, and now wants to see independent tests of the working model. "I
desperately want someone to analyse it," he says. "Assuming it works, it's
way better than quantum."

>From issue 2605 of New Scientist magazine, 23 May 2007, page 32


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R. A. Hettinga <mailto: rah at ibuc.com>
The Internet Bearer Underwriting Corporation <http://www.ibuc.com/>
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