20 November 2012
Secret Encryption Keys Distributed Via Weak Photons
Related previous papers:
Scientists Find Cheaper Way to Ensure Internet Security
By JOHN MARKOFF
Published: November 20, 2012
Scientists at Toshiba and Cambridge University have perfected a technique
that offers a less expensive way to ensure the security of the high-speed
fiber optic cables that are the backbone of the modern Internet.
The research, which will be published Tuesday in the science journal
Physical Review X, describes a technique
for making infinitesimally short time measurements needed to capture pulses
of quantum light hidden in streams of billions of photons transmitted each
second in data networks. Scientists used an advanced photodetector to extract
weak photons from the torrents of light pulses carried by fiber optic cables,
making it possible to safely distribute secret keys necessary to scramble
data over distances up to 56 miles.
Such data scrambling systems will most likely be used first for government
communications systems for national security. But they will also be valuable
for protecting financial data and ultimately all information transmitted
over the Internet.
The approach is based on quantum physics, which offers the ability to exchange
information in a way that the act of eavesdropping on the communication would
be immediately apparent. The achievement requires the ability to reliably
measure a remarkably small window of time to capture a pulse of light, in
this case lasting just 50 picoseconds the time it takes light to travel
The secure exchange of encryption keys used to scramble and unscramble data
is one of the most vexing aspects of modern cryptography.
Public key cryptography uses a key that is publicly distributed and a related
secret key that is held privately, allowing two people who have never met
physically to securely exchange information. But such systems have a number
of vulnerabilities, including potentially to computers powerful enough to
decode data protected by mathematical formulas.
If it is possible to reliably exchange secret keys, it is possible to use
an encryption system known as a one-time pad, one of the most secure forms.
Several commercially available quantum key distribution systems exist, but
they rely on the necessity of transmitting the quantum key separately from
communication data, frequently in a separate optical fiber, according to
Andrew J. Shields, one of the authors of the paper and the assistant managing
director for Toshiba Research Europe. This adds cost and complexity to the
cryptography systems used to protect the high-speed information that flows
over fiber optic networks.
Weaving quantum information into conventional networking data will lower
the cost and simplify the task of coding and decoding the data, making quantum
key distribution systems more attractive for commercial data networks, the
Modern optical data networking systems increase capacity by transmitting
multiple data streams simultaneously in different colors of light. The
Toshiba-Cambridge system sends the quantum information over the same fiber,
but isolates it in its own frequency.
We can pick out the quantum photons from the scattered light using
their expected arrival time at the detector, Dr. Shields said. The
quantum signals hit the detector at precisely known times every one
nanosecond, while the arrival time of the scattered light is random.
Despite their ability to carry prodigious amounts of data, fiber-optic cables
are also highly insecure. An eavesdropper needs only to bend a cable and
expose the fiber, Dr. Shields said. It is then possible to capture light
that leaks from the cable and convert it into digital ones and zeros.
The laws of quantum physics tell us that if someone tries to measure
those single photons, that measurement disturbs their state and it causes
errors in the information carried by the single photon, he said. By
measuring the error rate in the secret key, we can determine whether there
has been any eavesdropping in the fiber and in that way directly test the
secrecy of each key.