Tags: Scientists | Craft | Forgery-Proof | Signature

Scientists Craft Forgery-Proof Signature

Wednesday, 16 May 2001 12:00 AM

Massachusetts Institute of Technology physicist Isaac Chuang and University of California, Berkeley, computer scientist Daniel Gottesman have developed a quantum digital signature they say is "absolutely secure … no matter how advanced the enemy's computers."

Digital signatures both authenticate electronic documents and ensure they were not tampered with during transmission.

The security of presently available digital signatures depends on the ability of forgers to duplicate a signature key, a series of digital bits numbering either 0 or 1 used to decode messages or produce a valid signature. A signature key is much like a house key - the longer it is, and the more and varied its notches (in this case, 0s and 1s), the harder it is to replicate.

To replicate the key, current security methods force forgers to solve certain difficult mathematical problems. These mathematical problems involve finding "x," the key, given some complicated function f(x). If f(x) is a large prime number, for instance, factoring it into indivisible integers (x) is often one way to find the key.

Those signing a document provide values of f(x) to individuals who need to verify the signature, while keeping values of x itself secret. The technology's potential weak link is that many people might know values of f(x) - bankers, lawyers, bookkeepers. And even if f(x) is complicated, in theory a criminal could still derive x and thus covertly forge or alter a document.

But under the digital signature scheme Gottesman and Chuang propose, no one can ever discover x from f(x) - quantum mechanics won't permit it. The function f(x), which is actually a series of quantum states (like the spin direction of an atom) is called a "quantum one-way" function - one cannot reverse the reaction that gives f(x) from x.

Naval Postgraduate School computer science professor Wolfgang Baer expressed confidence in Chuang as a researcher, but is not certain the idea will work as promised.

"I would say you could probably trust Isaac Chuang to present ideas which are correct in this discipline," Baer told United Press International in a telephone interview from Monterey, Calif. "Chuang is a legitimate author with a good reputation. But it is very difficult to judge the validity of the concept he presents in his abstract."

Neil Rowe, associate chair of computer science research at the Naval Postgraduate School, doubts the absolute security of a quantum digital signature for reasons that have little to do with quantum mechanics.

"The authors' claim that their method is 'absolutely secure' is unfair since social-engineering methods can be used to entice a person to reveal their key just as much with the authors' scheme as with conventional keys," Rowe said. "That's the main threat to information security these days, as codes are virtually unbreakable in themselves."

Rowe also believes currently available digital signature schemes are secure.

"The authors seem to think there is some fundamental weakness of existing signature methods on digital computers. But no one has implemented anything close to an algorithm for factoring large prime numbers yet," Rowe said. He also sees a simple solution to problems with today's digital signatures.

"Security problems can be solved by increasing the size of the key," he explained. "A thousand bits for key length is probably secure against anything likely to be built in the next 20 years. Ten thousand bits should give enough security for the next 100 years."

Other experts were more enthusiastic, however.

Bert Kalish, director of laboratories for RSA, a leading corporate security provider, believes the Gottesman and Chuang's quantum-signature proposal represents a significant advance in cryptography if their results are correct, which Kalish said he has no reason to doubt.

"I'm delighted to see more progress being made in quantum cryptography - it is such a vast, still unexplored field with so much promise," Kalish told UPI from his Bedford, Mass., office. "If the complexity of this digital signature program is similar to the complexity of establishing quantum cryptography keys, then I could see practical implementation of quantum digital signatures in just a few years."

Chuang and Gottesman performed their research in conjunction with MIT Media Lab in Cambridge, Mass.

Copyright 2001 by United Press International.

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Massachusetts Institute of Technology physicist Isaac Chuang and University of California, Berkeley, computer scientist Daniel Gottesman have developed a quantum digital signature they say is absolutely secure … no matter how advanced the enemy's computers. Digital...
Wednesday, 16 May 2001 12:00 AM
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