The most common digital security technique used to protect both media copyright and Internet communications has a major weakness, University of Michigan computer scientists have discovered.

RSA authentication is a popular encryption method used in media players, laptop computers, smartphones, servers, and other devices. Retailers and banks also depend on it to ensure the safety of their customers' information online.

The scientists found they could foil the security system by varying the voltage supply to the holder of the "private key," which would be the consumer's device in the case of copy protection and the retailer or bank in the case of Internet communication. It is highly unlikely that a hacker could use this approach on a large institution, the researchers say.

The findings would be more likely to concern media companies and mobile device manufacturers, as well as those who use them.

Andrea Pellegrini, a doctoral student in the university's department of electrical engineering and computer science, presented a paper on the research in March at a Design, Automation and Test in Europe (DATE) conference in Dresden, Germany.

"The RSA algorithm gives security under the assumption that as long as the private key is private, you can't break in unless you guess it. We've shown that that's not true," says Valeria Bertacco, an associate professor in that department.

The private keys contain more than 1,000 digits of binary code. To guess a number that large would take longer than the age of the universe, Pellegrini says. Using their voltage tweaking scheme, the U-M researchers were able to extract the private key in about 100 hours.

They carefully manipulated the voltage with an inexpensive device built for that purpose. Varying the electric current essentially stresses out the computer and causes it to make small mistakes in its communications with other clients. Those faults reveal small pieces of the private key. Once the researchers caused enough faults, they were able to reconstruct the key offline.

This type of attack doesn't damage the device, so no tamper evidence is left.

"RSA authentication is so popular because it was thought to be so secure," says Todd Austin, a professor in the electrical engineering and computer science department. "Our work redefines the level of security it offers. It lowers the safety assurance by a significant amount."

Although the published paper on the topic discusses only the problem, the professors say they've identified a solution. It's a common cryptographic technique called "salting" that changes the order of the digits in a random way every time the key is requested.

"We've demonstrated that a fault-based attack on the RSA algorithm is possible," Austin says. "Hopefully, this will cause manufacturers to make a few small changes to their implementation of the algorithm. RSA is a good algorithm, and I think, ultimately, it will survive this type of attack."

The research was funded by the National Science Foundation and the Gigascale Systems Research Center.