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'Severe' OpenSSL vuln busts public key crypto

Private keys pilfered through power supply

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Computer scientists say they've discovered a "severe vulnerability" in the world's most widely used software encryption package that allows them to retrieve a machine's secret cryptographic key.

The bug in the OpenSSL cryptographic library is significant because the open-source package is used to protect sensitive data in countless applications and operating systems throughout the world. Although the attack technique is difficult to carry out, it could eventually be applied to a wide variety of devices, particularly media players and smartphones with anti-copying mechanisms.

"Wherever you need to verify the origin of a piece of software or a piece of information, those building blocks come in handy," said Karsten Nohl, an independent security researcher. Nohl has in unrelated attacks broken encryption in widely used smartcards and cordless phones. "The OpenSSL library provides much more than just SSL."

The scientists, from the University of Michigan's electrical engineering and computer science departments, said the bug is easily fixed by applying cryptographic "salt" to an underlying error-checking algorithm. The additional randomization would make the attack unfeasible.

An OpenSSL official, who asked that his name not be published, said engineers are in the process of pushing out a patch and stressed the attack is difficult to carry out in real-world settings.

The university scientists found that they could deduce tiny pieces of a private key by injecting slight fluctuations in a device's power supply as it was processing encrypted messages. In a little more than 100 hours, they fed the device enough "transient faults" that they were able to assemble the entirety of its 1024-bit key.

"This is probably not as much of a threat to a server system as it is to a consumer device," said Todd Austin, one of the scientists who devised the attack. "The place where this would be more applicable would be if you want to attack a Blu-ray player (where) you have an environment where someone is giving you a device that has a private key to protect intellectual property and you have physical access to the device."

Servers, by contrast, would be much harder to attack because they are generally located in places that prevent people from manipulating their power supply. But that doesn't mean they're immune to such exploits. In events where a machine was overheating or otherwise experiencing power fluctuations, the vulnerability will cause servers to leak secret data that could be intercepted by attackers.

The scientists are also experimenting with the possibility of exploiting the bug using lasers or natural radiation sources, they said.

The attack is enabled by what the researchers described as a "severe vulnerability" in the OpenSSL innards that carry out authentication based on the RSA public key encryption algorithm. It resides in the so-called fixed window exponentiation algorithm of the open-source crypto library, which is used when errors arise. By triggering a single-bit error in a multiplication operation, the scientists were able to force OpenSSL to divulge 4 bits of the secret key.

Once they gathered about 8,800 malformed messages from the targeted device, they fed the data into an 81-machine cluster of 2.4 GHz Pentium-4 systems running a custom-designed algorithm. They applied the technique to an embedded hardware device consisting of a Sparc processor running a Linux operating system and were able to extract its 1024-bit private key in 104 hours.

The researchers said it may be possible to apply the method to other crypto libraries, such as one offered by the Mozilla Foundation.

The other two scientists working on the project were Valeria Bertacco and Andrea Pellegrini. Their paper (pdf) will be presented next week in Dresden at the Design Automation and Test in Europe conference. ®

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