Pretty Good Privacy Flaw Reported

The security flaw centers on PGP's key recovery system, allowing someone to gain escrow capabilities over a sender's message, according to industry analysts.

PGP, a business unit of Network Associates (NAI), last week issued a public "regret" for the flaw on its Web site. The company is offering via download patches and new 6.5x freeware releases to fix the bug, according to Mike Wallach, president of PGP's security division.

Wallach described the potential PGP attack as "esoteric" and extremely difficult to pull off. He said the company has not found any user with compromised data because of the publicized flaw. The affected source code was written by PGP creator Phillip Zimmerman more than three years ago, before the company was acquired by NAI.

"This is the reason we have always submitted our source code for peer review. We want and expect people to find vulnerabilities so we can fix them," Wallach said.

Key recovery is a mechanism for third parties, such as law enforcement agencies, to unscramble encrypted information. In PGP, a user encrypts data with their private key, which is then decrypted by a public key.

PGP uses an Additional Decryption Key (ADK) that works in parallel with the user's private key to allow for third party decryption of data. Although use of ADKs is not required, the capability is in place with versions of PGP. Because the ADK does not use a digital signature, holders of non-signed ADKs -- which can be created without the real user's knowledge -- can intercept the message, said Shawn Hernan, team leader for vulnerability handling at the CERT (Computer Emergency Response Team) Coordination Center.

"If a sender of a message isn't paying attention, which is easy to do, it's possible to encrypt not only to the key you intended to, but also to the key of an intruder's choosing under some circumstances," Hernan said.

Charging into a pressure-intense security market, PGP is guilty of being a bit "too liberal" for keys it will encrypt to, Hernan added. "The problem here was just a simple oversight," he said. "The fix to the code was just a simple design. More rigorous design or more rigorous testing probably would have caught it."

The PGP vulnerability, which was discovered by German researcher Ralf Senderek, brings to light criticism of key escrow implementation that is often deployed at the corporate level, said Brad Arkin, a manager at Reliable Software Technologies' security group.

"Key escrow, at an executive-summary level, looks good on paper," Arkin said. "But as you introduce more complexity, it becomes more difficult to manage." It is a difficult and complicated process to decide who would have control over a digital signature in ADK, he added.

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