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Researchers devise new attack techniques against SSL

Lucian Constantin | Feb. 6, 2013
The developers of many SSL libraries are releasing patches for a vulnerability that could potentially be exploited to recover plaintext information, such as browser authentication cookies, from encrypted communications.

The TLS designers attempted to block such attacks in version 1.2 of the TLS specification, by reducing the timing variations to a level they thought would be too low to be exploitable. However, the Lucky Thirteen research from AlFardan and Paterson shows that this assumption was incorrect and that successful padding oracle attacks are still possible.

"The new AlFardan and Paterson result shows that it is indeed possible to distinguish the tiny timing differential caused by invalid padding, at least from a relatively close distance -- e.g., over a LAN," Matthew Green, a cryptographer and research professor at Johns Hopkins University in Baltimore, Maryland, said Monday in a blog post. "This is partly due to advances in computing hardware: most new computers now ship with an easily accessible CPU cycle counter. But it's also thanks to some clever statistical techniques that use many samples to smooth out and overcome the jitter and noise of a network connection."

In addition to being in close proximity to the targeted server, a successful Lucky Thirteen attack would also require a very high number -- millions -- of attempts in order to gather enough data to perform relevant statistical analysis of the timing differences and overcome network noise that might interfere with the process.

In order to achieve this, the attacker would need a way to force the victim's browser to make a very large number of HTTPS connections. This can be done by placing a piece of rogue JavaScript code on a website visited by the victim.

The secret plaintext targeted for decryption needs to have a fixed position in the HTTPS stream. This condition is met by authentication (session) cookies -- small strings of random text stored by websites in browsers to remember logged-in users. An authentication cookie can give the attacker access to the user's account on its corresponding website, making it a valuable piece of information worth stealing.

However, the biggest hurdle to be overcome by potential attackers is the fact that TLS kills the session after each failed decryption attempt, so the session needs to be renegotiated with the server. "TLS handshakes aren't fast, and this attack can take tens of thousands (or millions!) of connections per [recovered] byte," Green said. "So in practice the TLS attack would probably take days. In other words: don't panic."

DTLS on the other hand does not kill the session if the server fails to decrypt a record because it was altered, making the Lucky Thirteen attacks borderline practical against this protocol, Green said.

"The attacks can only be carried out by a determined attacker who is located close to the machine being attacked and who can generate sufficient sessions for the attacks," AlFardan and Paterson said. "In this sense, the attacks do not pose a significant danger to ordinary users of TLS in their current form. However, it is a truism that attacks only get better with time, and we cannot anticipate what improvements to our attacks, or entirely new attacks, may yet to be discovered."


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