Chink in encryption armor discovered: Via Tech News on ZDNet.

An underlying flaw in the widely used encryption protocol Open Secure Shell (OpenSSH) has been made public by researchers from the Royal Holloway, University of London.
The flaw, which lies in version 4.7 of OpenSSH on Debian/GNU Linux, allows 32 bits of encrypted text to be rendered in plaintext, according to a research team from the Royal Holloway Information Security Group (ISG).

An attacker has a 2^{-18} (that is, one in 262,144) chance of success. ISG lead professor Kenny Patterson told ZDNet UK last Monday that the flaw was more significant than previous vulnerabilities in OpenSSH.

"This is a design flaw in OpenSSH," said Patterson. "The other vulnerabilities have been more about coding errors."

According to Patterson, a man-in-the-middle attacker could sit on a network and grab blocks of encrypted text as they are sent from client to server. By re-transmitting the blocks to the server, an attacker can work out the first four bytes of corresponding plaintext. The attacker can do this by counting how many bytes the attacker sends until the server generates an error message and tears down the connection, then working backwards to deduce what was in the OpenSSH encryption field before encryption.

The attack relies on flaws in the RFC (Request for Comments) internet standards that define SSH, said Patterson.

Patterson gave a talk on Monday at the IEEE Symposium on Security and Privacy in California to explain his group's research findings. The three ISG academics involved in the research were Patterson, Martin Albrecht and Gaven Watson.

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