Crypto boffins question SSH security

Protocol isn't all it cracks up to be

By John Leyden, 23rd August 2001

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Cryptographic researchers have identified flaws in Secure Shell (SSH) which might allow hackers to obtain information about a user's password or traffic being sent using the secure protocol.

SSH has two weaknesses which might be exploited by traffic analysis that looked at the timing of keystrokes, according to a paper published by University of California, Berkeley researchers on the subject.

Firstly, if a block cipher is used, transmitted packets are packed with only an eight-bit boundary, which reveals the approximate size of original data. The second issue is that while in interactive mode every keystroke a user types is sent in a separate IP packet after a key is pressed, which gives information on a user's typing.

This may not seem to serious a problem at first but the paper (Timing analysis of keystrokes and Timing analysis of SSH) explains how a skilled cracker might derive information on the length of passwords. More sophisticated statistical traffic and timing analysis, which it is beyond our experience to understand, can yield information about what a user might have typed during a session.

Information on the attacks, and details of how they developed in a system the researchers christen as Herbivore, have been published in order to provide evidence that SSH (which is commonly used by Unix and network admins) is not as secure as people like to believe. Some countermeasures are also proposed.

The Researchers (Dawn Xiaodong Song, David Wagner and Xuqing Tian) hope their work will be consider by people designing secure protocols in future. ®