Timing Analysis of Keystrokes and Timing Attacks on SSH

Perry E. Metzger perry at piermont.com
Wed Aug 22 11:37:39 EDT 2001

What I find really neat here is that up until now, serious traffic
analysis has been fairly neglected in the open crypto community. Is
this the start of things to come?

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Date: Wed, 22 Aug 2001 08:53:30 -0600
From: aleph1 at securityfocus.com
To: secpapers at securityfocus.com
Cc: secureshell at securityfocus.com
Subject: Timing Analysis of Keystrokes and Timing Attacks on SSH
Message-ID: <20010822085330.J3366 at securityfocus.com>
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Timing Analysis of Keystrokes and Timing Attacks on SSH
Dawn Xiaodong Song, David Wagner, Xuqing Tian
University of California, Berkeley

SSH is designed to provide a security channel between two hosts. Despite the 
encryption and authentication mechanisms it uses, SSH has two weakness: 
First, the transmitted packets are padded only to an eight-byte boundary (if
a block cipher is in use), which reveals the approximate size of the 
original data. Second, in interactive mode, every individual keystroke that 
a user types is sent to the remote machine in a separate IP packet 
immediately after the key is pressed, which leaks the interkeystroke timing 
information of users' typing. In this paper, we show how these seemingly 
minor weaknesses result in serious security risks.

First we show that even very simply statistical techniques suffice to 
reveal sensitive information such as the length of users' passwords or even 
root passwords. More importantly, we further show that using more advanced 
statistical techniques on timing information collected from the network, 
the eavesdropped can learn significant information about what users type in 
SSH sessions. In particular, we perform a statistical study of users' 
typing patterns and show that these patterns reveal information about the 
keys typed. By developing a Hidden Markov Model and our key sequence 
prediction algorithm, we can predict key sequences from the interkeystroke 
timings. We further develop and attacker system, Herbivore, which tried to 
learn users' passwords by monitoring SSH sessions. By collecting timing 
information on the network, Herbivore can speed up exhaustive search for 
passwords by a factor of 50. We also propose some countermeasures.

In general our results apply not only to SSH, but also to general class of 
protocols for encrypting interactive traffic. We show that timing leaks 
open a new set of security risks, and hence caution must be taken when 
designing this type of protocol.


Elias Levy
Si vis pacem, para bellum

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