Timing analysis of keystrokes and timing attacks on SSH

Abstract

SSH is designed to provide a secure 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.