Date of Award


Degree Name

MS in Computer Science


Computer Science


College of Engineering


Bruce Edward DeBruhl

Advisor Department

Computer Science

Advisor College

College of Engineering


Secure Shell, a tool to securely access and run programs on a remote machine, is an important tool for both system administrators and developers alike. The technology landscape is becoming increasingly distributed and reliant on tools such as Secure Shell to protect information as a user works on a system remotely. While Secure Shell accounts for the abuses the security of older tools such as telnet overlook, it still has fundamental vulnerabilities which leak information about both the user and their activities through timing attacks. The OpenSSH client, the implementation included in all Linux, Mac, and Windows computers, sends each keystroke entered to the server as soon as it becomes available. As a result, an attacker can observe the network patterns to know when a user presses a key and draw conclusions based on that information such as what a user is typing or who they are. In this thesis, we demonstrate that such an attack allows a malicious observer to identify a user with a concerning level of accuracy without having direct access to either the client or server systems. Using machine learning classifiers, we identify individual users in a crowd based solely on the size and timing of packets traveling across the network. We find that our classifiers were able to identify users with 20\% accuracy using as little as one hour of network traffic. Two of them promise to scale well to the number of users.