Research Publications ~ 2010

A comparative study of the structure and behaviour of the operating system thumbnail caches used in Kubuntu and Ubuntu (9.10 and 10.04)

S.Morris, H.Chivers

Abstract

Browsing directories in thumbnail mode can assist the user in locating relevant documents quickly by providing a graphical representation of each file. Whilst thumbnail images can assist the user, they can be resource intensive to generate; therefore operating systems generally cache these images, along with associated metadata, to prevent unnecessary rendering. Thumbnail caches can provide a variety of information, including file paths and images of documents; however it is necessary to understand the user activity which resulted in the artefacts being created to understand their forensic significance.

This research used baseline versions of Ubuntu and Kubuntu (both versions 9.10 and 10.04) in virtual machines to determine the effects of a variety of user actions on the information stored in the thumbnail caches. A series of experiments were conducted to identify the structures used to store artefacts both in the thumbnail cache and any related file throughout the system, as well as determining the meaning of each artefact. Each experiment was performed on both operating systems and mimicked a variety of typical user behaviours, such as moving a file or accessing a USB stick.

Whilst both thumbnail caches implement the same structure for storing data, the user behaviour which leads to artefacts being stored in the thumbnail caches differs considerably between the two operating systems. Other information about user activity can be deduced from the thumbnail cache itself; for example, Kubuntu uses an RGB format for items cached without the directory being viewed and uses an RGBA format for standard record creation. This paper also identifies the user activity which led to artefacts being recovered and discusses the strengths and weaknesses of the thumbnail caches.

This research shows that similar artefacts from two closely related operating systems may nevertheless suggest different types of user activity, and hence have a different forensic significance.

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Reference

Morris, S; Chivers, H; (2010); "A comparative study of the structure and behaviour of the operating system thumbnail caches used in Kubuntu and Ubuntu (9.10 and 10.04)"; Proceedings from 4th Cybercrime Forensics Education & Training, Canterbury Christ Church University, Canterbury, UK.

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