SLAMorris Final Thesis After Corrections.pdf - Cranfield University

files is described on the Joint Photographic Experts Group website [JPEG,
2013].Using the brute force approach of searching for known signatures and
strings present in a file it is possible to identify a JPEG header by searching for
the signature string OxFFD8.
A thumbnail stored in JPEG format is likely to be stored over several clusters;
clusters which contain the middle of a JPEG file may not have any easily
identifiable features and therefore a tailored identification approach is required.
As a visual thumbnail may begin at any point in a cluster it is necessary to
perform an exhaustive byte by byte search for the header signature in order to
identify the start of the image.
It is possible to successfully decode a potential JPEG fragment that does not
belong in the current file sequence; identifying the wrong fragment increases in
likelihood when there is a substantial amount of similar images involved. One
possible solution to this problem is to analyse the changes in the intensity of the
pixels around the join of the clusters; a large change in pixel intensity suggests
the clusters should not be reassembled in that pattern [Pal, 2008]. The process
of identifying and reassembling JPEG file fragments can be considered process
intensive because of the requirement to decode each potential new cluster
sequence.
JPEG identification techniques typically combine identification and reassembly
of the file into a single phase where each cluster of the file is searched for in
order. When a potentially matching cluster is identified JPEG decoding is
performed on the current file fragments; if no error occurs during decoding the
fragment is added to the file sequence and the next cluster in the sequence is
searched for [Garfinkel, 2007].
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