SLAMorris Final Thesis After Corrections.pdf - Cranfield University

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Circumstances where information is added to the thumbnail cache A subrecord in the thumbnail cache is created when a file is viewed in thumbnail view, in a browser window, at a URI, for which no subrecord currently exists in the normal or large directories. A new record can be created under the following circumstance if the following 3 criteria are met: Case 1: No record available A directory is viewed using the file browser or the file browser within an application when it is in thumbnail mode The directory being viewed contains file types which are compatible with the thumbnail cache A file exists in the directory which does not currently have a thumbnail subrecord Case 2: A subrecord only exists in the fail directory if the following 4 criteria are met: A directory is viewed using the file browser or the file browser within an application when it is in thumbnail mode The directory being viewed contains file types which are compatible with the thumbnail cache A file exists in the directory which only has a subrecord in the fail directory The file is processed as if no record information is available Page 86
5.6.2 The modification of subrecords This subsection looks at the modification of information stored within the cache. In order to study the causes of modification to the thumbnail cache, clones of the virtual machines from Section 5.6.1will be used. Therefore the experiments start with data within the thumbnail cache; in order to assess how this data is modified the original source files will be modified during a variety of experiments described in this sub-section. Modifying the contents of a file A randomly selected group of files on the virtual machine were altered using appropriate pre-installed software. A list of the 500 user-generated files added to the virtual machine was stored in a text file. A small program was written to randomly select 10% of the files; the names of the selected files were stored in a second text file. Each of the selected files was opened using the default editing application for that file type; image files were modified by making the images entirely black, for text based file types the information within the file was copied and pasted underneath the original data. <strong>After</strong> the files were modified they were saved and the editor closed. Once all modifications had been completed the directories containing the user generated images were then viewed using the file manager. The modification caused the last modified time of the file to be updated. Both on Ubuntu and Kubuntu, this caused the affected subrecords to be altered, to show the current state of the file; all the files showed the modified black images or repeated text in the file browser. Modifying the files URI In order to ascertain the effect of modifying the file’s URI, the virtual machine was populated with 500 user generated files which were copied off a USB stick and into the user’s documents directory. A directory named ‘test’ was created Page 87
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CRANFIELD UNIVERSITY SARAH LOUISE A
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ABSTRACT This thesis establishes th
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ACKNOWLEDGEMENTS “And above all,
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TABLE OF CONTENTS ABSTRACT ........
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6 The structure and behaviour of th
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8.6.1 Stage 1: Preliminary Checks .
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LIST OF FIGURES Figure 1-1:A breakd
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LIST OF TABLES Table 5-1: A summary
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1 Introduction 1.1 Introduction In
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potential difficulty is the identif
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This is followed by a structural an
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Chapter 6 describes the structure a
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Chapter 11 evaluates the methodolog
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2 Related Research 2.1 Introduction
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Generally individuals, including so
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thought to be a disciplinary offenc
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2.2.2 Tools used in Forensic Comput
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structure and behaviour. Winhex pro
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with it are important aspects of an
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An important question for each arte
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For this research it is necessary t
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2007], which assist in building eve
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and therefore he did not have priva
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data may be affected by the behavio
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thumbnail cache is implemented in f
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Page 57 and 58: future. It is possible to establish
Page 59: structure and syntax the user and s
Page 62 and 63: Identify existing file carving tech
Page 64 and 65: throughout this research. Within th
Page 66 and 67: corroborate the results. Time const
Page 68 and 69: 4.2.1 Legal Constraints The law can
Page 70 and 71: to the evidence and to ensure any a
Page 72 and 73: To ensure the evidence is accurate
Page 74 and 75: evidence being extracted is crucial
Page 76 and 77: analyse human behaviour, it can sti
Page 78 and 79: 4.3 Criteria for evaluating the evi
Page 80 and 81: In order to determine an artefact w
Page 83 and 84: 5 The Structure and Behaviour of th
Page 85 and 86: the way data changes. The experimen
Page 87 and 88: 5.4 Default Installations This sect
Page 89 and 90: Figure 5-3: The structure of thumbn
Page 91 and 92: these can be identified by the addi
Page 93 and 94: The tEXt chunks contain the metadat
Page 95 and 96: holds information about sample dept
Page 97 and 98: Start of tEXt Chunk This tEXt chunk
Page 99 and 100: 00 00 00 2074 45 58 7453 6F 66 74 @
Page 101 and 102: order to establish if the software
Page 103 and 104: Each piece of software installed by
Page 105: which is present in the subrecords
Page 109 and 110: ascertain if unused subrecords woul
Page 111 and 112: in thumbnail view. This shows that
Page 113 and 114: 5.9.1 Metadata There are three dist
Page 115 and 116: only information which does not con
Page 117 and 118: information in the .thumbnails cach
Page 119 and 120: uses substantially more checks whic
Page 121 and 122: any manipulation to appear as stand
Page 123: highlighting the need for understan
Page 126 and 127: elationship between information con
Page 128 and 129: document the state of a default Win
Page 130 and 131: Figure 6-1: Directory structure for
Page 132 and 133: Clone 1-8: Directories at level 0 w
Page 134 and 135: Figure 6-2: The centralised thumbna
Page 136 and 137: Start of Record 87 F7 6A 62 4B 7B C
Page 138 and 139: Start of standard thumbnail cache s
Page 140 and 141: Start of subrecord 14F8CE010 43 4D
Page 142 and 143: 6.5 Identifying the behaviour The t
Page 144 and 145: greater than the maximum size image
Page 146 and 147: experiment was repeated with a non-
Page 148 and 149: to be shown [Douglas, 2009].Figure
Page 150 and 151: 6.5.1.2 Circumstances where informa
Page 152 and 153: thumbnail cache based solely upon t
Page 154 and 155: the cache to remove inactive record
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6.6.1 Windows.edb The database for
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Figure 6-10: Identifying the Defaul
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thumbnail view for 30 seconds. Upon
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checks are performed on the associa
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showing a relationship between info
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The type of a file can be identifie
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The event timeline created could th
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6.9.2 Media thumbnails In Windows 7
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the provenance of artefacts. This i
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6.10.2 Interpretation of Results An
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however it is still possible to tam
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implementations of the thumbnail ca
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valid results only when the file is
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7.3.1 Statistical Methods If docume
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ate for HTML and JPEG files [Veenma
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Having described current methods fo
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previous experiments or downloaded
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L2: The fragment is part of a visua
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Table 7-1: Breakdown of file fragme
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approximately half the data in Data
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Table 7-3: A list of thumbnail cach
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Figure 7-2: Classifications for thu
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7.7.2 H2 For the identification of
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Relative offset: 48 0B 09 0C 11 0F
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The Bayesian Network takes into acc
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Figure 7-3: Bayesian network for H1
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Actual Fragment Type(Percentage) 7.
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Actual Fragment Type(Percentage) Ta
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Table 7-19: Method 4 Results for th
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more suited to pattern recognition
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and 2. This is possible because the
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This chapter has successfully ident
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8.2 Problem Definition In the intro
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information to examine at the end o
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files is described on the Joint Pho
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were not from thumbnail cache files
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In order to identify potential H3 f
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Actual Fragment Type (Percentage) p
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Figure 8-3: A summary of the hybrid
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cache file fragments. The headers i
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follow the standard PNG specificati
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then be viewed by an analyst and us
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Actual Fragment Type (Percentage) T
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8.8 Discussion The end aim of Chapt
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identification method into stages l
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9 Thumbnail cache fragment reassemb
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Once the fragments have been identi
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9.3.2 Logging In order to maintain
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H1:Thumbcache_idx.db file H2: Image
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Therefore this research assumes tha
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ecords and 2 partial records. The n
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equirement for generating table siz
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100% success rate, with 0% false po
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then stored and marked as a complet
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Once a start of file fragment has b
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number of fragments which contain o
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Figure 9-7: Reassembling an image b
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As the number of fragments being an
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file fragment identification on sto
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Linux Category_4: The last fragment
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Table 9-1: Results from reassembly
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activity. All the non-standard subr
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which substantially reduces the inf
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10 Establishing the evidential valu
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The thumbnail cache artefact extrac
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and 6. Each stage of the method has
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adapted to extract artefact from ot
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10.7 Discussion Both live system an
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10.8 Conclusion This chapter evalua
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Figure 11-1: A breakdown of aim of
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analysing the file type it is possi
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singular file fragment; by combinin
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processing time of the reassembly m
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In Chapter 9.5 a decision was taken
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12 Conclusions and future work 12.1
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potential for improving the identif
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installations of the operating syst
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Carnagey, N.L., Anderson, C.A. & Bu
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Facebook, 2013. Facebook. Available
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Helix, 2011. Helix. Available at: h
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Noblett, M., Pollitt, M., Presley,
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Stone-kaplan, K., Roter, M., 2003.
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APPENDICES Cranfield University | 3
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A.1.3 Image File Header typedefstru
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Appendix B Bayesian probability tab
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B.8 Fragment contains the ASCII str
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B.15 32 byte record structure SR po
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B.19 H3 Each byte frequency is less
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B.21 H5 Stored in valid PNG chunks
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Appendix C Peer Reviewed Publicatio
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Cranfield University | 327
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