SLAMorris Final Thesis After Corrections.pdf - Cranfield University

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9.4.1 Thumbcache_idx file fragment categories The repeating and fixed size record structure of the thumbcache_idx.db file can assist with reassembling the various fragments of the file. The index file stores records in a hash table whose size is always a multiple of 101; each file fragment of 4096 bytes can hold up to 127 complete records as shown in figure 9.2. The figure shows there are four different categories of this file fragment type. Below each category is described: IDX Category_1: Fragment with 101 records and file header In this case the single fragment contains the entire 101 record index and therefore does not require further reassembly. The first check to identify these fragments is to check the file header identifier “IMMM”; this is followed by checking the header structure is a valid thumbcache_idx structure as identified in Appendix A. Within the header the number of records should be set to 101, if the number is larger, then the fragment belongs to Category_2. As a 4096 byte fragment can contain up to 127 complete records a further check is made to ensure that the fragment is empty after the 101 expected records; therefore the last 840 bytes of the fragment is checked to ensure they are0x00.If a fragment meets the criteria for this category it is logged as a reassembled file. As the entire 101 record file has been recovered the file is complete and the method moves on to the next fragment for identification. IDX Category_2: Fragment with 127 complete records and file header This fragment is the beginning of a larger index file which is stored over multiple fragments. The checks performed for this category are the same as in category_1; the difference between the two categories is the total number of records stored in the complete file. The file header contains the size of the index hash table and it is therefore possible to work out the total number of file fragments storing the index file; each fragment can hold up to 127 complete Page 240
ecords and 2 partial records. The number of allocated records in the hash table is also given in the file header this can assist in reassembly; in a situation where two fragments could fit into the same position of an index file, calculating the total number of records in the file could rule out a potential matching record. Figure 9-2: Thumbcache_IDX Classifications Page 241
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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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available for office documents; the
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future. It is possible to establish
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structure and syntax the user and s
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Identify existing file carving tech
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throughout this research. Within th
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corroborate the results. Time const
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4.2.1 Legal Constraints The law can
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to the evidence and to ensure any a
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To ensure the evidence is accurate
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evidence being extracted is crucial
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analyse human behaviour, it can sti
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4.3 Criteria for evaluating the evi
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In order to determine an artefact w
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5 The Structure and Behaviour of th
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the way data changes. The experimen
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5.4 Default Installations This sect
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Figure 5-3: The structure of thumbn
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these can be identified by the addi
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The tEXt chunks contain the metadat
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holds information about sample dept
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Start of tEXt Chunk This tEXt chunk
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00 00 00 2074 45 58 7453 6F 66 74 @
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order to establish if the software
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Each piece of software installed by
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which is present in the subrecords
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5.6.2 The modification of subrecord
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ascertain if unused subrecords woul
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in thumbnail view. This shows that
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5.9.1 Metadata There are three dist
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only information which does not con
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information in the .thumbnails cach
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uses substantially more checks whic
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any manipulation to appear as stand
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highlighting the need for understan
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elationship between information con
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document the state of a default Win
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Figure 6-1: Directory structure for
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Clone 1-8: Directories at level 0 w
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Figure 6-2: The centralised thumbna
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Start of Record 87 F7 6A 62 4B 7B C
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Start of standard thumbnail cache s
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Start of subrecord 14F8CE010 43 4D
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6.5 Identifying the behaviour The t
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greater than the maximum size image
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experiment was repeated with a non-
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to be shown [Douglas, 2009].Figure
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6.5.1.2 Circumstances where informa
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thumbnail cache based solely upon t
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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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Page 230 and 231: were not from thumbnail cache files
Page 232 and 233: In order to identify potential H3 f
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Page 236 and 237: Figure 8-3: A summary of the hybrid
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Page 246 and 247: 8.8 Discussion The end aim of Chapt
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Page 251 and 252: 9 Thumbnail cache fragment reassemb
Page 253 and 254: Once the fragments have been identi
Page 255 and 256: 9.3.2 Logging In order to maintain
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Page 259: Therefore this research assumes tha
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Page 269 and 270: Once a start of file fragment has b
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Page 281 and 282: Table 9-1: Results from reassembly
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Page 287 and 288: 10 Establishing the evidential valu
Page 289 and 290: The thumbnail cache artefact extrac
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Page 295 and 296: 10.7 Discussion Both live system an
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Page 300 and 301: Figure 11-1: A breakdown of aim of
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Page 308 and 309: 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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SLAMorris Final Thesis After Corrections.pdf - Cranfield University

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