SLAMorris Final Thesis After Corrections.pdf - Cranfield University

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were not from thumbnail cache files were randomly selected for comparison from data set 1. A small software program was implemented to analyse the frequencies of byte patterns for up to 4 bytes to ascertain if any characteristics could be identified for JPEG fragments. It was found that for single byte frequencies 0x00 is often quite high, with 0xFF generally being one of the less frequent bytes; four example images and their associated single byte frequencies are shown in figure 8.2. For two byte frequencies there are interesting characteristics for those starting with 0xFF; for example the set of bytes 0xFF FF does not appear in any of the JPEG image-only thumbnail cache fragments. However the sequence 0xFF FF did occur on multiple occasions in the non-thumbnail JPEG fragments. This interesting characteristic is due to the use of 0xFF as the first byte of a start of sequence marker; therefore only sequence markers used in the JPEG specification are valid in the JPEG clusters. A python program was written to extract each 2 byte sequence starting with 0xFF found in the Windows 7 JPEG visual thumbnails. The program found eight unique 2 byte sequences, which are documented in table 8.1;the function of each byte sequence is also described [Cuturicu,1999; Weeks, 1998]. Out of the identified byte pairs, six have static positions relative to the start of the visual thumbnail; therefore only 0xFF 00 and 0xFFD9 can occur after than the first 611 bytes of a JPEG visual thumbnail. As the first 623 bytes of a visual thumbnail are mostly consistent if a marker is identified it is possible to check the bytes around it and validate it occurs at the right relative offset in the visual thumbnail. If the byte sequence 0xFFD9 occurs this should be directly followed by a subrecord header; therefore this byte sequence is unlikely to occur in a H3 fragment. Page 210
Figure 8-2: Single byte frequencies for 4 visual thumbnails Page 211
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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
Page 180 and 181: valid results only when the file is
Page 182 and 183: 7.3.1 Statistical Methods If docume
Page 184 and 185: ate for HTML and JPEG files [Veenma
Page 186 and 187: Having described current methods fo
Page 188 and 189: previous experiments or downloaded
Page 190 and 191: L2: The fragment is part of a visua
Page 192 and 193: Table 7-1: Breakdown of file fragme
Page 194 and 195: approximately half the data in Data
Page 196 and 197: Table 7-3: A list of thumbnail cach
Page 198 and 199: Figure 7-2: Classifications for thu
Page 200 and 201: 7.7.2 H2 For the identification of
Page 202 and 203: Relative offset: 48 0B 09 0C 11 0F
Page 204 and 205: The Bayesian Network takes into acc
Page 206 and 207: Figure 7-3: Bayesian network for H1
Page 208 and 209: Actual Fragment Type(Percentage) 7.
Page 210 and 211: Actual Fragment Type(Percentage) Ta
Page 216 and 217: Table 7-19: Method 4 Results for th
Page 218 and 219: more suited to pattern recognition
Page 220 and 221: and 2. This is possible because the
Page 222 and 223: This chapter has successfully ident
Page 224 and 225: 8.2 Problem Definition In the intro
Page 226 and 227: information to examine at the end o
Page 228 and 229: files is described on the Joint Pho
Page 232 and 233: In order to identify potential H3 f
Page 234 and 235: Actual Fragment Type (Percentage) p
Page 236 and 237: Figure 8-3: A summary of the hybrid
Page 238 and 239: cache file fragments. The headers i
Page 240 and 241: follow the standard PNG specificati
Page 242 and 243: then be viewed by an analyst and us
Page 244 and 245: Actual Fragment Type (Percentage) T
Page 246 and 247: 8.8 Discussion The end aim of Chapt
Page 248 and 249: identification method into stages l
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
Page 257 and 258: H1:Thumbcache_idx.db file H2: Image
Page 259 and 260: Therefore this research assumes tha
Page 261 and 262: ecords and 2 partial records. The n
Page 263 and 264: equirement for generating table siz
Page 265 and 266: 100% success rate, with 0% false po
Page 267 and 268: then stored and marked as a complet
Page 269 and 270: Once a start of file fragment has b
Page 271 and 272: number of fragments which contain o
Page 273 and 274: Figure 9-7: Reassembling an image b
Page 275 and 276: As the number of fragments being an
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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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