1. Lecturer, University of Hull Centre for Internet Computing
Digital Evidence
Angus M. Marshall
BSc CEng MBCS FRSA
Lecturer, University of Hull Centre for Internet Computing
Director, n-gate ltd.
Programme Chair, FIDES 2004

2. Content Digital Evidence Forensic Computing Future Trends
Sources & Role
Forensic Computing
Principles & Practice
Future Trends
Challenges

3. Digital Evidence Evidence in digital form
Data recovered from digital devices
Data relating to digital devices

4. Source of digital evidence
More than the obvious
PCs
PDAs
Mobile Phones
GPS
Digital TV systems
CCTV
Other Embedded Devices

5. Use of digital evidence
Nature of crime determines probability of digital evidence & usefulness of evidence
Evidence of criminal act
Copyright theft, identity theft, blackmail etc.
Alibi / presence at crime scene
Habits & interests (propensity to commit crime)
“Malice aforethought”
Maps, knives ordered from e-bay......
Information retrieval
“H-bombs for dummies”

6. Taxonomy * Application guides investigative strategy
Potential sources & nature of evidence
Highlights challenges
*Marshall & Tompsett, “Spam 'n' Chips”, Science & Justice, 2002

7. Next steps
Once the nature of the activity is determined, investigation can proceed
Carefully

8. Principles and Practice
Forensic Computing
Principles and Practice

9. Forensic Computing – purpose
Forensic computing techniques may be deployed to :
Recover evidence from digital sources
Witness – factual only
Interpret recovered evidence
Expert witness – opinion & experience

10. Forensic Computing – definition
Relating to the recovery, examination and/or production of evidence for legal purposes
Computing
Through the application of computer-based techniques

11. Alternative definition
“...the application of science and engineering to the legal problem of digital evidence. It is a synthesis of science and law”
Special Agent Mark Pollitt, FBI – quoted in “Forensic Computing : A practitioner's guide” by Sammes & Jenkinson

12. Conventional Sources of Evidence
Magnetic Media
Disks, Tapes
Optical media
CD, DVD
Data
e.g. Log files, Deleted files, Swap space
Paper documents
printing, bills etc.
Handhelds, mobile phones etc.
(solid-state transient memory)

13. ACPO principles
Association of Chief Police Officers of England, Wales and Northern Ireland
Good Practice Guide for Computer Based Evidence, Version 2.
ACPO Crime Committee, 23 June 1999
Similar guidelines for Scotland
New version out November 2003

14. ACPO principles
4 principles relating to the recovery and investigation of computer based evidence
intended to guarantee the integrity of evidence and allow accurate replication of results
remove doubt / opportunity for challenge in court

15. Principle 1
No action taken by Police or their agents should change data held on a computer or other media which may subsequently be relied upon in Court.
Why ?

16. Principle 2
In exceptional circumstances where a person finds it necessary to access original data held on a target computer that person must be competent to do so and to give evidence explaining the relevance and implications of their actions.

17. Principle 3
An audit trail or other record of all processes applied to computer-based evidence should be created and preserved. An independent third party should be able to examine those processes and achieve the same result.

18. Principle 4
The Officer in charge of the case is responsible for ensuring that the law and these principles are adhered to. This applies to the possession of, and access to, information contained in a computer. They must be satisfied that anyone accessing the computer, or any use of a copying device, complies with these laws and principles.

19. Caveats Apply primarily to “single source of evidence” investigations
Networks cause problems
Locard's principle may not apply
Does not allow for ‘real-time’ investigation
Assumes that equipment can be seized and investigated offline

20. Constraints Human Rights Act Regulation of Investigatory Powers Act
P.A.C.E. & equivalents
Data Protection Act(s)
Computer Misuse Act
Direct impact on validity of evidence, rights of the suspect, ability to investigate

21. Internet Investigations – Special Features
Locality of Offence*
RIPA / HR / DP / CM contraventions
Covert nature
sysadmins unwilling to disclose
real time requirement
Network configuration
High disk activity systems
little coordination of “intelligence”
CERTs try
*Marshall & Tompsett, “Spam 'n' Chips”, Science & Justice, 2002

22. Static Evidence / Single Source
“Standard” case
Static Evidence / Single Source

23. Background Role of the forensic examiner Retrieve any and all evidence
Provide possible interpretations
How the evidence got there
What it may mean
Implication
The “illicit” activity has already been identified
Challenge is to determine who did it and how

24. Single source cases According to Marshall &Tompsett [1]
Any non-internet connected system can be treated as a single source of evidence, following the same examination principles as a single computer
Even a large network
Is this a valid proposition ?

25. Single source Implies that the locus of evidence can be determined
i.e. There are no unidentified or external entities involved
Even in a large network, all nodes can be identified
as long as the network is closed (i.e. The limit of extent of the network can be determined)
“Computer-assisted/enabled/only” categories.

26. Static Evidence Time is the enemy
Primary sources of evidence are 2o storage devices
Floppies, hard disks, CD, Zip etc.
Log files, swap files, slack space, temporary files
Data may be deleted, overwritten, damaged or compromised if not captured quickly
(See ACPO guidelines – No.1)

27. Standard seizure procedure [2]
Quarantine the scene
Move everyone away from the suspect equipment
Kill communications
Modem, network
Visual inspection
Photograph, notes
Screensavers ?
Kill power
Seize all associated equipment and removable media
Bag 'n' tag immediately
Record actions
Ask user/owner for passwords

28. Imaging and Checksumming
After seizure, before examination
Make forensically sound copies of media
Produce image files on trusted workstation
Produce checksums
For integrity checking

29. Why image ?
Why not just boot the suspect equipment and check it directly

30. Forensically sound copy
Byte by byte, block by block copy of ALL data on the medium, including deleted and/or bad blocks.
Device level and logical level (partitions)
Identical to the original
Specialist programs
(e.g. Encase)
Adapt standard tools
(e.g. “dd” on Unix/Linux/*BSD MacOS X)

31. Checksumming During/immediately after imaging
Calculate checksum files for the image. Ideally 1 per block.
Use later to verify that
Image file has not changed
Source media has not been modified
Difficult at device level – differences between devices. (manufacturing defects)
Possible algorithms
MD5, SHA, SNEFRU

32. Sources of evidence in the image
Image is a forensically sound copy
Can be treated as the original disk
Examine for
“live” files
Deleted files
Swap space
Slack space

33. Live Files “live” files Rely on suspect not having time to take action
Files in use on the system
Saved data
Temporary files
Cached files
Rely on suspect not having time to take action

34. Deleted files O/S rarely deletes all data associated with a file
More commonly marks space used by file as available for re-use
e.g.
In FAT systems, change 1st character of name to “deleted” marker
In Unix/Linux – add inodes to free list
Data may still be on disk, recoverable using sector-level tools

35. Swap space Both O/S and program swap
Areas of 10 memory swapped out to disk may contain usable data
Created by O/S during scheduling
Created by programs when required

36. Slack space Files rarely completely fill all allocated sectors
e.g. Sector size of 512 bytes, file size 514 bytes – 2 sectors, but one only contains 2 bytes of real data
Disk controller must write a complete sector.
Using DMA, grabs “spare” bytes from 10 memory and pads the sector
Padding may contain useful evidence, potentially from past programs – same rules apply to RAM as Disk! (unless powered down)

37. What about edited files ?
e.g.
Entries deleted from log files ?

38. Recovered data
Needs thorough analysis to reconstruct full or partial files
May not contain sufficient contextual information
e.g. missing file types, timestamps, filenames etc.

39. Challenges
Current & Future

40. Challenges - Current Recovered data may be Analytical challenges
Hashed
Encrypted
Steganographic
Analytical challenges

41. Hashed Data Non-reversible process
i.e. Original data cannot be determined from the hashed value
cf. Unix/Linux password files
Aka (erroneously) “one-way” encryption
“Brute Force” attack may be required
Is this good enough for legal purposes ?

42. Encryption Purpose Costly for criminal, costly for investigator
To increase the cost of recovery to a point where it is not worth the effort
Symmetric and Asymmetric
Reversible – encrypted version contains full representation of original
Costly for criminal, costly for investigator

43. Steganography Information hiding e.g.
Maps tattooed on heads
Books with pinpricks through letters
Low-order bits in image files
Difficult to detect, plenty of free tools
Often combined with cryptographic techniques.

44. Worse yet CryptoSteg SteganoCrypt Combination of two techniques...
layered

45. Additional challenges
Emerging technologies
Wireless
Bluetooth
“Bluejacking”, bandwidth theft
b/g/a
Insecure networks, Insecure devices
Bandwidth theft, storage space theft
Forms of identity theft

46. Additional challenges
Viral propagation
Proxy implantation
Sobig, SuperZonda
Pornography, SPAM
Evidence “planting”
Proven defence

47. Case studies Choose from : IPR theft Identity theft & financial fraud
Murder
Street crime (mugging)
Blackmail
Fraudulent trading
etc. etc. etc.

48. Conclusion
Digital Evidence now forms an almost essential adjunct to other investigative sciences
Can be a source of “prima facie” evidence
Requires specialist knowledge
Will continue to evolve
Current research areas :
Silicon DNA profile, Steg. Detection, ID theft