Transcript Lecture8 - The University of Texas at Dallas

Digital Forensics
Dr. Bhavani Thuraisingham
The University of Texas at Dallas
File Systems and Forensics Tools
September 20, 2013
Outline
 File Systems
- File Systems Overview
- Windows File System (for Forensics)
 Tools and Standards
- Forensics Tools
- Standards
- Revisiting File Systems
- http://www.cftt.nist.gov/NISTIR_7490.pdf
Windows File System
 Overview of File Systems
 Microsoft File Structures
 NTFS Disks (New Technology File System)
- Partitions, disks, etc.
 Other concepts (Registries, startup tasks)
 Virtual Machines
File Systems
 What is it?
- Structure of the data that is stored
- Linear file system, Hierarchical file system, etc.
 Type of file system determines how the data is stored on disk
 File system is part of the OS; a file system is a way for storing
and organizing computer files and the data they contain to
make it easy to find and access them.
 Key aspects of file system include
- Boot sequence
- Disk drives
- File name, metadata, security access
- Different types of file systems
File Systems - 2
 Boot sequence
- When a suspect’s computer starts, make sure it boots to a
forensic floppy disk/CD and not to the hard disk
- Booting to the hard disk may overwrite evidence
Make modifications to CMOS setup
 Disk drives
- Geometry, Head, Tracks, Cylinders, Sectors
 Every file has a file name; metadata consists of information
about a file, access control policies may be defined on a file
 Types of file systems include disk file system, flash file
systems, database file systems, network file systems, - - -
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File Systems - 3
 File systems typically have directories which associate file names
with files, usually by connecting the file name to an index in a file
allocation table (FAT in Windows, Inode in Unix)
 Directory structures may be flat, or allow hierarchies where
directories may contain subdirectories.
 In some file systems, file names are structured, with special syntax
for filename extensions and version numbers. In others, file names
are simple strings
 Metadata
- The length of the data contained in a file may be stored as the
number of blocks allocated for the file or as an exact byte count.
- The time that the file was last modified may be stored as the
file's timestamp; also file creation time, the time it was last
accessed
Microsoft File Structures
 Sectors
- Sectors are groped to for clusters which are the storage
allocations units.
- Cluster numbers are logical addresses and section numbers are
physical addresses.
 Disk Partitions
 Hard drive is partitioned. A partition is a logical drive.
 Master Boot Record (MBR)
- Stores information about the partitions in a disk and their
locations, sizes etc.
 FAT (File Allocation Table) Disks
- Original Microsoft file structure database
 NTFS
- New Technology File System
NTFS Disks
 Overview of NTFS Disks
- Newer Microsoft products are based on new Technology
File System
- Everything written to a disk is considered s file
First data set is the Partition Boot Sector
Next is the Master File Table (similar to FAT)
- Uses Unicode
 NTFS System Files
- The first file MFT ahs information in all the files
Records in MFT are called metadata
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NTFS Disks - 2
 NTFS Data Streams
- Ways data can be appended to existing files
- Can obscure evidence; only way to know there is a data
stream is by looking at MFT
 NTFS Compressed Files
Provides compression to improve data storage
 Encryption
- Implements public key/private key method
- Whole disk encryption (Chapter 4) for extra protection for
certain information such as personal identity numbers.
 Performance
- tune some of global NTFS parameters to achieve
significant increase of disk performance. Other
techniques like disk defragmentation could help
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NTFS Disks – 3 (Summary)
 File Storage Hardware and Disk Organization
 Hard Disk Drive Basics
- Making Tracks
- Sectors and Clusters
 Master Boot Record (MBR)
- Viruses Can Infect the Master Boot Record
 Partition Table
- Boot Indicator Field
- System ID Field
- Starting and Ending Head, Sector, and Cylinder Field
- Relative Sectors and Number of Sectors Fields
- Logical Drives and Extended Partitions
NTFS Recovery

Why id Partition recovery needed
- MBR (Master Boot Record) is damaged
- Partition is deleted or Partition Table is damaged
- Partition Boot Sector is damaged
- Missing or Corrupted System Files
 Partition/Drive Recovery
- "Physical partition recovery". The goal is to find out the
problem and write some information to the proper place
on HDD and after that partition becomes visible to OS
again.
- "Virtual partition recovery". The goal is to determine the
critical parameters of the deleted/damaged/overwritten
partition and after that enable to scan it and display its
content.
NTFS Recovery - 2

NTFS File Recovery
- Disk Scan for deleted entries
 Disk Scan is a process of low-level enumeration of
all entries in the Root Folders; The goal is to find and
display deleted entries.
- Defining clusters chain for the deleted entry
 To define clusters chain scan drive, going through
one by one all allocated and free clusters belonging
to the file until the file size equals to the total size of
the selected clusters. If the file is fragmented,
clusters chain will be composed of several extents.
- Clusters chain recovery
 After clusters chain is defined read and save
contents of the defined clusters to another place
verifying their contents.
Other Concepts
 Registry
- Registry is a database that stores initialization files such
as hardware/software configuration, network connections,
user preferences, setup information
- Set of tools (e.g., Registry editor) to view and modify the
data
 Start-up tasks
- Forensics examiner must have a very good understanding
of what happens to the data during start-up.
- E.g., What is the process, what are the files involved, etc.
Virtual Machines
 An examiner may need lot more space than he has on the
machine he is using. The concept of Virtual machine is used it
overcome this limitation.
 Virtual machine addresses the need for having a variety of
resources by creating the representation of another computer
on an existing physical computer.
 A few files from the other computer is on the examiner’s
machine and space ahs to be allocated for this.
 Also useful when one upgrades computer, but still has some
old applications. Therefore virtual machine of the old OS is
created.
Reading for Lecture for September 20th
Intelligence Digital Forensics
 http://dfrws.org/2006/proceedings/7-Alink.pdf
 XIRAF – XML-based indexing and querying for digital forensics
http://dfrws.org/2006/proceedings/8-Turner.pdf
 Selective and intelligent imaging using digital evidence bags
 http://dfrws.org/2006/proceedings/9-Lee.pdf
 Detecting false captioning using common-sense reasoning
Forensics Tools
 Hardware Forensics Tools
- Range from single purpose components (e.g., devices) to
complete systems (forensics workstations)
 Software Forensics Tools
Analysis tools such ProDiscover and EnCase
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Functions of Forensics Tools
 Acquisition
 Validation and Discrimination
 Extraction
 Reconstruction
 Reporting
 Comparison of some forensics tools are given on page 277 of
Textbook (ProDiscover, AccessData, EnCase)
Functions of Forensics Tools - 2
 Acquisition
- Tools for data acquisition
- Physical data copy, logical data copy, data acquiring
format, GUI acquisition
 Validation and Discrimination
Integrity of the data, Also includes hashing, filtering,
analyzing file headers
 Extraction
- Recovery task
Data viewing, Keyword searching, Decompressing
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 Reconstruction
 Reporting
Functions of Forensics Tools - 3
 Reconstruction
- Recreate the crime scene (suspect drive)
- Disk to disk copy, Image to disk copy, etc.
 Reporting
- Reporting generation tools help the examiner the prepare
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report
Also helps to log reports
Software Tools
 Command line forensics tools
 Unix/Linux forensics tools
- SMART, Helix, Autopsy and Sleuth Kit
 GUI Forensics Tools
- Visualizing the data is important to understand the data
Hardware Tools
 Forensics workstations
- How to build a workstation
- What are the components
- How are the workstations connected in a lab
- How can distributed forensics be carried out
 Write Blockers
- Write blocker devoices to protect evidence disks (see the
discussion in Chapter 4 under data acquisition)
Validating Forensics Tools
 NIST (National Institute of Standards and Technology) is
coming up with standards for validation (will be discussed
under standards)
- Establish categories for forensics tools, Identify forensics
category requirements, Develop test assertions
Identify test cases
- Establish test method
- Report test results
- NIST (National Institute of Standards and Technology) is
coming up with standards for validation (will be discussed
under standards
 Chapter 7 discusses validation protocols as well as some
examination protocols
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NIST Standards
 There are three digital forensics projects at the National Institute of
Standards and Technology (NIST).
 These projects are supported by the U.S. Department of Justice's
National Institute of Justice (NIJ), federal, state, and local law
enforcement, and the National Institute of Standards and
Technology Office of Law Enforcement Standards (OLES) to
promote efficient and effective use of computer technology in the
investigation of crimes involving computers.
 These projects are the following:
- • National Software Reference Library (NSRL)
• Computer Forensic Tool Testing (CFTT)
- • Computer Forensic Reference Data Sets (CFReDS)
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NSRL
 The NSRL is designed to collect software from various sources and
incorporate file profiles computed from this software into a
Reference Data Set (RDS) including hashes of known files created
when software is installed on a computer. The law enforcement
community approached NIST requesting a software library and
signature database that meets four criteria:
- • The organizations involved in the implementation of the file
profiles must be unbiased and neutral.
- • Control over the quality of data provided by the database must
be maintained.
- • A repository of original software must be made available from
which data can be reproduced.
- • The database must provide a wide range of capabilities with
respect to the information that can be obtained from file systems
under investigation.
NSRL
 The primary focus of the NSRL is to aid computer forensics
examiners in their investigations of computer systems.
 The majority of stakeholders are in federal, state and local law
enforcement in the United States and internationally.
 These organizations typically use the NSRL data to aid in criminal
investigations.
CFTT
 The goal of the CFTT project at NIST is to establish a
methodology for testing computer forensic software tools through
the development of general tool specifications, test procedures,
test criteria, test sets, and test hardware. The results provide the
information necessary for toolmakers to improve tools, for users to
make informed choices about acquiring and using computer
forensics tools, and for interested parties to understand the tools
capabilities.
 The testing methodology developed by NIST is functionality driven.
The activities of forensic investigations are separated into discrete
functions, such as hard disk write protection, disk imaging, string
searching, etc. A test methodology is then developed for each
category. After a test methodology is developed it is posted to the
web.
CFReDS
 The Computer Forensic Reference Data Sets (CFReDS)
provide to an investigator documented sets of simulated digital
evidence for examination.
 Since CFReDS has documented contents, such as target search
strings seeded in known locations, investigators can compare the
results of searches for the target strings with the known placement
of the strings.
 Investigators can use CFReDS in several ways including validating
the software tools used in their investigations, equipment check
out, training investigators, and proficiency testing of investigators
as part of laboratory accreditation.
 The CFReDS site is a repository of images. Some images are
produced by NIST, often from the CFTT (tool testing) project, and
some are contributed by other organizations.
CFReDS
 In addition to test images, the CFReDS site contains resources to
aid in creating test images.
 These creation aids are in the form of interesting data files, useful
software tools and procedures for specific tasks.
 The CFReDS web site is http://www.cfreds.nist.gov.
International Standards
 The Scientific Working Group on Digital Evidence (SWGDE) was established
in February 1998 through a collaborative effort of the Federal Crime
Laboratory Directors. SWGDE, as the U.S.-based component of
standardization efforts conducted by the International Organization on
Computer Evidence (IOCE), was charged with the development of crossdisciplinary guidelines and standards for the recovery, preservation, and
examination of digital evidence, including audio, imaging, and electronic
devices.
 The following document was drafted by SWGDE and presented at the
International Hi-Tech Crime and Forensics Conference (IHCFC) held in
London, United Kingdom, October 4-7, 1999. It proposes the establishment of
standards for the exchange of digital evidence between sovereign nations
and is intended to elicit constructive discussion regarding digital evidence.
This document has been adopted as the draft standard for U.S. law
enforcement agencies.
 http://www.fbi.gov/hq/lab/fsc/backissu/april2000/swgde.htm
Macintosh Operating System (MAC OS X)
 Early MAC OS used HFS (Hierarchical file system) OS X uses
HFS+ (optional) and also supports Unix File System
 OS 9 supports Volumes. Volume can be all or part of the
storage media for hard disks
 Newer MACs booted can be booted from CD, DVD, Firewire
drive. Older systems booted from hard drive
 Some forensics tools special for OS X. Some other Windows
tools can also be used
Unix/Linux Operating System
 Everything is a file including disk drives, monitors, tape
drives, network interface cards, etc.
 Unix has four components for its file system
- Boot block, superblock, Inode, data block
Block is smallest disk allocation
Boot clock has bootstrap code, superblock has system
information, Inode is assignee to every file allocation
unit., data blocks store directories and files
 Forensic examiner must understand the boot process of the
operating system
 Disk partitions in Unix/Linus is very different from Windows.
In Unix/Linux partitions are labeled as paths.
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Papers for Lecture on September 24, 2009
Tamper Detection and Forensics Analysis
 Richard T. Snodgrass, Stanley Yao and Christian Collberg,
"Tamper Detection in Audit Logs," In Proceedings of the
International Conference on Very Large Databases, Toronto,
Canada, August–September 2004, pp. 504–515.
Tamper Detection in Audit Logs
 Did the problem occur? (e.g. similar to intrusion
detection)
 Kyri Pavlou and Richard T. Snodgrass, "Forensic Analysis of
Database Tampering," in Proceedings of the ACM SIGMOD
International Conference on Management of Data (SIGMOD),
pages 109-120, Chicago, June, 2006.
 Who caused the problem (e.g., similar to digital
forensics analysis)
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