1. Lecture 4 Forensic Analysis of Windows Systems
Prof. Shamik Sengupta
Office 4210N
Fall 2010

2. What we will cover today
Understanding Hard Disk
File concept
Structure of a file Concept and detecting file types
File system traces (Understanding MAC traces)
Hands-on Practice.

3. Understanding Hard Disk
Richest source of digital evidence on computers among various storage media
Disk drives are made up of one or more platters coated with magnetic material
Hard disk drives are organized as a concentric stack of disks (platters)

4. Understanding Disk Drives
Disk drive components
Platter or disk
Each disk has two surfaces: top and bottom
Accessed by two heads
Tracks
Cylinders
Sectors

5. Data are recorded on a platter in concentric circles called tracks
Hard Disk (Continued)
Data are recorded on a platter in concentric circles called tracks
Tracks are numbered, starting from zero, starting at the outside of the platter and increasing as you go in
A modern hard disk has tens of thousands of tracks on each platter
Each track broken into smaller units called sectors (typically 512 bytes)

7. Hard Disk (Continued): Cylinders
A hard disk is usually made up of multiple platters
Because of this arrangement, often the track location of the heads is not referred to as a track number but rather as a cylinder number
A cylinder is basically the set of all tracks that all the heads are currently located at

9. Hard Disk (Continued) Disk capacity? CHS
Multiply the number of cylinders, heads, sectors and 512(block size)
Ex) What’s the size of disk which has 12,495 cylinders, 16 heads, 63 sectors?
12,495 cylinders * 16 heads * 63 sectors * 512 bytes = about 6GB
CHS
Each storage unit on a disk can be identified by a 3-coordinate system identifying
Cylinder
Head
Sector

10. Zone Bit Recording (ZBR) in Hard Disks
The earlier concept of uniform sectoring was not efficient
every track had the same number of sectors
But the ones on the outside of the platter are much larger than the ones on the inside
Outer tracks are underutilized
New technique
Zoned bit recording (ZBR)

11. Determines how data is stored on the disk
Understanding Files
File system
Gives OS a road map to data on a disk
Determines how data is stored on the disk

12. File system Basics File File systems Named collection of data objects
Typically stored on secondary storage devices, though they can exist exclusively in volatile main memory
Operations
Open, close, create, destroy, copy, rename, list, etc
File systems
Organizes files and manages access to data
Directory
File containing the name and location of other files in the file system

13. File system Basics: FAT
FAT file systems
Simplest windows file systems:
A FAT basically is an array that keeps track of what clusters belong to which file
To locate data on a volume, directories and a FAT are used
FAT: Older, for management of smaller disks
Compatible with 9X/2000/NT, MS-DOS, OS2, etc
The original FAT had many limitations
No subdirectories
Limited to very small disks
Hard to recover the disk if allocation tables were damaged
FAT32: enhanced version
Available with Win95

14. Examining FAT Disks (continued)
When the OS stores data in a FAT file system, it assigns a starting cluster position to a file
Data for the file is written to the first sector of the first assigned cluster
When this first assigned cluster is filled and runs out of room
FAT assigns the next available cluster to the file
If the next available cluster isn’t contiguous to the current cluster
File becomes fragmented

15. FAT-based File system: Schematic

16. In Microsoft OSs, when a file is deleted
Deleting FAT Files
In Microsoft OSs, when a file is deleted
Directory entry is marked as a deleted file
With the HEX E5 (σ) character replacing the first letter of the filename
Data in the file remains on the disk drive
Area of the disk where the deleted file resides becomes unallocated disk space
Available to receive new data from newly created files or other files needing more space

17. FAT: Characteristics FAT12 FAT16 (MS-DOS, Win95/98/NT/2000/XP)
For floppy disks
12-bit fields for each entry in FAT
FAT16 (MS-DOS, Win95/98/NT/2000/XP)
16-bit fields to identify a particular cluster in the FAT
For hard disks
FAT32 (Win95/98/2000/XP)
28-bit fields to identify a particular cluster in the FAT
4 bits of the 32-bit fields are “reserved”
For larger hard disks
No limit on root directory size
Limited time/date information

18. New Technology File System (NTFS)
Examining NTFS Disks
New Technology File System (NTFS)
Introduced with Windows NT
Primary file system for Windows Vista
Improvements over FAT file systems
NTFS provides more information about a file
NTFS gives more control over files and folders
Different from FAT file system
Storing information in MFT (Master File Table)
In NTFS, everything written to the disk is considered a file
NTFS was Microsoft’s move toward a journaling file system

19. File system Basics: NTFS (Continued)
NTFS uses several metadata files to keep track of both files and folders on a given volume
Virtually everything is a file in NTFS including its component parts
NTFS represents all characters using 16-bit Unicode including file and directory name
FAT file systems use the 8-bit ASCII/ANSI character set for the most part
Long file names are represented with 16-bit Unicode characters

20. Supported File Systems for MS OS

21. File Allocation and Deletion in NTFS
Master File Table (MFT)
System file created during the formatting of an NTFS volume
Table that describes files and directories
Each file and directory has an entry in MFT
Including an entry for itself and other metadata files
These metadata files are located in the root folder of a volume
Have names beginning with ‘$’ and not generally visible
E.g. Windows 2000 metadata file (In earlier versions of NTFS, these files are visible with, e.g., “dir /ah” but invisible in XP)
MFT File Record No.
File Name
Description
$MFT
Master File Table (MFT) 1
$MFTMIRR
Copy of the first 16 records of the MFT 2
$LOGFILE
List of file system transactions 3
$VOLUME
Information about the volume, including NTFS version, volume name and creation time 4
$ATTRDEF
Table of attribute definitions 5 .
Root folder 6
$BITMAP
Bitmap representation of used and unused clusters on volume 7
$BOOT
Boot record with bootstrap loader code if the volume is bootable 8
$BADCLUS
List of the bad clusters in the volume 9
$SECURE
Stores security descriptions (Windows 2000 only) 10
$UPCASE
Conversion table for converting lowercase characters to matching uppercase Unicode characters 11
$EXTEND
Enables file system extensions such as volume quotas (Windows 2000 only)

22. File Allocation and Deletion in NTFS (Continued)
MFT records store attributes of files and folder including
the name in the $FILENAME
MAC times and other file characteristics in the $STANDARD_INFORMATION
some or all of the data in a file using the $DATA attribute
A small file may exist entirely within its MFT record with no associated data elsewhere on disk
E.g. Simplified MFT record for a file
Header
$FILENAME
$STANDARD-INFORMATION
$DATA
Attribute list

23. File Allocation and Deletion in NTFS (Continued)
$BITMAP file
System file created during formatting of NTFS volume to keep track of cluster usage
Uses one bit to record the status of each cluster on the volume
E.g. If a cluster in an NTFS volume is used, the corresponding bit in the $BITMAP file is changed to 1

24. File Allocation and Deletion in NTFS (Continued)
Steps to allocate a file on an NTFS volume
1) $BITMAP file must be modified to reflect that the used clusters are allocated
2) Allocated MFT record must be created for the file
3) An index entry must be created for the file name in the parent’s folder’s MFT record
4) cluster extent entries must be created in the file’s MFT record if the file is non-resident
When a file is deleted
1) Its cluster reference in the $BITMAP file are changed to 0
2) MFT record for that file is marked for deletion
3) Its index entry is deleted

25. Analyzing the structure of a file: Hands-on investigation
Understanding the structure of a file for digital investigation
Data are stored on computers as files
But files can be corrupted meaningfully to hide data
To understand the corruption, we must understand the structure of a file

26. Analyzing the structure of a file
The simplest structure of a file consists of:
Filename
filename is a unique identifier which allows the computer to correctly identify each file
File header
information which describes the type of the file
Important for file header signature
Important for OS to understand which application to be associated
File content
Actual data
files can be corrupted meaningfully to hide data
Change or delete the file type (today’s lab)
Delete the data!

27. Case Study: Hiding a pdf
There are simple ways to hide a pdf from OS
Change the file type
Delete the file type
Let’s check the file type by looking at Hexadecimal signature values

28. Fun lab
Download the test.zip from the class website and see if there is any corruption done with the files. If yes, can you detect the file types?

29. An individual’s actions on a computer leave traces
File System Traces
An individual’s actions on a computer leave traces
Temporal traces is very important to understand the file’s life
When it was created/downloaded
When accessed
When modified etc.
An understanding of the cause-effect relationship in a reverse manner is particularly important for digital investigator
What is cause-effect relationship in forward manner?
You changed something, you see the changed result
You see the changed result, you have to guess what caused it

30. Windows records the date and time of a file’s
MAC Times in NTFS
Windows records the date and time of a file’s
Creation : created
The date and time that a file was created on the current volume
Last modification: modified
The date and time that a file was last modified
Last accessed: accessed
The date that a file was last accessed
An examination of the MAC times of a file can provide insight into
the file’s history on a computer
the extent of the user’s knowledge of the file’s existence and contents

31. MAC Times in NTFS (Continued)
Case Example – MAC times
In a prosecution for the possession and distribution of “CP”, a Supervising Special Agent for the California Department of Justice charted the MAC times of several files.
The defendant asserted at trial that he merely downloaded files of unknown content and then forwarded them to others without having viewed them himself.
During a trial recess, the prosecution and defense attorneys held a conference to disclose and discuss the Supervising Special Agent’s scheduled testimony regarding the results of his forensic examination.
The agent informed and described how many files had been accessed at times far remote from their dates of creation.
As a result of this discussion, the defendant entered a plea of guilty prior to the agent’s testimony

32. MAC Times in NTFS (Continued)
Example – MAC times
Can be viewed by
dir /TC
dir /TA
dir /TW
MAC times can also be
viewed and sorted using
Forensic Tools

33. Understanding the temporal traces
File moved within a volume
No change in last-modified, last-accessed, last-created
File moved/copied across volume
Last-accessed and last-created time change
NO change in Last-modified!
However, for parent directory listing
Last-accessed and last-modified updated. WHY?
Homework Assignment #1 will be up by tomorrow
Next up, more forensic examination of Windows and EnCase