Transcript Real-Time Compilation Technologies and Instruction Parallelism

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Lecture 4
Forensic Analysis of Windows Systems
Prof. Shamik Sengupta
Office 4210N
[email protected]
http://jjcweb.jjay.cuny.edu/ssengupta/
Fall 2010
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What we will cover today
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Understanding Hard Disk
File concept
Structure of a file Concept and detecting file types
File system traces (Understanding MAC traces)
 Hands-on Practice.
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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)
Understanding Disk Drives
 Disk drive components
– Platter or disk
– Each disk has two surfaces: top and bottom
– Accessed by two heads
– Tracks
– Cylinders
– Sectors
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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)
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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
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Hard Disk (Continued)
 Disk capacity?
– 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 3coordinate system identifying
– Cylinder
– Head
– Sector
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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)
Understanding Files
 File system
– Gives OS a road map to data on a disk
 Determines how data is stored on the disk
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File system Basics
 File
– 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
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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
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
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FAT-based File system: Schematic
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
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FAT: Characteristics
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FAT12
– For floppy disks
– 12-bit fields for each entry in FAT
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FAT16 (MS-DOS, Win95/98/NT/2000/XP)
– 16-bit fields to identify a particular cluster in the FAT
– For hard disks
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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
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
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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
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Supported File Systems for MS OS
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File Allocation and Deletion in NTFS
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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
MFT File
Record No.
– These metadata files are located in the root folder of a volume
– Have names beginning with ‘$’ and not generally visible
– E.g. Windows 2000 metadata file (In earlier versions of NTFS, these files are visible with,
e.g., “dir /ah” but invisible in XP)
File Name
Description
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$MFT
Master File Table (MFT)
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$MFTMIRR
Copy of the first 16 records of the MFT
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$LOGFILE
List of file system transactions
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$VOLUME
Information about the volume, including NTFS version, volume name and creation time
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$ATTRDEF
Table of attribute definitions
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Root folder
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$BITMAP
Bitmap representation of used and unused clusters on volume
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$BOOT
Boot record with bootstrap loader code if the volume is bootable
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$BADCLUS
List of the bad clusters in the volume
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$SECURE
Stores security descriptions (Windows 2000 only)
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$UPCASE
Conversion table for converting lowercase characters to matching uppercase Unicode
characters
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$EXTEND
Enables file system extensions such as volume quotas (Windows 2000 only)
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File Allocation and Deletion in NTFS
(Continued)
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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
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E.g. Simplified MFT record for a file
Header
$FILENAME
$STANDARDINFORMATION
$DATA
Attribute list
File Allocation and Deletion in NTFS
(Continued)
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$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
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File Allocation and Deletion in NTFS
(Continued)
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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
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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
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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
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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!
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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
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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?
http://www.garykessler.net/library/file_sigs.html
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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
– What is cause-effect relationship in forward manner?
– You see the changed result, you have to guess what caused it
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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
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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
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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
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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