Difference between pages "FAT" and "Disk Imaging"

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The FAT allocation system is primarly concerned with a descrete method of organizing files. In order to protect the file system, two copies are stored: FAT1 and FAT2. With two copies available redundancy is achieved in case one fails. The partition Boot Sector stores information critical to the file system. This information includes the number of sectors, and number of clustors, the number of sectors per cluster and more. When a computer user wants to read any type of file, the FAT file system also reassembles each piece of the file into one complete unit for viewing.
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{{expand}}
The Root Folder contains a small piece of information for each file and directory in the system. Unlike other files in the system the Root Folder has a fixed size.
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*  FAT Partition Boot Sector
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Disk imaging is the process of making a bit-by-bit copy of a disk. Imaging (in more general terms) can apply to anything that can be considered as a bit-stream, e.g. a physical or logical volumes, network streams, etc.
*  FAT File System
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*  FAT Root Folder
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*  FAT Folder Structure
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[[Image:Recover-FAT-volume-structur.jpg]]
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The most straight-forward disk imaging method is reading a disk from start to end and writing the data to a [[:Category:Forensics_File_Formats|Forensics image format]].
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This can be a time consuming process especially for disks with a large capacity.
  
'''History'''
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The process of disk imaging is also referred to as disk duplication.
  
----
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== Disk Imaging Solutions ==
 +
See: [[:Category:Disk Imaging|Disk Imaging Solutions]]
  
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== Common practice ==
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It common practice to use a [[Write Blockers|Write Blocker]] when imaging a pyhical disk. The write blocker is an additional measure to prevent write access to the disk.
  
Originally developed by Bill Gates in 1976 as a way to store data on floppy disks for a version of Basic, the file allocation table system was quickly incorporated into an early version of Tim Patterson's (of Seattle Computer Products fame) operating system, QDOS ("Quick and Dirty Operating System"). Gates later bought the rights to QDOS and released it under Microsoft as PC-DOS and later, MS-DOS.
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Also see: [[DCO and HPA|Device Configuration Overlay (DCO) and Host Protected Area (HPA)]]
  
----
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== Integrity ==
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Often when creating a disk image a [http://en.wikipedia.org/wiki/Cryptographic_hash_function cryptographic hash] is calculated of the entire disk. Commonly used cryptographic hashes are MD5, SHA1 and/or SHA256.
  
 
'''FAT32''':
 
FAT32 is the enhanced version of the FAT system implemented beginning with Windows 95 OSR2, Windows 98, and Windows Me.
 
Features include:
 
*  Drives of up to 2 terabytes are supported (Windows 2000 only supports up to 32 gigabytes)
 
*  Since FAT32 uses smaller clusters (of 4 kilobytes each), it uses hard drive space more efficiently. This is a 10 to 15 percent improvement over FAT or FAT16.
 
*  The limitations of FAT or FAT 16 on the number of root folder entries have been eliminated. In FAT32, the root folder is an ordinary cluster chain, and can be located anywhere on the drive.
 
*  File allocation mirroring can be disabled in FAT32. This allows a different copy of the file allocation table then the default to be active.
 
  
Currently the FAT file system has become the ubiquitous format that is used for interchange of media between computers. Since the advent of less expensive, removable flash memory, the FAT file system has become the format that is used between digital devices. Some items in which you might find the FAT file format are:
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By recalculating the integrity hash at a later time, one can determine if the data in the disk image has been changed. This by itself provides no protection against intentional tampering, but can indicate that the data was altered, e.g. due to corruption. The integrity hash does not indicate where int he data the alteration has occurred. Therefore some image tools and/or formats provide for additional integrity checks like:
 +
* A checksum
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* Parity data
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* [[Piecewise hashing]]
  
* Thumb drives
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== Smart imaging ==
* Portable digital still/video cameras
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Smart imaging is a combination of techniques to make the imaging process more intelligent.
* Portable digital audio and video players
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* Compressed storage
* Multifunction printers
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* Deduplication
* Electronic photo frames
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* Selective imaging
* Electronic musical instruments
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* Decryption while imaging
* Standard televisions
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References:
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=== Compressed storage ===
ntfs.com
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microsoft.com
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A common technique to reduce the size of an image file is to compress the data. Where the compression method should be [http://en.wikipedia.org/wiki/Lossless_data_compression lossless].
http://support.microsoft.com/kb/q154997/#XSLTH3126121123120121120120
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On modern computers, with multiple cores, the compression can be done in parallel reducing the output without prolonging the imaging process.
 +
Since the write speed of the target disk can be a bottleneck in imaging process, parallel compression can reduce the total time of the imaging process.
 +
[[Guymager]] was one of the first imaging tools to implement the concept of multi-process compression for the [[Encase image file format]]. This technique is now used by various imaging tools including [http://www.tableau.com/index.php?pageid=products&model=TSW-TIM Tableau Imager (TIM)]
 +
 
 +
Other techniques like storing the data sparse, using '''empty-block compression''' or '''pattern fill''', can reduce the total time of the imaging process and the resulting size of new non-encrypted (0-byte filled) disks.
 +
 
 +
=== Deduplication ===
 +
Deduplication is the process of determining and storing data that occurs more than once on-disk, only once in the image.
 +
It is even possible to store the data once for a corpus of images using techniques like hash based imaging.
 +
 
 +
=== Selective imaging ===
 +
Selective imaging is a technique to only make a copy of certain information on a disk like the $MFT on an [[NTFS]] volume with the necessary contextual information.
 +
 
 +
[[EnCase]] Logical Evidence Format (LEF) is an example of a selective image; although only file related contextual information is stored in the format by [[EnCase]].
 +
 
 +
=== Decryption while imaging ===
 +
Encrypted data is worst-case scenario for compression. Because the encryption process should be deterministic, a solution to reduce the size of an encrypted image is to store it non-encrypted and compressed and encrypt it again on-the-fly if required. Although this should be rare since the non-encrypted data is what undergoes analysis.
 +
 
 +
== Also see ==
 +
* [[:Category:Forensics_File_Formats|Forensics File Formats]]
 +
* [[Write Blockers]]
 +
* [[Piecewise hashing]]
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* [[Memory Imaging]]
 +
 
 +
== External Links ==
 +
* [http://www.tableau.com/pdf/en/Tableau_Forensic_Disk_Perf.pdf Benchmarking Hard Disk Duplication Performance in Forensic Applications], by [[Robert Botchek]]
 +
 
 +
=== Hash based imaging ===
 +
* [http://www.dfrws.org/2010/proceedings/2010-314.pdf Hash based disk imaging using AFF4], by [[Michael Cohen]], [[Bradley Schatz]]
 +
 
 +
[[Category:Disk Imaging]]

Revision as of 04:29, 28 July 2012

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Please help to improve this article by expanding it.
Further information might be found on the discussion page.

Disk imaging is the process of making a bit-by-bit copy of a disk. Imaging (in more general terms) can apply to anything that can be considered as a bit-stream, e.g. a physical or logical volumes, network streams, etc.

The most straight-forward disk imaging method is reading a disk from start to end and writing the data to a Forensics image format. This can be a time consuming process especially for disks with a large capacity.

The process of disk imaging is also referred to as disk duplication.

Contents

Disk Imaging Solutions

See: Disk Imaging Solutions

Common practice

It common practice to use a Write Blocker when imaging a pyhical disk. The write blocker is an additional measure to prevent write access to the disk.

Also see: Device Configuration Overlay (DCO) and Host Protected Area (HPA)

Integrity

Often when creating a disk image a cryptographic hash is calculated of the entire disk. Commonly used cryptographic hashes are MD5, SHA1 and/or SHA256.


By recalculating the integrity hash at a later time, one can determine if the data in the disk image has been changed. This by itself provides no protection against intentional tampering, but can indicate that the data was altered, e.g. due to corruption. The integrity hash does not indicate where int he data the alteration has occurred. Therefore some image tools and/or formats provide for additional integrity checks like:

Smart imaging

Smart imaging is a combination of techniques to make the imaging process more intelligent.

  • Compressed storage
  • Deduplication
  • Selective imaging
  • Decryption while imaging

Compressed storage

A common technique to reduce the size of an image file is to compress the data. Where the compression method should be lossless. On modern computers, with multiple cores, the compression can be done in parallel reducing the output without prolonging the imaging process. Since the write speed of the target disk can be a bottleneck in imaging process, parallel compression can reduce the total time of the imaging process. Guymager was one of the first imaging tools to implement the concept of multi-process compression for the Encase image file format. This technique is now used by various imaging tools including Tableau Imager (TIM)

Other techniques like storing the data sparse, using empty-block compression or pattern fill, can reduce the total time of the imaging process and the resulting size of new non-encrypted (0-byte filled) disks.

Deduplication

Deduplication is the process of determining and storing data that occurs more than once on-disk, only once in the image. It is even possible to store the data once for a corpus of images using techniques like hash based imaging.

Selective imaging

Selective imaging is a technique to only make a copy of certain information on a disk like the $MFT on an NTFS volume with the necessary contextual information.

EnCase Logical Evidence Format (LEF) is an example of a selective image; although only file related contextual information is stored in the format by EnCase.

Decryption while imaging

Encrypted data is worst-case scenario for compression. Because the encryption process should be deterministic, a solution to reduce the size of an encrypted image is to store it non-encrypted and compressed and encrypt it again on-the-fly if required. Although this should be rare since the non-encrypted data is what undergoes analysis.

Also see

External Links

Hash based imaging