Difference between pages "Windows Prefetch File Format" and "Disk Imaging"

From Forensics Wiki
(Difference between pages)
Jump to: navigation, search
(Section D)
 
(Selective imaging)
 
Line 1: Line 1:
A Windows Prefetch file consists of one file header and multiple file sections with different content. Not all content has an obvious forensic value.
+
{{expand}}
  
As far as have been possible to ascertain, there is no public description of the format. The description below has been synthesised from examination
+
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.
of multiple prefetch files.  
+
  
== Header ==
+
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]].
 +
This can be a time consuming process especially for disks with a large capacity.
  
This format has been observed on Windows XP, ...  will need to be modified for Vista/Win7 format
+
== Hardware solutions ==
  
{| class="wikitable"
+
== Software solutions ==
|-
+
! Field
+
! Offset
+
! Length
+
! Type
+
! Notes
+
|-
+
| H1
+
| 0x0000
+
| 4
+
| DWORD
+
| ? Probably a version number, identifying the file structure. Observed values: 0x11 - Windows XP; 0x17 - Vista, Windows 7
+
|-
+
| H2
+
| 0x0004
+
| 4
+
| DWORD
+
| ? Probably a file magic number. Only observed value: 0x41434353
+
|-
+
| H3
+
| 0x0008
+
| 4
+
| DWORD?
+
| ? Observed values: 0x0F - Windows XP, 0x11 - Windows 7
+
|-
+
| H4
+
| 0x000C
+
| 4
+
| DWORD
+
| Prefetch file length.
+
|-
+
| H5
+
|0x0010
+
| 60
+
| USTR
+
| Name of executable as Unicode string, truncated after character 29 if necessary, and terminated by U+0000. As it appears in the prefetch file file name.
+
|-
+
| H6
+
|0x004C
+
|4
+
|DWORD
+
|The prefetch hash, as it appears in the pf file name.
+
|-
+
| H7
+
|0x0050
+
|4
+
|?
+
|? Observed values: 0 for almost all prefetch files (XP); 1 for NTOSBOOT-B00DFAAD.pf (XP)
+
|-
+
| H8
+
| 0x0054
+
| 4
+
| DWORD
+
| Offset to section A
+
|-
+
| H9
+
| 0x0058
+
| 4
+
| DWORD
+
| ? Nr of entries in section A
+
|-
+
| H10
+
| 0x005C
+
| 4
+
| DWORD
+
| Offset to section B
+
|-
+
| H11
+
| 0x0060
+
| 4
+
| DWORD
+
| Nr of entries in section B
+
|-
+
| H12
+
| 0x0064
+
| 4
+
| DWORD
+
| Offset to section C
+
|-
+
| H13
+
| 0x0068
+
| 4
+
| DWORD
+
| Length of section C
+
|-
+
| H14
+
| 0x006C
+
| 4
+
| DWORD
+
| Offset to section D
+
|-
+
| H15
+
| 0x0070
+
| 4
+
| DWORD
+
| ? Probably the number of entries in the D section header
+
|-
+
| H16
+
| 0x0074
+
| 4
+
| DWORD
+
| Length of section D
+
|-
+
| H17
+
| 0x0078
+
| 8
+
| FTIME
+
| Latest execution time of executable (FILETIME)
+
|-
+
| H18
+
| 0x0080
+
| 16
+
| ?
+
| ? Possibly structured as 4 DWORD. Observed values: /0x00000000 0x00000000 0x00000000 0x00000000/, /0x47868c00 0x00000000 0x47860c00 0x00000000/
+
|-
+
| H19
+
| 0x0090
+
| 4
+
| DWORD
+
| Execution counter
+
|-
+
| H20
+
| 0x0094
+
| 4
+
| DWORD?
+
| ? Observed values: 1, 2, 3, 4, 5, 6 (XP)
+
|-
+
|}
+
  
It's worth noting that the name of a carved prefetch file can be restored using the information in field H5 and H6, and its size can be determined by field H4.
+
== Compressed storage ==
  
== Section A and B ==
+
A common technique to reduce the size of an image file is to compress the data.
 +
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)]
  
The content of these two sections is unknown.
+
Other techniques like storing the data sparse or '''empty-block compression''' can reduce the total time of the imaging process and the resulting size of new non-encrypted (0-byte filled) disks.
  
== Section C ==
+
== Error tolerance and recovery ==
  
== Section D ==
+
== Smart imaging ==
 +
Smart imaging is a combination of techniques to make the imaging process more intelligent.
 +
* Deduplication
 +
* Selective imaging
 +
* Decryption while imaging
  
Section D begins with one or more headers. The number is (most likely) determined by the DWORD at file offset 0x0070. Each header refers to a hard drive.
+
=== 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.
  
In this sections, all offsets are assumed to be counted from the start of the D section.
+
=== 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.
  
{| class="wikitable"
+
[[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]].
|-
+
 
! Field
+
=== Decryption while imaging ===
! Offset
+
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 on-the-fly if required. Although this should be rare since the non-encrypted data is what undergoes analysis.
! Length
+
 
! Type
+
== Also see ==
! Notes
+
[[:Category:Forensics_File_Formats|Forensics File Formats]]
|-
+
 
| DH1
+
== External Links ==
| 0x0000
+
* [http://www.tableau.com/pdf/en/Tableau_Forensic_Disk_Perf.pdf Benchmarking Hard Disk Duplication Performance in Forensic Applications], by [[Robert Botchek]]
| 4
+
 
| DWORD
+
=== Hash based imaging ===
| Offset to volume string (Unicode, terminated by U+0000)
+
* [http://www.dfrws.org/2010/proceedings/2010-314.pdf Hash based disk imaging using AFF4], by [[Michael Cohen]], [[Bradley Schatz]]
|-
+
|}
+

Revision as of 09:23, 21 July 2012

Information icon.png

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.

Contents

Hardware solutions

Software solutions

Compressed storage

A common technique to reduce the size of an image file is to compress the data. 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 or empty-block compression can reduce the total time of the imaging process and the resulting size of new non-encrypted (0-byte filled) disks.

Error tolerance and recovery

Smart imaging

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

  • Deduplication
  • Selective imaging
  • Decryption while imaging

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 on-the-fly if required. Although this should be rare since the non-encrypted data is what undergoes analysis.

Also see

Forensics File Formats

External Links

Hash based imaging