Encase image file format

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The Encase image file format is used by EnCase used to store various types of digital evidence e.g.

  • disk image (physical bitstream of an acquired disk)
  • volume image
  • memory
  • logical files

History

Expert Witness (for Windows) was the original name for EnCase (dating back to 1998). More info about this can be found on the Internet Archive [1] including a demo of the original software [2].

(presumably) the product was renamed because it intruded the Expert Wittness trademark held by ASR Data [3].

The Encase image file format therefore is also referred to as the Expert Witness (Compression) Format.


Currently there are 2 versions of the format:

  • version 1 is (reportedly) based on ASR Data's Expert Witness Compression Format
  • version 2 was introduced in EnCase 7, for which a format specification (at least non-encrypted Ex01) is available, but requires registration.

The libewf project indicates that the January 2012 version of the version 2 format specification, besides Lx01 not being specified, is sufficient to read non-encrypted Ex01 files.


Although the format specification is not complete, at the moment Guidance Software is working on an update. This will not include:

  • encrypted Ex01
  • Lx01


So in contrast to other claims [4] both versions of the EWF file format are:

  • proprietary
  • partially open specification

For more information about these definitions see: File formats

Version 1

The media data can be stored in multiple evidence files, which are called segment files. Each segment file consist of multiple sections, which has a distinct section start definition containing a section type. Up to EnCase 5 the segment file were limited to 2 GiB, due to the internal 31-bit file offset representation. This limitation was lifted by adding a base offset value in EnCase 6.


EnCase allows to store the data compressed either using a fast or best level of the deflate compression method. EnCase 7 no longer distinguishes between fast or best compression and just provides for either uncompressed or compressed.


Besides digital evidence the evidence files, or segment files, contain a header containing case information. The case information which entails date and time of acquisition, an examiner's name, notes on the acquisition, and an optional password.

  • In EnCase 3 the case information header is stored in the "header" section, which is defined twice within the file and contain the same information.
  • As of EnCase 4 an additional "header2" section was added. The "header" section now appears only once, but the new "header2" section twice.


The format adds error detection by storing the data with checksums (Adler32), for both the metadata as the data blocks, which are by default 64 x 512 byte sectors (32 KiB). As of EnCase 5 the number of sectors per block (chunk) can vary. EnCase 3F introduced an "error2" section that it uses to record the location and number of bad sector chunks. The way it handles the sections it can't read is that those areas are filled with zero. Then EnCase displays to the user the areas that could not be read when the image was acquired. The granularity of unreadable chunks appears to be 32K. As of EnCase 5 the granularity of unreadable chunks can vary.


EnCase 3 can store a one-way hash of the data. For a bitstream it does so by calculating e.g. a MD5 hash of the original media data and adds a hash section to the last of the segment file. As of EnCase 6 the option to store a SHA1 hash was added.


EnCase 5 and later have the option to store single files into the EnCase Logical Evidence File (LEF) or EWF-L01. This format changed slightly in EnCase 6 and 7.

Version 2

In EnCase 7 the EWF format was succeeded by the EnCase Evidence File Format Version 2 (EWF2-EX01 and EWF2-LX01). EWF2-EX01 is at it's lower levels a different format then EWF-E01 and provides support for:

  • bzip2 compression
  • direct encryption (AES-256) of the section data

The same features are added to the new logical evidence file format (EWF2-LX01) with the exception of encryption. The actual encryption method and corresponding key derivation are, currently, not open.

EWF2-EX01, EWF2-LX01 are not backwards compatible with previous EnCase products.

See Also

External Links