Encase image file format

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EnCase uses a closed format for images which is reportedly based on ASR Data's Expert Witness Compression Format. The evidence files, or E01 files, contain a physical bitstream of an acquired disk, prefixed with a '"Case Info" header, interlaced with checksums (Adler32) for every block of 64 x 512 byte sectors (32 KiB), and followed by a footer containing an MD5 hash for the entire bitstream. Contained in the header are the date and time of acquisition, an examiner's name, notes on the acquisition, and an optional password; the header concludes with its own checksum.

EnCase can store media data into multiple evidence files, which are called segment files. Each segment file consist of multiple sections. Each section consist of a section start definition. This contains 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 using a base offset work around in EnCase 6.

At least from Encase 3 the case info header is contained in the "header" section, which is defined twice within the file and contain the same information.

With Encase 4 an additional "header2" section was added. The "header" section now appears only once, but the new "header2" section twice.

Version 3 of The Encase F introduced an "error2" sections 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.

Within Encase 5 the number of sectors per block (chunk) can vary.

Encase from at least in version 3 can hash the data of the media it acquires. It does this by calculating a MD5 hash of the original media data and adds a hash section to the last of the segment files.

Later versions of Encase 6 included the option to store a MD5 and/or SHA1 hash.

In EnCase 7 the EWF format was succeeded by the EnCase Evidence File Format Version 2 (EWF-EX01 and EWF-LX01). EnCase 7 also uses a different version of EWF-L01 then its predecessors. EnCase 7 no longer distinguishes between fast or best compression.

EWF-EX01 is at it's lower levels a different format then EWF-E01 and provides support for:

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

The same features are added to the new logical evidence file format (Lx01) with the exception of encryption.

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

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