ASR Data's Expert Witness Compression Format
Expert Witness File Format Specification Revised: April 7, 2002 Original URL: http://www.asrdata.com/SMART/whitepaper.html Redistributed with permission
This page is intended to preserve the original material, including errors and typos. It is wiki-style formatted as closely as possible to the original.
Developed by ASR Data, the Expert Witness file format (aka E01 format aka EnCase file format) is an industry standard format for storing "forensic" images. The format allows a user to access arbitrary offsets in the uncompressed data without requiring decompression of the entire data stream. The specification does NOT provide for quantifyable assurance of integrity, it is up to the implementation to provide meaningful authentication for any data contained in an "evidence file".
- File Header
- The Section
- Section Types
- 'header' section
- 'volume' section
- 'table' section
- 'next' and 'done' sections
The Expert Witness Compression format can store a single image in one or more segment files. Each file consists of a standard 13-byte header, followed by a series of sections. The sections are typically arranged back-to-back. A section cannot span two files.
A Chunk is a 32k run of data (64 standard sectors). All offsets are relative to the beginning of the segment file, unless otherwise noted.
File Header Each file begins with the following 13-byte header. (This is not to be confused with the header section, below.)
Signature Part (8 bytes)...
Fields Part (5 bytes)...
|Data||0x01||1 or higher||0x0000|
Every section begins with the same standard data, with the following layout.
|0 (0x0)||16||"volume", "header", etc.||Section type string|
|16 (0x10)||8||64-bit offset in current file to the next section|
|24 (0x18)||8||64-bit byte-size of the section|
|72 (0x48)||4||CRC of all previous setion data|
Expert Witness Compression uses the following section types: header, volume, table, next, and done. Some of these section types have unique data that begins directly after the standard section structure above.
|76 (0x4c)||to end of section||zlib compress()'ed data||Comments structure (see below)|
Comment structure is simply a text string in the following tab- and newline-delimited format. (The data in each cell is separated by a tab character, and each row is separated by a newline character.) The first three lines are standard and must not change. The characters in the third line serve as reminders for the content of the fields in the fourth line. (The fourth line is the only line that needs to be customized.)
|Case Number||Evidence Number||Unique Description||Examiner Name||Notes||Acquired Date||System Date||pwhash||char|
- Case Number, Evidence Number, Unique Description, Examiner Name, and Notes are free-form (provided they don't contain tab or newline characters).
- Acquired Date and System Date are in the form of: "2002 3 4 10 19 59" (March 4, 2002 10:19:59).
- pwhash should simply be the character '0'.
- char should be the one of these three characters: 'b', 'f', or 'n'. This represents "best", "fastest", or "no compression". Expert Witness Compression uses 'f'.
The header section should appear in the first segment file only.
|80 (0x50)||4||Chunk Count|
|84 (0x54)||4||64||Sectors per Chunk|
|88 (0x58)||4||512||Bytes per Sector|
|92 (0x5c)||4||Sector Count|
|161 (0xa1)||5||Reserved (Signature)|
|166 (0xa6)||4||CRC of all previous 'volume' data, starting with offset 76|
The volume section should appear in the first segment file only.
|76 (0x4c)||4||1||Chunk Count (for this table)|
|96 (0x60)||4||CRC of all previous 'table' data, starting with offset 76|
|100 (0x64)||as long as necessary||Offset array (see below)|
|from end of offset array||to end of section||zlib compress()'ed data Chunks|
The offset array is a series of back-to-back 4-byte unsigned integer values. Each entry is an offset to the start of a compressed 'Chunk'. The high bit of each value must be set! There must be one entry per Chunk.
Each table section can hold 16375 entries. If more entries are needed, you must create multiple table sections per file.
'next' and 'done' sections...
Each file ends with a 'next' or 'done' section. If the file is the last segment in an Expert Witness compressed image, the section will be named 'done'. Otherwise, the section will be named 'next' to indicate that another segment file must be read. The "next section" pointer for these section types points to the beginning of the section itself, since it is the last section in a file. These section types have no unique data.