CP Shrink

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File Format
Name CP Shrink
Extension(s) .cpz
Released ≤1992

CP Shrink (or CPSHRINK) is a compressed archive program for DOS, and its native .CPZ file format. It was developed by Central Point Software. It usually uses PKWARE DCL Implode compression.

File structure

CPSHRINK CPZ archive format overview

 This information is based on reverse engineering. It may be incorrect
  or incomplete.
 Byte order is little-endian.
 CRC algorithm is CRC-32-IEEE.
 Sizes and offsets are in bytes, unless otherwise noted.

File structure

 Offset         Size         Description
 ------         ----         -----------
 0              8            Archive Header segment
 8              32×nmembers  Member File Headers segment
 8+32×nmembers               File Data area

Archive Header segment

 Offset   Size Label     Description
 ------   ---- -----     -----------
 0        4    nmembers  Number of member files
 4        4              CRC of the 32×nmembers bytes following this

 Note: The nmembers field might actually be a 2-byte field, followed
 by 2 bytes of unknown use.

Member File Headers segment

 Sequence of Member File Headers, each 32 bytes, starts at offset 8.

Member File Header

 Offset   Size   Description
 ------   ----   -----------
 +0       4      CRC of *compressed* file data
 +4       15     Filename, NUL terminated
 +19      1      Compression method:
                  0 = PKWARE DCL Implode, Binary
                  1 = PKWARE DCL Implode, ASCII
                  2 = not compressed
 +20      4      Original size
 +24      4      Compressed size
 +28      4      Timestamp (DOS date, time)

File Data area

 The first file's compressed data starts at 8+32×nmembers.

 Location of other files' data is implied by the sum of the compressed
 sizes of the preceding files.


There's no quick and easy way to identify a CPZ file from its contents. Validating the CRC of the file headers segment could work, but that segment does not have a fixed size.


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