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Analysis of the 624 (Six-2-Four) packer

The 624 packer is a executable packer that got released in 1997 by Kim Holviala. It only supports DOS .com files and was targeted to compress 4kb demoscene intros but offers a decent compression of files from 1 kb to 20 kb size.

  • Uses LZSS (Lempel–Ziv–Storer–Szymanski) compression algorithm
  • The assembly unpacking stub is 127 byte small
  • Fixed length huffmann codes are used to store the length and offset of a match
  • Packed data is stored as a bit stream. This allows for a shorter unpacker stub and higher compression ratio but slightly slower decompression
  • 1 byte matches are stored using 6 bit, 1 to mark a match, 1 to store the length as a marker for 1 byte length and the remaining 4 bit to encode the offset of the match, therefor up to 16 byte backwards in the output buffer
  • The storing of output bytes is using a byte addressing instruction. This allows the compressor to use RLE to compress a run of a literal by storing offset 1 and the amount of bytes to copy as match length

Later someone released a rewrite of this packer in assembly as version 1.1. In this version the unpacking stub had been optimized to 116 byte and the compression was notable faster.

Ideas to improve the compression ratio:

  • With the current implementation the first byte is always a literal, therefor the bit in front to mark it as literal is not needed. The unpacking stub could jump at beginning of execution directly to the code to copy the literal to the output buffer
  • A match will probably be followed by a literal. The bit to mark the literal can be skipped and the unpacker adjusted to expect a literal after a match without changing the size of the unpacker stub
  • A run of literals could be encoded with the length of it. The following compressed flag of the match following the literal run would not need to be saved
  • Huffman codes can be replaced by [gamma encoding](https://en.wikipedia.org/wiki/Universal_code_(data_compression)), which would result in a smaller unpacker stub but may depend on the input file to compress to improve the overall compression ratio

The packer versions to download:

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