File Format Reverse Engineering

File-format reverse engineering derives a falsifiable structural specification from representative files and controlled input/output experiments.

Acquire evidence

  1. Collect multiple versions, sizes and known application states.
  2. Create known input/output pairs by changing one value at a time.
  3. Preserve originals, hashes and provenance.
  4. Compare signatures, headers, offsets and repeated structures.
  5. Distinguish fixed records, length-prefixed/chunked structures and offset tables.

Interpret cautiously

Text-like bytesTest encoding, terminators and fixed/variable length.
Numeric candidatesTest endian order, signedness, scale, dates and floats.
High entropyMay indicate compression, encryption or packed data.
Changing tail/headerLength, checksum, directory or version metadata.

Implement a defensive parser that validates lengths/offsets and rejects impossible structures. Test unseen files and document uncertain hypotheses rather than forcing every byte into a field.

This methodology differs from proprietary-file recovery, which evaluates extraction and damage. Use executable analysis to observe producers/consumers and the pillar for project scope.

Provide a representative corpus, producing application, known values and the required fields/operations.

Describe your legacy system or problem

Share what still works, what failed, and what must be preserved.

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