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Aperi'Solve v.3.2.4

PCRT

PCRT (PNG Check & Repair Tool) detects and automatically repairs corrupted PNG files. Aperi'Solve embeds its own Python port, based on sherlly/PCRT and the PCRT3 forks by indonumberone and Etr1x. Where pngcheck only diagnoses, PCRT fixes.

What Aperi'Solve runs

The port runs in-process (no external command). The upload must at least contain IHDR, IDAT and IEND markers to be treated as a PNG; it is then checked and rebuilt section by section:

  • Signature — a tampered first 8 bytes is restored to 89 50 4E 47 0D 0A 1A 0A.
  • IHDR — the CRC is verified. On mismatch, the stored CRC is looked up in a database of known IHDR configurations (common resolutions and bit depths); if nothing matches, an exhaustive search recomputes the CRC for every width x height from 1 to 4999 (capped at 30 seconds) until it matches, recovering zeroed or falsified dimensions.
  • Ancillary chunks (PLTE, tRNS, gAMA, pHYs...) — copied over with their CRCs validated and fixed.
  • IDAT — a length/data mismatch triggers a DOS-to-Unix recovery (re-inserting \x0d bytes before \x0a candidates until the CRC matches); a plain CRC mismatch is recomputed.
  • IEND — a missing or malformed trailer is replaced by the standard 12-byte chunk, and any bytes after IEND are extracted.

Reading the output

The log lists each check with offsets, for example:

Error IHDR CRC found at offset 0x1d
Chunk crc: 00000000, Correct crc: 575943df
Found correct dimensions via exhaustive search: 800x600

When any fix succeeds, the repaired image is saved as pcrt_recovered_<name>.png and offered on the result page — open it to see what the corruption was hiding. Data found after IEND is saved as extra_data.bin in a downloadable archive.

Installing locally

$ git clone https://github.com/sherlly/PCRT
$ python PCRT.py -i image.png

The original is Python 2 era; the PCRT3 forks run on Python 3.

Limitations

  • PNG only, and the file must still contain IHDR/IDAT/IEND markers — for a fully mangled file, rebuild the header by hand in a hex editor.
  • The exhaustive dimension search covers 1–4999 pixels per side within a time budget; extreme sizes may not be recovered.
  • Repairing structure does not extract LSB payloads — run zsteg on the recovered image too.

Common CTF patterns

  • Zeroed width/height in IHDR: the image displays as 0x0 or refuses to open, but the untouched CRC lets PCRT brute-force the true dimensions.
  • Height shrunk to crop the flag off the bottom — the recovered image shows the full picture.
  • Signature bytes overwritten so file misidentifies the upload.
  • A flag or archive appended after IEND — also visible to binwalk.