cscg ist super

2026/cscg/forensics/intro3/restore_file.py

@@ -0,0 +1,33 @@
+from pngparser import PngParser, chunktypes
+import zlib
+
+
+def calc_crc(chunk):
+    return zlib.crc32(chunk.type + chunk.data).to_bytes(chunktypes.CHUNK_CRC_SIZE, 'big')
+
+
+def reconstruct_image():
+    parser = PngParser("./intro-forensics-3")
+    header = parser.get_header()
+    header.crc = calc_crc(header)
+
+    data_chunks = sorted([chunk  for chunk in parser.get_all() if chunk.type not in  [b"IHDR", b"IEND"]], key=lambda x: x.crc)
+
+    for chunk in data_chunks:
+        chunk.crc = calc_crc(chunk)
+
+    end_chunk = parser.get_by_type(chunktypes.TYPE_IEND)
+    end_chunk[0].crc = calc_crc(end_chunk[0])
+
+    chunks = [header] + data_chunks + end_chunk
+
+    png_file = bytes.fromhex("89 50 4E 47 0D 0A 1A 0A".replace(" ", ""))
+
+    for chunk in chunks:
+        png_file += chunk.to_bytes()
+
+    with open("reconstructed.png", "wb") as f:
+        f.write(png_file)
+
+if __name__ == "__main__":
+    reconstruct_image()