changed repo structure

2025/cscg/forensics/intro3/test.py

@@ -0,0 +1,38 @@
+from pngparser import PngParser, chunktypes, ChunkIHDR
+import zlib
+
+
+def calc_crc(chunk):
+    return zlib.crc32(chunk.type + chunk.data).to_bytes(chunktypes.CHUNK_CRC_SIZE, 'big')
+
+parser = PngParser("./test.png")
+
+header = parser.get_header()
+header.crc = calc_crc(header)
+header = ChunkIHDR(chunktypes.TYPE_IHDR, header.data, header.crc)
+
+print(header)
+
+data_chunks = [chunk  for chunk in parser.get_all() if chunk.type not in  [b"IHDR", b"IEND"]]
+
+data_chunks = data_chunks[:10]
+
+for chunk in data_chunks:
+    chunk.crc = calc_crc(chunk)
+
+print([chunk.type for chunk in data_chunks])
+
+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("test_cut.png", "wb") as f:
+    f.write(png_file)
+
+print(len(data_chunks))