Added watermark decryption helper

watermarked/wm_decrypt.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python3
+# Parses and decrypts watermarks from Red Star OS 3.0.
+# Currently supports only image watermarks.
+import sys
+import struct
+try:
+    from Crypto.Cipher import DES
+except ImportError:
+    print('Run: pip install pycrypto')
+    sys.exit(1)
+
+
+KEY = b'\x07\x06\x05\x04\x03\x02\x01\x00'
+
+
+def wm_decrypt(wm):
+    c = DES.new(KEY, DES.MODE_ECB)
+    return c.decrypt(wm)
+
+
+if __name__ == '__main__':
+    f = open(sys.argv[1], 'rb')
+    f.seek(-3, 2)
+    if f.read(3) != b'EOF':
+        print('File does not include a watermark')
+        sys.exit(1)
+    f.seek(-7, 2)
+    # The total length of the watermarks is
+    # a 32 Bit little endian integer right
+    # before the 'EOF' sequence.
+    size = struct.unpack('<I', f.read(4))[0]
+    if not size:
+        print('Invalid size')
+        sys.exit(1)
+    f.seek(0)
+    wms = f.read(size)
+    # Watermarks are always 24 Byte long.
+    if len(wms) % 24 != 0:
+        print('Invalid watermark count')
+        sys.exit(1)
+
+    for i in [wms[i:i+24] for i in range(0, len(wms), 24)]:
+        print(wm_decrypt(i))
+