Add an old tool I found on my HDD to decrypt obfuscated EXEs from Twinkle games

twinkle/twinkle_iidx_mgp_genkey.py

@@ -0,0 +1,47 @@
+import argparse
+
+def generate_key(filename):
+    key = 0
+
+    for c in filename[-6:][::-1]: # Not necessary to reverse the string, but the real game reads from the tail backward so recreate for accuracy
+        key += ord(c)
+
+        if ord(c) - 0x61 >= 0x1a:
+            # I'm not sure of anything that hits this path so I can't verify, but it's in the ASM code
+            key += 0xffe0
+
+    return key & 0xffff
+
+
+def decrypt_data(input_key, input_filename, output_filename):
+    key = generate_key(input_key)
+
+    print("Key: %s -> %04x" % (input_key, key))
+
+    print("Decrypting %s..." % input_filename)
+
+    with open(input_filename, "rb") as infile:
+        data = bytearray(infile.read())
+
+    for i in range(0, len(data), 2):
+        curdata = ((data[i+1] << 8) | data[i]) ^ key
+
+        data[i] = (((curdata & 0x1c00) >> 5) | (curdata & 0x1f)) & 0xff
+        data[i+1] = (((curdata & 0xe000) >> 8) | ((curdata & 0x3e0) >> 5)) & 0xff
+
+        key = (key + 0x71) & 0xffff
+
+    with open(output_filename, "wb") as outfile:
+        outfile.write(data)
+
+    print("Saved to %s!" % output_filename)
+
+
+if __name__ == '__main__':
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--input', help='Input file', required=True)
+    parser.add_argument('--output', help='Output file', required=True)
+    parser.add_argument('--key', help='String to be used for key (usually the file path from root, such as "/DATA/511.MGP")', required=True)
+    args = parser.parse_args()
+
+    decrypt_data(args.key, args.input, args.output)