Add a simple script to decrypt ulog files

This now only decrypts xchacha20 encrypted logs, where keys are
exchanged with rsa_oaep_sha256 and nonce appended to the end of the
key

Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>

Tools/decrypt_ulog.py

@@ -0,0 +1,44 @@
+#!/usr/bin/env python3
+
+from Crypto.PublicKey import RSA
+from Crypto.Cipher import PKCS1_OAEP
+from Crypto.Cipher import ChaCha20
+from Crypto.Hash import SHA256
+import binascii
+import argparse
+#from pathlib import Path
+import sys
+
+if __name__ == "__main__":
+
+    parser = argparse.ArgumentParser(description="""CLI tool to decrypt an ulog file\n""")
+    parser.add_argument("ulog_file", help=".ulog file", nargs='?', default=None)
+    parser.add_argument("ulog_key", help=".ulogk, encrypted key", nargs='?', default=None)
+    parser.add_argument("rsa_key", help=".pem format key for decrypting the ulog key", nargs='?', default=None)
+
+    args = parser.parse_args()
+
+    # Only generate a key pair, don't sign
+    if not args.ulog_file or not args.ulog_key or not args.rsa_key:
+        print('Need all arguments, the encrypted ulog file, the key and the key decryption key')
+        sys.exit(1);
+
+    # Read the private RSA key to decrypt the cahcha key
+    with open(args.rsa_key, 'rb') as f:
+        r = RSA.importKey(f.read(), passphrase='')
+
+    # Read the encrypted xchacha key and the nonce
+    with open(args.ulog_key, 'rb') as f:
+        ulog_key_cipher = f.read(256)
+        nonce = f.read(24)
+
+    # Decrypt the xchacha key
+    cipher_rsa = PKCS1_OAEP.new(r,SHA256)
+    ulog_key = cipher_rsa.decrypt(ulog_key_cipher)
+    #print(binascii.hexlify(ulog_key))
+
+    # Read and decrypt the .ulgc
+    cipher = ChaCha20.new(key=ulog_key, nonce=nonce)
+    with open(args.ulog_file, 'rb') as f:
+        with open(args.ulog_file.rstrip(args.ulog_file[-1]), 'wb') as out:
+            out.write(cipher.decrypt(f.read()))