A more concrete way to determine database offset

Whatsapp_Chat_Exporter/extract.py

@@ -112,7 +112,8 @@ def decrypt_backup(database, key, output, crypt=Crypt.CRYPT14, show_crypt15=Fals
             raise ValueError("The crypt15 file must be at least 131 bytes")
         t1 = t2 = None
         iv = database[8:24]
-        db_ciphertext = database[131:]
+        db_offset = database[0] + 2 # Skip protobuf + protobuf size and backup type
+        db_ciphertext = database[proto_len:]
 
     if t1 != t2:
         raise ValueError("The signature of key file and backup file mismatch")