Move Android backup decryption to a standalone module

Whatsapp_Chat_Exporter/__main__.py

@@ -7,23 +7,24 @@
 import json
 import string
 import glob
+import importlib.metadata
+from Whatsapp_Chat_Exporter import android_crypt, exported_handler, android_handler
+from Whatsapp_Chat_Exporter import ios_handler, ios_media_handler
+from Whatsapp_Chat_Exporter.data_model import ChatStore
+from Whatsapp_Chat_Exporter.utility import APPLE_TIME, Crypt, check_update, DbType
+from Whatsapp_Chat_Exporter.utility import readable_to_bytes, sanitize_filename
+from Whatsapp_Chat_Exporter.utility import import_from_json, bytes_to_readable
+from argparse import ArgumentParser, SUPPRESS
+from datetime import datetime
+from getpass import getpass
+from sys import exit
 try:
     import vobject
 except ModuleNotFoundError:
     vcards_deps_installed = False
 else:
     from Whatsapp_Chat_Exporter.vcards_contacts import ContactsFromVCards
     vcards_deps_installed = True
-from Whatsapp_Chat_Exporter import exported_handler, android_handler
-from Whatsapp_Chat_Exporter import ios_handler, ios_media_handler
-from Whatsapp_Chat_Exporter.data_model import ChatStore
-from Whatsapp_Chat_Exporter.utility import APPLE_TIME, Crypt, DbType, readable_to_bytes, check_update
-from Whatsapp_Chat_Exporter.utility import import_from_json, sanitize_filename, bytes_to_readable
-from argparse import ArgumentParser, SUPPRESS
-from datetime import datetime
-from getpass import getpass
-from sys import exit
-import importlib.metadata
 
 
 def main():
@@ -448,12 +449,12 @@ def main():
             db = open(args.backup, "rb").read()
             if args.wab:
                 wab = open(args.wab, "rb").read()
-                error_wa = android_handler.decrypt_backup(wab, key, contact_db, crypt, args.showkey, DbType.CONTACT, key_stream)
+                error_wa = android_crypt.decrypt_backup(wab, key, contact_db, crypt, args.showkey, DbType.CONTACT, key_stream)
                 if isinstance(key, io.IOBase):
                     key.seek(0)
             else:
                 error_wa = 0
-            error_message = android_handler.decrypt_backup(db, key, msg_db, crypt, args.showkey, DbType.MESSAGE, key_stream)
+            error_message = android_crypt.decrypt_backup(db, key, msg_db, crypt, args.showkey, DbType.MESSAGE, key_stream)
             if error_wa != 0:
                 error = error_wa
             elif error_message != 0:

Whatsapp_Chat_Exporter/android_crypt.py

@@ -0,0 +1,211 @@
+import hmac
+import io
+import zlib
+import javaobj
+from typing import Tuple, Union
+from hashlib import sha256
+from Whatsapp_Chat_Exporter.utility import CRYPT14_OFFSETS, Crypt, DbType
+try:
+    import zlib
+    from Crypto.Cipher import AES
+except ModuleNotFoundError:
+    support_backup = False
+else:
+    support_backup = True
+try:
+    import javaobj
+except ModuleNotFoundError:
+    support_crypt15 = False
+else:
+    support_crypt15 = True
+
+
+def _derive_main_enc_key(key_stream: bytes) -> Tuple[bytes, bytes]:
+    """
+    Derive the main encryption key for the given key stream. The key is derived using HMAC of HMAC of the provided key stream.
+
+    Args:
+        key_stream (bytes): The key stream to generate HMAC of HMAC.
+
+    Returns:
+        Tuple[bytes, bytes]: A tuple containing the main encryption key and the original key stream.
+    """
+    key = hmac.new(
+        hmac.new(
+            b'\x00' * 32,
+            key_stream,
+            sha256
+        ).digest(),
+        b"backup encryption\x01",
+        sha256
+    )
+    return key.digest(), key_stream
+
+
+def _extract_enc_key(keyfile: bytes) -> Tuple[bytes, bytes]:
+    """
+    Extract the encryption key from the keyfile.
+
+    Args:
+        keyfile (bytes): The keyfile containing the encrypted key.
+
+    Returns:
+        Tuple[bytes, bytes]: values from _derive_main_enc_key()
+    """
+    key_stream = b""
+    for byte in javaobj.loads(keyfile):
+        key_stream += byte.to_bytes(1, "big", signed=True)
+
+    return _derive_main_enc_key(key_stream)
+
+
+def brute_force_offset(max_iv: int = 200, max_db: int = 200):
+    """
+    Brute force the offsets for IV and database start position in WhatsApp backup files.
+    Used when common offsets are not applicable to a backup file.
+
+    Args:
+        max_iv (int, optional): Maximum value to try for IV offset. Defaults to 200.
+        max_db (int, optional): Maximum value to try for database start offset. Defaults to 200.
+
+    Yields:
+        tuple: A tuple containing:
+            - int: Start position of IV
+            - int: End position of IV (start + 16)
+            - int: Start position of database
+    """
+    for iv in range(0, max_iv):
+        for db in range(0, max_db):
+            yield iv, iv + 16, db
+
+
+def decrypt_backup(
+    database: bytes,
+    key: Union[str, io.IOBase],
+    output: str = None,
+    crypt: Crypt = Crypt.CRYPT14,
+    show_crypt15: bool = False,
+    db_type: DbType = DbType.MESSAGE,
+    dry_run: bool = False,
+    key_stream: bool = False
+) -> int:
+    """
+    Decrypt the WhatsApp backup database.
+
+    Args:
+        database (bytes): The encrypted database file.
+        key (str or io.IOBase): The key to decrypt the database. The key should either be a string (32 bytes hex key) or a file object (encryption key file).
+        key_stream (bool, optional): Whether the key is a key stream. False for hex key. True for key stream.
+        output (str, optional): The path to save the decrypted database. Defaults to None. When dry_run is True, this parameter is ignored.
+        crypt (Crypt, optional): The encryption version of the database. Defaults to Crypt.CRYPT14.
+        show_crypt15 (bool, optional): Whether to show the HEX key of the crypt15 backup. Defaults to False.
+        db_type (DbType, optional): The type of database (MESSAGE or CONTACT). Defaults to DbType.MESSAGE.
+        dry_run (bool, optional): Whether to perform a dry run without saving the decrypted database. Defaults to False.
+
+    Returns:
+        int: The status code of the decryption process.
+            - 0: The decryption process was successful.
+            - 1: The decryption process failed because the necessary dependencies for backup decryption are not available.
+            - 2: The decryption process failed because the common offsets for the IV and database are not applicable, and the brute force attempt to find the correct offsets also failed.
+            - 3: The decryption process failed due to unknown error
+    """
+    if not support_backup:
+        return 1
+    if not dry_run and output is None:
+        ValueError("The path to the decrypted database must be specified unless dry_run is true.")
+    if isinstance(key, io.IOBase):
+        key = key.read()
+        if crypt is not Crypt.CRYPT15:
+            t1 = key[30:62]
+    if crypt is not Crypt.CRYPT15 and len(key) != 158:
+        raise ValueError("The key file must be 158 bytes")
+    # Determine the IV and database offsets
+    if crypt == Crypt.CRYPT14:
+        if len(database) < 191:
+            raise ValueError("The crypt14 file must be at least 191 bytes")
+        current_try = 0
+        offsets = CRYPT14_OFFSETS[current_try]
+        t2 = database[15:47]
+        iv = database[offsets["iv"]:offsets["iv"] + 16]
+        db_ciphertext = database[offsets["db"]:]
+    elif crypt == Crypt.CRYPT12:
+        if len(database) < 67:
+            raise ValueError("The crypt12 file must be at least 67 bytes")
+        t2 = database[3:35]
+        iv = database[51:67]
+        db_ciphertext = database[67:-20]
+    elif crypt == Crypt.CRYPT15:
+        if not support_crypt15:
+            return 1
+        if len(database) < 131:
+            raise ValueError("The crypt15 file must be at least 131 bytes")
+        t1 = t2 = None
+        if db_type == DbType.MESSAGE:
+            iv = database[8:24]
+            db_offset = database[0] + 2  # Skip protobuf + protobuf size and backup type
+        elif db_type == DbType.CONTACT:
+            iv = database[7:23]
+            db_offset = database[0] + 1  # Skip protobuf + protobuf size
+        db_ciphertext = database[db_offset:]
+
+    if t1 != t2:
+        raise ValueError("The signature of key file and backup file mismatch")
+
+    if crypt == Crypt.CRYPT15:
+        if key_stream:
+            main_key, hex_key = _extract_enc_key(key)
+        else:
+            main_key, hex_key = _derive_main_enc_key(key)
+        if show_crypt15:
+            hex_key = [hex_key.hex()[c:c+4] for c in range(0, len(hex_key.hex()), 4)]
+            print("The HEX key of the crypt15 backup is: " + ' '.join(hex_key))
+    else:
+        main_key = key[126:]
+    decompressed = False
+    while not decompressed:
+        cipher = AES.new(main_key, AES.MODE_GCM, iv)
+        db_compressed = cipher.decrypt(db_ciphertext)
+        try:
+            db = zlib.decompress(db_compressed)
+        except zlib.error:
+            if crypt == Crypt.CRYPT14:
+                current_try += 1
+                if current_try < len(CRYPT14_OFFSETS):
+                    offsets = CRYPT14_OFFSETS[current_try]
+                    iv = database[offsets["iv"]:offsets["iv"] + 16]
+                    db_ciphertext = database[offsets["db"]:]
+                    continue
+                else:
+                    print("Common offsets are not applicable to "
+                          "your backup. Trying to brute force it...")
+                    for start_iv, end_iv, start_db in brute_force_offset():
+                        iv = database[start_iv:end_iv]
+                        db_ciphertext = database[start_db:]
+                        cipher = AES.new(main_key, AES.MODE_GCM, iv)
+                        db_compressed = cipher.decrypt(db_ciphertext)
+                        try:
+                            db = zlib.decompress(db_compressed)
+                        except zlib.error:
+                            continue
+                        else:
+                            decompressed = True
+                            print(
+                                f"The offsets of your IV and database are {start_iv} and "
+                                f"{start_db}, respectively. To include your offsets in the "
+                                "program, please report it by creating an issue on GitHub: "
+                                "https://github.com/KnugiHK/Whatsapp-Chat-Exporter/discussions/47"
+                            )
+                            break
+                    if not decompressed:
+                        return 2
+            else:
+                return 3
+        else:
+            decompressed = True
+        if db[0:6].upper() == b"SQLITE":
+            if not dry_run:
+                with open(output, "wb") as f:
+                    f.write(db)
+            return 0
+        else:
+            raise ValueError("The plaintext is not a SQLite database. Did you use the key to encrypt something...")