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# Chrysalide - Outil d'analyse de fichiers binaires
# backup.py - gestionnaire du format de fichier des sauvegardes Android
#
# Copyright (C) 2019 Cyrille Bagard
#
# This file is part of Chrysalide.
#
# Chrysalide is free software; you can redistribute it and/or modify
# it under the terms of the GNU General Public License as published by
# the Free Software Foundation; either version 3 of the License, or
# (at your option) any later version.
#
# Chrysalide is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program; if not, write to the Free Software
# Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
try:
from Crypto.Cipher import AES
except:
AES = None
import hashlib
import io
import pychrysalide
from pychrysalide.analysis.contents import FileContent
from pychrysalide.arch import vmpa
import zlib
PBKDF2_KEY_SIZE = 32
class AndroidBackup():
"""Reader for Android backups."""
def _read_lines(self, pos, lcount):
"""Read lcount lines from a given pos in a binary content."""
count = 0
buf = ''
while count < lcount:
got = chr(self._content.read_u8(pos))
buf += got
if got == '\n':
count += 1
if pos.phys == self._content.size:
break
return buf
def __init__(self, content):
"""Create an Android backup reader from a binary content."""
# Refs from https://android.googlesource.com/platform/frameworks/base.git/+/refs/heads/master/ :
# - versions : services/backup/java/com/android/server/backup/BackupManagerService.java#184
# - content : services/backup/java/com/android/server/backup/fullbackup/PerformAdbBackupTask.java#335
self._content = content
pos = vmpa(0, vmpa.VMPA_NO_VIRTUAL)
# Read the global file header
self._header = self._read_lines(pos, 4)
lines = self._header.split('\n')
if len(lines) != 5 or lines[0] != 'ANDROID BACKUP':
raise ValueError('Invalid header')
self._version = lines[1]
if not(lines[2] in ['0', '1']):
raise ValueError('Content should be compressed or uncompressed')
self._compressed = (lines[2] == '1')
if not(lines[3] in ['none', 'AES-256']):
raise ValueError('Encryption not supported!')
self._encrypted = (lines[3] == 'AES-256')
# Read the encryption header
if not(self._encrypted):
self._enc_header = None
self._user_password_salt = None
self._master_key_checksum_salt = None
self._pbkdf2_rounds_numer = None
self._user_key_iv = None
self._master_key_blob = None
else:
self._enc_header = self._read_lines(pos, 5)
lines = self._enc_header.split('\n')
if len(lines) != 6:
raise ValueError('Invalid encryption header')
self._user_password_salt = bytes.fromhex(lines[0])
self._master_key_checksum_salt = bytes.fromhex(lines[1])
self._pbkdf2_rounds_numer = int(lines[2])
self._user_key_iv = bytes.fromhex(lines[3])
self._master_key_blob = bytes.fromhex(lines[4])
self._backup_start = pos.phys
def is_encrypted(self):
"""Tell if the backup is encrypted."""
return self._encrypted
def _convert_master_key(self, input_bytes):
"""Convert a master key into an UTF-8 byte array."""
output = []
for byte in input_bytes:
if byte < ord(b'\x80'):
output.append(byte)
else:
output.append(ord('\xef') | (byte >> 12))
output.append(ord('\xbc') | ((byte >> 6) & ord('\x3f')))
output.append(ord('\x80') | (byte & ord('\x3f')))
return bytes(output)
def get_master_key(self, password):
"""Get a verified master key and its IV for the backup encryption."""
if AES is None:
raise OSError('No AES support')
key = hashlib.pbkdf2_hmac('sha1', password.encode('utf-8'),
self._user_password_salt,
self._pbkdf2_rounds_numer, PBKDF2_KEY_SIZE)
alg = AES.new(key, AES.MODE_CBC, self._user_key_iv)
master_key = alg.decrypt(self._master_key_blob)
blob = io.BytesIO(master_key)
master_iv_length = ord(blob.read(1))
master_iv = blob.read(master_iv_length)
master_key_length = ord(blob.read(1))
master_key = blob.read(master_key_length)
master_key_checksum_length = ord(blob.read(1))
master_key_checksum = blob.read(master_key_checksum_length)
checksum = hashlib.pbkdf2_hmac('sha1', self._convert_master_key(master_key),
self._master_key_checksum_salt,
self._pbkdf2_rounds_numer, PBKDF2_KEY_SIZE)
if not master_key_checksum == checksum:
raise ValueError('Invalid decryption password')
return master_key, master_iv
def get_content(self, password=None):
"""Extract the backup content as a simple tarball content."""
data = self._content.data[self._backup_start:]
if self._encrypted:
assert(password)
# Cf. https://f-o.org.uk/2017/decrypting-android-backups-with-python.html
master_key, master_iv = self.get_master_key(password)
alg = AES.new(master_key, AES.MODE_CBC, master_iv)
data = alg.decrypt(data)
if self._compressed:
d = zlib.decompressobj()
extracted = d.decompress(data)
else:
extracted = data
return extracted
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