cryptomator-utils/lib_cryptomator_utils/cryptomator.py

#   cryptomator-utils: Python utilities for inspecting Cryptomator drives
#   Copyright (C) 2024  Lee Yingtong Li (RunasSudo)
#
#   This program is free software: you can redistribute it and/or modify
#   it under the terms of the GNU Affero General Public License as published by
#   the Free Software Foundation, either version 3 of the License, or
#   (at your option) any later version.
#
#   This program 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 Affero General Public License for more details.
#
#   You should have received a copy of the GNU Affero General Public License
#   along with this program.  If not, see <https://www.gnu.org/licenses/>.

from .aes import aes_siv_decrypt, aes_siv_encrypt
from .b64url import b64url_decode, b64url_encode

from Crypto.Cipher import AES
from cryptography.hazmat.primitives.kdf.scrypt import Scrypt
from cryptography.hazmat.primitives.keywrap import aes_key_unwrap, InvalidUnwrap

import base64
import getpass
import hashlib
import hmac
import json
import os
import struct
import sys

def load_vault_config(vault_config_path):
	"""
	Load and verify a vault.cryptomator configuration file and associated master key
	
	Returns (primary_master_key, hmac_master_key)
	"""
	
	# ------------------
	# Parse vault config
	
	with open(vault_config_path, 'r') as vault_config_file:
		vault_config_data = vault_config_file.read()
	
	# Split JWT header, payload and signature
	vault_config_jwt_parts = vault_config_data.split('.')
	
	if len(vault_config_jwt_parts) != 3:
		print('Error: Invalid JWT (got {} parts, expected 3)'.format(len(vault_config_jwt_parts)))
		sys.exit(1)
	
	# Parse JWT header and payload
	try:
		vault_config_header = json.loads(b64url_decode(vault_config_jwt_parts[0]).decode('utf-8'))
		vault_config_payload = json.loads(b64url_decode(vault_config_jwt_parts[1]).decode('utf-8'))
	except json.JSONDecodeError as ex:
		print('Error: Malformed JWT (invalid JSON)')
		import traceback; traceback.print_exc()
		sys.exit(1)
	
	# Validate settings
	if vault_config_header['typ'] != 'JWT':
		print('Error: Malformed JWT (no "typ" in header)')
		sys.exit(1)
	if vault_config_header['alg'] != 'HS256':
		print('Error: Unsupported JWT algorithm (got {}, expected HS256)'.format(vault_config_header['alg']))
		sys.exit(1)
	if vault_config_payload['format'] != 8:  # Current Cryptomator vault format
		print('Error: Unsupported vault format (got {}, expected 8)'.format(vault_config_payload['format']))
		sys.exit(1)
	if vault_config_payload['cipherCombo'] != 'SIV_GCM':
		print('Error: Unsupported vault cipher mode (got {}, expected SIV_GCM)'.format(vault_config_payload['cipherCombo']))
		sys.exit(1)
	
	# -------------
	# Read key file
	
	if not vault_config_header['kid'].startswith('masterkeyfile:'):
		print('Error: Unsupported vault master key ID (got {}, expected masterkeyfile)'.format(vault_config_header['kid']))
		sys.exit(1)
	
	master_key_path = os.path.join(os.path.split(vault_config_path)[0], vault_config_header['kid'][len('masterkeyfile:'):])
	
	with open(master_key_path, 'r') as master_key_file:
		master_key_config = json.load(master_key_file)
	
	# ----------
	# Derive KEK
	
	master_password = getpass.getpass()
	
	kek = Scrypt(
		base64.b64decode(master_key_config['scryptSalt']),
		32,  # Masterkey.SUBKEY_LEN_BYTES in cryptolib
		master_key_config['scryptCostParam'],
		master_key_config['scryptBlockSize'],
		1  # Scrypt.P in cryptolib
	).derive(master_password.encode('utf-8'))
	
	# -----------------------------------------
	# Unwrap encryption keys using AES Key Wrap
	
	try:
		primary_master_key = aes_key_unwrap(kek, base64.b64decode(master_key_config['primaryMasterKey']))
		hmac_master_key = aes_key_unwrap(kek, base64.b64decode(master_key_config['hmacMasterKey']))
	except InvalidUnwrap:
		print('Error: Incorrect password')
		sys.exit(1)
	
	# -------------------------------
	# Validate vault config signature
	
	# HMAC secret is the combined "masterkey" (primary_master_key + hmac_master_key) - see Masterkey class in cryptolib
	expected_signature = hmac.digest(primary_master_key + hmac_master_key, (vault_config_jwt_parts[0] + '.' + vault_config_jwt_parts[1]).encode('utf-8'), 'SHA256')
	
	if b64url_encode(expected_signature) != vault_config_jwt_parts[2]:
		print('Error: Invalid vault configuration file signature')
		sys.exit(1)
	
	return primary_master_key, hmac_master_key

def hash_directory_id(primary_master_key, hmac_master_key, directory_id):
	"""
	Compute a hashed encrypted directory ID for the given plaintext directory ID
	"""
	
	# Directory ID is encrypted with AES-SIV and hashed
	encrypted_directory_id = aes_siv_encrypt(primary_master_key, hmac_master_key, directory_id.encode('utf-8'), None)
	hashed_directory_id = base64.b32encode(hashlib.sha1(encrypted_directory_id).digest()).decode('utf-8')
	return hashed_directory_id

def directory_path_to_id(vault_path, primary_master_key, hmac_master_key, directory_path):
	"""
	Recurse the drive to resolve the directory ID for the directory at the given plaintext path
	"""
	
	directory_path_parts = directory_path.strip('/').split('/')
	
	# Begin in root directory
	# The root directory in Cryptomator has an empty directory ID
	directory_id = ''
	
	# Traverse path
	for path_part in directory_path_parts:
		if not path_part:
			continue
		
		# Hash the current directory ID
		hashed_directory_id = hash_directory_id(primary_master_key, hmac_master_key, directory_id)
		
		# Look up the encrypted path_part in the current directory
		encrypted_filename = encrypt_filename(primary_master_key, hmac_master_key, directory_id, path_part)
		
		# Get the directory ID of the part_path
		subdirectory_dir_file = os.path.join(vault_path, 'd', hashed_directory_id[:2], hashed_directory_id[2:], encrypted_filename, 'dir.c9r')
		with open(subdirectory_dir_file, 'r') as f:
			new_directory_id = f.read()
		
		# Traverse to the new directory ID
		directory_id = new_directory_id
	
	return directory_id

def list_directory(vault_path, primary_master_key, hmac_master_key, directory_id):
	"""
	Return a list of files and directories in the directory with the given plaintext directory ID
	"""
	
	hashed_directory_id = hash_directory_id(primary_master_key, hmac_master_key, directory_id)
	
	# Check directory ID given by dirid.c9r matches the expected directory ID
	stored_directory_id = decrypt_file(vault_path, primary_master_key, hashed_directory_id, 'dirid.c9r')
	if stored_directory_id != directory_id.encode('utf-8'):
		print('Error: Unexpected dirid.c9r (got "{}", expected "{}")'.format(stored_directory_id.decode('utf-8'), directory_id))
		sys.exit(1)
	
	# List directory contents
	directory_contents = []
	
	for entry in os.scandir(os.path.join(vault_path, 'd', hashed_directory_id[:2], hashed_directory_id[2:])):
		if entry.name == 'dirid.c9r':
			continue
		elif entry.name.endswith('.c9r'):
			directory_contents.append(decrypt_filename(primary_master_key, hmac_master_key, directory_id, entry.name))
		elif entry.name.endswith('.c9s'):
			print('Warning: Unsupported entry with long filename "{}" - not yet implemented'.format(entry.name))
		else:
			print('Warning: Unknown file "{}" - ignoring'.format(entry.name))
	
	return directory_contents

def encrypt_filename(primary_master_key, hmac_master_key, directory_id, filename):
	"""
	Return the encrypted filename for a given plaintext filename within a directory with the given plaintext directory ID
	"""
	
	# Filename is encrypted with AES-SIV, passing the plaintext directory ID as AAD
	return b64url_encode(aes_siv_encrypt(primary_master_key, hmac_master_key, filename.encode('utf-8'), [directory_id.encode('utf-8')]), strip_padding=False) + '.c9r'

def decrypt_filename(primary_master_key, hmac_master_key, directory_id, encrypted_filename):
	"""
	Return the plaintext filename for a given encrypted filename within a directory with the given plaintext directory ID
	"""
	
	if not encrypted_filename.endswith('.c9r'):
		raise ValueError('Encrypted filename must end with .c9r')
	
	# Reverse the AES-SIV encryption, passing the plaintext directory ID as AAD
	ciphertext_filename = b64url_decode(encrypted_filename[:-len('.c9r')])
	plaintext_filename = aes_siv_decrypt(primary_master_key, hmac_master_key, ciphertext_filename, [directory_id.encode('utf-8')]).decode('utf-8')
	return plaintext_filename

def decrypt_file(vault_path, primary_master_key, hashed_directory_id, filename):
	"""
	Return the decrypted contents of the file given by the plaintext filename and hashed directory ID
	"""
	
	# Read the encrypted contents
	encrypted_file_path = os.path.join(vault_path, 'd', hashed_directory_id[:2], hashed_directory_id[2:], filename)
	with open(encrypted_file_path, 'rb') as f:
		ciphertext = f.read()
	
	# Read header
	ciphertext_header = ciphertext[:68]
	header_nonce = ciphertext_header[:12]
	header_payload = ciphertext_header[12:-16]
	header_tag = ciphertext_header[-16:]
	
	# Decrypt header to obtain the content key
	cipher = AES.new(primary_master_key, AES.MODE_GCM, nonce=header_nonce)
	plaintext_header = cipher.decrypt_and_verify(header_payload, header_tag)
	content_key = plaintext_header[8:]
	
	# Decrypt file in chunks corresponding to 32 KiB plaintext
	plaintext = bytearray()
	
	for chunk_num, idx in enumerate(range(68, len(ciphertext), 32*1024 + 28)):  # Skip 68 byte header; each encrypted chunk is 32 KiB + 28 byte overhead
		ciphertext_chunk = ciphertext[idx:idx+32*1024+28]
		chunk_nonce = ciphertext_chunk[:12]
		chunk_payload = ciphertext_chunk[12:-16]
		chunk_tag = ciphertext_chunk[-16:]
		
		# Chunk is encrypted with AES-GCM
		cipher = AES.new(content_key, AES.MODE_GCM, nonce=chunk_nonce)
		
		# Chunk number and header nonce are passed as AAD
		# Contrary to the Cryptomator documentation, chunk number is a 64-bit not 32-bit integer - https://github.com/cryptomator/docs/pull/54
		cipher.update(struct.pack('>Q', chunk_num))
		cipher.update(header_nonce)
		
		plaintext_chunk = cipher.decrypt_and_verify(chunk_payload, chunk_tag)
		plaintext.extend(plaintext_chunk)
	
	return bytes(plaintext)