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# -*- coding: utf-8 -*-
# crypto.py
# Copyright (C) 2013 LEAP
#
# This program 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.
#
# 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 General Public License for more details.
#
# You should have received a copy of the GNU General Public License
# along with this program. If not, see <http://www.gnu.org/licenses/>.
"""
Cryptographic utilities for Soledad.
"""
import os
import binascii
import hmac
import hashlib
from Crypto.Cipher import AES
from Crypto.Util import Counter
from leap.soledad import (
soledad_assert,
soledad_assert_type,
)
class EncryptionMethods(object):
"""
Representation of encryption methods that can be used.
"""
AES_256_CTR = 'aes-256-ctr'
class UnknownEncryptionMethod(Exception):
"""
Raised when trying to encrypt/decrypt with unknown method.
"""
pass
class NoSymmetricSecret(Exception):
"""
Raised when trying to get a hashed passphrase.
"""
class SoledadCrypto(object):
"""
General cryptographic functionality.
"""
MAC_KEY_LENGTH = 64
def __init__(self, soledad):
"""
Initialize the crypto object.
@param soledad: A Soledad instance for key lookup.
@type soledad: leap.soledad.Soledad
"""
self._soledad = soledad
def encrypt_sym(self, data, key,
method=EncryptionMethods.AES_256_CTR):
"""
Encrypt C{data} using a {password}.
Currently, the only encryption method supported is AES-256 CTR mode.
@param data: The data to be encrypted.
@type data: str
@param key: The key used to encrypt C{data} (must be 256 bits long).
@type key: str
@param method: The encryption method to use.
@type method: str
@return: A tuple with the initial value and the encrypted data.
@rtype: (long, str)
"""
soledad_assert_type(key, str)
# AES-256 in CTR mode
if method == EncryptionMethods.AES_256_CTR:
soledad_assert(
len(key) == 32, # 32 x 8 = 256 bits.
'Wrong key size: %s bits (must be 256 bits long).' %
(len(key) * 8))
iv = os.urandom(8)
ctr = Counter.new(64, prefix=iv)
cipher = AES.new(key=key, mode=AES.MODE_CTR, counter=ctr)
return binascii.b2a_base64(iv), cipher.encrypt(data)
# raise if method is unknown
raise UnknownEncryptionMethod('Unkwnown method: %s' % method)
def decrypt_sym(self, data, key,
method=EncryptionMethods.AES_256_CTR, **kwargs):
"""
Decrypt data using symmetric secret.
Currently, the only encryption method supported is AES-256 CTR mode.
@param data: The data to be decrypted.
@type data: str
@param key: The key used to decrypt C{data} (must be 256 bits long).
@type key: str
@param method: The encryption method to use.
@type method: str
@param kwargs: Other parameters specific to each encryption method.
@type kwargs: dict
@return: The decrypted data.
@rtype: str
"""
soledad_assert_type(key, str)
# AES-256 in CTR mode
if method == EncryptionMethods.AES_256_CTR:
# assert params
soledad_assert(
len(key) == 32, # 32 x 8 = 256 bits.
'Wrong key size: %s (must be 256 bits long).' % len(key))
soledad_assert(
'iv' in kwargs,
'AES-256-CTR needs an initial value given as.')
ctr = Counter.new(64, prefix=binascii.a2b_base64(kwargs['iv']))
cipher = AES.new(key=key, mode=AES.MODE_CTR, counter=ctr)
return cipher.decrypt(data)
# raise if method is unknown
raise UnknownEncryptionMethod('Unkwnown method: %s' % method)
def doc_passphrase(self, doc_id):
"""
Generate a passphrase for symmetric encryption of document's contents.
The password is derived using HMAC having sha256 as underlying hash
function. The key used for HMAC are the first
C{soledad.REMOTE_STORAGE_SECRET_KENGTH} bytes of Soledad's storage
secret stripped from the first MAC_KEY_LENGTH characters. The HMAC
message is C{doc_id}.
@param doc_id: The id of the document that will be encrypted using
this passphrase.
@type doc_id: str
@return: The passphrase.
@rtype: str
@raise NoSymmetricSecret: if no symmetric secret was supplied.
"""
if self.secret is None:
raise NoSymmetricSecret()
return hmac.new(
self.secret[
self.MAC_KEY_LENGTH:
self._soledad.REMOTE_STORAGE_SECRET_LENGTH],
doc_id,
hashlib.sha256).digest()
def doc_mac_key(self, doc_id):
"""
Generate a key for calculating a MAC for a document whose id is
C{doc_id}.
The key is derived using HMAC having sha256 as underlying hash
function. The key used for HMAC is the first MAC_KEY_LENGTH characters
of Soledad's storage secret. The HMAC message is C{doc_id}.
@param doc_id: The id of the document.
@type doc_id: str
@return: The key.
@rtype: str
@raise NoSymmetricSecret: if no symmetric secret was supplied.
"""
if self.secret is None:
raise NoSymmetricSecret()
return hmac.new(
self.secret[:self.MAC_KEY_LENGTH],
doc_id,
hashlib.sha256).digest()
#
# secret setters/getters
#
def _get_secret(self):
return self._soledad.storage_secret
secret = property(
_get_secret, doc='The secret used for symmetric encryption')
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