path: root/src/leap/soledad/client/_crypto.py

# -*- coding: utf-8 -*-
# _crypto.py
# Copyright (C) 2016 LEAP Encryption Access Project
#
# 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 operations for the soledad client.

This module implements streaming crypto operations.
It replaces the old client.crypto module, that will be deprecated in soledad
0.12.

The algorithm for encrypting and decrypting is as follow:

The KEY is a 32 bytes value.
The IV is a random 16 bytes value.
The PREAMBLE is a packed_structure with encryption metadata, such as IV.
The SEPARATOR is a space.

Encryption
----------

IV = os.urandom(16)
PREAMBLE = MAGIC, ENC_SCHEME, ENC_METHOD, time, IV, doc_id, rev,
and size.

PREAMBLE = base64_encoded(PREAMBLE)
CIPHERTEXT = base64_encoded(AES_GCM(KEY, cleartext) + resulting_tag) if armor

CIPHERTEXT = AES_GCM(KEY, cleartext) + resulting_tag if not armor
# "resulting_tag" came from AES-GCM encryption. It will be the last 16 bytes of
# our ciphertext.

encrypted_payload = PREAMBLE + SEPARATOR + CIPHERTEXT

Decryption
----------

Ciphertext and Tag CAN come encoded in base64 (with armor=True) or raw (with
armor=False). Preamble will always come encoded in base64.

PREAMBLE, CIPHERTEXT = PAYLOAD.SPLIT(' ', 1)

PREAMBLE = base64_decode(PREAMBLE)
CIPHERTEXT = base64_decode(CIPHERTEXT) if armor else CIPHERTEXT

CIPHERTEXT, TAG = CIPHERTEXT[:-16], CIPHERTEXT[-16:]
CLEARTEXT = aes_gcm_decrypt(KEY, IV, CIPHERTEXT, TAG, associated_data=PREAMBLE)

AES-GCM will check preamble authenticity as well, since we are using
Authenticated Encryption with Associated Data (AEAD). Ciphertext and associated
data (PREAMBLE) authenticity will both be checked together during decryption.
PREAMBLE consistency (if it matches the desired document, for instance) is
checked during PREAMBLE reading.
"""


import base64
import hashlib
import hmac
import os

from io import BytesIO
from collections import namedtuple

from twisted.internet import defer
from twisted.internet import interfaces
from twisted.web.client import FileBodyProducer

from leap.soledad.common import soledad_assert
from cryptography.exceptions import InvalidTag
from cryptography.hazmat.primitives.ciphers import Cipher, algorithms, modes
from cryptography.hazmat.backends import default_backend

from zope.interface import implementer

from leap.soledad.common.blobs.preamble import InvalidPreambleException
from leap.soledad.common.blobs.preamble import decode_preamble
from leap.soledad.common.blobs.preamble import Preamble
from leap.soledad.common.blobs.preamble import ENC_SCHEME, ENC_METHOD
from leap.soledad.common.blobs.preamble import MAGIC, PREAMBLE_SIZE


SECRET_LENGTH = 64
SEPARATOR = ' '  # Anything that doesn't belong to base64 encoding

CRYPTO_BACKEND = default_backend()
DocInfo = namedtuple('DocInfo', 'doc_id rev')


class EncryptionDecryptionError(Exception):
    pass


class InvalidBlob(Exception):
    pass


class EncryptionSchemeNotImplementedException(Exception):
    pass


class SoledadCrypto(object):
    """
    This class provides convenient methods for document encryption and
    decryption using BlobEncryptor and BlobDecryptor classes.
    """
    def __init__(self, secret):
        """
        Initialize the crypto object.

        :param secret: The Soledad remote storage secret.
        :type secret: str
        """
        self.secret = secret

    def encrypt_doc(self, doc):
        """
        Creates and configures a BlobEncryptor, asking it to start encryption
        and wrapping the result as a simple JSON string with a "raw" key.

        :param doc: the document to be encrypted.
        :type doc: Document
        :return: A deferred whose callback will be invoked with a JSON string
            containing the ciphertext as the value of "raw" key.
        :rtype: twisted.internet.defer.Deferred
        """

        def put_raw(blob):
            raw = blob.getvalue()
            return '{"raw": "' + raw + '"}'

        content = BytesIO(str(doc.get_json()))
        info = DocInfo(doc.doc_id, doc.rev)
        del doc
        encryptor = BlobEncryptor(info, content, secret=self.secret)
        d = encryptor.encrypt()
        d.addCallback(put_raw)
        return d

    def decrypt_doc(self, doc):
        """
        Creates and configures a BlobDecryptor, asking it decrypt and returning
        the decrypted cleartext content from the encrypted document.

        :param doc: the document to be decrypted.
        :type doc: Document
        :return: The decrypted cleartext content of the document.
        :rtype: str
        """
        info = DocInfo(doc.doc_id, doc.rev)
        ciphertext = BytesIO()
        payload = doc.content['raw']
        del doc
        ciphertext.write(str(payload))
        decryptor = BlobDecryptor(info, ciphertext, secret=self.secret)
        return decryptor.decrypt()


def encrypt_sym(data, key, method=ENC_METHOD.aes_256_gcm):
    """
    Encrypt data using AES-256 cipher in selected mode.

    :param data: The data to be encrypted.
    :type data: str
    :param key: The key used to encrypt data (must be 256 bits long).
    :type key: str

    :return: A tuple with the initialization vector and the ciphertext, both
        encoded as base64.
    :rtype: (str, str)
    """
    mode = _mode_by_method(method)
    encryptor = AESWriter(key, mode=mode)
    encryptor.write(data)
    _, ciphertext = encryptor.end()
    iv = base64.b64encode(encryptor.iv)
    tag = encryptor.tag or ''
    return iv, ciphertext + tag


def decrypt_sym(data, key, iv, method=ENC_METHOD.aes_256_gcm):
    """
    Decrypt data using AES-256 cipher in selected mode.

    :param data: The data to be decrypted.
    :type data: str
    :param key: The symmetric key used to decrypt data (must be 256 bits
                long).
    :type key: str
    :param iv: The base64 encoded initialization vector.
    :type iv: str

    :return: The decrypted data.
    :rtype: str
    """
    _iv = base64.b64decode(str(iv))
    mode = _mode_by_method(method)
    tag = None
    if mode == modes.GCM:
        data, tag = data[:-16], data[-16:]
    decryptor = AESWriter(key, _iv, tag=tag, mode=mode)
    decryptor.write(data)
    _, plaintext = decryptor.end()
    return plaintext


# TODO maybe rename this to Encryptor, since it will be used by blobs an non
# blobs in soledad.
class BlobEncryptor(object):
    """
    Produces encrypted data from the cleartext data associated with a given
    Document using AES-256 cipher in GCM mode.

    The production happens using a Twisted's FileBodyProducer, which uses a
    Cooperator to schedule calls and can be paused/resumed. Each call takes at
    most 65536 bytes from the input.

    Both the production input and output are file descriptors, so they can be
    applied to a stream of data.
    """
    # TODO
    # This class needs further work to allow for proper streaming.
    # Right now we HAVE TO WAIT until the end of the stream before encoding the
    # result. It should be possible to do that just encoding the chunks and
    # passing them to a sink, but for that we have to encode the chunks at
    # proper alignment (3 bytes?) with b64 if armor is defined.

    def __init__(self, doc_info, content_fd, secret=None, armor=True,
                 sink=None):
        if not secret:
            raise EncryptionDecryptionError('no secret given')

        self.doc_id = doc_info.doc_id
        self.rev = doc_info.rev
        self.armor = armor

        self._content_fd = content_fd
        self._content_size = self._get_rounded_size(content_fd)
        self._producer = FileBodyProducer(content_fd, readSize=2**16)

        self.sym_key = _get_sym_key_for_doc(doc_info.doc_id, secret)
        self._aes = AESWriter(self.sym_key, _buffer=sink)
        self._aes.authenticate(self._encode_preamble())

    def _get_rounded_size(self, fd):
        """
        Returns a rounded value in order to minimize information leaks due to
        the original size being exposed.
        """
        fd.seek(0, os.SEEK_END)
        size = _ceiling(fd.tell())
        fd.seek(0)
        return size

    @property
    def iv(self):
        return self._aes.iv

    @property
    def tag(self):
        return self._aes.tag

    def encrypt(self):
        """
        Starts producing encrypted data from the cleartext data.

        :return: A deferred which will be fired when encryption ends and whose
                 callback will be invoked with the resulting ciphertext.
        :rtype: twisted.internet.defer.Deferred
        """
        # XXX pass a sink to aes?
        d = self._producer.startProducing(self._aes)
        d.addCallback(lambda _: self._end_crypto_stream_and_encode_result())
        return d

    def _encode_preamble(self):
        scheme = ENC_SCHEME.symkey
        method = ENC_METHOD.aes_256_gcm
        content_size = self._content_size

        return Preamble(self.doc_id, self.rev, scheme, method, iv=self.iv,
                        content_size=content_size).encode()

    def _end_crypto_stream_and_encode_result(self):

        # TODO ---- this needs to be refactored to allow PROPER streaming
        # We should write the preamble as soon as possible,
        # Is it possible to write the AES stream as soon as it is encrypted by
        # chunks?
        # FIXME also, it needs to be able to encode chunks with base64 if armor

        raw_preamble, encrypted = self._aes.end()
        result = BytesIO()
        result.write(
            base64.urlsafe_b64encode(raw_preamble))
        result.write(SEPARATOR)

        if self.armor:
            result.write(
                base64.urlsafe_b64encode(encrypted + self.tag))
        else:
            result.write(encrypted + self.tag)

        result.seek(0)
        return defer.succeed(result)


# TODO maybe rename this to just Decryptor, since it will be used by blobs
# and non blobs in soledad.
class BlobDecryptor(object):
    """
    Decrypts an encrypted blob associated with a given Document.

    Will raise an exception if the blob doesn't have the expected structure, or
    if the GCM tag doesn't verify.
    """
    def __init__(self, doc_info, ciphertext_fd, result=None,
                 secret=None, armor=True, start_stream=True, tag=None):
        if not secret:
            raise EncryptionDecryptionError('no secret given')

        self.doc_id = doc_info.doc_id
        self.rev = doc_info.rev
        self.fd = ciphertext_fd
        self.armor = armor
        self._producer = None
        self.result = result or BytesIO()
        sym_key = _get_sym_key_for_doc(doc_info.doc_id, secret)
        self.size = None
        self.tag = None

        preamble, iv = self._consume_preamble()
        soledad_assert(preamble)
        soledad_assert(iv)

        self._aes = AESWriter(sym_key, iv, self.result, tag=tag or self.tag)
        self._aes.authenticate(preamble)
        if start_stream:
            self._start_stream()

    @property
    def decrypted_content_size(self):
        return self._aes.written

    def _start_stream(self):
        self._producer = FileBodyProducer(self.fd, readSize=2**16)

    def _consume_preamble(self):
        """
        Consume the preamble and write remaining bytes as ciphertext. This
        function is called during a stream and can be holding both, so we need
        to consume only preamble and store the remaining.
        """
        self.fd.seek(0)
        try:
            parts = self.fd.getvalue().split(SEPARATOR, 1)
            encoded_preamble = base64.urlsafe_b64decode(parts[0])
            if len(parts) == 2:
                ciphertext = parts[1]
                if self.armor:
                    ciphertext = base64.urlsafe_b64decode(ciphertext)
                self.tag, ciphertext = ciphertext[-16:], ciphertext[:-16]
                self.fd.seek(0)
                self.fd.write(ciphertext)
                self.fd.seek(len(ciphertext))
                self.fd.truncate()
                self.fd.seek(0)

        except (TypeError, ValueError):
            raise InvalidBlob

        try:
            preamble = decode_preamble(encoded_preamble)
        except InvalidPreambleException as e:
            raise InvalidBlob(e)

        if preamble.magic != MAGIC:
            raise InvalidBlob
        # TODO check timestamp. Just as a sanity check, but for instance
        # we can refuse to process something that is in the future or
        # too far in the past (1984 would be nice, hehe)
        if preamble.scheme != ENC_SCHEME.symkey:
            raise EncryptionSchemeNotImplementedException(preamble.scheme)
        if preamble.method != ENC_METHOD.aes_256_gcm:
            method = preamble.method
            raise InvalidBlob('Invalid encryption scheme: %s' % method)
        if preamble.rev != self.rev:
            rev = preamble.rev
            msg = 'Invalid revision. Expected: %s, was: %s' % (self.rev, rev)
            raise InvalidBlob(msg)
        if preamble.doc_id != self.doc_id:
            msg = 'Invalid doc_id. '
            + 'Expected: %s, was: %s' % (self.doc_id, preamble.doc_id)
            raise InvalidBlob(msg)

        return encoded_preamble, preamble.iv

    def _end_stream(self):
        try:
            self._aes.end()
        except InvalidTag:
            raise InvalidBlob('Invalid Tag. Blob authentication failed.')
        fd = self.result
        fd.seek(0)
        return self.result

    def decrypt(self):
        """
        Starts producing encrypted data from the cleartext data.

        :return: A deferred which will be fired when encryption ends and whose
            callback will be invoked with the resulting ciphertext.
        :rtype: twisted.internet.defer.Deferred
        """
        d = self.startProducing()
        d.addCallback(lambda _: self._end_stream())
        return d

    def startProducing(self):
        if not self._producer:
            self._start_stream()
        return self._producer.startProducing(self._aes)

    def endStream(self):
        return self._end_stream()

    def write(self, data):
        self._aes.write(data)

    def close(self):
        result = self._aes.end()
        return result


@implementer(interfaces.IConsumer)
class AESWriter(object):
    """
    A Twisted's Consumer implementation that takes an input file descriptor and
    applies AES-256 cipher in GCM mode.

    It is used both for encryption and decryption of a stream, depending of the
    value of the tag parameter. If you pass a tag, it will operate in
    decryption mode, verifying the authenticity of the preamble and ciphertext.
    If no tag is passed, encryption mode is assumed, which will generate a tag.
    """

    def __init__(self, key, iv=None, _buffer=None, tag=None, mode=modes.GCM):
        if len(key) != 32:
            raise EncryptionDecryptionError('key is not 256 bits')

        if tag is not None:
            # if tag, we're decrypting
            assert iv is not None

        self.iv = iv or os.urandom(16)
        self.buffer = _buffer or BytesIO()
        cipher = _get_aes_cipher(key, self.iv, tag, mode)
        cipher = cipher.decryptor() if tag else cipher.encryptor()
        self.cipher, self.aead = cipher, ''
        self.written = 0

    def authenticate(self, data):
        self.aead += data
        self.cipher.authenticate_additional_data(data)

    @property
    def tag(self):
        return getattr(self.cipher, 'tag', None)

    def write(self, data):
        self.written += len(data)
        self.buffer.write(self.cipher.update(data))

    def end(self):
        self.buffer.write(self.cipher.finalize())
        return self.aead, self.buffer.getvalue()


def is_symmetrically_encrypted(content):
    """
    Returns True if the document was symmetrically encrypted.
    'EzcB' is the base64 encoding of \x13\x37 magic number and 1 (symmetrically
    encrypted value for enc_scheme flag).

    :param doc: The document content as string
    :type doc: str

    :rtype: bool
    """
    sym_signature = '{"raw": "EzcB'
    return content and content.startswith(sym_signature)


# utils


def _hmac_sha256(key, data):
    return hmac.new(key, data, hashlib.sha256).digest()


def _get_sym_key_for_doc(doc_id, secret):
    key = secret[SECRET_LENGTH:]
    return _hmac_sha256(key, doc_id)


def _get_aes_cipher(key, iv, tag, mode=modes.GCM):
    mode = mode(iv, tag) if mode == modes.GCM else mode(iv)
    return Cipher(algorithms.AES(key), mode, backend=CRYPTO_BACKEND)


def _mode_by_method(method):
    if method == ENC_METHOD.aes_256_gcm:
        return modes.GCM
    else:
        return modes.CTR


def _ceiling(size):
    """
    Some simplistic ceiling scheme that uses powers of 2.
    We report everything below 4096 bytes as that minimum threshold.
    See #8759 for research pending for less simplistic/aggresive strategies.
    """
    for i in xrange(12, 31):
        step = 2 ** i
        if size < step:
            return step


def get_unarmored_ciphertext_size(cleartext_size):
    """
    Get the (unarmored) ciphertext size from the cleartext size.

    In some cases (when streaming uploads, for example) we need to tell the
    server how much data we are sending before starting to encrypt the stream.
    This function returns the final size of what will be streamed from the
    total size of the cleartext.
    """
    tag_size = 16  # AES-GCM 16-byte tag appended to ciphertext
    separator_size = 1  # space separating content from preamble
    size = tag_size + PREAMBLE_SIZE + separator_size
    # without considering the preamble and tag, the ciphertext size is equal to
    # the cleartext size.
    return cleartext_size + size