Refactor fetch into leap.mail.incoming IService

1 files changed, 760 insertions, 0 deletions

diff --git a/src/leap/mail/incoming/service.py b/src/leap/mail/incoming/service.py
new file mode 100644
index 0000000..e52c727
--- /dev/null
+++ b/src/leap/mail/incoming/service.py
@@ -0,0 +1,760 @@
+# -*- coding: utf-8 -*-
+# service.py
+# Copyright (C) 2015 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/>.
+"""
+Incoming mail fetcher.
+"""
+import copy
+import logging
+import shlex
+import threading
+import time
+import traceback
+import warnings
+
+from email.parser import Parser
+from email.generator import Generator
+from email.utils import parseaddr
+from StringIO import StringIO
+from urlparse import urlparse
+
+from twisted.application.service import Service
+from twisted.python import log
+from twisted.internet import defer, reactor
+from twisted.internet.task import LoopingCall
+from twisted.internet.task import deferLater
+from u1db import errors as u1db_errors
+
+from leap.common import events as leap_events
+from leap.common.check import leap_assert, leap_assert_type
+from leap.common.events.events_pb2 import IMAP_FETCHED_INCOMING
+from leap.common.events.events_pb2 import IMAP_MSG_PROCESSING
+from leap.common.events.events_pb2 import IMAP_MSG_DECRYPTED
+from leap.common.events.events_pb2 import IMAP_MSG_SAVED_LOCALLY
+from leap.common.events.events_pb2 import IMAP_MSG_DELETED_INCOMING
+from leap.common.events.events_pb2 import IMAP_UNREAD_MAIL
+from leap.common.events.events_pb2 import SOLEDAD_INVALID_AUTH_TOKEN
+from leap.common.mail import get_email_charset
+from leap.keymanager import errors as keymanager_errors
+from leap.keymanager.openpgp import OpenPGPKey
+from leap.mail.adaptors import soledad_indexes as fields
+from leap.mail.decorators import deferred_to_thread
+from leap.mail.utils import json_loads, empty, first
+from leap.soledad.client import Soledad
+from leap.soledad.common.crypto import ENC_SCHEME_KEY, ENC_JSON_KEY
+from leap.soledad.common.errors import InvalidAuthTokenError
+
+
+logger = logging.getLogger(__name__)
+
+MULTIPART_ENCRYPTED = "multipart/encrypted"
+MULTIPART_SIGNED = "multipart/signed"
+PGP_BEGIN = "-----BEGIN PGP MESSAGE-----"
+PGP_END = "-----END PGP MESSAGE-----"
+
+# The period between succesive checks of the incoming mail
+# queue (in seconds)
+INCOMING_CHECK_PERIOD = 60
+
+
+class MalformedMessage(Exception):
+    """
+    Raised when a given message is not well formed.
+    """
+    pass
+
+
+class IncomingMail(Service):
+    """
+    Fetches and process mail from the incoming pool.
+
+    This object implements IService interface, has public methods
+    startService and stopService that will actually initiate a
+    LoopingCall with check_period recurrency.
+    The LoopingCall itself will invoke the fetch method each time
+    that the check_period expires.
+
+    This loop will sync the soledad db with the remote server and
+    process all the documents found tagged as incoming mail.
+    """
+
+    name = "IncomingMail"
+
+    RECENT_FLAG = "\\Recent"
+    CONTENT_KEY = "content"
+
+    LEAP_SIGNATURE_HEADER = 'X-Leap-Signature'
+    """
+    Header added to messages when they are decrypted by the IMAP fetcher,
+    which states the validity of an eventual signature that might be included
+    in the encrypted blob.
+    """
+    LEAP_SIGNATURE_VALID = 'valid'
+    LEAP_SIGNATURE_INVALID = 'invalid'
+    LEAP_SIGNATURE_COULD_NOT_VERIFY = 'could not verify'
+
+    fetching_lock = threading.Lock()
+
+    def __init__(self, keymanager, soledad, inbox, userid,
+                 check_period=INCOMING_CHECK_PERIOD):
+
+        """
+        Initialize IncomingMail..
+
+        :param keymanager: a keymanager instance
+        :type keymanager: keymanager.KeyManager
+
+        :param soledad: a soledad instance
+        :type soledad: Soledad
+
+        :param inbox: the inbox where the new emails will be stored
+        :type inbox: IMAPMailbox
+
+        :param check_period: the period to fetch new mail, in seconds.
+        :type check_period: int
+        """
+
+        leap_assert(keymanager, "need a keymanager to initialize")
+        leap_assert_type(soledad, Soledad)
+        leap_assert(check_period, "need a period to check incoming mail")
+        leap_assert_type(check_period, int)
+        leap_assert(userid, "need a userid to initialize")
+
+        self._keymanager = keymanager
+        self._soledad = soledad
+        self._inbox = inbox
+        self._userid = userid
+
+        self._loop = None
+        self._check_period = check_period
+
+        # initialize a mail parser only once
+        self._parser = Parser()
+
+    #
+    # Public API: fetch, start_loop, stop.
+    #
+
+    def fetch(self):
+        """
+        Fetch incoming mail, to be called periodically.
+
+        Calls a deferred that will execute the fetch callback
+        in a separate thread
+        """
+        def mail_compat(failure):
+            if failure.check(u1db_errors.InvalidGlobbing):
+                # It looks like we are a dealing with an outdated
+                # mx. Fallback to the version of the index
+                warnings.warn("JUST_MAIL_COMPAT_IDX will be deprecated!",
+                              DeprecationWarning)
+                return self._soledad.get_from_index(
+                    fields.JUST_MAIL_COMPAT_IDX, "*")
+            return failure
+
+        def syncSoledadCallback(_):
+            d = self._soledad.get_from_index(
+                fields.JUST_MAIL_IDX, "*", "0")
+            d.addErrback(mail_compat)
+            d.addCallback(self._process_doclist)
+            return d
+
+        logger.debug("fetching mail for: %s %s" % (
+            self._soledad.uuid, self._userid))
+        if not self.fetching_lock.locked():
+            d1 = self._sync_soledad()
+            d = defer.gatherResults([d1], consumeErrors=True)
+            d.addCallbacks(syncSoledadCallback, self._errback)
+            d.addCallbacks(self._signal_fetch_to_ui, self._errback)
+            return d
+        else:
+            logger.debug("Already fetching mail.")
+
+    def startService(self):
+        """
+        Starts a loop to fetch mail.
+        """
+        Service.startService(self)
+        if self._loop is None:
+            self._loop = LoopingCall(self.fetch)
+            self._loop.start(self._check_period)
+        else:
+            logger.warning("Tried to start an already running fetching loop.")
+
+    def stopService(self):
+        """
+        Stops the loop that fetches mail.
+        """
+        if self._loop and self._loop.running is True:
+            self._loop.stop()
+            self._loop = None
+        Service.stopService(self)
+
+    #
+    # Private methods.
+    #
+
+    # synchronize incoming mail
+
+    def _errback(self, failure):
+        logger.exception(failure.value)
+        traceback.print_exc()
+
+    @deferred_to_thread
+    def _sync_soledad(self):
+        """
+        Synchronize with remote soledad.
+
+        :returns: a list of LeapDocuments, or None.
+        :rtype: iterable or None
+        """
+        with self.fetching_lock:
+            try:
+                log.msg('FETCH: syncing soledad...')
+                self._soledad.sync()
+                log.msg('FETCH soledad SYNCED.')
+            except InvalidAuthTokenError:
+                # if the token is invalid, send an event so the GUI can
+                # disable mail and show an error message.
+                leap_events.signal(SOLEDAD_INVALID_AUTH_TOKEN)
+
+    def _signal_fetch_to_ui(self, doclist):
+        """
+        Send leap events to ui.
+
+        :param doclist: iterable with msg documents.
+        :type doclist: iterable.
+        :returns: doclist
+        :rtype: iterable
+        """
+        doclist = first(doclist)  # gatherResults pass us a list
+        if doclist:
+            fetched_ts = time.mktime(time.gmtime())
+            num_mails = len(doclist) if doclist is not None else 0
+            if num_mails != 0:
+                log.msg("there are %s mails" % (num_mails,))
+            leap_events.signal(
+                IMAP_FETCHED_INCOMING, str(num_mails), str(fetched_ts))
+            return doclist
+
+    def _signal_unread_to_ui(self, *args):
+        """
+        Sends unread event to ui.
+        """
+        leap_events.signal(
+            IMAP_UNREAD_MAIL, str(self._inbox.getUnseenCount()))
+
+    # process incoming mail.
+
+    def _process_doclist(self, doclist):
+        """
+        Iterates through the doclist, checks if each doc
+        looks like a message, and yields a deferred that will decrypt and
+        process the message.
+
+        :param doclist: iterable with msg documents.
+        :type doclist: iterable.
+        :returns: a list of deferreds for individual messages.
+        """
+        log.msg('processing doclist')
+        if not doclist:
+            logger.debug("no docs found")
+            return
+        num_mails = len(doclist)
+
+        deferreds = []
+        for index, doc in enumerate(doclist):
+            logger.debug("processing doc %d of %d" % (index + 1, num_mails))
+            leap_events.signal(
+                IMAP_MSG_PROCESSING, str(index), str(num_mails))
+
+            keys = doc.content.keys()
+
+            # TODO Compatibility check with the index in pre-0.6 mx
+            # that does not write the ERROR_DECRYPTING_KEY
+            # This should be removed in 0.7
+
+            has_errors = doc.content.get(fields.ERROR_DECRYPTING_KEY, None)
+            if has_errors is None:
+                warnings.warn("JUST_MAIL_COMPAT_IDX will be deprecated!",
+                              DeprecationWarning)
+
+            if has_errors:
+                logger.debug("skipping msg with decrypting errors...")
+            elif self._is_msg(keys):
+                d = self._decrypt_doc(doc)
+                d.addCallback(self._extract_keys)
+                d.addCallbacks(self._add_message_locally, self._errback)
+                deferreds.append(d)
+        return defer.gatherResults(deferreds, consumeErrors=True)
+
+    #
+    # operations on individual messages
+    #
+
+    #FIXME: @deferred_to_thread
+    def _decrypt_doc(self, doc):
+        """
+        Decrypt the contents of a document.
+
+        :param doc: A document containing an encrypted message.
+        :type doc: SoledadDocument
+
+        :return: A Deferred that will be fired with the document and the
+                 decrypted message.
+        :rtype: SoledadDocument, str
+        """
+        log.msg('decrypting msg')
+
+        def process_decrypted(res):
+            if isinstance(res, tuple):
+                decrdata, _ = res
+                success = True
+            else:
+                decrdata = ""
+                success = False
+
+            leap_events.signal(IMAP_MSG_DECRYPTED, "1" if success else "0")
+            return self._process_decrypted_doc(doc, decrdata)
+
+        d = self._keymanager.decrypt(
+            doc.content[ENC_JSON_KEY],
+            self._userid, OpenPGPKey)
+        d.addErrback(self._errback)
+        d.addCallback(process_decrypted)
+        d.addCallback(lambda data: (doc, data))
+        return d
+
+    def _process_decrypted_doc(self, doc, data):
+        """
+        Process a document containing a succesfully decrypted message.
+
+        :param doc: the incoming message
+        :type doc: SoledadDocument
+        :param data: the json-encoded, decrypted content of the incoming
+                     message
+        :type data: str
+
+        :return: a Deferred that will be fired with an str of the proccessed
+                 data.
+        :rtype: Deferred
+        """
+        log.msg('processing decrypted doc')
+
+        # XXX turn this into an errBack for each one of
+        # the deferreds that would process an individual document
+        try:
+            msg = json_loads(data)
+        except UnicodeError as exc:
+            logger.error("Error while decrypting %s" % (doc.doc_id,))
+            logger.exception(exc)
+
+            # we flag the message as "with decrypting errors",
+            # to avoid further decryption attempts during sync
+            # cycles until we're prepared to deal with that.
+            # What is the same, when Ivan deals with it...
+            # A new decrypting attempt event could be triggered by a
+            # future a library upgrade, or a cli flag to the client,
+            # we just `defer` that for now... :)
+            doc.content[fields.ERROR_DECRYPTING_KEY] = True
+            deferLater(reactor, 0, self._update_incoming_message, doc)
+
+            # FIXME this is just a dirty hack to delay the proper
+            # deferred organization here...
+            # and remember, boys, do not do this at home.
+            return []
+
+        if not isinstance(msg, dict):
+            defer.returnValue(False)
+        if not msg.get(fields.INCOMING_KEY, False):
+            defer.returnValue(False)
+
+        # ok, this is an incoming message
+        rawmsg = msg.get(self.CONTENT_KEY, None)
+        if not rawmsg:
+            return ""
+        return self._maybe_decrypt_msg(rawmsg)
+
+    @deferred_to_thread
+    def _update_incoming_message(self, doc):
+        """
+        Do a put for a soledad document. This probably has been called only
+        in the case that we've needed to update the ERROR_DECRYPTING_KEY
+        flag in an incoming message, to get it out of the decrypting queue.
+
+        :param doc: the SoledadDocument to update
+        :type doc: SoledadDocument
+        """
+        log.msg("Updating SoledadDoc %s" % (doc.doc_id))
+        self._soledad.put_doc(doc)
+
+    @deferred_to_thread
+    def _delete_incoming_message(self, doc):
+        """
+        Delete document.
+
+        :param doc: the SoledadDocument to delete
+        :type doc: SoledadDocument
+        """
+        log.msg("Deleting Incoming message: %s" % (doc.doc_id,))
+        self._soledad.delete_doc(doc)
+
+    def _maybe_decrypt_msg(self, data):
+        """
+        Tries to decrypt a gpg message if data looks like one.
+
+        :param data: the text to be decrypted.
+        :type data: str
+
+        :return: a Deferred that will be fired with an str of data, possibly
+                 decrypted.
+        :rtype: Deferred
+        """
+        leap_assert_type(data, str)
+        log.msg('maybe decrypting doc')
+
+        # parse the original message
+        encoding = get_email_charset(data)
+        msg = self._parser.parsestr(data)
+
+        fromHeader = msg.get('from', None)
+        senderAddress = None
+        if (fromHeader is not None
+            and (msg.get_content_type() == MULTIPART_ENCRYPTED
+                 or msg.get_content_type() == MULTIPART_SIGNED)):
+                senderAddress = parseaddr(fromHeader)
+
+        def add_leap_header(ret):
+            decrmsg, signkey = ret
+            if (senderAddress is None or
+                    isinstance(signkey, keymanager_errors.KeyNotFound)):
+                decrmsg.add_header(
+                    self.LEAP_SIGNATURE_HEADER,
+                    self.LEAP_SIGNATURE_COULD_NOT_VERIFY)
+            elif isinstance(signkey, keymanager_errors.InvalidSignature):
+                decrmsg.add_header(
+                    self.LEAP_SIGNATURE_HEADER,
+                    self.LEAP_SIGNATURE_INVALID)
+            else:
+                decrmsg.add_header(
+                    self.LEAP_SIGNATURE_HEADER,
+                    self.LEAP_SIGNATURE_VALID,
+                    pubkey=signkey.key_id)
+            return decrmsg.as_string()
+
+        if msg.get_content_type() == MULTIPART_ENCRYPTED:
+            d = self._decrypt_multipart_encrypted_msg(
+                msg, encoding, senderAddress)
+        else:
+            d = self._maybe_decrypt_inline_encrypted_msg(
+                msg, encoding, senderAddress)
+        d.addCallback(add_leap_header)
+        return d
+
+    def _decrypt_multipart_encrypted_msg(self, msg, encoding, senderAddress):
+        """
+        Decrypt a message with content-type 'multipart/encrypted'.
+
+        :param msg: The original encrypted message.
+        :type msg: Message
+        :param encoding: The encoding of the email message.
+        :type encoding: str
+        :param senderAddress: The email address of the sender of the message.
+        :type senderAddress: str
+
+        :return: A Deferred that will be fired with a tuple containing a
+                 decrypted Message and the signing OpenPGPKey if the signature
+                 is valid or InvalidSignature or KeyNotFound.
+        :rtype: Deferred
+        """
+        log.msg('decrypting multipart encrypted msg')
+        msg = copy.deepcopy(msg)
+        self._msg_multipart_sanity_check(msg)
+
+        # parse message and get encrypted content
+        pgpencmsg = msg.get_payload()[1]
+        encdata = pgpencmsg.get_payload()
+
+        # decrypt or fail gracefully
+        def build_msg(res):
+            decrdata, signkey = res
+
+            decrmsg = self._parser.parsestr(decrdata)
+            # remove original message's multipart/encrypted content-type
+            del(msg['content-type'])
+
+            # replace headers back in original message
+            for hkey, hval in decrmsg.items():
+                try:
+                    # this will raise KeyError if header is not present
+                    msg.replace_header(hkey, hval)
+                except KeyError:
+                    msg[hkey] = hval
+
+            # all ok, replace payload by unencrypted payload
+            msg.set_payload(decrmsg.get_payload())
+            return (msg, signkey)
+
+        d = self._keymanager.decrypt(
+            encdata, self._userid, OpenPGPKey,
+            verify=senderAddress)
+        d.addCallbacks(build_msg, self._decryption_error, errbackArgs=(msg,))
+        return d
+
+    def _maybe_decrypt_inline_encrypted_msg(self, origmsg, encoding,
+                                            senderAddress):
+        """
+        Possibly decrypt an inline OpenPGP encrypted message.
+
+        :param origmsg: The original, possibly encrypted message.
+        :type origmsg: Message
+        :param encoding: The encoding of the email message.
+        :type encoding: str
+        :param senderAddress: The email address of the sender of the message.
+        :type senderAddress: str
+
+        :return: A Deferred that will be fired with a tuple containing a
+                 decrypted Message and the signing OpenPGPKey if the signature
+                 is valid or InvalidSignature or KeyNotFound.
+        :rtype: Deferred
+        """
+        log.msg('maybe decrypting inline encrypted msg')
+        # serialize the original message
+        buf = StringIO()
+        g = Generator(buf)
+        g.flatten(origmsg)
+        data = buf.getvalue()
+
+        def decrypted_data(res):
+            decrdata, signkey = res
+            return data.replace(pgp_message, decrdata), signkey
+
+        def encode_and_return(res):
+            data, signkey = res
+            if isinstance(data, unicode):
+                data = data.encode(encoding, 'replace')
+            return (self._parser.parsestr(data), signkey)
+
+        # handle exactly one inline PGP message
+        if PGP_BEGIN in data:
+            begin = data.find(PGP_BEGIN)
+            end = data.find(PGP_END)
+            pgp_message = data[begin:end + len(PGP_END)]
+            d = self._keymanager.decrypt(
+                pgp_message, self._userid, OpenPGPKey,
+                verify=senderAddress)
+            d.addCallbacks(decrypted_data, self._decryption_error,
+                           errbackArgs=(data,))
+        else:
+            d = defer.succeed((data, None))
+        d.addCallback(encode_and_return)
+        return d
+
+    def _decryption_error(self, failure, msg):
+        """
+        Check for known decryption errors
+        """
+        if failure.check(keymanager_errors.DecryptError):
+            logger.warning('Failed to decrypt encrypted message (%s). '
+                           'Storing message without modifications.'
+                           % str(failure.value))
+            return (msg, None)
+        elif failure.check(keymanager_errors.KeyNotFound):
+            logger.error('Failed to find private key for decryption (%s). '
+                         'Storing message without modifications.'
+                         % str(failure.value))
+            return (msg, None)
+        else:
+            return failure
+
+    def _extract_keys(self, msgtuple):
+        """
+        Retrieve attached keys to the mesage and parse message headers for an
+        *OpenPGP* header as described on the `IETF draft
+        <http://tools.ietf.org/html/draft-josefsson-openpgp-mailnews-header-06>`
+        only urls with https and the same hostname than the email are supported
+        for security reasons.
+
+        :param msgtuple: a tuple consisting of a SoledadDocument
+                         instance containing the incoming message
+                         and data, the json-encoded, decrypted content of the
+                         incoming message
+        :type msgtuple: (SoledadDocument, str)
+
+        :return: A Deferred that will be fired with msgtuple when key
+                 extraction finishes
+        :rtype: Deferred
+        """
+        OpenPGP_HEADER = 'OpenPGP'
+        doc, data = msgtuple
+
+        # XXX the parsing of the message is done in mailbox.addMessage, maybe
+        #     we should do it in this module so we don't need to parse it again
+        #     here
+        msg = self._parser.parsestr(data)
+        _, fromAddress = parseaddr(msg['from'])
+
+        header = msg.get(OpenPGP_HEADER, None)
+        dh = defer.succeed(None)
+        if header is not None:
+            dh = self._extract_openpgp_header(header, fromAddress)
+
+        da = defer.succeed(None)
+        if msg.is_multipart():
+            da = self._extract_attached_key(msg.get_payload(), fromAddress)
+
+        d = defer.gatherResults([dh, da])
+        d.addCallback(lambda _: msgtuple)
+        return d
+
+    def _extract_openpgp_header(self, header, address):
+        """
+        Import keys from the OpenPGP header
+
+        :param header: OpenPGP header string
+        :type header: str
+        :param address: email address in the from header
+        :type address: str
+
+        :return: A Deferred that will be fired when header extraction is done
+        :rtype: Deferred
+        """
+        d = defer.succeed(None)
+        fields = dict([f.strip(' ').split('=') for f in header.split(';')])
+        if 'url' in fields:
+            url = shlex.split(fields['url'])[0]  # remove quotations
+            urlparts = urlparse(url)
+            addressHostname = address.split('@')[1]
+            if (urlparts.scheme == 'https'
+                    and urlparts.hostname == addressHostname):
+                def fetch_error(failure):
+                    if failure.check(keymanager_errors.KeyNotFound):
+                        logger.warning("Url from OpenPGP header %s failed"
+                                       % (url,))
+                    elif failure.check(keymanager_errors.KeyAttributesDiffer):
+                        logger.warning("Key from OpenPGP header url %s didn't "
+                                       "match the from address %s"
+                                       % (url, address))
+                    else:
+                        return failure
+
+                d = self._keymanager.fetch_key(address, url, OpenPGPKey)
+                d.addCallback(
+                    lambda _:
+                    logger.info("Imported key from header %s" % (url,)))
+                d.addErrback(fetch_error)
+            else:
+                logger.debug("No valid url on OpenPGP header %s" % (url,))
+        else:
+            logger.debug("There is no url on the OpenPGP header: %s"
+                         % (header,))
+        return d
+
+    def _extract_attached_key(self, attachments, address):
+        """
+        Import keys from the attachments
+
+        :param attachments: email attachment list
+        :type attachments: list(email.Message)
+        :param address: email address in the from header
+        :type address: str
+
+        :return: A Deferred that will be fired when all the keys are stored
+        :rtype: Deferred
+        """
+        MIME_KEY = "application/pgp-keys"
+
+        deferreds = []
+        for attachment in attachments:
+            if MIME_KEY == attachment.get_content_type():
+                logger.debug("Add key from attachment")
+                d = self._keymanager.put_raw_key(
+                    attachment.get_payload(),
+                    OpenPGPKey,
+                    address=address)
+                deferreds.append(d)
+        return defer.gatherResults(deferreds)
+
+    def _add_message_locally(self, msgtuple):
+        """
+        Adds a message to local inbox and delete it from the incoming db
+        in soledad.
+
+        :param msgtuple: a tuple consisting of a SoledadDocument
+                         instance containing the incoming message
+                         and data, the json-encoded, decrypted content of the
+                         incoming message
+        :type msgtuple: (SoledadDocument, str)
+
+        :return: A Deferred that will be fired when the messages is stored
+        :rtype: Defferred
+        """
+        doc, data = msgtuple
+        log.msg('adding message %s to local db' % (doc.doc_id,))
+
+        if isinstance(data, list):
+            if empty(data):
+                return False
+            data = data[0]
+
+        def msgSavedCallback(result):
+            if not empty(result):
+                leap_events.signal(IMAP_MSG_SAVED_LOCALLY)
+                deferLater(reactor, 0, self._delete_incoming_message, doc)
+                leap_events.signal(IMAP_MSG_DELETED_INCOMING)
+
+        d = self._inbox.addMessage(data, (self.RECENT_FLAG,))
+        d.addCallbacks(msgSavedCallback, self._errback)
+        return d
+
+    #
+    # helpers
+    #
+
+    def _msg_multipart_sanity_check(self, msg):
+        """
+        Performs a sanity check against a multipart encrypted msg
+
+        :param msg: The original encrypted message.
+        :type msg: Message
+        """
+        # sanity check
+        payload = msg.get_payload()
+        if len(payload) != 2:
+            raise MalformedMessage(
+                'Multipart/encrypted messages should have exactly 2 body '
+                'parts (instead of %d).' % len(payload))
+        if payload[0].get_content_type() != 'application/pgp-encrypted':
+            raise MalformedMessage(
+                "Multipart/encrypted messages' first body part should "
+                "have content type equal to 'application/pgp-encrypted' "
+                "(instead of %s)." % payload[0].get_content_type())
+        if payload[1].get_content_type() != 'application/octet-stream':
+            raise MalformedMessage(
+                "Multipart/encrypted messages' second body part should "
+                "have content type equal to 'octet-stream' (instead of "
+                "%s)." % payload[1].get_content_type())
+
+    def _is_msg(self, keys):
+        """
+        Checks if the keys of a dictionary match the signature
+        of the document type we use for messages.
+
+        :param keys: iterable containing the strings to match.
+        :type keys: iterable of strings.
+        :rtype: bool
+        """
+        return ENC_SCHEME_KEY in keys and ENC_JSON_KEY in keys