stanza_session.py

# -*- coding:utf-8 -*-
## src/common/stanza_session.py
##
## Copyright (C) 2007-2010 Yann Leboulanger <asterix AT lagaule.org>
## Copyright (C) 2007 Julien Pivotto <roidelapluie AT gmail.com>
## Copyright (C) 2007-2008 Brendan Taylor <whateley AT gmail.com>
##                         Jean-Marie Traissard <jim AT lapin.org>
## Copyright (C) 2008 Jonathan Schleifer <js-gajim AT webkeks.org>
##
## This file is part of Gajim.
##
## Gajim 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; version 3 only.
##
## Gajim 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 Gajim. If not, see <http://www.gnu.org/licenses/>.
##

from common import gajim
from common import xmpp
from common.exceptions import DecryptionError, NegotiationError
import xmpp.c14n

import itertools
import random
import string
import time
import base64
import os
from hashlib import sha256
from hmac import HMAC
from common import crypto

import logging
log = logging.getLogger('gajim.c.stanza_session')

if gajim.HAVE_PYCRYPTO:
    from Crypto.Cipher import AES
    from Crypto.PublicKey import RSA

    from common import dh
    import secrets

XmlDsig = 'http://www.w3.org/2000/09/xmldsig#'

class StanzaSession(object):
    '''
    '''
    def __init__(self, conn, jid, thread_id, type_):
        '''
        '''
        self.conn = conn
        self.jid = jid
        self.type = type_
        self.resource = jid.getResource()

        if thread_id:
            self.received_thread_id = True
            self.thread_id = thread_id
        else:
            self.received_thread_id = False
            if type_ == 'normal':
                self.thread_id = None
            else:
                self.thread_id = self.generate_thread_id()

        self.loggable = True

        self.last_send = 0
        self.last_receive = 0
        self.status = None
        self.negotiated = {}

    def is_loggable(self):
        return self.loggable and gajim.config.should_log(self.conn.name,
            self.jid.getStripped())

    def get_to(self):
        to = str(self.jid)
        if self.resource and not to.endswith(self.resource):
            to += '/' + self.resource
        return to

    def remove_events(self, types):
        """
        Remove events associated with this session from the queue

        Returns True if any events were removed (unlike events.py remove_events)
        """
        any_removed = False

        for j in (self.jid, self.jid.getStripped()):
            for event in gajim.events.get_events(self.conn.name, j, types=types):
                # the event wasn't in this session
                if (event.type_ == 'chat' and event.parameters[8] != self) or \
                (event.type_ == 'printed_chat' and event.parameters[0].session != \
                self):
                    continue

                # events.remove_events returns True when there were no events
                # for some reason
                r = gajim.events.remove_events(self.conn.name, j, event)

                if not r:
                    any_removed = True

        return any_removed

    def generate_thread_id(self):
        return ''.join([f(string.ascii_letters) for f in itertools.repeat(
                random.choice, 32)])

    def send(self, msg):
        if self.thread_id:
            msg.NT.thread = self.thread_id

        msg.setAttr('to', self.get_to())
        self.conn.send_stanza(msg)

        if isinstance(msg, xmpp.Message):
            self.last_send = time.time()

    def reject_negotiation(self, body=None):
        msg = xmpp.Message()
        feature = msg.NT.feature
        feature.setNamespace(xmpp.NS_FEATURE)

        x = xmpp.DataForm(typ='submit')
        x.addChild(node=xmpp.DataField(name='FORM_TYPE', value='urn:xmpp:ssn'))
        x.addChild(node=xmpp.DataField(name='accept', value='0'))

        feature.addChild(node=x)

        if body:
            msg.setBody(body)

        self.send(msg)

        self.cancelled_negotiation()

    def cancelled_negotiation(self):
        """
        A negotiation has been cancelled, so reset this session to its default
        state
        """
        if self.control:
            self.control.on_cancel_session_negotiation()

        self.status = None
        self.negotiated = {}

    def terminate(self, send_termination = True):
        # only send termination message if we've sent a message and think they
        # have XEP-0201 support
        if send_termination and self.last_send > 0 and \
        (self.received_thread_id or self.last_receive == 0):
            msg = xmpp.Message()
            feature = msg.NT.feature
            feature.setNamespace(xmpp.NS_FEATURE)

            x = xmpp.DataForm(typ='submit')
            x.addChild(node=xmpp.DataField(name='FORM_TYPE', value='urn:xmpp:ssn'))
            x.addChild(node=xmpp.DataField(name='terminate', value='1'))

            feature.addChild(node=x)

            self.send(msg)

        self.status = None

    def acknowledge_termination(self):
        # we could send an acknowledgement message to the remote client here
        self.status = None


class ArchivingStanzaSession(StanzaSession):
    def __init__(self, conn, jid, thread_id, type_='chat'):
        StanzaSession.__init__(self, conn, jid, thread_id, type_='chat')
        self.archiving = False

    def archiving_logging_preference(self, initiator_options=None):
        return self.conn.logging_preference(self.jid, initiator_options)

    def negotiate_archiving(self):
        self.negotiated = {}

        request = xmpp.Message()
        feature = request.NT.feature
        feature.setNamespace(xmpp.NS_FEATURE)

        x = xmpp.DataForm(typ='form')

        x.addChild(node=xmpp.DataField(name='FORM_TYPE', value='urn:xmpp:ssn',
                typ='hidden'))
        x.addChild(node=xmpp.DataField(name='accept', value='1', typ='boolean',
                required=True))

        x.addChild(node=xmpp.DataField(name='logging', typ='list-single',
                options=self.archiving_logging_preference(), required=True))

        x.addChild(node=xmpp.DataField(name='disclosure', typ='list-single',
                options=['never'], required=True))
        x.addChild(node=xmpp.DataField(name='security', typ='list-single',
                options=['none'], required=True))

        feature.addChild(node=x)

        self.status = 'requested-archiving'

        self.send(request)

    def respond_archiving(self, form):
        field = form.getField('logging')
        options = [x[1] for x in field.getOptions()]
        values = field.getValues()

        logging = self.archiving_logging_preference(options)
        self.negotiated['logging'] = logging

        response = xmpp.Message()
        feature = response.NT.feature
        feature.setNamespace(xmpp.NS_FEATURE)

        x = xmpp.DataForm(typ='submit')

        x.addChild(node=xmpp.DataField(name='FORM_TYPE', value='urn:xmpp:ssn'))
        x.addChild(node=xmpp.DataField(name='accept', value='true'))

        x.addChild(node=xmpp.DataField(name='logging', value=logging))

        self.status = 'responded-archiving'

        feature.addChild(node=x)

        if not logging:
            response = xmpp.Error(response, xmpp.ERR_NOT_ACCEPTABLE)

            feature = xmpp.Node(xmpp.NS_FEATURE + ' feature')

            n = xmpp.Node('field')
            n['var'] = 'logging'
            feature.addChild(node=n)

            response.T.error.addChild(node=feature)

        self.send(response)

    def we_accept_archiving(self, form):
        if self.negotiated['logging'] == 'mustnot':
            self.loggable = False
        log.debug('archiving session accepted: %s' % self.loggable)
        self.status = 'active'
        self.archiving = True
        if self.control:
            self.control.print_archiving_session_details()

    def archiving_accepted(self, form):
        negotiated = {}
        ask_user = {}
        not_acceptable = []

        if form['logging'] not in self.archiving_logging_preference():
            raise

        self.negotiated['logging'] = form['logging']

        accept = xmpp.Message()
        feature = accept.NT.feature
        feature.setNamespace(xmpp.NS_FEATURE)

        result = xmpp.DataForm(typ='result')

        result.addChild(node=xmpp.DataField(name='FORM_TYPE',
                value='urn:xmpp:ssn'))
        result.addChild(node=xmpp.DataField(name='accept', value='1'))

        feature.addChild(node=result)

        self.send(accept)
        if self.negotiated['logging'] == 'mustnot':
            self.loggable = False
        log.debug('archiving session accepted: %s' % self.loggable)
        self.status = 'active'
        self.archiving = True
        if self.control:
            self.control.print_archiving_session_details()

    def stop_archiving_for_session(self):
        self.conn.stop_archiving_session(self.thread_id)


class EncryptedStanzaSession(ArchivingStanzaSession):
    """
    An encrypted stanza negotiation has several states. They arerepresented as
    the following values in the 'status' attribute of the session object:

            1. None:
                            default state
            2. 'requested-e2e':
                            this client has initiated an esession negotiation and is waiting
                            for a response
            3. 'responded-e2e':
                            this client has responded to an esession negotiation request and
                            is waiting for the initiator to identify itself and complete the
                            negotiation
            4. 'identified-alice':
                            this client identified itself and is waiting for the responder to
                            identify itself and complete the negotiation
            5. 'active':
                            an encrypted session has been successfully negotiated. messages
                            of any of the types listed in 'encryptable_stanzas' should be
                            encrypted before they're sent.

    The transition between these states is handled in gajim.py's
    handle_session_negotiation method.
    """

    def __init__(self, conn, jid, thread_id, type_='chat'):
        ArchivingStanzaSession.__init__(self, conn, jid, thread_id,
            type_='chat')

        self.xes = {}
        self.es = {}
        self.n = 128
        self.enable_encryption = False

        # _s denotes 'self' (ie. this client)
        self._kc_s = None
        # _o denotes 'other' (ie. the client at the other end of the session)
        self._kc_o = None

        # has the remote contact's identity ever been verified?
        self.verified_identity = False

    def _get_contact(self):
        c = gajim.contacts.get_contact(self.conn.name, self.jid, self.resource)
        if not c:
            c = gajim.contacts.get_contact(self.conn.name, self.jid)
        return c

    def _is_buggy_gajim(self):
        c = self._get_contact()
        if c and c.supports(xmpp.NS_ROSTERX):
            return False
        return True

    def set_kc_s(self, value):
        """
        Keep the encrypter updated with my latest cipher key
        """
        self._kc_s = value
        self.encrypter = self.cipher.new(self._kc_s, self.cipher.MODE_CTR,
                counter=self.encryptcounter)

    def get_kc_s(self):
        return self._kc_s

    def set_kc_o(self, value):
        """
        Keep the decrypter updated with the other party's latest cipher key
        """
        self._kc_o = value
        self.decrypter = self.cipher.new(self._kc_o, self.cipher.MODE_CTR,
                counter=self.decryptcounter)

    def get_kc_o(self):
        return self._kc_o

    kc_s = property(get_kc_s, set_kc_s)
    kc_o = property(get_kc_o, set_kc_o)

    def encryptcounter(self):
        self.c_s = (self.c_s + 1) % (2 ** self.n)
        return crypto.encode_mpi_with_padding(self.c_s)

    def decryptcounter(self):
        self.c_o = (self.c_o + 1) % (2 ** self.n)
        return crypto.encode_mpi_with_padding(self.c_o)

    def sign(self, string):
        if self.negotiated['sign_algs'] == (XmlDsig + 'rsa-sha256'):
            hash_ = crypto.sha256(string)
            return crypto.encode_mpi(gajim.pubkey.sign(hash_, '')[0])

    def encrypt_stanza(self, stanza):
        encryptable = [x for x in stanza.getChildren() if x.getName() not in
                ('error', 'amp', 'thread')]

        # FIXME can also encrypt contents of <error/> elements in stanzas @type =
        # 'error'
        # (except for <defined-condition
        # xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/> child elements)

        old_en_counter = self.c_s

        for element in encryptable:
            stanza.delChild(element)

        plaintext = ''.join(map(str, encryptable))

        m_compressed = self.compress(plaintext)
        m_final = self.encrypt(m_compressed)

        c = stanza.NT.c
        c.setNamespace('http://www.xmpp.org/extensions/xep-0200.html#ns')
        c.NT.data = base64.b64encode(m_final)

        # FIXME check for rekey request, handle <key/> elements

        m_content = ''.join(map(str, c.getChildren()))
        c.NT.mac = base64.b64encode(self.hmac(self.km_s, m_content + \
                crypto.encode_mpi(old_en_counter)))

        msgtxt = '[This is part of an encrypted session. ' \
                'If you see this message, something went wrong.]'
        lang = os.getenv('LANG')
        if lang is not None and lang != 'en': # we're not english
            msgtxt = _('[This is part of an encrypted session. '
                    'If you see this message, something went wrong.]') + ' (' + \
                    msgtxt + ')'
        stanza.setBody(msgtxt)

        return stanza

    def is_xep_200_encrypted(self, msg):
        msg.getTag('c', namespace=xmpp.NS_STANZA_CRYPTO)

    def hmac(self, key, content):
        return HMAC(key, content, self.hash_alg).digest()

    def generate_initiator_keys(self, k):
        return (self.hmac(k, 'Initiator Cipher Key'),
                self.hmac(k, 'Initiator MAC Key'),
                self.hmac(k, 'Initiator SIGMA Key'))

    def generate_responder_keys(self, k):
        return (self.hmac(k, 'Responder Cipher Key'),
                self.hmac(k, 'Responder MAC Key'),
                self.hmac(k, 'Responder SIGMA Key'))

    def compress(self, plaintext):
        if self.compression is None:
            return plaintext

    def decompress(self, compressed):
        if self.compression is None:
            return compressed

    def encrypt(self, encryptable):
        padded = crypto.pad_to_multiple(encryptable, 16, ' ', False)

        return self.encrypter.encrypt(padded)

    def decrypt_stanza(self, stanza):
        """
        Delete the unencrypted explanation body, if it exists
        """
        orig_body = stanza.getTag('body')
        if orig_body:
            stanza.delChild(orig_body)

        c = stanza.getTag(name='c',
                namespace='http://www.xmpp.org/extensions/xep-0200.html#ns')

        stanza.delChild(c)

        # contents of <c>, minus <mac>, minus whitespace
        macable = ''.join(str(x) for x in c.getChildren() if x.getName() != 'mac')

        received_mac = base64.b64decode(c.getTagData('mac'))
        calculated_mac = self.hmac(self.km_o, macable + \
                crypto.encode_mpi_with_padding(self.c_o))

        if not calculated_mac == received_mac:
            raise DecryptionError('bad signature')

        m_final = base64.b64decode(c.getTagData('data'))
        m_compressed = self.decrypt(m_final)
        plaintext = self.decompress(m_compressed)

        try:
            parsed = xmpp.Node(node='<node>' + plaintext + '</node>')
        except Exception:
            raise DecryptionError('decrypted <data/> not parseable as XML')

        for child in parsed.getChildren():
            stanza.addChild(node=child)

        # replace non-character unicode
        stranza = self.conn.connection.Dispatcher.replace_non_character(stanza)

        return stanza

    def decrypt(self, ciphertext):
        return self.decrypter.decrypt(ciphertext)

    def logging_preference(self):
        if gajim.config.get_per('accounts', self.conn.name,
        'log_encrypted_sessions'):
            return ['may', 'mustnot']
        else:
            return ['mustnot', 'may']

    def get_shared_secret(self, e, y, p):
        if (not 1 < e < (p - 1)):
            raise NegotiationError('invalid DH value')

        return crypto.sha256(crypto.encode_mpi(crypto.powmod(e, y, p)))

    def c7lize_mac_id(self, form):
        kids = form.getChildren()
        macable = [x for x in kids if x.getVar() not in ('mac', 'identity')]
        return ''.join(xmpp.c14n.c14n(el, self._is_buggy_gajim()) for el in \
                macable)

    def verify_identity(self, form, dh_i, sigmai, i_o):
        m_o = base64.b64decode(form['mac'])
        id_o = base64.b64decode(form['identity'])

        m_o_calculated = self.hmac(self.km_o, crypto.encode_mpi(self.c_o) + id_o)

        if m_o_calculated != m_o:
            raise NegotiationError('calculated m_%s differs from received m_%s' %
                    (i_o, i_o))

        if i_o == 'a' and self.sas_algs == 'sas28x5':
            # we don't need to calculate this if there's a verified retained secret
            # (but we do anyways)
            self.sas = crypto.sas_28x5(m_o, self.form_s)

        if self.negotiated['recv_pubkey']:
            plaintext = self.decrypt(id_o)
            parsed = xmpp.Node(node='<node>' + plaintext + '</node>')

            if self.negotiated['recv_pubkey'] == 'hash':
                # fingerprint = parsed.getTagData('fingerprint')
                # FIXME find stored pubkey or terminate session
                raise NotImplementedError()
            else:
                if self.negotiated['sign_algs'] == (XmlDsig + 'rsa-sha256'):
                    keyvalue = parsed.getTag(name='RSAKeyValue', namespace=XmlDsig)

                    n, e = (crypto.decode_mpi(base64.b64decode(
                            keyvalue.getTagData(x))) for x in ('Modulus', 'Exponent'))
                    eir_pubkey = RSA.construct((n, long(e)))

                    pubkey_o = xmpp.c14n.c14n(keyvalue, self._is_buggy_gajim())
                else:
                    # FIXME DSA, etc.
                    raise NotImplementedError()

            enc_sig = parsed.getTag(name='SignatureValue',
                    namespace=XmlDsig).getData()
            signature = (crypto.decode_mpi(base64.b64decode(enc_sig)), )
        else:
            mac_o = self.decrypt(id_o)
            pubkey_o = ''

        c7l_form = self.c7lize_mac_id(form)

        content = self.n_s + self.n_o + crypto.encode_mpi(dh_i) + pubkey_o

        if sigmai:
            self.form_o = c7l_form
            content += self.form_o
        else:
            form_o2 = c7l_form
            content += self.form_o + form_o2

        mac_o_calculated = self.hmac(self.ks_o, content)

        if self.negotiated['recv_pubkey']:
            hash_ = crypto.sha256(mac_o_calculated)

            if not eir_pubkey.verify(hash_, signature):
                raise NegotiationError('public key signature verification failed!')

        elif mac_o_calculated != mac_o:
            raise NegotiationError('calculated mac_%s differs from received mac_%s'
                    % (i_o, i_o))

    def make_identity(self, form, dh_i):
        if self.negotiated['send_pubkey']:
            if self.negotiated['sign_algs'] == (XmlDsig + 'rsa-sha256'):
                pubkey = secrets.secrets().my_pubkey(self.conn.name)
                fields = (pubkey.n, pubkey.e)

                cb_fields = [base64.b64encode(crypto.encode_mpi(f)) for f in
                        fields]

                pubkey_s = '<RSAKeyValue xmlns="http://www.w3.org/2000/09/xmldsig#"'
                '><Modulus>%s</Modulus><Exponent>%s</Exponent></RSAKeyValue>' % \
                        tuple(cb_fields)
        else:
            pubkey_s = ''

        form_s2 = ''.join(xmpp.c14n.c14n(el, self._is_buggy_gajim()) for el in \
                form.getChildren())

        old_c_s = self.c_s
        content = self.n_o + self.n_s + crypto.encode_mpi(dh_i) + pubkey_s + \
                self.form_s + form_s2

        mac_s = self.hmac(self.ks_s, content)

        if self.negotiated['send_pubkey']:
            signature = self.sign(mac_s)

            sign_s = '<SignatureValue xmlns="http://www.w3.org/2000/09/xmldsig#">'
            '%s</SignatureValue>' % base64.b64encode(signature)

            if self.negotiated['send_pubkey'] == 'hash':
                b64ed = base64.b64encode(self.hash(pubkey_s))
                pubkey_s = '<fingerprint>%s</fingerprint>' % b64ed

            id_s = self.encrypt(pubkey_s + sign_s)
        else:
            id_s = self.encrypt(mac_s)

        m_s = self.hmac(self.km_s, crypto.encode_mpi(old_c_s) + id_s)

        if self.status == 'requested-e2e' and self.sas_algs == 'sas28x5':
            # we're alice; check for a retained secret
            # if none exists, prompt the user with the SAS
            self.sas = crypto.sas_28x5(m_s, self.form_o)

            if self.sigmai:
                # FIXME save retained secret?
                self.check_identity(tuple)

        return (xmpp.DataField(name='identity', value=base64.b64encode(id_s)),
                xmpp.DataField(name='mac', value=base64.b64encode(m_s)))

    def negotiate_e2e(self, sigmai):
        self.negotiated = {}

        request = xmpp.Message()
        feature = request.NT.feature
        feature.setNamespace(xmpp.NS_FEATURE)

        x = xmpp.DataForm(typ='form')

        x.addChild(node=xmpp.DataField(name='FORM_TYPE', value='urn:xmpp:ssn',
                typ='hidden'))
        x.addChild(node=xmpp.DataField(name='accept', value='1', typ='boolean',
                required=True))

        # this field is incorrectly called 'otr' in XEPs 0116 and 0217
        x.addChild(node=xmpp.DataField(name='logging', typ='list-single',
                options=self.logging_preference(), required=True))

        # unsupported options: 'disabled', 'enabled'
        x.addChild(node=xmpp.DataField(name='disclosure', typ='list-single',
                options=['never'], required=True))
        x.addChild(node=xmpp.DataField(name='security', typ='list-single',
                options=['e2e'], required=True))
        x.addChild(node=xmpp.DataField(name='crypt_algs', value='aes128-ctr',
                typ='hidden'))
        x.addChild(node=xmpp.DataField(name='hash_algs', value='sha256',
                typ='hidden'))
        x.addChild(node=xmpp.DataField(name='compress', value='none',
                typ='hidden'))

        # unsupported options: 'iq', 'presence'
        x.addChild(node=xmpp.DataField(name='stanzas', typ='list-multi',
                options=['message']))

        x.addChild(node=xmpp.DataField(name='init_pubkey', options=['none', 'key',
                'hash'], typ='list-single'))

        # FIXME store key, use hash
        x.addChild(node=xmpp.DataField(name='resp_pubkey', options=['none',
                'key'], typ='list-single'))

        x.addChild(node=xmpp.DataField(name='ver', value='1.0', typ='hidden'))

        x.addChild(node=xmpp.DataField(name='rekey_freq', value='4294967295',
                typ='hidden'))

        x.addChild(node=xmpp.DataField(name='sas_algs', value='sas28x5',
                typ='hidden'))
        x.addChild(node=xmpp.DataField(name='sign_algs',
                value='http://www.w3.org/2000/09/xmldsig#rsa-sha256', typ='hidden'))

        self.n_s = crypto.generate_nonce()

        x.addChild(node=xmpp.DataField(name='my_nonce',
                value=base64.b64encode(self.n_s), typ='hidden'))

        modp_options = [ int(g) for g in gajim.config.get('esession_modp').split(
                ',') ]

        x.addChild(node=xmpp.DataField(name='modp', typ='list-single',
                options=[[None, y] for y in modp_options]))

        x.addChild(node=self.make_dhfield(modp_options, sigmai))
        self.sigmai = sigmai

        self.form_s = ''.join(xmpp.c14n.c14n(el, self._is_buggy_gajim()) for el \
                in x.getChildren())

        feature.addChild(node=x)

        self.status = 'requested-e2e'

        self.send(request)

    def verify_options_bob(self, form):
        """
        4.3 esession response (bob)
        """
        negotiated = {'recv_pubkey': None, 'send_pubkey': None}
        not_acceptable = []
        ask_user = {}

        fixed = { 'disclosure': 'never', 'security': 'e2e',
                'crypt_algs': 'aes128-ctr', 'hash_algs': 'sha256', 'compress': 'none',
                'stanzas': 'message', 'init_pubkey': 'none', 'resp_pubkey': 'none',
                'ver': '1.0', 'sas_algs': 'sas28x5' }

        self.encryptable_stanzas = ['message']

        self.sas_algs = 'sas28x5'
        self.cipher = AES
        self.hash_alg = sha256
        self.compression = None

        for name in form.asDict():
            field = form.getField(name)
            options = [x[1] for x in field.getOptions()]
            values = field.getValues()

            if not field.getType() in ('list-single', 'list-multi'):
                options = values

            if name in fixed:
                if fixed[name] in options:
                    negotiated[name] = fixed[name]
                else:
                    not_acceptable.append(name)
            elif name == 'rekey_freq':
                preferred = int(options[0])
                negotiated['rekey_freq'] = preferred
                self.rekey_freq = preferred
            elif name == 'logging':
                my_prefs = self.logging_preference()

                if my_prefs[0] in options: # our first choice is offered, select it
                    pref = my_prefs[0]
                    negotiated['logging'] = pref
                else:   # see if other acceptable choices are offered
                    for pref in my_prefs:
                        if pref in options:
                            ask_user['logging'] = pref
                            break

                    if not 'logging' in ask_user:
                        not_acceptable.append(name)
            elif name == 'init_pubkey':
                for x in ('key'):
                    if x in options:
                        negotiated['recv_pubkey'] = x
                        break
            elif name == 'resp_pubkey':
                for x in ('hash', 'key'):
                    if x in options:
                        negotiated['send_pubkey'] = x
                        break
            elif name == 'sign_algs':
                if (XmlDsig + 'rsa-sha256') in options:
                    negotiated['sign_algs'] = XmlDsig + 'rsa-sha256'
            else:
                # FIXME some things are handled elsewhere, some things are
                # not-implemented
                pass

        return (negotiated, not_acceptable, ask_user)

    def respond_e2e_bob(self, form, negotiated, not_acceptable):
        """
        4.3 esession response (bob)
        """
        response = xmpp.Message()
        feature = response.NT.feature
        feature.setNamespace(xmpp.NS_FEATURE)

        x = xmpp.DataForm(typ='submit')

        x.addChild(node=xmpp.DataField(name='FORM_TYPE', value='urn:xmpp:ssn'))
        x.addChild(node=xmpp.DataField(name='accept', value='true'))

        for name in negotiated:
            # some fields are internal and should not be sent
            if not name in ('send_pubkey', 'recv_pubkey'):
                x.addChild(node=xmpp.DataField(name=name, value=negotiated[name]))

        self.negotiated = negotiated

        # the offset of the group we chose (need it to match up with the dhhash)
        group_order = 0
        self.modp = int(form.getField('modp').getOptions()[group_order][1])
        x.addChild(node=xmpp.DataField(name='modp', value=self.modp))

        g = dh.generators[self.modp]
        p = dh.primes[self.modp]

        self.n_o = base64.b64decode(form['my_nonce'])

        dhhashes = form.getField('dhhashes').getValues()
        self.negotiated['He'] = base64.b64decode(dhhashes[group_order].encode(
                'utf8'))

        bytes = int(self.n / 8)

        self.n_s = crypto.generate_nonce()

        # n-bit random number
        self.c_o = crypto.decode_mpi(crypto.random_bytes(bytes))
        self.c_s = self.c_o ^ (2 ** (self.n - 1))

        self.y = crypto.srand(2 ** (2 * self.n - 1), p - 1)
        self.d = crypto.powmod(g, self.y, p)

        to_add = {'my_nonce': self.n_s,
                'dhkeys': crypto.encode_mpi(self.d),
                'counter': crypto.encode_mpi(self.c_o),
                'nonce': self.n_o}

        for name in to_add:
            b64ed = base64.b64encode(to_add[name])
            x.addChild(node=xmpp.DataField(name=name, value=b64ed))

        self.form_o = ''.join(xmpp.c14n.c14n(el, self._is_buggy_gajim()) for el \
                in form.getChildren())
        self.form_s = ''.join(xmpp.c14n.c14n(el, self._is_buggy_gajim()) for el \
                in x.getChildren())

        self.status = 'responded-e2e'

        feature.addChild(node=x)

        if not_acceptable:
            response = xmpp.Error(response, xmpp.ERR_NOT_ACCEPTABLE)

            feature = xmpp.Node(xmpp.NS_FEATURE + ' feature')

            for f in not_acceptable:
                n = xmpp.Node('field')
                n['var'] = f
                feature.addChild(node=n)

            response.T.error.addChild(node=feature)

        self.send(response)

    def verify_options_alice(self, form):
        """
        'Alice Accepts'
        """
        negotiated = {}
        ask_user = {}
        not_acceptable = []

        if not form['logging'] in self.logging_preference():
            not_acceptable.append(form['logging'])
        elif form['logging'] != self.logging_preference()[0]:
            ask_user['logging'] = form['logging']
        else:
            negotiated['logging'] = self.logging_preference()[0]

        for r, a in (('recv_pubkey', 'resp_pubkey'), ('send_pubkey',
        'init_pubkey')):
            negotiated[r] = None

            if a in form.asDict() and form[a] in ('key', 'hash'):
                negotiated[r] = form[a]

        if 'sign_algs' in form.asDict():
            if form['sign_algs'] in (XmlDsig + 'rsa-sha256', ):
                negotiated['sign_algs'] = form['sign_algs']
            else:
                not_acceptable.append(form['sign_algs'])

        return (negotiated, not_acceptable, ask_user)

    def accept_e2e_alice(self, form, negotiated):
        """
        'Alice Accepts', continued
        """
        self.encryptable_stanzas = ['message']
        self.sas_algs = 'sas28x5'
        self.cipher = AES
        self.hash_alg = sha256
        self.compression = None

        self.negotiated = negotiated

        accept = xmpp.Message()
        feature = accept.NT.feature
        feature.setNamespace(xmpp.NS_FEATURE)

        result = xmpp.DataForm(typ='result')

        self.c_s = crypto.decode_mpi(base64.b64decode(form['counter']))
        self.c_o = self.c_s ^ (2 ** (self.n - 1))
        self.n_o = base64.b64decode(form['my_nonce'])

        mod_p = int(form['modp'])
        p = dh.primes[mod_p]
        x = self.xes[mod_p]
        e = self.es[mod_p]

        self.d = crypto.decode_mpi(base64.b64decode(form['dhkeys']))
        self.k = self.get_shared_secret(self.d, x, p)

        result.addChild(node=xmpp.DataField(name='FORM_TYPE',
                value='urn:xmpp:ssn'))
        result.addChild(node=xmpp.DataField(name='accept', value='1'))
        result.addChild(node=xmpp.DataField(name='nonce',
                value=base64.b64encode(self.n_o)))

        self.kc_s, self.km_s, self.ks_s = self.generate_initiator_keys(self.k)

        if self.sigmai:
            self.kc_o, self.km_o, self.ks_o = self.generate_responder_keys(self.k)
            self.verify_identity(form, self.d, True, 'b')
        else:
            srses = secrets.secrets().retained_secrets(self.conn.name,
                    self.jid.getStripped())
            rshashes = [self.hmac(self.n_s, rs[0]) for rs in srses]

            if not rshashes:
                # we've never spoken before, but we'll pretend we have
                rshash_size = self.hash_alg().digest_size
                rshashes.append(crypto.random_bytes(rshash_size))

            rshashes = [base64.b64encode(rshash) for rshash in rshashes]
            result.addChild(node=xmpp.DataField(name='rshashes', value=rshashes))
            result.addChild(node=xmpp.DataField(name='dhkeys',
                    value=base64.b64encode(crypto.encode_mpi(e))))

            self.form_o = ''.join(xmpp.c14n.c14n(el, self._is_buggy_gajim()) for \
                    el in form.getChildren())

        # MUST securely destroy K unless it will be used later to generate the
        # final shared secret

        for datafield in self.make_identity(result, e):
            result.addChild(node=datafield)

        feature.addChild(node=result)
        self.send(accept)

        if self.sigmai:
            self.status = 'active'
            self.enable_encryption = True
        else:
            self.status = 'identified-alice'

    def accept_e2e_bob(self, form):
        """
        4.5 esession accept (bob)
        """
        response = xmpp.Message()

        init = response.NT.init
        init.setNamespace(xmpp.NS_ESESSION_INIT)

        x = xmpp.DataForm(typ='result')

        for field in ('nonce', 'dhkeys', 'rshashes', 'identity', 'mac'):
            # FIXME: will do nothing in real world...
            assert field in form.asDict(), "alice's form didn't have a %s field" \
                    % field

        # 4.5.1 generating provisory session keys
        e = crypto.decode_mpi(base64.b64decode(form['dhkeys']))
        p = dh.primes[self.modp]

        if crypto.sha256(crypto.encode_mpi(e)) != self.negotiated['He']:
            raise NegotiationError('SHA256(e) != He')

        k = self.get_shared_secret(e, self.y, p)
        self.kc_o, self.km_o, self.ks_o = self.generate_initiator_keys(k)

        # 4.5.2 verifying alice's identity
        self.verify_identity(form, e, False, 'a')

        # 4.5.4 generating bob's final session keys
        srs = ''

        srses = secrets.secrets().retained_secrets(self.conn.name,
                self.jid.getStripped())
        rshashes = [base64.b64decode(rshash) for rshash in form.getField(