summaryrefslogtreecommitdiff
path: root/src/leap/bitmask/core/mail_services.py
blob: a3cf692e1853d7eb3b066dc2d0cdd83e3d00bc17 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
# -*- coding: utf-8 -*-
# mail_services.py
# Copyright (C) 2016 LEAP Encryption Acess 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/>.
"""
Mail services.

This is quite moving work still.
This should be moved to the different packages when it stabilizes.
"""
import json
import os
import shutil
import tempfile
from collections import defaultdict
from collections import namedtuple

from twisted.application import service
from twisted.internet import defer
from twisted.internet import reactor
from twisted.internet import task
from twisted.python import log

# TODO move to bitmask.common
from leap.common.service_hooks import HookableService
from leap.bitmask.bonafide import config
from leap.bitmask.keymanager import KeyManager
from leap.bitmask.keymanager.errors import KeyNotFound
from leap.bitmask.keymanager.validation import ValidationLevels
from leap.bitmask.mail.constants import INBOX_NAME
from leap.bitmask.mail.mail import Account
from leap.bitmask.mail.imap.service import imap
from leap.bitmask.mail.incoming.service import IncomingMail
from leap.bitmask.mail.incoming.service import INCOMING_CHECK_PERIOD
from leap.bitmask.mail import smtp
from leap.bitmask.util import get_gpg_bin_path
from leap.soledad.client.api import Soledad

from leap.bitmask.core.uuid_map import UserMap
from leap.bitmask.core.configurable import DEFAULT_BASEDIR


class Container(object):

    def __init__(self, service=None):
        self._instances = defaultdict(None)
        if service is not None:
            self.service = service

    def get_instance(self, key):
        return self._instances.get(key, None)

    def add_instance(self, key, data):
        self._instances[key] = data


class ImproperlyConfigured(Exception):
    pass


class SoledadContainer(Container):

    def __init__(self, service=None, basedir=DEFAULT_BASEDIR):
        self._basedir = os.path.expanduser(basedir)
        self._usermap = UserMap()
        super(SoledadContainer, self).__init__(service=service)

    def add_instance(self, userid, passphrase, uuid=None, token=None):

        if not uuid:
            bootstrapped_uuid = self._usermap.lookup_uuid(userid, passphrase)
            uuid = bootstrapped_uuid
            if not uuid:
                return
        else:
            self._usermap.add(userid, uuid, passphrase)

        user, provider = userid.split('@')

        soledad_path = os.path.join(self._basedir, 'soledad')
        soledad_url = _get_soledad_uri(self._basedir, provider)
        cert_path = _get_ca_cert_path(self._basedir, provider)

        soledad = self._create_soledad_instance(
            uuid, passphrase, soledad_path, soledad_url,
            cert_path, token)

        super(SoledadContainer, self).add_instance(userid, soledad)

        data = {'user': userid, 'uuid': uuid, 'token': token,
                'soledad': soledad}
        self.service.trigger_hook('on_new_soledad_instance', **data)

    def _create_soledad_instance(self, uuid, passphrase, soledad_path,
                                 server_url, cert_file, token):
        # setup soledad info
        secrets_path = os.path.join(soledad_path, '%s.secret' % uuid)
        local_db_path = os.path.join(soledad_path, '%s.db' % uuid)

        if token is None:
            syncable = False
            token = ''
        else:
            syncable = True

        return Soledad(
            uuid,
            unicode(passphrase),
            secrets_path=secrets_path,
            local_db_path=local_db_path,
            server_url=server_url,
            cert_file=cert_file,
            auth_token=token,
            defer_encryption=True,
            syncable=syncable)

    def set_remote_auth_token(self, userid, token):
        self.get_instance(userid).token = token

    def set_syncable(self, userid, state):
        # TODO should check that there's a token!
        self.get_instance(userid).set_syncable(bool(state))

    def sync(self, userid):
        self.get_instance(userid).sync()


def _get_provider_from_full_userid(userid):
    _, provider_id = config.get_username_and_provider(userid)
    # TODO -- this autoconf should be passed from the
    # command flag. workaround to get cli workinf for now.
    return config.Provider(provider_id, autoconf=True)


def is_service_ready(service, provider):
    """
    Returns True when the following conditions are met:
       - Provider offers that service.
       - We have the config files for the service.
       - The service is enabled.
    """
    has_service = provider.offers_service(service)
    has_config = provider.has_config_for_service(service)
    is_enabled = provider.is_service_enabled(service)
    return has_service and has_config and is_enabled


class SoledadService(HookableService):

    def __init__(self, basedir):
        service.Service.__init__(self)
        self._basedir = basedir

    def startService(self):
        log.msg('Starting Soledad Service')
        self._container = SoledadContainer(service=self)
        super(SoledadService, self).startService()

    # hooks

    def hook_on_passphrase_entry(self, **kw):
        userid = kw.get('username')
        provider = _get_provider_from_full_userid(userid)
        provider.callWhenReady(self._hook_on_passphrase_entry, provider, **kw)

    def _hook_on_passphrase_entry(self, provider, **kw):
        if is_service_ready('mx', provider):
            userid = kw.get('username')
            password = kw.get('password')
            uuid = kw.get('uuid')
            container = self._container
            log.msg("on_passphrase_entry: New Soledad Instance: %s" % userid)
            if not container.get_instance(userid):
                container.add_instance(userid, password, uuid=uuid, token=None)
        else:
            log.msg('Service MX is not ready...')

    def hook_on_bonafide_auth(self, **kw):
        userid = kw['username']
        provider = _get_provider_from_full_userid(userid)
        provider.callWhenReady(self._hook_on_bonafide_auth, provider, **kw)

    def _hook_on_bonafide_auth(self, provider, **kw):
        if provider.offers_service('mx'):
            userid = kw['username']
            password = kw['password']
            token = kw['token']
            uuid = kw['uuid']

            container = self._container
            if container.get_instance(userid):
                log.msg("Passing a new SRP Token to Soledad: %s" % userid)
                container.set_remote_auth_token(userid, token)
                container.set_syncable(userid, True)
            else:
                log.msg("Adding a new Soledad Instance: %s" % userid)
                container.add_instance(
                    userid, password, uuid=uuid, token=token)


class KeymanagerContainer(Container):

    def __init__(self, service=None, basedir=DEFAULT_BASEDIR):
        self._basedir = os.path.expanduser(basedir)
        super(KeymanagerContainer, self).__init__(service=service)

    def add_instance(self, userid, token, uuid, soledad):
        log.msg("Adding Keymanager instance for: %s" % userid)
        keymanager = self._create_keymanager_instance(
            userid, token, uuid, soledad)
        super(KeymanagerContainer, self).add_instance(userid, keymanager)
        d = self._get_or_generate_keys(keymanager, userid)
        d.addCallback(self._on_keymanager_ready_cb, userid, soledad)
        return d

    def set_remote_auth_token(self, userid, token):
        self.get_instance(userid).token = token

    def _on_keymanager_ready_cb(self, keymanager, userid, soledad):
        data = {'userid': userid, 'soledad': soledad, 'keymanager': keymanager}
        self.service.trigger_hook('on_new_keymanager_instance', **data)

    def _get_or_generate_keys(self, keymanager, userid):

        def _get_key(_):
            log.msg("looking up private key for %s" % userid)
            return keymanager.get_key(userid, private=True, fetch_remote=False)

        def _found_key(key):
            log.msg("found key: %r" % key)

        def _if_not_found_generate(failure):
            failure.trap(KeyNotFound)
            log.msg("key not found, generating key for %s" % (userid,))
            d = keymanager.gen_key()
            d.addCallbacks(_send_key, _log_key_error("generating"))
            return d

        def _send_key(ignored):
            # ----------------------------------------------------------------
            # It might be the case that we have generated a key-pair
            # but this hasn't been successfully uploaded. How do we know that?
            # XXX Should this be a method of bonafide instead?
            # -----------------------------------------------------------------
            log.msg("key generated for %s" % userid)

            if not keymanager.token:
                log.msg("token not available, scheduling new send attempt...")
                return task.deferLater(reactor, 5, _send_key, None)

            log.msg("sending public key to server")
            d = keymanager.send_key()
            d.addCallbacks(
                lambda _: log.msg("key sent to server"),
                _log_key_error("sending"))
            return d

        def _log_key_error(step):
            def log_error(failure):
                log.err("Error while %s key!" % step)
                log.err(failure)
                return failure
            return log_error

        def _maybe_sync(ever_synced):
            if ever_synced:
                log.msg("soledad has synced in the past")
                return defer.succeed(None)

            log.msg("soledad has never synced")

            if not keymanager.token:
                log.msg("no token to sync now, scheduling a new check")
                d = task.deferLater(reactor, 5, keymanager.ever_synced)
                d.addCallback(_maybe_sync)
                return d

            log.msg("syncing soledad for the first time...")
            return keymanager._soledad.sync()

        log.msg("checking if soledad has ever synced...")
        d = keymanager.ever_synced()
        d.addCallback(_maybe_sync)
        d.addCallback(_get_key)
        d.addCallbacks(_found_key, _if_not_found_generate)
        d.addCallback(lambda _: keymanager)
        return d

    def _create_keymanager_instance(self, userid, token, uuid, soledad):
        user, provider = userid.split('@')
        nickserver_uri = self._get_nicknym_uri(provider)

        cert_path = _get_ca_cert_path(self._basedir, provider)
        api_uri = self._get_api_uri(provider)

        if not token:
            token = self.service.tokens.get(userid)

        km_args = (userid, nickserver_uri, soledad)

        km_kwargs = {
            "token": token, "uid": uuid,
            "api_uri": api_uri, "api_version": "1",
            "ca_cert_path": cert_path,
            "gpgbinary": get_gpg_bin_path()
        }
        keymanager = KeyManager(*km_args, **km_kwargs)
        return keymanager

    def _get_api_uri(self, provider):
        # TODO get this from service.json (use bonafide service)
        api_uri = "https://api.{provider}:4430".format(
            provider=provider)
        return api_uri

    def _get_nicknym_uri(self, provider):
        return 'https://nicknym.{provider}:6425'.format(
            provider=provider)


class KeymanagerService(HookableService):

    def __init__(self, basedir=DEFAULT_BASEDIR):
        service.Service.__init__(self)
        self._basedir = basedir

    def startService(self):
        log.msg('Starting Keymanager Service')
        self._container = KeymanagerContainer(self._basedir)
        self._container.service = self
        self.tokens = {}
        super(KeymanagerService, self).startService()

    # hooks

    def hook_on_new_soledad_instance(self, **kw):
        container = self._container
        user = kw['user']
        token = kw['token']
        uuid = kw['uuid']
        soledad = kw['soledad']
        if not container.get_instance(user):
            log.msg('Adding a new Keymanager instance for %s' % user)
            if not token:
                token = self.tokens.get(user)
            container.add_instance(user, token, uuid, soledad)

    def hook_on_bonafide_auth(self, **kw):
        userid = kw['username']
        provider = _get_provider_from_full_userid(userid)
        provider.callWhenReady(self._hook_on_bonafide_auth, provider, **kw)

    def _hook_on_bonafide_auth(self, provider, **kw):
        if provider.offers_service('mx'):
            userid = kw['username']
            token = kw['token']

            container = self._container
            if container.get_instance(userid):
                log.msg('Passing a new SRP Token to Keymanager: %s' % userid)
                container.set_remote_auth_token(userid, token)
            else:
                log.msg('storing the keymanager token... %s ' % token)
                self.tokens[userid] = token

    # commands

    def do_list_keys(self, userid, private=False):
        km = self._container.get_instance(userid)
        d = km.get_all_keys(private=private)
        d.addCallback(lambda keys: [dict(key) for key in keys])
        return d

    def do_export(self, userid, address, private=False):
        km = self._container.get_instance(userid)
        d = km.get_key(address, private=private, fetch_remote=False)
        d.addCallback(lambda key: dict(key))
        return d

    def do_insert(self, userid, address, rawkey, validation='Fingerprint'):
        km = self._container.get_instance(userid)
        validation = ValidationLevels.get(validation)
        d = km.put_raw_key(rawkey, address, validation=validation)
        d.addCallback(lambda _: km.get_key(address, fetch_remote=False))
        d.addCallback(lambda key: dict(key))
        return d

    @defer.inlineCallbacks
    def do_delete(self, userid, address, private=False):
        km = self._container.get_instance(userid)
        key = yield km.get_key(address, private=private, fetch_remote=False)
        km.delete_key(key)
        defer.returnValue(key.fingerprint)


class StandardMailService(service.MultiService, HookableService):
    """
    A collection of Services.

    This is the parent service, that launches 3 different services that expose
    Encrypted Mail Capabilities on specific ports:

        - SMTP service, on port 2013
        - IMAP service, on port 1984
        - The IncomingMail Service, which doesn't listen on any port, but
          watches and processes the Incoming Queue and saves the processed mail
          into the matching INBOX.
    """

    name = 'mail'

    # TODO factor out Mail Service to inside mail package.

    subscribed_to_hooks = ('on_new_keymanager_instance',)

    def __init__(self, basedir):
        self._basedir = basedir
        self._soledad_sessions = {}
        self._keymanager_sessions = {}
        self._sendmail_opts = {}
        self._service_tokens = {}
        self._active_user = None
        super(StandardMailService, self).__init__()
        self.initializeChildrenServices()

    def initializeChildrenServices(self):
        self.addService(IMAPService(self._soledad_sessions))
        self.addService(SMTPService(
            self._soledad_sessions, self._keymanager_sessions,
            self._sendmail_opts))
        # TODO adapt the service to receive soledad/keymanager sessions object.
        # See also the TODO before IncomingMailService.startInstance
        self.addService(IncomingMailService(self))

    def startService(self):
        log.msg('Starting Mail Service...')
        super(StandardMailService, self).startService()

    def stopService(self):
        super(StandardMailService, self).stopService()

    def startInstance(self, userid, soledad, keymanager):
        username, provider = userid.split('@')

        self._soledad_sessions[userid] = soledad
        self._keymanager_sessions[userid] = keymanager

        sendmail_opts = _get_sendmail_opts(self._basedir, provider, username)
        self._sendmail_opts[userid] = sendmail_opts

        incoming = self.getServiceNamed('incoming_mail')
        incoming.startInstance(userid)

        def registerToken(token):
            self._service_tokens[userid] = token
            self._active_user = userid
            return token

        d = soledad.get_or_create_service_token('mail_auth')
        d.addCallback(registerToken)
        d.addCallback(self._write_tokens_file, userid)
        return d

    def stopInstance(self):
        pass

    # hooks

    def hook_on_new_keymanager_instance(self, **kw):
        # XXX we can specify this as a waterfall, or just AND the two
        # conditions.
        userid = kw['userid']
        soledad = kw['soledad']
        keymanager = kw['keymanager']

        # TODO --- only start instance if "autostart" is True.
        self.startInstance(userid, soledad, keymanager)

    # commands

    def do_status(self):
        status = 'running' if self.running else 'disabled'
        return {'mail': status}

    def get_token(self):
        active_user = self._active_user
        if not active_user:
            return defer.succeed({'user': None})
        token = self._service_tokens.get(active_user)
        return defer.succeed({'user': active_user, 'token': token})

    def do_get_smtp_cert_path(self, userid):
        username, provider = userid.split('@')
        return _get_smtp_client_cert_path(self._basedir, provider, username)

    # access to containers

    def get_soledad_session(self, userid):
        return self._soledad_sessions.get(userid)

    def get_keymanager_session(self, userid):
        return self._keymanager_sessions.get(userid)

    def _write_tokens_file(self, token, userid):
        tokens_folder = os.path.join(tempfile.gettempdir(), "bitmask_tokens")
        if os.path.exists(tokens_folder):
            try:
                shutil.rmtree(tokens_folder)
            except OSError as e:
                log.msg("Can't remove tokens folder %s: %s"
                        % (tokens_folder, e))
                return
        os.mkdir(tokens_folder, 0700)

        tokens_path = os.path.join(tokens_folder,
                                   "%s.json" % (userid,))
        token_dict = {'mail_auth': self._service_tokens[userid]}
        with open(tokens_path, 'w') as ftokens:
            json.dump(token_dict, ftokens)


class IMAPService(service.Service):

    name = 'imap'

    def __init__(self, soledad_sessions):
        port, factory = imap.run_service(soledad_sessions)

        self._port = port
        self._factory = factory
        self._soledad_sessions = soledad_sessions
        super(IMAPService, self).__init__()

    def startService(self):
        log.msg('Starting IMAP Service')
        super(IMAPService, self).startService()

    def stopService(self):
        self._port.stopListening()
        self._factory.doStop()
        super(IMAPService, self).stopService()


class SMTPService(service.Service):

    name = 'smtp'

    def __init__(self, soledad_sessions, keymanager_sessions, sendmail_opts,
                 basedir=DEFAULT_BASEDIR):

        self._basedir = os.path.expanduser(basedir)
        port, factory = smtp.run_service(
            soledad_sessions, keymanager_sessions, sendmail_opts)
        self._port = port
        self._factory = factory
        self._soledad_sessions = soledad_sessions
        self._keymanager_sessions = keymanager_sessions
        self._sendmail_opts = sendmail_opts
        super(SMTPService, self).__init__()

    def startService(self):
        log.msg('Starting SMTP Service')
        super(SMTPService, self).startService()

    def stopService(self):
        # TODO cleanup all instances
        super(SMTPService, self).stopService()


class IncomingMailService(service.Service):

    name = 'incoming_mail'

    def __init__(self, mail_service):
        super(IncomingMailService, self).__init__()
        self._mail = mail_service
        self._instances = {}

    def startService(self):
        log.msg('Starting IncomingMail Service')
        super(IncomingMailService, self).startService()

    def stopService(self):
        super(IncomingMailService, self).stopService()

    # Individual accounts

    # TODO IncomingMail *IS* already a service.
    # I think we should better model the current Service
    # as a startInstance inside a container, and get this
    # multi-tenant service inside the leap.mail.incoming.service.
    # ... or just simply make it a multiService and set per-user
    # instances as Child of this parent.

    def startInstance(self, userid):
        soledad = self._mail.get_soledad_session(userid)
        keymanager = self._mail.get_keymanager_session(userid)

        log.msg('Starting Incoming Mail instance for %s' % userid)
        self._start_incoming_mail_instance(
            keymanager, soledad, userid)

    def stopInstance(self, userid):
        # TODO toggle offline!
        pass

    def _start_incoming_mail_instance(self, keymanager, soledad,
                                      userid, start_sync=True):

        def setUpIncomingMail(inbox):
            incoming_mail = IncomingMail(
                keymanager, soledad,
                inbox, userid,
                check_period=INCOMING_CHECK_PERIOD)
            return incoming_mail

        def registerInstance(incoming_instance):
            self._instances[userid] = incoming_instance
            if start_sync:
                incoming_instance.startService()

        acc = Account(soledad, userid)
        d = acc.callWhenReady(
            lambda _: acc.get_collection_by_mailbox(INBOX_NAME))
        d.addCallback(setUpIncomingMail)
        d.addCallback(registerInstance)
        d.addErrback(log.err)
        return d

# --------------------------------------------------------------------
#
# config utilities. should be moved to bonafide
#

SERVICES = ('soledad', 'smtp', 'eip')


Provider = namedtuple(
    'Provider', ['hostname', 'ip_address', 'location', 'port'])

SendmailOpts = namedtuple(
    'SendmailOpts', ['cert', 'key', 'hostname', 'port'])


def _get_ca_cert_path(basedir, provider):
    path = os.path.join(
        basedir, 'providers', provider, 'keys', 'ca', 'cacert.pem')
    return path


def _get_sendmail_opts(basedir, provider, username):
    cert = _get_smtp_client_cert_path(basedir, provider, username)
    key = cert
    prov = _get_provider_for_service('smtp', basedir, provider)
    hostname = prov.hostname
    port = prov.port
    opts = SendmailOpts(cert, key, hostname, port)
    return opts


def _get_smtp_client_cert_path(basedir, provider, username):
    path = os.path.join(
        basedir, 'providers', provider, 'keys', 'client', 'stmp_%s.pem' %
        username)
    return path


def _get_config_for_service(service, basedir, provider):
    if service not in SERVICES:
        raise ImproperlyConfigured('Tried to use an unknown service')

    config_path = os.path.join(
        basedir, 'providers', provider, '%s-service.json' % service)
    try:
        with open(config_path) as config:
            config = json.loads(config.read())
    except IOError:
        # FIXME might be that the provider DOES NOT offer this service!
        raise ImproperlyConfigured(
            'could not open config file %s' % config_path)
    else:
        return config


def first(xs):
    return xs[0]


def _pick_server(config, strategy=first):
    """
    Picks a server from a list of possible choices.
    The service files have a  <describe>.
    This implementation just picks the FIRST available server.
    """
    servers = config['hosts'].keys()
    choice = config['hosts'][strategy(servers)]
    return choice


def _get_subdict(d, keys):
    return {key: d.get(key) for key in keys}


def _get_provider_for_service(service, basedir, provider):

    if service not in SERVICES:
        raise ImproperlyConfigured('Tried to use an unknown service')

    config = _get_config_for_service(service, basedir, provider)
    p = _pick_server(config)
    attrs = _get_subdict(p, ('hostname', 'ip_address', 'location', 'port'))
    provider = Provider(**attrs)
    return provider


def _get_smtp_uri(basedir, provider):
    prov = _get_provider_for_service('smtp', basedir, provider)
    url = 'https://{hostname}:{port}'.format(
        hostname=prov.hostname, port=prov.port)
    return url


def _get_soledad_uri(basedir, provider):
    prov = _get_provider_for_service('soledad', basedir, provider)
    url = 'https://{hostname}:{port}'.format(
        hostname=prov.hostname, port=prov.port)
    return url