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Mobile push notification service

Zulip's iOS and Android mobile apps support receiving push notifications from Zulip servers to let users know when new messages have arrived. This is an important feature to having a great experience using the Zulip mobile apps.

For technical reasons (explained below), in order to deliver mobile push notifications in the app store versions of our mobile apps, you will need to register your Zulip server with the Zulip mobile push notification service. This service will forward push notifications generated by your server to the Zulip mobile app automatically.

Starting with Zulip 1.6 for both Android and iOS, Zulip servers support forwarding push notifications to a central push notification forwarding service. You can enable this for your Zulip server as follows:

Uncomment the PUSH_NOTIFICATION_BOUNCER_URL = 'https://push.zulipchat.com' line in your /etc/zulip/settings.py file (i.e. remove the # at the start of the line), and restart your Zulip server. If you installed your Zulip server with a version older than 1.6, you'll need to add the line (it won't be there to uncomment).
If you're running Zulip 1.8.1 or newer, you can run the registration command:
```
# As root:
su zulip -c '/home/zulip/deployments/current/manage.py register_server'
# Or as the zulip user, you can skip the `su zulip -c`:
/home/zulip/deployments/current/manage.py register_server
```
This command will print the registration data it would send to the mobile push notifications service, ask you to accept the terms of service, and if you accept, register your server. If you have trouble, email support@zulipchat.com with the output of this command.
If you or your users have already set up the Zulip mobile app, you'll each need to log out and log back in again in order to start getting push notifications.

Congratulations! You've successfully setup the service.

If you'd like to verify that everything is working, you can do the following. Please follow the instructions carefully:

Configure mobile push notifications to always be sent (normally they're only sent if you're idle, which isn't ideal for this sort of testing).
On an Android device, download and login to the Zulip Android app. If you were already logged in before configuring the server, you'll need to logout first, since the app only registers for push notifications on login.
Hit the home button, so Zulip is running in the background, and then have another user send you a private message (By default, Zulip only sends push notifications for private messages sent by other users and messages mentioning you). A push notification should appear in the Android notification area.

Updating your server's registration

Your server's registration includes the server's hostname and contact email address (from EXTERNAL_HOST and ZULIP_ADMINISTRATOR in /etc/zulip/settings.py, aka the --hostname and --email options in the installer). You can update your server's registration data by running manage.py register_server again.

If you'd like to rotate your server's API key for this service (zulip_org_key), you need to use manage.py register_server --rotate-key option; it will automatically generate a new zulip_org_key and store that new key in /etc/zulip/zulip-secrets.conf.

Why this is necessary

Both Google's and Apple's push notification services have a security model that does not support mutually untrusted self-hosted servers sending push notifications to the same app. In particular, when an app is published to their respective app stores, one must compile into the app a secret corresponding to the server that will be able to publish push notifications for the app. This means that it is impossible for a single app in their stores to receive push notifications from multiple, mutually untrusted, servers.

Zulip's solution to this problem is to provide a central push notification forwarding service, which allows registered Zulip servers to send push notifications to the Zulip app indirectly (through the forwarding service).

Security and privacy

Use of the push notification bouncer is subject to the Zulipchat Terms of Service. By using push notifications, you agree to those terms.

We've designed this push notification bouncer service with security and privacy in mind:

A central design goal of the the Push Notification Service is to avoid any message content being stored or logged by the service, even in error cases.
The Push Notification Service only stores the necessary metadata for delivering the notifications to the appropriate devices, and nothing else:
- The APNS/FCM tokens needed to securely send mobile push notifications to iOS and Android devices, one per device registered to be notified by your Zulip server.
- User ID numbers generated by your Zulip server, needed to route a given notification to the appropriate set of mobile devices. These user ID numbers are are opaque to the Push Notification Service and Kandra Labs.
The Push Notification Service receives (but does not store) the contents of individual mobile push notifications:
- The numeric message ID generated by your Zulip server.
- Metadata on the message's sender (name and avatar URL).
- Metadata on the message's recipient (stream name + ID, topic, private message recipients, etc.).
- A timestamp.
- The message's content.
There's a PUSH_NOTIFICATION_REDACT_CONTENT setting available to disable any message content being sent via the push notification bouncer (i.e. message content will be replaced with ***REDACTED***). Note that this setting makes push notifications significantly less usable.

We plan to replace that setting with end-to-end encryption which would eliminate that usability tradeoff and additionally allow us to not have any access to the other details mentioned in this section.
All of the network requests (both from Zulip servers to the Push Notification Service and from the Push Notification Service to the relevant Google and Apple services) are encrypted over the wire with SSL/TLS.
The code for the push notification forwarding service is 100% open source and available as part of the Zulip server project on GitHub.
The push notification forwarding servers are professionally managed by a small team of security expert engineers.

If you have any questions about the security model, contact support@zulipchat.com.

Submitting statistics

Systems using the Mobile Push Notifications Service will, by default, submit basic usage statistics (e.g. Zulip version, number of users, number of messages sent) to the service. These statistics help the Zulip open source project understand how many people are using Zulip, and help us allocate resources towards supporting self-hosted installations.

Our use of these statistics is governed by the same ToS and Privacy Policy that covers the Mobile Push Notifications Service itself. If your organization does not want to submit these statistics, you can disable this feature at any time by setting SUBMIT_USAGE_STATISTICS=False in /etc/zulip/settings.py.

Sending push notifications directly from your server

This section documents an alternative way to send push notifications that does not involve using the Mobile Push Notifications Service at the cost of needing to compile and distribute modified versions of the Zulip mobile apps.

We don't recommend this path -- patching and shipping a production mobile app can take dozens of hours to setup even for an experienced developer, and even more time to maintain. And it doesn't provide material privacy benefits -- your organization's push notification data would still go through Apple/Google's servers, just not Kandra Labs'. Our view is the correct way to optimize for privacy is end-to-end encryption of push notifications. But in the interest of transparency, we document in this section roughly what's involved in doing so.

As we discussed above, it is impossible for a single app in their stores to receive push notifications from multiple, mutually untrusted, servers. The Mobile Push Notification Service is one of the possible solutions to this problem. The other possible solution is for an individual Zulip server's administrators to build and distribute their own copy of the Zulip mobile apps, hardcoding a key that they possess.

This solution is possible with Zulip, but it requires the server administrators to publish their own copies of the Zulip mobile apps (and there's nothing the Zulip team can do to eliminate this onerous requirement).

The main work is distributing your own copies of the Zulip mobile apps configured to use APNS/FCM keys that you generate. This is not for the faint of heart! If you haven't done this before, be warned that one can easily spend hundreds of dollars (on things like a DUNS number registration) and a week struggling through the hoops Apple requires to build and distribute an app through the Apple app store, even if you're making no code modifications to an app already present in the store (as would be the case here). The Zulip mobile app also gets frequent updates that you will have to either forgo or republish to the app stores yourself.

If you've done that work, the Zulip server configuration for sending push notifications through the new app is quite straightforward:

Create a FCM push notifications key in the Google Developer console and set android_gcm_api_key in /etc/zulip/zulip-secrets.conf to that key.
Register for a mobile push notification certificate from Apple's developer console. Set APNS_SANDBOX=False and APNS_CERT_FILE to be the path of your APNS certificate file in /etc/zulip/settings.py.
Set the APNS_TOPIC and ZULIP_IOS_APP_ID settings to the ID for your app (for the official Zulip apps, they are both org.zulip.Zulip).
Restart the Zulip server.

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