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16/PUSH-NOTIFICATION-SERVER

Version: 0.1

Status: Raw

Authors: Andrea Maria Piana [email protected]

Reason

Push notifications for iOS devices and some Android devices can only be implemented by relying on APN service for iOS or Firebase.

This is useful for Android devices that do not support foreground services or that often kill the foreground service.

iOS only allows certain kind of applications to keep a connection open when in the background, VoIP for example, which current status client does not qualify for.

Applications on iOS can also request execution time when they are in the background but it has a limited set of use cases, for example it won’t schedule any time if the application was force quit, and generally is not responsive enough to implement a push notification system.

Therefore Status provides a set of Push notification services that can be used to achieve this functionality.

Because this can’t be safely implemented in a privacy preserving manner, clients MUST be given an option to opt-in to receiving and sending push notifications. They are disabled by default.

Requirements

The party releasing the app MUST possess a certificate for the Apple Push Notification service and its has to run a gorush publicly accessible server for sending the actual notification. The party releasing the app, Status in this case, needs to run its own gorush

Components

Gorush instance

A gorush instance MUST be publicly available, this will be used only by push notification servers.

Push notification server

A push notification server used by clients to register for receiving and sending push notifications.

Registering client

A Status client that wants to receive push notifications

Sending client

A Status client that wants to send push notifications

Registering with the push notification service

A client MAY register with one or more Push Notification services of their choice.

A PNR message (Push Notification Registration) MUST be sent to the partitioned topic for the public key of the node, encrypted with this key.

The message MUST be wrapped in a ApplicationMetadataMessage with type set to PUSH_NOTIFICATION_REGISTRATION.

The marshaled protobuf payload MUST also be encrypted with AES-GCM using the Diffie–Hellman key generated from the client and server identity.

This is done in order to ensure that the extracted key from the signature will be considered invalid if it can’t decrypt the payload.

The content of the message MUST contain the following protobuf record:

message PushNotificationRegistration {
  enum TokenType {
    UNKNOWN_TOKEN_TYPE = 0;
    APN_TOKEN = 1;
    FIREBASE_TOKEN = 2;
  }
  TokenType token_type = 1;
  string device_token = 2;
  string installation_id = 3;
  string access_token = 4;
  bool enabled = 5;
  uint64 version = 6;
  repeated bytes allowed_key_list = 7;
  repeated bytes blocked_chat_list = 8;
  bool unregister = 9;
  bytes grant = 10;
  bool allow_from_contacts_only = 11;
}

A push notification server will handle the message according to the following rules:

  • it MUST extract the public key of the sender from the signature and verify that the payload can be decrypted successfully.
  • it MUST verify that token_type is supported
  • it MUST verify that device_token is non empty
  • it MUST verify that installation_id is non empty
  • it MUST verify that version is non-zero and greater than the currently stored version for the public key and installation id of the sender, if any
  • it MUST verify that grant is non empty and according to the specs
  • it MUST verify that access_token is a valid uuid

If the message can’t be decrypted, the message MUST be discarded.

If token_type is not supported, a response MUST be sent with error set to UNSUPPORTED_TOKEN_TYPE.

If token,installation_id,device_tokens,version are empty, a response MUST be sent with error set to MALFORMED_MESSAGE.

If the version is equal or less than the currently stored version, a response MUST be sent with error set to VERSION_MISMATCH.

If any other error occurs the error should be set to INTERNAL_ERROR.

If the response is successful success MUST be set to true otherwise a response MUST be sent with success set to false.

request_id should be set to the SHAKE-256 of the encrypted payload.

The response MUST be sent on the [partitioned topic][./10-waku-usage.md#partitioned-topic] of the sender and MUST not be encrypted using the secure transport to facilitate the usage of ephemeral keys.

The payload of the response is:

message PushNotificationRegistrationResponse {
  bool success = 1;
  ErrorType error = 2;
  bytes request_id = 3;

  enum ErrorType {
    UNKNOWN_ERROR_TYPE = 0;
    MALFORMED_MESSAGE = 1;
    VERSION_MISMATCH = 2;
    UNSUPPORTED_TOKEN_TYPE = 3;
    INTERNAL_ERROR = 4;
  }
}

The message MUST be wrapped in a ApplicationMetadataMessage with type set to PUSH_NOTIFICATION_REGISTRATION_RESPONSE.

A client SHOULD listen for a response sent on the [partitioned topic][./10-waku-usage.md#partitioned-topic] that the key used to register.

If success is true the client has registered successfully.

If success is false:

  • If MALFORMED_MESSAGE is returned, the request SHOULD NOT be retried without ensuring that it is correctly formed.
  • If INTERNAL_ERROR is returned, the request MAY be retried, but the client MUST backoff exponentially

A client MAY register with multiple Push Notification Servers in order to increase availability.

A client SHOULD make sure that all the notification services they registered with have the same information about their tokens.

If no response is returned the request SHOULD be considered failed and MAY be retried with the same server or a different one, but clients MUST exponentially backoff after each trial.

If the request is successful the token SHOULD be advertised as described below

Query topic

On successful registration the server MUST be listening to the topic derived from:

   0XHexEncode(Shake256(CompressedClientPublicKey))

Using the topic derivation algorithm described here and listen for client queries.

Server grant

A push notification server needs to demonstrate to a client that it was authorized by the client to send them push notifications. This is done by building a grant which is specific to a given client-server pair. The grant is built as follow:

   Signature(Keccak256(CompressedPublicKeyOfClient . CompressedPublicKeyOfServer . AccessToken), PrivateKeyOfClient)

When receiving a grant the server MUST be validate that the signature matches the registering client.

Re-registering with the push notification server

A client SHOULD re-register with the node if the APN or FIREBASE token changes.

When re-registering a client SHOULD ensure that it has the most up-to-date PushNotificationRegistration and increment version if necessary.

Once re-registered, a client SHOULD advertise the changes.

Changing options

This is handled in exactly the same way as re-registering above.

Unregistering from push notifications

To unregister a client MUST send a PushNotificationRegistration request as described above with unregister set to true, or removing their device information.

The server MUST remove all data about this user if unregistering is true, apart from the hash of the public key and the version of the last options, in order to make sure that old messages are not processed.

A client MAY unregister from a server on explicit logout if multiple chat keys are used on a single device.

Advertising a push notification server

Each user registered with one or more push notification servers SHOULD advertise periodically the push notification services that they have registered with for each device they own.

message PushNotificationQueryInfo {
  string access_token = 1;
  string installation_id = 2;
  bytes public_key = 3;
  repeated bytes allowed_user_list = 4;
  bytes grant = 5;
  uint64 version = 6;
  bytes server_public_key = 7;
}

message ContactCodeAdvertisement {
  repeated PushNotificationQueryInfo push_notification_info = 1;
}

The message MUST be wrapped in a ApplicationMetadataMessage with type set to PUSH_NOTIFICATION_QUERY_INFO.

If no filtering is done based on public keys, the access token SHOULD be included in the advertisement. Otherwise it SHOULD be left empty.

This SHOULD be advertised on the contact code topic and SHOULD be coupled with normal contact-code advertisement.

Every time a user register or re-register with a push notification service, their contact-code SHOULD be re-advertised.

Multiple servers MAY be advertised for the same installation_id for redundancy reasons.

Discovering a push notification server

To discover a push notification service for a given user, their contact code topic SHOULD be listened to. A mailserver can be queried for the specific topic to retrieve the most up-to-date contact code.

Querying the push notification server

If a token is not present in the latest advertisement for a user, the server SHOULD be queried directly.

To query a server a message:

message PushNotificationQuery {
  repeated bytes public_keys = 1;
}

The message MUST be wrapped in a ApplicationMetadataMessage with type set to PUSH_NOTIFICATION_QUERY.

MUST be sent to the server on the topic derived from the hashed public key of the key we are querying, as described above.

An ephemeral key SHOULD be used and SHOULD NOT be encrypted using the secure transport.

If the server has information about the client a response MUST be sent:

message PushNotificationQueryInfo {
  string access_token = 1;
  string installation_id = 2;
  bytes public_key = 3;
  repeated bytes allowed_user_list = 4;
  bytes grant = 5;
  uint64 version = 6;
  bytes server_public_key = 7;
}

message PushNotificationQueryResponse {
  repeated PushNotificationQueryInfo info = 1;
  bytes message_id = 2;
  bool success = 3;
}

A PushNotificationQueryResponse message MUST be wrapped in a ApplicationMetadataMessage with type set to PUSH_NOTIFICATION_QUERY_RESPONSE.

Otherwise a response MUST NOT be sent.

If allowed_key_list is not set access_token MUST be set and allowed_key_list MUST NOT be set.

If allowed_key_list is set allowed_key_list MUST be set and access_token MUST NOT be set.

If access_token is returned, the access_token SHOULD be used to send push notifications.

If allowed_key_list are returned, the client SHOULD decrypt each token by generating an AES-GCM symmetric key from the Diffie–Hellman between the target client and itself If AES decryption succeeds it will return a valid uuid which is what is used for access_token. The token SHOULD be used to send push notifications.

The response MUST be sent on the [partitioned topic][./10-waku-usage.md#partitioned-topic] of the sender and MUST not be encrypted using the secure transport to facilitate the usage of ephemeral keys.

On receiving a response a client MUST verify grant to ensure that the server has been authorized to send push notification to a given client.

Sending a push notification

When sending a push notification, only the installation_id for the devices targeted by the message SHOULD be used.

If a message is for all the user devices, all the installation_id known to the client MAY be used.

The number of devices MAY be capped in order to reduce resource consumption.

At least 3 devices SHOULD be targeted, ordered by last activity.

For any device that a token is available, or that a token is successfully queried, a push notification message SHOULD be sent to the corresponding push notification server.

message PushNotification {
  string access_token = 1;
  string chat_id = 2;
  bytes public_key = 3;
  string installation_id = 4;
  bytes message = 5;
}

message PushNotificationRequest {
  repeated PushNotification requests = 1;
  bytes message_id = 2;
}

A PushNotificationRequest message MUST be wrapped in a ApplicationMetadataMessage with type set to PUSH_NOTIFICATION_REQUEST.

Where message is the encrypted payload of the message and chat_id is the SHAKE-256 of the chat_id. message_id is the id of the message

If multiple server are available for a given push notification, only one notification MUST be sent.

If no response is received a client SHOULD wait at least 3 seconds, after which the request MAY be retried against a different server

This message SHOULD be sent using an ephemeral key.

On receiving the message, the push notification server MUST validate the access token. If the access token is valid, a notification MUST be sent to the gorush instance with the following data:

{
  "notifications": [
    {
      "tokens": ["token_a", "token_b"],
      "platform": 1,
      "message": "You have a new message",
      "data": {
        "chat_id": chat_id,
        "message": message,
        "installation_ids": [installation_id_1, installation_id_2]
      }
    }
  ]
}

Where platform is 1 for IOS and 2 for Firebase, according to the gorush documentation

A server MUST return a response message:

message PushNotificationReport {
  bool success = 1;
  ErrorType error = 2;
  enum ErrorType {
    UNKNOWN_ERROR_TYPE = 0;
    WRONG_TOKEN = 1;
    INTERNAL_ERROR = 2;
    NOT_REGISTERED = 3;
  }
  bytes public_key = 3;
  string installation_id = 4;
}

message PushNotificationResponse {
  bytes message_id = 1;
  repeated PushNotificationReport reports = 2;
}

A PushNotificationResponse message MUST be wrapped in a ApplicationMetadataMessage with type set to PUSH_NOTIFICATION_RESPONSE.

Where message_id is the message_id sent by the client.

The response MUST be sent on the [partitioned topic][./10-waku-usage.md#partitioned-topic] of the sender and MUST not be encrypted using the secure transport to facilitate the usage of ephemeral keys.

If the request is accepted success MUST be set to true. Otherwise success MUST be set to false.

If error is BAD_TOKEN the client MAY query again the server for the token and retry the request.

If error is INTERNAL_ERROR the client MAY retry the request.

Flow

Registration process

  • A client will generate a notification token through APN or Firebase.
  • The client will register with one or more push notification server of their choosing.
  • The server should process the response and respond according to the success of the operation
  • If the request is not successful it might be retried, and adjusted according to the response. A different server can be also used.
  • Once the request is successful the client should advertise the new coordinates

Sending a notification

  • A client should prepare a message and extract the targeted installation-ids
  • It should retrieve the most up to date information for a given user, either by querying a push notification server, a mailserver if not listening already to the given topic, or checking the database locally
  • It should then send a push notification according to the rules described
  • The server should then send a request to the gorush server including all the required information

Receiving a push notification

  • On receiving the notification, a client can open the right account by checking the installation_id included. The chat_id MAY be used to open the chat if present.
  • message can be decrypted and presented to the user. Otherwise messages can be pulled from the mailserver if the message_id is no already present.

Protobuf description

PushNotificationRegistration

token_type: the type of token. Currently supported is APN_TOKEN for Apple Push device_token: the actual push notification token sent by Firebase or APN and FIREBASE_TOKEN for firebase. installation_id: the installation_id of the device access_token: the access token that will be given to clients to send push notifications enabled: whether the device wants to be sent push notifications version: a monotonically increasing number identifying the current PushNotificationRegistration. Any time anything is changed in the record it MUST be increased by the client, otherwise the request will not be accepted. allowed_key_list: a list of access_token encrypted with the AES key generated by Diffie–Hellman between the publisher and the allowed contact. blocked_chat_list: a list of SHA2-256 hashes of chat ids. Any chat id in this list will not trigger a notification. unregister: whether the account should be unregistered grant: the grant for this specific server allow_from_contacts_only: whether the client only wants push notifications from contacts

Data disclosed

  • Type of device owned by a given user
  • The FIREBASE or APN push notification token
  • Hash of the chat_id a user is not interested in for notifications
  • The times a push notification record has been modified by the user
  • The number of contacts a client has, in case allowed_key_list is set

PushNotificationRegistrationResponse

success: whether the registration was successful error: the error type, if any request_id: the SHAKE-256 hash of the signature of the request preferences: the server stored preferences in case of an error

ContactCodeAdvertisement

push_notification_info: the information for each device advertised

Data disclosed

  • The chat key of the sender

PushNotificationQuery

public_keys: the SHAKE-256 of the public keys the client is interested in

Data disclosed

  • The hash of the public keys the client is interested in

PushNotificationQueryInfo

access_token: the access token used to send a push notification installation_id: the installation_id of the device associated with the access_token public_key: the SHAKE-256 of the public key associated with this access_token and installation_id allowed_key_list: a list of encrypted access tokens to be returned to the client in case there’s any filtering on public keys in place. grant: the grant used to register with this server. version: the version of the registration on the server. server_public_key: the compressed public key of the server.

PushNotificationQueryResponse

info: a list of PushNotificationQueryInfo. message_id: the message id of the PushNotificationQueryInfo the server is replying to. success: whether the query was successful.

PushNotification

access_token: the access token used to send a push notification. chat_id: the SHAKE-256 of the chat_id. public_key: the SHAKE-256 of the compressed public key of the receiving client. installation_id: the installation id of the receiving client. message: the encrypted message that is being notified on.

Data disclosed

  • The SHAKE-256 of the chat_id the notification is to be sent for
  • the cypher text of the message

PushNotificationRequest

requests: a list of PushNotification message_id: the status message id

Data disclosed

  • The status message id for which the notification is for

PushNotificationResponse

message_id: the message_id being notified on. reports: a list of PushNotificationReport

PushNotificationReport

success: whether the push notification was successful. error: the type of the error in case of failure. public_key: the public key of the user being notified. installation_id: the installation id of the user being notified.

Anonymous mode of operations

An anonymous mode of operations MAY be provided by the client, where the responsibility of propagating information about the user is left to the client, in order to preserve privacy.

A client in anonymous mode can register with the server using a key different from their chat key. This will hide their real chat key.

This public key is effectively a secret and SHOULD only be disclosed to clients that you the user wants to be notified by.

A client MAY advertise the access token on the contact-code topic of the key generated. A client MAY share their public key through contact updates

A client receiving a push notification public key SHOULD listen to the contact code topic of the push notification public key for updates.

The method described above effectively does not share the identity of the sender nor the receiver to the server, but MAY result in missing push notifications as the propagation of the secret is left to the client.

This can be mitigated by device syncing, but not completely addressed.

Security considerations

If no anonymous mode is used, when registering with a push notification service a client discloses:

  • The chat key
  • The devices that will receive notifications

A client MAY disclose:

  • The hash of the chat_ids they want to filter out

When running in anonymous mode, the client’s chat key is not disclosed.

When querying a push notification server a client will disclose:

  • That it is interested in sending push notification to another client, but the querying client’s chat key is not disclosed

When sending a push notification a client discloses:

  • The SHAKE-256 of the chat id

review process. Point can be integrated, suggestion welcome.

FAQ

Why having ACL done at the server side and not the client?

We looked into silent notification for IOS (android has no equivalent) but can’t be used as it’s expected to receive maximum 2/3 per hour, so not our use case. There are also issue when the user force quit the app.

Why using an access token?

The access token is used to decouple the requesting information from the user from actually sending the push notification.

Some ACL is necessary otherwise it would be too easy to spam users (it’s still fairly trivial, but with this method you could allow only contacts to send you push notifications).

Therefore your identity must be revealed to the server either when sending or querying.

By using an access token we increase deniability, as the server would know who requested the token but not necessarily who sent a push notification. Correlation between the two can be trivial in some cases.

This also allows a mode of use as we had before, where the server does not propagate info at all, and it’s left to the user to propagate the token, through contact requests for example.

Why advertise with the bundle?

Advertising with the bundle allows us to piggy-back on an already implemented behavior and save some bandwidth in cases where is not filtering by public keys

What’s the bandwidth impact for this?

Generally speaking, for each 1-to-1 message and group chat message you will sending 1 and number of participants push notifications. This can be optimized if multiple users are using the same push notification server. Queries have also a bandwidth impact but they are made only when actually needed

What’s the information disclosed?

The data disclosed with each message sent by the client is above, but for a summary:

When you register with a push notification service you may disclose:

1) Your chat key 2) Which devices you have 3) The hash of the chat_ids you want to filter out 4) The hash of the public keys you are interested/not interested in

When you query a notification service you may disclose:

1) Your chat key 2) The fact that you are interested in sending push notification to a given user

Effectively this is fairly revealing if the user has a whitelist implemented. Therefore sending notification should be optional.

What prevents a user from generating a random key and getting an access token and spamming?

Nothing really, that’s the same as the status app as a whole. the only mechanism that prevents this is using a white-list as described above, but that implies disclosing your true identity to the push notification server.

Why not 0-knowledge proofs/quantum computing

We start simple, we can iterate

How to handle backward/forward compatibility

Most of the request have a target, so protocol negotiation can happen. We cannot negotiated the advertisement as that’s effectively a broadcast, but those info should not change and we can always accrete the message.

Why ack_key?

That’s necessary to avoid duplicated push notifications and allow for the retry in case the notification is not successful.

Deduplication of the push notification is done on the client side, to reduce a bit of centralization and also in order not to have to modify gorush.

Can I run my own node?

Sure, the methods allow that

Can I register with multiple nodes for redundancy

Yep

What does my node disclose?

Your node will disclose the IP address is running from, as it makes an HTTP post to gorush. A waku adapter could be used, but please not now.

Does this have high-reliability requirements?

The gorush server yes, no way around it.

The rest, kind of, at least one node having your token needs to be up for you to receive notifications. But you can register with multiple servers (desktop, status, etc) if that’s a concern.

Can someone else (i.e not status) run this?

Push notification servers can be run by anyone. Gorush can be run by anyone I take, but we are in charge of the certificate, so they would not be able to notify status-clients.

Changelog

Version 0.1

Released

  • Initial version

Copyright and related rights waived via CC0.