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README.md

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Content filtering

Protocol identifier*: /vac/waku/filter/2.0.0-beta1

Content filtering is a way to do message-based filtering. Currently the only content filter being applied is on contentTopic. This corresponds to topics in Waku v1.

Rationale

Unlike the store protocol for historical messages, this protocol allows for native lower latency scenarios such as instant messaging. It is thus complementary to it.

Strictly speaking, it is not just doing basic request response, but performs sender push based on receiver intent. While this can be seen as a form of light pub/sub, it is only used between two nodes in a direct fashion. Unlike the Gossip domain, this is meant for light nodes which put a premium on bandwidth. No gossiping takes place.

It is worth noting that a light node could get by with only using the store protocol to query for a recent time window, provided it is acceptable to do frequent polling.

Design Requirements

The effectiveness and reliability of the content filtering service enabled by WakuFilter protocol rely on the high availability of the full nodes as the service providers. To this end, full nodes must feature high uptime (to persistently listen and capture the network messages) as well as high Bandwidth (to provide timely message delivery to the light nodes).

Security Consideration

Note that while using WakuFilter allows light nodes to save bandwidth, it comes with a privacy cost in the sense that they need to disclose their liking topics to the full nodes to retrieve the relevant messages. Currently, anonymous subscription is not supported by the WakuFilter, however, potential solutions in this regard are sketched below in Future Work section.

Terminology

The term Personally identifiable information (PII) refers to any piece of data that can be used to uniquely identify a user. For example, the signature verification key, and the hash of one's static IP address are unique for each user and hence count as PII.

Adversarial Model

Any node running the WakuFilter protocol i.e., both the subscriber node and the queried node are considered as an adversary. Furthermore, we consider the adversary as a passive entity that attempts to collect information from other nodes to conduct an attack but it does so without violating protocol definitions and instructions. For example, under the passive adversarial model, no malicious node intentionally hides the messages matching to one's subscribed content filter as it is against the description of the WakuFilter protocol.

The following are not considered as part of the adversarial model:

An adversary with a global view of all the nodes and their connections.
An adversary that can eavesdrop on communication links between arbitrary pairs of nodes (unless the adversary is one end of the communication). In specific, the communication channels are assumed to be secure.

Protobuf

message FilterRequest {
  bool subscribe = 1;
  string topic = 2;
  repeated ContentFilter contentFilters = 3;

  message ContentFilter {
    string contentTopic = 1;
  }
}

message MessagePush {
  repeated WakuMessage messages = 1;
}

message FilterRPC {
  string requestId = 1;
  FilterRequest request = 2;
  MessagePush push = 3;
}

FilterRPC

A node MUST send all Filter messages (FilterRequest, MessagePush) wrapped inside a FilterRPC this allows the node handler to determine how to handle a message as the Waku Filter protocol is not a request response based protocol but instead a push based system.

The requestId MUST be a uniquely generated string. When a MessagePush is sent the requestId MUST match the requestId of the subscribing FilterRequest whose filters matched the message causing it to be pushed.

FilterRequest

A FilterRequest contains an optional topic, zero or more content filters and a boolean signifying whether to subscribe or unsubscribe to the given filters. True signifies 'subscribe' and false signifies 'unsubscribe'.

A node that sends the RPC with a filter request and subscribe set to 'true' requests that the filter node SHOULD notify the light requesting node of messages matching this filter.

A node that sends the RPC with a filter request and subscribe set to 'false' requests that the filter node SHOULD stop notifying the light requesting node of messages matching this filter if it is currently doing so.

The filter matches when content filter and, optionally, a topic is matched. Content filter is matched when a WakuMessage contentTopic field is the same.

A filter node SHOULD honor this request, though it MAY choose not to do so. If it chooses not to do so it MAY tell the light why. The mechanism for doing this is currently not specified. For notifying the light node a filter node sends a MessagePush message.

Since such a filter node is doing extra work for a light node, it MAY also account for usage and be selective in how much service it provides. This mechanism is currently planned but underspecified.

MessagePush

A filter node that has received a filter request SHOULD push all messages that match this filter to a light node. These WakuMessage's are likely to come from the relay protocol and be kept at the Node, but there MAY be other sources or protocols where this comes from. This is up to the consumer of the protocol.

A filter node MUST NOT send a push message for messages that have not been requested via a FilterRequest.

If a specific light node isn't connected to a filter node for some specific period of time (e.g. a TTL), then the filter node MAY choose to not push these messages to the node. This period is up to the consumer of the protocol and node implementation, though a reasonable default is one minute.

Future Work

Anonymous filter subscription: This feature guarantees that nodes can anonymously subscribe for a message filter (i.e., without revealing their exact content filter). As such, no adversary in the WakuFilter protocol would be able to link nodes to their subscribed content filers. The current version of the WakuFilter protocol does not provide anonymity as the subscribing node has a direct connection to the full node and explicitly submits its content filter to be notified about the matching messages. However, one can consider preserving anonymity through one of the following ways:

By hiding the source of the subscription i.e., anonymous communication. That is the subscribing node shall hide all its PII in its filter request e.g., its IP address. This can happen by the utilization of a proxy server or by using Tor. Note that the current structure of filter requests i.e., FilterRPC does not embody any piece of PII, otherwise, such data fields must be treated carefully to achieve anonymity.
By deploying secure 2-party computations in which the subscribing node obtains the messages matching a content filter whereas the full node learns nothing about the content filter as well as the messages pushed to the subscribing node. Examples of such 2PC protocols are Oblivious Transfers and one-way Private Set Intersections (PSI).

Changelog

Added initial threat model and security analysis.

2.0.0-beta2

Initial draft version. Released 2020-10-28

Fix: Ensure contentFilter is a repeated field, on implementation
Change: Add ability to unsubscribe from filters. Make subscribe an explicit boolean indication. Edit protobuf field order to be consistent with libp2p.

2.0.0-beta1

Initial draft version. Released 2020-10-05