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README.md

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Abstract

This specification explains the 13/WAKU2-STORE protocol which enables querying of messages received through the relay protocol and stored by other nodes. It also supports pagination for more efficient querying of historical messages.

Protocol identifier*: /vac/waku/store/2.0.0-beta4

Terminology

The term PII, Personally Identifiable Information, 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.

Design Requirements

The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in RFC2119.

Nodes willing to provide the storage service using 13/WAKU2-STORE protocol, SHOULD provide a complete and full view of message history. As such, they are required to be highly available and specifically have a high uptime to consistently receive and store network messages. The high uptime requirement makes sure that no message is missed out hence a complete and intact view of the message history is delivered to the querying nodes. Nevertheless, in case storage provider nodes cannot afford high availability, the querying nodes may retrieve the historical messages from multiple sources to achieve a full and intact view of the past.

The concept of ephemeral messages introduced in 14/WAKU2-MESSAGE affects 13/WAKU2-STORE as well. Nodes running 13/WAKU2-STORE SHOULD support ephemeral messages as specified in 14/WAKU2-MESSAGE. Nodes running 13/WAKU2-STORE SHOULD NOT store messages with the ephemeral flag set to true.

Adversarial Model

Any peer running the 13/WAKU2-STORE protocol, i.e. both the querying node and the queried node, are considered as an adversary. Furthermore, we currently consider the adversary as a passive entity that attempts to collect information from other peers to conduct an attack but it does so without violating protocol definitions and instructions. As we evolve the protocol, further adversarial models will be considered. For example, under the passive adversarial model, no malicious node hides or lies about the history of messages as it is against the description of the 13/WAKU2-STORE protocol.

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

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

Wire Specification

Peers communicate with each other using a request / response API. The messages sent are Protobuf RPC messages which are implemented using protocol buffers v3. The following are the specifications of the Protobuf messages.

Payloads

syntax = "proto3";

message Index {
  bytes digest = 1;
  sint64 receiverTime = 2;
  sint64 senderTime = 3;
  string pubsubTopic = 4;
}

message PagingInfo {
  uint64 pageSize = 1;
  Index cursor = 2;
  enum Direction {
    BACKWARD = 0;
    FORWARD = 1;
  }
  Direction direction = 3;
}

message ContentFilter {
  string contentTopic = 1;
}

message HistoryQuery {
  // the first field is reserved for future use
  string pubsubtopic = 2;
  repeated ContentFilter contentFilters = 3;
  PagingInfo pagingInfo = 4;
}

message HistoryResponse {
  // the first field is reserved for future use
  repeated WakuMessage messages = 2;
  PagingInfo pagingInfo = 3;
  enum Error {
    NONE = 0;
    INVALID_CURSOR = 1;
  }
  Error error = 4;
}

message HistoryRPC {
  string request_id = 1;
  HistoryQuery query = 2;
  HistoryResponse response = 3;
}

Index

To perform pagination, each WakuMessage stored at a node running the 13/WAKU2-STORE protocol is associated with a unique Index that encapsulates the following parts.

digest: a sequence of bytes representing the SHA256 hash of a WakuMessage. The hash is computed over the concatenation of contentTopic and payload fields of a WakuMessage (see 14/WAKU2-MESSAGE).
receiverTime: the UNIX time in nanoseconds at which the WakuMessage is received by the receiving node.
senderTime: the UNIX time in nanoseconds at which the WakuMessage is generated by its sender.
pubsubTopic: the pubsub topic on which the WakuMessage is received.

PagingInfo

PagingInfo holds the information required for pagination. It consists of the following components.

pageSize: A positive integer indicating the number of queried WakuMessages in a HistoryQuery (or retrieved WakuMessages in a HistoryResponse).
cursor: holds the Index of a WakuMessage.
direction: indicates the direction of paging which can be either FORWARD or BACKWARD.

ContentFilter

ContentFilter carries the information required for filtering historical messages.

contentTopic represents the content topic of the queried historical WakuMessage. This field maps to the contentTopic field of the 14/WAKU2-MESSAGE.

HistoryQuery

RPC call to query historical messages.

The pubsubTopic field MUST indicate the pubsub topic of the historical messages to be retrieved. This field denotes the pubsub topic on which WakuMessages are published. This field maps to topicIDs field of Message in 11/WAKU2-RELAY. Leaving this field empty means no filter on the pubsub topic of message history is requested. This field SHOULD be left empty in order to retrieve the historical WakuMessage regardless of the pubsub topics on which they are published.
The contentFilters field MUST indicate the list of content filters based on which the historical messages are to be retrieved. Leaving this field empty means no filter on the content topic of message history is required. This field SHOULD be left empty in order to retrieve historical WakuMessage regardless of their content topics.
PagingInfo holds the information required for pagination.
Its pageSize field indicates the number of WakuMessages to be included in the corresponding HistoryResponse. It is RECOMMENDED that the queried node defines a maximum page size internally. If the querying node leaves the pageSize unspecified, or if the pageSize exceeds the maximum page size, the queried node SHOULD auto-paginate the HistoryResponse to no more than the configured maximum page size. This allows mitigation of long response time for HistoryQuery. In the forward pagination request, the messages field of the HistoryResponse SHALL contain, at maximum, the pageSize amount of WakuMessage whose Index values are larger than the given cursor (and vise versa for the backward pagination). Note that the cursor of a HistoryQuery MAY be empty (e.g., for the initial query), as such, and depending on whether the direction is BACKWARD or FORWARD the last or the first pageSize WakuMessage SHALL be returned, respectively.

Sorting Messages

The queried node MUST sort the WakuMessage based on their Index, where the senderTime constitutes the most significant part and the digest comes next, and then perform pagination on the sorted result. As such, the retrieved page contains an ordered list of WakuMessage from the oldest messages to the most recent one. Alternatively, the receiverTime (instead of senderTime ) MAY be used to sort messages during the paging process. However, it is RECOMMENDED the use of the senderTime for sorting as it is invariant and consistent across all the nodes. This has the benefit of cursor reusability i.e., a cursor obtained from one node can be consistently used to query from another node. However, this cursor reusability does not hold when the receiverTime is utilized as the receiver time is affected by the network delay and nodes' clock asynchrony.

HistoryResponse

RPC call to respond to a HistoryQuery call.

The messages field MUST contain the messages found, these are 14/WAKU2-MESSAGE types.
PagingInfo holds the paging information based on which the querying node can resume its further history queries. The pageSize indicates the number of returned Waku messages (i.e., the number of messages included in the messages field of HistoryResponse). The direction is the same direction as in the corresponding HistoryQuery. In the forward pagination, the cursor holds the Index of the last message in the HistoryResponse messages (and the first message in the backward paging). Regardless of the paging direction, the retrieved messages are always sorted in ascending order based on their timestamp as explained in the sorting messages section, that is, from the oldest to the most recent. The requester SHALL embed the returned cursor inside its next HistoryQuery to retrieve the next page of the 14/WAKU2-MESSAGE.
The cursor obtained from one node SHOULD NOT be used in a request to another node because the result may be different.
The error field contains information about any error that has occurred while processing the corresponding HistoryQuery. NONE stands for no error. This is also the default value. INVALID_CURSOR means that the cursor field of HistoryQuery does not match with the Index of any of the WakuMessage persisted by the queried node.

Security Consideration

The main security consideration to take into account while using this protocol is that a querying node have to reveal their content filters of interest to the queried node, hence potentially compromising their privacy.

Future Work

Anonymous query: This feature guarantees that nodes can anonymously query historical messages from other nodes i.e., without disclosing the exact topics of 14/WAKU2-MESSAGE they are interested in.
As such, no adversary in the 13/WAKU2-STORE protocol would be able to learn which peer is interested in which content filters i.e., content topics of 14/WAKU2-MESSAGE. The current version of the 13/WAKU2-STORE protocol does not provide anonymity for historical queries, as the querying node needs to directly connect to another node in the 13/WAKU2-STORE protocol and explicitly disclose the content filters of its interest to retrieve the corresponding messages. However, one can consider preserving anonymity through one of the following ways:
- By hiding the source of the request i.e., anonymous communication. That is the querying node shall hide all its PII in its history 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 historical requests does not embody any piece of PII, otherwise, such data fields must be treated carefully to achieve query anonymity.
- By deploying secure 2-party computations in which the querying node obtains the historical messages of a certain topic, the queried node learns nothing about the query. Examples of such 2PC protocols are secure one-way Private Set Intersections (PSI).

Robust and verifiable timestamps: Messages timestamp is a way to show that the message existed prior to some point in time. However, the lack of timestamp verifiability can create room for a range of attacks, including injecting messages with invalid timestamps pointing to the far future.
To better understand the attack, consider a store node whose current clock shows 2021-01-01 00:00:30 (and assume all the other nodes have a synchronized clocks +-20seconds). The store node already has a list of messages, (m1,2021-01-01 00:00:00), (m2,2021-01-01 00:00:01), ..., (m10:2021-01-01 00:00:20), that are sorted based on their timestamp.
An attacker sends a message with an arbitrary large timestamp e.g., 10 hours ahead of the correct clock (m',2021-01-01 10:00:30). The store node places m' at the end of the list, (m1,2021-01-01 00:00:00), (m2,2021-01-01 00:00:01), ..., (m10:2021-01-01 00:00:20), (m',2021-01-01 10:00:30). Now another message arrives with a valid timestamp e.g., (m11, 2021-01-01 00:00:45). However, since its timestamp precedes the malicious message m', it gets placed before m' in the list i.e., (m1,2021-01-01 00:00:00), (m2,2021-01-01 00:00:01), ..., (m10:2021-01-01 00:00:20), (m11, 2021-01-01 00:00:45), (m',2021-01-01 10:00:30). In fact, for the next 10 hours, m' will always be considered as the most recent message and served as the last message to the querying nodes irrespective of how many other messages arrive afterward.

A robust and verifiable timestamp allows the receiver of a message to verify that a message has been generated prior to the claimed timestamp. One solution is the use of open timestamps e.g., block height in Blockchain-based timestamps. That is, messages contain the most recent block height perceived by their senders at the time of message generation. This proves accuracy within a range of minutes (e.g., in Bitcoin blockchain) or seconds (e.g., in Ethereum 2.0) from the time of origination.