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title	version	status	authors
WakuStore	2.0.0-beta2	Draft	Oskar Thorén <oskar@status.im>, Dean Eigenmann <dean@status.im>, Sanaz Taheri <sanaz@status.im>

Table of Contents
Abstract
Design Requirements
Security Consideration
- Terminology
Adversarial Model
Wire Specification
- Protobuf
Future Work
Changelog
Copyright

Abstract

This specification explains the Waku WakuStore protocol which enables querying of messages received through 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-beta2

Design Requirements

Nodes willing to provide storage service in WakuStore protocol are required to be highly available and in specific 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 that 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.

Security Consideration

The main security consideration to be taken into account while using WakuStore is to notice that the querying nodes have to reveal their topics of interest to the queried nodes hence compromising their privacy.

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 peer running the WakuStore protocol i.e., both the querying 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 peers to conduct an attack but it does so without violating protocol definitions and instructions. 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 WakuStore 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. The followings are the specifications of the Protobuf messages.

Protobuf

message Index {
  bytes digest = 1;
  float receivedTime = 2;
}

message PagingInfo {
  int64 pageSize = 1;
  Index cursor = 2;
  enum Direction {
    FORWARD = 0;
    BACKWARD = 1;
  }
  Direction direction = 3;
}

message HistoryQuery {
  repeated string topics = 2;
  optional PagingInfo pagingInfo = 3; // used for pagination
}

message HistoryResponse {
  repeated WakuMessage messages = 2;
  optional PagingInfo pagingInfo = 3; // used for pagination
}

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

Index

To perform pagination, each WakuMessage stored at a node running the WakuStore protocol is associated with a unique Index that encapsulates the following parts.

digest: a sequence of bytes representing the hash of a WakuMessage.
receivedTime: the UNIX time at which the waku message is received by the node running the WakuStore protocol.

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.

HistoryQuery

RPC call to query historical messages.

The topics field MUST indicate the list of topics to query.
PagingInfo holds the information required for pagination. Its pageSize field indicates the number of WakuMessages to be included in the corresponding HistoryResponse. If the pageSize is zero then no pagination is required. If the pageSize exceeds a threshold then the threshold value shall be used instead. In the forward pagination request, the messages field of the HistoryResponse shall contain at maximum the pageSize amount of waku messages 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 waku messages shall be returned, respectively. The queried node MAY sort the WakuMessages based on their Index, where the receivedTime 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 WakuMessages from the oldest message to the most recent one.

HistoryResponse

RPC call to respond to a HistoryQuery call.

The messages field MUST contain the messages found, these are [WakuMessage] types as defined in the corresponding specification.
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). The requester shall embed the returned cursor inside its next HistoryQuery to retrieve the next page of the waku messages. The cursor obtained from one node SHOULD NOT be used in a request to another node because the result MAY be different.

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 waku messages they are interested in). As such, no adversary in the WakuStore protocol would be able to learn which peer is interested in which topics of waku message. The current version of the WakuStore protocol does not provide anonymity for historical queries as the querying node needs to directly connect to another node in the WakuStore protocol and explicitly disclose the topics 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 whereas the queried node learns nothing about the query. Examples of such 2PC protocols are secure one-way Private Set Intersections (PSI). .

Changelog

Added the initial threat model and security analysis.

2.0.0-beta2

Released 2020-11-05

Added pagination support.

2.0.0-beta1

Released 2020-10-06

Initial draft version.

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