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11/WAKU2-RELAY

11/WAKU2-RELAY specifies a Publish/Subscribe approach to peer-to-peer messaging with a strong focus on privacy, censorship-resistance, security and scalability. Its current implementation is a minor extension of the libp2p GossipSub protocol and prescribes gossip-based dissemination. As such the scope is limited to defining a separate protocol id for 11/WAKU2-RELAY, establishing privacy and security requirements, and defining how the underlying GossipSub is to be interpreted and implemented within the Waku and cryptoeconomic domain. 11/WAKU2-RELAY should not be confused with libp2p circuit relay.

Protocol identifier: /vac/waku/relay/2.0.0

Security Requirements​

The 11/WAKU2-RELAY protocol is designed to provide the following security properties under a static Adversarial Model. Note that data confidentiality, integrity, and authenticity are currently considered out of scope for 11/WAKU2-RELAY and must be handled by higher layer protocols such as 14/WAKU2-MESSAGE.

  • Publisher-Message Unlinkability: This property indicates that no adversarial entity can link a published Message to its publisher. This feature also implies the unlinkability of the publisher to its published topic ID as the Message embodies the topic IDs.

  • Subscriber-Topic Unlinkability: This feature stands for the inability of any adversarial entity from linking a subscriber to its subscribed topic IDs.

Terminology​

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 entity running the 11/WAKU2-RELAY protocol is considered an adversary. This includes publishers, subscribers, and all the peers' direct connections. Furthermore, we consider the adversary as a passive entity that attempts to collect information from others to conduct an attack but it does so without violating protocol definitions and instructions. For example, under the passive adversarial model, no malicious subscriber hides the messages it receives from other subscribers as it is against the description of 11/WAKU2-RELAY. However, a malicious subscriber may learn which topics are subscribed to by which peers.
  • 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 other words, the communication channels are assumed to be secure.

Wire Specification​

The PubSub interface specification defines the protobuf RPC messages exchanged between peers participating in a GossipSub network. We republish these messages here for ease of reference and define how 11/WAKU2-RELAY uses and interprets each field.

Protobuf definitions​

The PubSub RPC messages are specified using protocol buffers v2

syntax = "proto2";

message RPC {
  repeated SubOpts subscriptions = 1;
  repeated Message publish = 2;

  message SubOpts {
    optional bool subscribe = 1;
    optional string topicid = 2;
  }

  message Message {
    optional string from = 1;
    optional bytes data = 2;
    optional bytes seqno = 3;
    repeated string topicIDs = 4;
    optional bytes signature = 5;
    optional bytes key = 6;
  }
}

> NOTE: The various control messages defined for GossipSub are used as specified there.

> NOTE: The TopicDescriptor is not currently used by 11/WAKU2-RELAY.

Message fields​

The Message protobuf defines the format in which content is relayed between peers. 11/WAKU2-RELAY specifies the following usage requirements for each field:

SubOpts fields​

The SubOpts protobuf defines the format in which subscription options are relayed between peers. A 11/WAKU2-RELAY node MAY decide to subscribe or unsubscribe from topics by sending updates using SubOpts. The following usage requirements apply:

  • The subscribe field MUST contain a boolean, where true indicates subscribe and false indicates unsubscribe to a topic.

  • The topicid field MUST contain the pubsub topic.

> Note: The topicid refering to pubsub topic and topicId refering to content-topic are detailed in 23/WAKU2-TOPICS.

Signature Policy​

The StrictNoSign option MUST be used, to ensure that messages are built without the signature, key, from and seqno fields. Note that this does not merely imply that these fields be empty, but that they MUST be absent from the marshalled message.

Security Analysis​

  • Publisher-Message Unlinkability: To address publisher-message unlinkability, one should remove any PII from the published message. As such, 11/WAKU2-RELAY follows the StrictNoSign policy as described in libp2p PubSub specs. As the result of the StrictNoSign policy, Messages should be built without the from, signature and key fields since each of these three fields individually counts as PII for the author of the message (one can link the creation of the message with libp2p peerId and thus indirectly with the IP address of the publisher). Note that removing identifiable information from messages cannot lead to perfect unlinkability. The direct connections of a publisher might be able to figure out which Messages belong to that publisher by analyzing its traffic. The possibility of such inference may get higher when the data field is also not encrypted by the upper-level protocols.
  • Subscriber-Topic Unlinkability: To preserve subscriber-topic unlinkability, it is recommended by 10/WAKU2 to use a single PubSub topic in the 11/WAKU2-RELAY protocol. This allows an immediate subscriber-topic unlinkability where subscribers are not re-identifiable from their subscribed topic IDs as the entire network is linked to the same topic ID. This level of unlinkability / anonymity is known as k-anonymity where k is proportional to the system size (number of participants of Waku relay protocol). However, note that 11/WAKU2-RELAY supports the use of more than one topic. In case that more than one topic id is utilized, preserving unlinkability is the responsibility of the upper-level protocols which MAY adopt partitioned topics technique to achieve K-anonymity for the subscribed peers.

Future work​

  • Economic spam resistance: In the spam-protected 11/WAKU2-RELAY protocol, no adversary can flood the system with spam messages (i.e., publishing a large number of messages in a short amount of time). Spam protection is partly provided by GossipSub v1.1 through scoring mechanism. At a high level, peers utilize a scoring function to locally score the behavior of their connections and remove peers with a low score. 11/WAKU2-RELAY aims at enabling an advanced spam protection mechanism with economic disincentives by utilizing Rate Limiting Nullifiers. In a nutshell, peers must conform to a certain message publishing rate per a system-defined epoch, otherwise, they get financially penalized for exceeding the rate. More details on this new technique can be found in 17/WAKU2-RLN-RELAY.
  • Providing Unlinkability, Integrity and Authenticity simultaneously: Integrity and authenticity are typically addressed through digital signatures and Message Authentication Code (MAC) schemes, however, the usage of digital signatures (where each signature is bound to a particular peer) contradicts with the unlinkability requirement (messages signed under a certain signature key are verifiable by a verification key that is bound to a particular publisher). As such, integrity and authenticity are missing features in 11/WAKU2-RELAY in the interest of unlinkability. In future work, advanced signature schemes like group signatures can be utilized to enable authenticity, integrity, and unlinkability simultaneously. In a group signature scheme, a member of a group can anonymously sign a message on behalf of the group as such the true signer is indistinguishable from other group members.

Copyright and related rights waived via CC0.

References​

  1. 10/WAKU2

  2. 14/WAKU2-MESSAGE

  3. 17/WAKU-RLN

  4. GossipSub v1.0

  5. GossipSub v1.1

  6. K-anonimity

  7. libp2p concepts: Publish/Subscribe

  8. libp2p protocol negotiation

  9. Partitioned topics

  10. Protocol Buffers

  11. PubSub interface for libp2p (r2, 2019-02-01)

  12. Waku v1 spec

  13. Whisper spec (EIP627)