rfc/content/docs/rfcs/17/README.md

slug	title	name	status	tags	editor
17	17/WAKU-RLN-RELAY	Waku v2 RLN Relay	raw	waku-core	Sanaz Taheri <sanaz@status.im>

The current specification embodies the details of the spam-protected version of relay protocol empowered by Rate Limiting Nullifiers (RLN).

The security objective is to control the number of PubSub messages that each peer can publish per epoch where epoch is a system design parameter, regardless of the published topic.

Protocol identifier*: /vac/waku/waku-rln-relay/2.0.0-alpha1

Motivation

In open p2p messaging networks, one big problem is spam-resistance. Existing solutions, such as Whisper’s proof of work, are insufficient, especially for heterogeneous nodes. Other reputation-based approaches might not be desirable, due to issues around arbitrary exclusion and privacy.

We augment the relay protocol with a novel, light, and effective spam prevention mechanism which also suits the resource-constrained nodes.

Flow

SetUp and Registration

A peer willing to publish a message is required to register. Registration is moderated through a smart contract deployed on the Ethereum blockchain. The state of the contract contains the list of registered members. An overview of registration is illustrated in Figure 1.

For the registration, a peer creates a transaction that sends x (TODO to be specified) ETH to the contract. The peer who has the "private key" sk associated with that deposit would be able to withdraw x ETH by providing valid proof. Note that sk is initially only known by the owning peer however it may get exposed to other peers in case the owner attempts spamming the system i.e., sending more than one message per epoch.

Publishing

In order to publish at a given epoch, the publishing peer proceeds based on the regular relay protocol.
However, in order to protect against spamming, each PubSub message must carry a proof. At a high level, the proof is a zero-knowledge proof signifying that the publishing peer is a registered member, and she has not exceeded the messaging rate at the given epoch.

The proof is embedded inside the data field of the PubSub message, which, in the 11/WAKU2-RELAY protocol, corresponds to the 14/WAKU2-MESSAGE.

The proof generation relies on the knowledge of two pieces of private information i.e., sk and authPath. authPath is the information by which one can prove its membership in the group. To construct authPath, peers need to locally store a Merkle tree out of the group members public keys. Peers need to keep the tree updated with the recent state of the group.
Further inputs to the proof generation which are public are tree's root, epoch and payload||contentTopic where payload and contentTopic come from the WakuMessage. The tree root can be obtained from the locally maintained Merkle tree. The proof generation results in the following data items which are encoded inside the proof:

1. share_x
2. share_y
3. nullifier
4. zkSNARKs

The preceding values as well as the tree root (based on which the proof is generated) are encoded inside the proof as |zkSNARKs<256>|root<32>|epoch<32>|share_x<32>|share_y<32>|nullifier<32>|. The numbers enclosed in angle brackets indicate the bit length of the corresponding data item. The tuple of (nullifier, share_x, share_y) can be seen as partial disclosure of peer's sk for the intended epoch. Given two such tuples with identical nullifier but distinct share_x, share_y results in full disclosure of peer's sk and hence burning the associated deposit. Note that the nullifier is a deterministic value derived from sk and epoch therefore any two messages issued by the same peer (i.e., sing the same sk) for the same epoch are guaranteed to have identical nullifiers.

Note that the authPath of each peer depends on the current status of the registration tree (hence changes when new peers register). As such, it is recommended (and necessary for anonymity) that the publisher updates her authPath based on the latest status of the group and attempts the proof using her updated authPath.

Routing

Upon the receipt of a PubSub message, the routing peer needs to extract and parse the proof from the data field.
If the epoch attached to the message has a non-reasonable gap (TODO: the gap should be defined) with the routing peer's current epoch then the message must be dropped (this is to prevent a newly registered peer spamming the system by messaging for all the past epochs). Furthermore, the routing peers MUST check whether the proof is valid and the message is not spam. If both checks are passed successfully, then the message is relayed. If proof is invalid then the message is dropped. If spamming is detected, the publishing peer gets slashed. An overview of routing procedure is depicted in Figure 2.

Spam Detection and Slashing

In order to enable local spam detection and slashing, routing peers MUST record the nullifier, share_x, and share_y of any incoming message conditioned that it is not spam and has valid proof. To do so, the peer should follow the following steps.

1. The routing peer first verifies the zkSNARKs and drops the message if not verified.
2. Otherwise, it checks whether a message with an identical nullifier has already been relayed.
   1. If such message exists and its share_x and share_y components are different from the incoming message, then slashing takes place (if the share_x and share_y fields of the previously relayed message is identical to the incoming message, then the message is a duplicate and shall be dropped).
   2. If none found, then the message gets relayed.

An overview of slashing procedure is provided in Figure 2.

Security Considerations

---

Payloads

Payloads are protobuf messages implemented using protocol buffers v3. Nodes MAY extend the 14/WAKU2-MESSAGE with a proof field to indicate that their message is not a spam.

syntax = "proto3";

message RateLimitProof {
  bytes proof = 1;
  bytes merkle_root = 2;
  bytes epoch = 3;
  bytes share_x = 4;
  bytes share_y = 5;
  bytes nullifier = 6;
}

message WakuMessage {
  bytes payload = 1;
  string contentTopic = 2;
  uint32 version = 3;
  double timestamp = 4;
+ RateLimitProof rate_limit_proof = 21;
}

WakuMessage

rate_limit_proof holds the information required to prove that the message owner has not exceeded the message rate limit.

RateLimitProof

The proof field is an array of 256 bytes and carries the zkSNARK proof as explained in the Publishing process. The proof asserts that:

1. The message publisher is the current member of the group i.e., her/his identity commitment key is part of the membership group Merkle tree with the root merkle_root.
2. share_x and share_y are correctly computed.
3. The nullifier is constructed correctly.

Other fields of the RateLimitProof message are the public inputs to the rln circuit and used for the generation of the proof.

The merkle_root is an array of 32 bytes which holds the root of membership group Merkle tree at the time of publishing the message.

The epoch is an array of 32 bytes that represents the epoch in which the message is published.

share_x and share_y are shares of the user's identity key. These shares are created using Shamir secret sharing scheme. share_x is an array of 32 bytes and contains the hash of the WakuMessage's payload concatenated with its contentTopic.

share_y is also an array of 32 bytes which is calculated using Shamir secret sharing scheme.

The nullifier is an internal nullifier which allows specifying whether two messages are published by the same publisher during the same epoch. It is an array of 32 bytes.

Copyright

Copyright and related rights waived via CC0.

References

1. RLN documentation
2. Public inputs to the rln circuit
3. Shamir secret sharing scheme used in RLN
4. RLN internal nullifier