The current specification embodies the details of the spam-protected version of `relay` protocol empowered by Rate Limiting Nullifiers (RLN). More details on RLN can be found in [this spec]() (TODO: to link the spec).
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.
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.
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 (realized by a Merkle Tree). 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.
TODO: the interaction with the contract is subject to change depending on the final implementation
Once registered, the peer obtains the root of the tree (after the registration of the current peer) i.e., `root` as well as the authenticity path `authPath`. A peer can prove her membership using the `authPath`.
`sk` and `authPath` are secret data and MUST be permanently and locally stored by the peer.
TODO: To specify the details of protobuf messages for the interaction with the contract
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 `proofBundle`. At a high level, the `proofBundle` is a zero-knowledge proof (TODO: to clarify what a zero-knowledge proof means) signifying that the publishing peer is a registered member, and she has not exceeded the messaging rate at the given `epoch`.
The `proofBundle` is embedded inside the `data` field of the PubSub message, which, in the `relay` protocol, corresponds to the `WakuMessage`. More details on the `proofBundle`'s message fields are provided under the Protobuf section.
The proof generation relies on the knowledge of `sk` and `authPath` (that is why they should be permanently and privately stored by the owning peer). Further inputs to the proof generation are `root`, `epoch` and `payload||contentTopic` where `payload` and `contentTopic` come from the `WakuMessage` (TODO: the inputs of the proof generation may change). The proof generation results in the following data items which are included as part of the `ProofBundle`:
The tuple of (`nullifier`, `shareX`, `ShareY`) can be seen as partial disclosure of peer's `sk` for the intended `epoch`. Given two such tuples with identical `nullifier` but distinct `shareX`, `ShareY` 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 `nullifier`s.
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 tree 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 `proofBundle` 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 `proofBundle` is valid and the message is not spam. If both checks are passed successfully, then the message is relayed. If `proofBundle` 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.
In order to enable local spam detection and slashing, routing peers MUST record the `nullifier`, `shareX`, and `shareY` 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. If such message exists and its `shareX` and `shareY` components are different from the incoming message, then slashing takes place (if the `shareX` and `shareY` fields of the previously relayed message is identical to the incoming message, then the message is a duplicate and shall be dropped).
TODO: may shorten or delete the Spam detection and slashing process
TODO: may consider [validator functions](https://github.com/libp2p/specs/tree/master/pubsub#topic-validation) or [extended validators](https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#extended-validators) for the spam detection
