Waku's Requirements on a Consensus Mechanism

requirements/consensus.md

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+# Waku's Requirements on a Consensus Mechanism
+
+**Note:** It is unclear at this stage whether these requirements should be fulfilled by Nomos, Status Network, both, or neither.
+
+This document outlines limitations in Waku's current reliance on smart contracts and explains how they impede the delivery of Waku's desired properties: privacy, anonymity, and censorship-resistance.
+
+## RLN Protocol Dependency
+
+Waku relies on the RLN (Rate Limit Nullifier) protocol to rate-limit message publishers in a permissionless, censorship-resistant, and privacy-preserving manner.
+This ensures unlinkability between wallet addresses and messages, as well as between separate messages.
+
+### RPC API Usage
+
+Commitments are added to a Merkle tree. The tree’s `root` is used to validate incoming messages (proof verification), while Merkle proofs enable proof generation (for sending messages).
+The `root` and `getMerkleProof` functions are available on the RLN EVM smart contract ABI. Note that a user’s RLN identity (commitment) is required when calling `getMerkleProof`.
+
+All Waku network nodes must enforce identical message validation rules to prevent network splits at the libp2p-gossipsub layer.
+This necessitates consensus on the Merkle tree’s state across the network.
+
+To achieve this, range validation is employed: a node verifies messages against the current root and a set of previous roots, accommodating proofs generated on recent prior roots.
+However, this requires Waku nodes to constantly track the smart contract’s root, which updates whenever a user registers or withdraws membership.
+On L2 networks, roots may change every few seconds, making Waku a heavy consumer of Web3 RPC APIs.
+
+**Mitigation Strategy:**  
+We intend to enhance the smart contract to expose a set of historical `root` values.
+This would reduce RPC call frequency, though scalability remains unproven.
+Event subscriptions (e.g., WebSocket) could also minimize RPC usage, but we abandoned this approach due to RPC provider instability (shift from WebSocket to HTTP long polling).
+Re-evaluation may occur during migration to Status Network, given potential closer relation with RPC providers.
+
+*Note:* These constraints primarily affect Waku Relay nodes (cloud/laptop-based).
+Edge nodes (mobile/browser) require less frequent RPC access due to lower message volume and relaxed time constraints—since they verify messages without forwarding them (unlike relays, which must validate before propagation).
+
+### Deposits and RLN Entrypoints
+
+While RLN mitigates network DoS attacks by limiting message propagation, the smart contract itself requires protection against membership influx surges. Proposed strategies include:
+
+- zk-based proof-of-humanity protocols (e.g., zkPassport)
+- On-chain heuristics (e.g., Karma, ENS, or POAP ownership)
+
+The initial implementation uses an ERC-20 deposit: users lock DAI proportional to their desired rate limit for a fixed membership period (e.g., 6–12 months). Deposits are refundable post-expiry.
+
+## Waku’s Desired Privacy Properties
+
+Waku aims to provide:
+- **Participation anonymity:** Observers can detect Waku usage by an IP but cannot identify which Waku application is in use (assuming a shared network).
+- **Receiver anonymity:** External observers cannot determine which specific messages a Waku user (IP) is interested in.
+- **Sender anonymity (work in progress):** External observers cannot identify messages sent by a specific Waku user (IP).
+- **Message relation unlinkability:** External observers cannot link messages as originating from the same user or targeting the same receiver.
+- **Wallet-to-message unlinkability:** External observers or RPC endpoints cannot associate messages with specific wallet addresses (e.g., those used for RLN membership registration).
+
+## Risks of Current Implementation
+
+The use of EVM smart contracts with Web3 RPC API endpoints introduces critical risks:
+- **RPC scalability burden:** All Waku applications—especially Relay nodes—require frequent Web3 RPC access. This affects even casual desktop users.
+- **Smart contract censorship:** Major RPC providers blocking access to the RLN contract would cause a network-wide outage.
+- **Privacy leak: IP-to-wallet linkage:** RPC interactions risk exposing unintended PII, such as correlating user IPs with wallet addresses. (Note: Waku does not conceal "IP X uses Waku" by design.)
+- **Privacy leak: Wallet-to-Waku association:** Smart contract interactions on transparent chains publicly reveal that a wallet uses Waku via RLN.
+- **Privacy leak: IP-to-RLN-credentials:** RPC interactions risk a given IP revealing their specific membership commitment. Note it is unclear how important this point is at this stage.
+
+### FURPS
+
+In summary, we could summarize Waku's needs for a consensus mechanism, due to RLN, as follows:
+
+TODO
+
+
+
+## de-MLS
+
+TODO: This section will most likely be filled at a later stage in 2025.
+We believe that de-MLS is likely to be the right technology to scale the Waku Chat SDK.
+However, its implementation is not yet planned.
+
+We already understand that message ordering for group management purposes is required,
+and hence a consensus mechanism will be required.
+It may be possible for such mechanism to be done within Waku.
+Alternatively, it needs to be out-of-band, hence the present section.