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Railgun

Trust Me, I’m Reliable: Railgun Broadcasters and the Self-Scored Scoreboard

Manuel Fernandez

• 3 min read
Trust Me, I’m Reliable: Railgun Broadcasters and the Self-Scored Scoreboard

Railgun promises private transfers on public chains (Currently Ethereum, Arbitrum, BSC, and Polygon) using Zero-Knowledge proofs—neat math, fewer prying eyes. In this post I zoom in on a smaller but telling piece of the stack: public broadcasters. They relay your encrypted TX so your public 0x address doesn’t dox you. That’s the idea. The funny part? Their ““reliability”” score is self-reported. Yes, the scoreboard lets players keep their own score. Below I show how that works in code and why a broadcaster can crown itself king with a one-line change.

Before starting, it’s important to understand the protocol’s basic architecture and the broadcaster component.
When someone uses the Railgun protocol, they create a 0zk address, which is later used to privately send assets between 0zk addresses or to transfer assets from a 0zk (private) address to a 0x (public) address. For any of these operations, a user can either self-broadcast the transaction using their public 0x address or let a broadcaster submit the transaction on their behalf (a “Public Broadcaster”), at the cost of a small fee paid to the broadcaster:

Broadcasters are the recommended move—especially for 0zk → 0x exits—because they add a privacy buffer between your wallet and the chain. Pick Public Broadcaster and the wallet shows a lineup ordered by fee ratio; if two tie on fees, the tiebreaker is their Reliability score:

Reliability ranges from 0 to 1 . It starts at 0 and ticks up with each successfully submitted TX; errors knock it down. To ground it, here’s the current broadcaster flow at a glance:

High level. Your wallet (Railway or Terminal) runs the Railgun SDK, which talks to core.rootedinprivacy.com—a gateway into the Waku pub/sub network. Through that pipe, the wallet fetches the list of public broadcasters along with each one’s fee ratio and “reliability” score.

When you transfer or withdraw assets using a broadcaster, the wallet signs the TX, encrypts it to the chosen broadcaster’s public key, and publishes it on Waku. The broadcaster picks it up and submits it on-chain.

The wobbly part. Where does “reliability” come from? From the broadcaster itself. In /src/server/waku-broadcaster/waku-broadcaster.ts, the broadcastFeesForChain() function grabs the reliability via getReliabilityRatio(chain), which is persisted in the operator’s local DB. Self-scored reputation—what could possibly go wrong?

async broadcastFeesForChain(chain: BroadcasterChain): Promise<void> {
    // Map from tokenAddress to BigNumber hex string
    const { fees, feeCacheID } = getAllUnitTokenFeesForChain(chain);
    const reliability = await getReliabilityRatio(chain);
    const feeBroadcastData = await promiseTimeout(
      this.createFeeBroadcastData(fees, feeCacheID, chain, reliability),
      3 * 1000,
    )
    ...omitted for brevity...
}

Swap the line that calls the getReliabilityRatio(chain) function for a constant, and you’ve crowned yourself. For example—hardcode 99:

async broadcastFeesForChain(chain: BroadcasterChain): Promise<void> {
    // Map from tokenAddress to BigNumber hex string
    const { fees, feeCacheID } = getAllUnitTokenFeesForChain(chain);
    
    const reliability = 99; // Modified reliability
    
    const feeBroadcastData = await promiseTimeout(
      this.createFeeBroadcastData(fees, feeCacheID, chain, reliability),
      3 * 1000,
    )
    ...omitted for brevity...
}

Then I spun up my broadcaster. Railway wallet happily displayed Reliability: 99 (even outside the intended 0-1 range) because the reputation source is the broadcaster itself.

That’s the bug in a byte. A reputation system that trusts the subject of the reputation. Practically, it means a broadcaster can float to the top of the fee list without earning it. A healthier design would derive reliability from observer data instead.
This one is low impact, but it’s a canary: never outsource trust to the party being judged. I’ve got a couple of sharper issues coming next—same tone. Stay tuned.

Manuel

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