SIP 65: Decentralized Circuit Breaker Source

AuthorJustin J Moses
Discussions-To<https://discordapp.com/invite/AEdUHzt>
StatusProposed
Created2020-06-17

Simple Summary

Integrate a circuit breaker into each exchange, if the price of either Synth being exchanged has changed by more than a specific percentage the Synth will be suspended.

Abstract

When any exchange is attempted, prior to executing the exchange the protocol will check to see the price difference on both the source and destination synths since the last exchange of either. If the price difference on either is over a certain threshold (SCCP modifible), then the synth will be suspended (via SIP-44) and the exchange prevented. It will then be up to the protocolDAO to investigate the issue and to activate the synth once again after reviewing the incident.

Motivation

Both of the major outages of the Synthetix protocol have been the result of an incorrect price being reported via an oracle. The first was the KRW case from the Synthetix oracle in June 2019 and was exploited by a bot. The second was the mispricing of silver (XAG) to gold (XAU) from the Chainlink pricing network in February 2020. Even with robust data integrity checking, both instances reflect situations where multiple failures occurred simultaneously and prices were published that were incorrect and exploitable.

Both of these should have immediately caused the synth to be suspended and unusable until the protocolDAO had time to investigate.

Because the debt pool is shared among all SNX stakers, and because of Synthetix’s infinite liquidity feature whereby 100% of any synth can be traded into the same USD value of any other synth, a mispriced synth could have catastropic consequences for the debt pool - inflating it to a point where the SNX collateral was not sufficient to back all outstanding debt.

This change will ensure that at the moment of exploit (trying to exchange one synth to another), a check is performed. Note that sUSD is fixed to 1 so by tackling this problem at the exchange action we capture the vast majority of the exploits (more on this below).

Specification

Overview

The Exchanger contract will be amended to include a new priceDeviationThreshold parameter (configurable by SCCP).

Every time an exchange occurs, we will check that both the source and destination synth prices have not changed by more than the threshold. We will then persist these rates as the lastExchangedRate for both synths. If there is not lastExchangeRate for either synth, the contract will lookup the last three price changes on chain (which is more gas intensive but is a rare edge-case).

The function to check and potentially suspend will be publicly available, so that anyone may invoke it without needing to attempt an exchange.

In addition, we need to handle the settlement of a trade (see SIP-37 for more details on trade settlement). Because settlement is called to process some past event (i.e. how much is owed when the price of the oracle after the waiting period ends is taken into account) - we cannot nor would not want to do any suspension during settlement. However, we also cannot leave the trade in an unsettled state and block future user exchanges. As such, we propose to waive any reclaims or rebates in the event that the amount received deviates from the amount that should have been received by more than the priceDeviationThreshold.

There is a remote possibility that an exchange gets in before a spike, fronrunning a real rate change, but by the time the waiting period expires N minutes later, a spike occurs, and the exchange is settled with no fee reclaim. As such, the protocolDAO, when investigating suspended synths via price spikes, must also look through the unsettled exchanges performed right before the spike and determine the necessary course of action before resuming the synth in question.

Finally, as the suspension is limited to the synth, even in a case of a false positive - where a synth is suspended when it shouldn’t be - the only concern is increased downtime for any user to exchange or transfer that synth. It will be on the protocolDAO to investigate and resume the synth after a thorough investigation.

Rationale

A cleaner way to solve this problem would be to suspend the system on price updates, not on exchanges. However as Synthetix partially uses decentralized price feeds from Chainlink (and is planning to migrate to them fully in the near term with SIP-36), it cannot hook into actions from contracts it reads due to the limitations of smart contract interactions.

Note: a future version of this will instead incorporate upcoming changes in Chainlink Aggregators to read circuit breaker switches from them, instead of having to rely on previous prices from exchanges.

In the meantime, checking at the moment of exchange is the optimal strategy. Unfortunately this means slightly more gas for the user, which in the current climate is a difficult decision, but necessary to prevent collapse of the system.

In order to reduce gas as much as possible, this SIP proposes to store a lastExchangeRate mapping locally on Exchanger and use that as the source of truth, rather than looking back through ExchangeRates for some amount of predetermined time to determine if an invalid price occurred.

Caveats

  1. Imposing the suspension on a user’s exchange will incur slightly more gas cost per successful exchange (~5%). This is unfortunate in the current Ethereum gas climate but necessary.

  2. If the suspension is hit, then the user who performs the action will pay the gas cost (which is much less than an exchange) and not have their exchange processed. However, this suspension action is expected to be a very rare edge case, and adding extra development work to repay them for their efforts is not worth it given the unlikeliness of this being needed. Additionally we are investigating the implementation of a keeper network to ensure that actions like this are incentivised and do not fall onto the user.

  3. If we only check exchange actions, this does not prevent SNX stakers from issuing or burning sUSD while an invalid price is on-chain. However, the primary risk is that a staker can pay off their debt using the exploit. That is, that the debt pool has reduced and they now need to burn far less sUSD to unlock their SNX. This exploit is only possible if one or more prices are returned much lower than is accurate (otherwise the debt pool would expand rather than contract), and that those synths consist of a large enough proportion of the debt pool. Additionally, since sUSD is always fixed to 1, the proportion of the debt pool denominated in sUSD will never change regardless of the other synths. As such, this extreme edge case does not seem worthy of also performing the check on issue, burn and claim actions given the additional gas costs per action.

  4. Checking the last price from an exchange isn’t a perfect solution, it may miss scenarios where the price fluctuates in and out of a reasonable band - but it’s a acceptable compromise until such time as Chainlink have integrated circuit breakers onto all their Aggregators and we have migrated to Chainlink completely.

Technical Specification

IExchanger additions:


// Views
function isSynthPricingInvalid(bytes32 currencyKey) external view returns (bool);

// Mutative functions
function suspendInvalidSynth(bytes32 currencyKey) external;

In order to save gas, each time a new exchange occurs, the price of both the src and dest synths will be stored locally in Exchanger (rather than looking back in ExchangeRates for some amount of time).

Additionally, Exchanger.exchange will be amended to perform suspendInvalidSynth(currencyKey) for either (or both) source or destination synth when isSynthPricingInvalid(currencyKey) is true.

Workflow

  • Synthetix.exchange(onBehalf)? invoked from synth src to dest by user for amount

    • For both src and dest synths:
      • Is there a previous rate for the synth?
        • Yes:
          • Is the absolute % difference in rate now compared to the previous rate >= priceDeviationThreshold?
            • Yes: ✅🔚 Settle any unsettled trades into src as per usual (if src was the breach, then settle with no reclaim or rebate - see below), then suspend the synth and return immediately.
            • No: Persist the current rate as the last
        • No:
          • For each of the last 3 rounds,
            • Is the absolute % difference in rate now compared to the rate at current round >= priceDeviationThreshold?
              • Yes: ✅🔚 Suspend the synth and return immediately.
              • No: Persist the current rate as the last
    • Then
      • ✅ Continue with exchange
  • Synthetix.settle invoked on dest for user

    • For each unsettled exchange from some synth to dest:
      • Is the absolute % difference in amountReceived compared amountShouldHaveReceived >= priceDeviationThreshold?
        • Yes: Settle the exchange with 0 reclaim and 0 rebate
        • No: Settle the exchange as per usual

Test Cases

Preconditions

  • Given the priceDeviationThreshold is set to 50%
  • And the waitingPeriodSecs is set to 180 (3 minutes)

Common cases

  • Given there was previously an exchange for sUSD (or any synth) to sETH, recording the last sETH price as 200
    • And the current market price of sETH is returning 500
      • When a user attempts to exchange sBTC (or any other synth) into sETH (or from any synth into sETH)
        • Then as 500 is more than 50% away from 200 (i.e. it’s more than 300), sETH will be suspended, and the exchange will be prevented.
    • And the current market price of sETH is returning 105
      • When a user attempts to exchange sBTC (or any other synth) into sETH
        • Then as 105 is less than 50% away from 200 (i.e. it’s above 100), then the exchange will continue, and 105 will be persisted as the last price.
          • When one minute elapses
          • And a new price for sETH is returned at 5
          • And five minutes elapses (thus ending their waiting period)
          • And a new price for sETH is returned at 100
          • And the user attempts to exchange sETH into any other synth
            • Then as the price of 5 (the price at three minutes after their exchange) is more than 50% away from 105 (i.e. below 52.5), then the settlement will process with no reclaim or rebate.

Edge cases

  • Given there is no previous exchange recorded for sETH
    • And the protocol upgrades Exchanger contract
      • And there are 10 previous rates for sETH price, all within 1% of each other
        • When a user attempts to exchange sETH for sUSD,
          • Then the current rate will be within 1% of the previous three rates, and the exchange will continue, persisting the current rate
    • And the protocol upgrades ExchangeRates contract or a Chainlink Aggregator is replaced for a price
      • And there is no previous rates for sETH
        • Then the exchange will continue, persisting the current rate

Configurable Values (Via SCCP)

  • priceDeviationThreshold the absolute % amount that a price must deviate (from it’s last exchange) to trigger the suspension.

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