Vault Liquidations

Vaults are collateralized entities in the system responsible for keeping BTC in custody. If Vaults fail to behave according to protocol rules, they face punishment through slashing of collateral. There are two types of failures: safety failures and crash failures.

Safety Failures

A safety failure occurs in two cases:

  1. Theft: a Vault is considered to have committed theft if it moves/spends BTC unauthorized by the interBTC bridge. Theft is detected and reported by Relayers via an SPV proof.

  2. Severe Undercollteralization: a Vaults drops below the LiquidationThreshold.

In both cases, the Vault’s entire BTC holdings are liquidated and its collateral is slashed - up to the SecureCollateralThreshold of the liquidated BTC value.

Consequently, the bridge offers users to burn (“Burn Event”, see liquidationRedeem) their tokens to restore the 1:1 balance between the issued (e.g., interBTC) and locked asset (e.g., BTC).

Crash Failures

If Vaults go offline and fail to execute redeem, they are:

  • Penalized (punishment fee slashed) and

  • Temporarily banned for 24 hours from accepting further issue, redeem, and replace requests.

The punishment fee is calculated based on the amount to be redeemed:

  • Punishment Fee: 10% of the failed redeem value.

Liquidations (Safety Failures)

When a Vault is liquidated, its issued and toBeIssued tokens are moved to the Liquidation Vault. In contrast, the Vault’s toBeRedeemed tokens are copied over. The Vault loses access to at least part of its backing collateral:

  • The Vault loses confiscatedCollateral = min(SECURE_THRESHOLD * (issued + toBeIssued), backingCollateral), and any leftover amount is released to its free balance.

  • Of the confiscated collateral, an amount of confiscatedCollateral * (toBeRedeemed / (issued + toBeIssued)) stays locked in the Vault, and the rest is moved to the Liquidation Vault. This is in anticipation of vaults being able to complete ongoing redeem and replace requests. When these requests succeed, the liquidated Vault’s collateral is returned. When the requests fail (i.e., the cancel calls are being made), the remaining collateral is slashed to the Liquidation Vault.

When the Liquidation Vault contains tokens, users can do a liquidation_redeem (“burn event”). Users can call this function to burn interBTC and receive DOT in return.

  • The user receives liquidationVault.collateral * (burnedTokens / (issued + toBeIssued) in its free balance.

  • At most liquidationVault.issued - liquidationVault.toBeRedeemed tokens can be burned.

Vault liquidation affects Vault interactions is the following ways:

  • Operations that increase toBeIssued or toBeRedeemed are disallowed. This means that no new issue/redeem/replace request can be made.

  • Any operation that would decrease toBeIssued or change issued on a user Vault instead changes it on the Liquidation Vault

  • Any operation that would decrease toBeRedeemed tokens on a user Vault additionally decreases it on the Liquidation Vault

Issue

  • requestIssue

    • disallowed

  • executeIssue

    • Overpayment protection is disabled; if a user transfers too many BTC, the user loses it.

  • cancelIssue

    • User’s griefing collateral is released back to the user, rather than slashed to the Vault.

Redeem

  • requestRedeem

    • disallowed

  • executeRedeem

    • Part of the Vault’s collateral is released. Amount: Vault.backingCollateral * (redeem.amount / Vault.toBeRedeemed), where toBeRedeemed is read before it is decreased

    • The premium, if any, is not transferred to the user.

  • cancelRedeem

    • Calculates slashedCollateral = Vault.backingCollateral * (redeem.amount / Vault.toBeRedeemed), where toBeRedeemed is read before it is decreased, and then:

    • If reimburse:

      • transfers slashedCollateral to user.

    • Else if not reimburse:

      • transfers slashedCollateral to Liquidation Vault.

    • Fee pool does not receive anything.

Replace

  • requestReplace, acceptReplace, withdrawReplace

    • disallowed

  • executeReplace
    • if oldVault is liquidated

      • oldVVault’s collateral is released as in executeRedeem above

    • if newVault is liquidated

      • newVault’s remaining collateral is slashed as in executeIssue above

  • cancelReplace
    • if oldVault is liquidated

      • collateral is slashed to Liquidation Vault, as in cancelRedeem above

    • if newVault is liquidated

      • griefing collateral is slashed to newVault’s free balance rather than to its backing collateral

Implementation Notes

  • In cancelIssue, when the griefing collateral is slashed, it is forwarded to the fee pool.

  • In cancelReplace, when the griefing collateral is slashed, it is forwarded to the backing collateral to the Vault. In case the Vault is liquidated, it is forwarded to the free balance of the Vault.

  • In premiumRedeem, the griefing collateral is set as 0.

  • In executeReplace, the oldVault’s griefing collateral is released, regardless of whether or not it is liquidated.