Bridge-in & Bridge-out

Bridge-in process


  • Alice: user that wants to bridge in

  • SideCar: some kind of dapp server or Botanix server that helps users with pegins and has access to an index of the bitcoin chain (like an esplora or so)

  • Minting contract on the Spiderchain EVM

  • Botanix chain validators: anyone running a Spiderchain EVM in validating mode (not applicable yet)


  1. Alice creates her ethereum public key.

  2. Alice sends [ethAddrress] to SideCar. SideCar will query the Spiderchain EVM via RPC to get the current aggregated public key i.e FROST pubkey.

  3. SideCar will send all necessary components for a Gateway Address (GA) to the Spiderchain EVM via RPC to get a GA. Note that the RPC to get a GA is purely a utility that abstracts away the complexity of generating the taproot address. To verify, SideCar can generate the same taproot address. Additinally this RPC node should be authenticated or rate-limited. Without a rate-limiting method this utlity method is subject to spam attack.

  4. The Spiderchain EVM will combine this info with the FROST pubkey to create the internal key for her taproot gateway address: I = FROST + H(FROST | ethAddress ) * G. The taproot would then be calculated using the taproot equation Q = I + TapTweak(I | S) * G. And she sends her pegin transaction to that taproot address on the Bitcoin chain. Additional tapscripts will include the safe spend path. More to come on that in a different spec.

  5. Alice sends her sats to her gateway address.

  6. After the bitcoin transaction reaches N confirmations, the SideCar takes the transaction and constructs the pegin proof by combining the tx, the block it was confirmed (merkle proof), the block height at which the tx was confirmed and the most recent block headers between the block the tx got confirmed in and the tip.

  7. The SideCar generates a Spiderchain pegin tx calling into the Minting contract providing Alice's information and the pegin proof. SideCar will manage its own Spiderchain Eth account and provide the gas for the pegin tx.

    NOTE: It's important to note that the above two steps could be done by Alice, but the helper just makes it so that Alice doesn't have to construct her pegin proof, doesn't need a Bitcoin node and doesn't need to know how to call into the Minting contract. Advanced peginners might want to do this themselves, but in this simple UX flow, some stateless helper can easily do it.

  8. The contract will check if the pegin increments Alice's pegin block height and emit a Mint event containing the pegin data on success. It will also mint the amount to Alice.

  9. Any chain validator, both when applying the tx to mempool or when validating a block, monitors for events of this type from this contract and performs a check to validate the proof. This can happen outside of EVM execution, probably right after a tx or block executions summary is communicated to some upstream layer.

NOTE: Calls into the Minting contract are supposed to somehow be free of gas, but are required to pass these additional consensus checks.

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