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Bitcoin Light Client

We have chosen to use the production ready tBTC-v2 (summa / keep-network) relay contracts and supporting libraries to support the initial development of the BOB stack. The contracts are already well-optimized for gas consumption and have been used on mainnet Ethereum for quite some time.

A specific advantage of using the Simple Payment Verification (SPV) "Light Relay" developed for tBTC is that we do not need to store all block headers from the genesis / initialization height. It uses stateless SPV proofs and provides some recency guarantee using Bitcoin's difficulty adjustment based on the latest retarget.

How does it work?​

  • The light relay is initialized to the beginning of a difficulty period (epoch)
  • A "maintainer" submits proofLength block headers before and after the retarget
  • The relay validates the chain and updates the expected difficulty for blocks in that epoch
  • A user can then submit a transaction proof in that or the last period
    • Requires header chain of at least txProofDifficultyFactor

Notes​

There was only one issue highlighted in the Least Authority audit related to the SPV client which was also identified in the interBTC (Substrate / Polkadot) code here. We can solve this issue by checking the coinbase proof as was implemented there. Since BOB will be deployed as a rollup we can make some tradeoffs with regard to gas consumption.

Using The Relay​

The code for the light relay is in src/relay/LightRelay.sol which stores the difficulty for the current and previous epoch. To update this it is possible to use retarget(headers) with proofLength * 2 block headers from Bitcoin (before and after the retarget) serialized sequentially.

Adding BOB contracts as dependency​

To add the BOB contracts to your own projects, if your project is using Foundry, you can simply run forge install bob-collective/bob to add BOB contracts as a dependency to your project.

Build the code​

To build all the contracts, run forge build.

Run the tests​

To run the built-in tests, run forge test.

Using the Contracts from TypeScript​

BOB SDK

To get the required input data for the contract, use the getBitcoinHeaders function to automatically read numBlocks from the configured Electrs REST API.

Validating Merkle Proofs (SPV)​

To check the inclusion of a specific transaction, the BitcoinTx.validateProof function can be used. See test/LightRelay.t.sol for an example. This requires the serialized transaction and merkle proof with txProofDifficultyFactor block headers to prove sufficient work has been built on top.

BOB SDK

Refer to the getBitcoinTxProof and getBitcoinTxInfo functions to encode the expected arguments.

Validating Merkle Proofs (SPV + Witness)​

info

Why might you want to do this? Under normal SPV assumptions it is not possible to prove witness data (such as Ordinal inscriptions) are included on the main chain.

To check that witness data is also included according to the relay we need to do the following:

  1. Verify coinbase is included (tx + merkle proof)
  2. Verify payment is included (tx + merkle proof)
  3. Validate witness commitment (extract root from coinbase, provide merkle proof for wtxids)

Use the WitnessTx.validateWitnessProof function to verify witness data is included. See test/WitnessTx.t.sol for an example. As above, this requires the serialized transaction and merkle proof for the coinbase transaction. To verify the witness data is included we need to encode the payment arguments differently. Check the expected structs in src/bridge/WitnessTx.sol, it requires a witnessVector and separate witness merkle root hash built using the block's "wtxids" - transactions serialized with the witness data and then hashed according to Bitcoin's double sha2.

BOB SDK

This approach is still experimental and not yet fully supported by the SDK. To construct the arguments as before use getBitcoinTxProof but set forWitness to true for getBitcoinTxInfo to get the witnessVector. To construct the witness merkle proof follow the test in sdk/test/utils.test.ts using getMerkleProof with the full raw block data.

Checking Output Amounts​

To extract the output amount BitcoinTx.getTxOutputValue can be be used to extract the amount transfered to a specific address. See test/BitcoinTx.t.sol for an example. The address is the keccak256 hash of the expected scriptPubKey.

BOB SDK

Use getBitcoinTxInfo and pass the outputVector.