BTC Relay

BTC Relay is an Ethereum contract for Bitcoin SPV. The main functionality it provides are:

1. verification of a Bitcoin transaction
2. optionally relay the Bitcoin transaction to any Ethereum contract
3. storage of Bitcoin block headers
4. inspection of the latest Bitcoin block header stored in the contract

BTC Relay contract address and ABI:

• mainnet
• testnet Morden

The address and ABI is all that's needed to use BTC Relay, in addition to the API documentation below.

API

verifyTx(rawTransaction, transactionIndex, merkleSibling, blockHash)

Verifies the presence of a transaction on the Bitcoin blockchain, primarily that the transaction is on Bitcoin's main chain and has at least 6 confirmations.

• rawTransaction - raw bytes of the transaction
• transactionIndex - transaction's index within the block, as int256
• merkleSibling - array of the sibling hashes comprising the Merkle proof, as int256[]
• blockHash - hash of the block that contains the transaction, as int256

Returns uint256

• hash of the verified Bitcoin transaction
• 0 if rawTransaction is exactly 64 bytes in length or fails verification

Note: See examples/sampleCall.html including use of bitcoin-proof for constructing merkleSibling.

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relayTx(rawTransaction, transactionIndex, merkleSibling, blockHash, contractAddress)

Verifies a Bitcoin transaction per verifyTx() and relays the verified transaction to the specified Ethereum contract.

• rawTransaction - raw bytes of the transaction
• transactionIndex - transaction's index within the block, as int256
• merkleSibling - array of the sibling hashes comprising the Merkle proof, as int256[]
• blockHash - hash of the block that contains the transaction, as int256
• contractAddress - address of the processor contract that will receive the verified Bitcoin transaction, as int256

The processor contract at contractAddress should have a function of signature processTransaction(bytes rawTransaction, uint256 transactionHash) returns (int256) and is what will be invoked by relayTx if the transaction passes verification. For examples, see BitcoinProcessor.sol and testnetSampleRelayTx.html.

Returns int256

• value returned by the processor contract's processTransaction function
• or ERR_RELAY_VERIFY, see constants.se

Note: Callers cannot be 100% certain when an ERR_RELAY_VERIFY occurs because it may also have been returned by processTransaction(). Callers should be aware of the contract that they are relaying transactions to, and understand what the processor contract's processTransaction method returns.

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storeBlockHeader(blockHeader)

Store a single block header if it is valid, such as a valid Proof-of-Work and the previous block it reference exists.

• blockHeader - raw bytes of the block header (not the hex string, but the actual bytes).

Returns int256

• block height of the header if it was successfully stored
• 0 otherwise

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bulkStoreHeader(bytesOfHeaders, numberOfHeaders)

Store multiple block headers if they are valid.

• bytesOfHeaders - raw bytes of the block headers (not the hex string, but the actual bytes), with one following immediately the other.
• numberOfHeaders - int256 count of the number of headers being stored.

Returns int256

• block height of the last header if all block headers were successfully stored
• 0 if any of the block headers were not successfully stored

Note: See deploy/relayTest/testBulkDeploy.yaml for an example of the data for storing multiple headers. Also, to avoid exceeding Ethereum's block gas limit, a guideline is to store only 5 headers at time.

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getBlockHeader(blockHash)

Get the 80 byte block header for a given blockHash. A payment value of getFeeAmount(blockHash) must be provided in the transaction.

• blockHash - hash of the block as int256

Returns bytes

• block header, always as 80 bytes (all zeros if header does not exist)
• or 0 (as a single byte) if insufficient payment is provided

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getBlockHash(blockHeight)

Get the block hash for a given blockHeight.

• blockHeight - height of the block as int256. Minimum value is 1.

Returns int256

• block hash
• 0 if not found

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getAverageChainWork()

Returns the difference between the chainWork of the latest block and the 10th block prior.

This is provided in case an Ethereum contract wants to use the chainWork or Bitcoin network difficulty (which can be derived) as a data feed.

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getBlockchainHead(), getLastBlockHeight(), others

getBlockchainHead - returns the hash of the latest block, asint256

getLastBlockHeight - returns the block height of the latest block, as int256

See BitcoinRelayAbi.js for other APIs and testnetContractStatus.html for an example of calling some of them.

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Incentives

The following APIs are described in Incentives for Relayers below.

storeBlockWithFee(), changeFeeRecipient(), getFeeRecipient(), getFeeAmount(), getChangeRecipientFee()

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Examples

Examples for how to use BTC Relay include:

• testnetSampleCall.html for calling verifyTx including use of bitcoin-proof for constructing merkleSibling.

• mainnet sampleCall.html for calling verifyTx (very similar to above.)

• testnetSampleRelayTx.html shows relayTx relaying a Bitcoin transaction from the frontend to an Ethereum contract.

• testnetContractStatus.html for calling other basic functions.

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How to use BTC Relay

The easiest way to use BTC Relay is via relayTx because the ABI can remain on the frontend.

testnetSampleRelayTx.html shows how a Bitcoin transaction from the frontend can be passed (relayed) to an Ethereum contract.

See other examples for other ways to use BTC Relay and the docs for FAQ.

Libs/utils

Thanks to those who wrote these:

• https://github.com/rainbeam/solidity-btc-parser
• https://github.com/tjade273/BTCRelay-tools
• https://github.com/MrChico/verifyIPFS/blob/master/contracts/verifyIPFS.sol (hex to base58)

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Incentives for Relayers

Relayers are those who submit block headers to BTC Relay. To incentivize the community to be relayers, and thus allow BTC Relay to be autonomous and up-to-date with the Bitcoin blockchain, Relayers can call storeBlockWithFee. The Relayer will be the getFeeRecipient() for the block they submit, and when any transactions are verified in the block, or the header is retrieved via getBlockHeader, the Relayer will be rewarded with getFeeAmount().

To avoid a relayer R1 from setting excessive fees, it is possible for a relayer R2 to changeFeeRecipient(). R2 must specify a fee lower than what R1 specified, and pay getChangeRecipientFee() to R1, but now R2 will be the getFeeRecipient() for the block and will earn all future getFeeAmount().

With this background, here are API details for incentives.

storeBlockWithFee(blockHeader, fee)

Store a single block header (like storeBlockHeader) and set a fee that will be charged for verifications that use blockHeader.

• blockHeader - raw bytes of the block header (not the hex string, but the actual bytes).
• fee - int256 amount in wei.

Returns int256

• block height of the header if it was successfully stored
• 0 otherwise

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changeFeeRecipient(blockHash, fee, recipient)

Set the fee and recipient for a given blockHash. The call must have msg.value of at least getChangeRecipientFee(), and must also specify a fee lower than the current getFeeAmount(blockHash).

• blockHash - hash of the block as int256.
• fee - int256 amount in wei.
• recipient - int256 address of the recipient of fees.

Returns int256

• 1 if the fee and recipient were successfully set
• 0 otherwise

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getFeeRecipient(blockHash)

Get the address that receives the fees for a given blockHash.

• blockHash - hash of the block as int256.

Returns int256

• address of the recipient

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getFeeAmount(blockHash)

Get the fee amount in wei for verifications using a given blockHash.

• blockHash - hash of the block as int256.

Returns int256

• amount of the fee in wei.

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getChangeRecipientFee()

Get the amount of wei required that must be sent to BTC Relay when calling changeFeeRecipient.

Returns int256

• amount of wei

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Development

Requirements

• Serpent
• pyethereum (for tests)
• pyepm (for deployment)