Accepted Bitcoin Transaction Format

The Functions: Parser module of BTC-Relay can in theory be used to parse arbitrary Bitcoin transactions. However, the interBTC component of the BTC Parachain restricts the format of Bitcoin transactions to ensure consistency and prevent protocol failure due to parsing errors.

As such, Bitcoin transactions for which transaction inclusion proofs are submitted to BTC-Relay as part of the in the interBTC Issue, Redeem, and Replace protocols must be P2PKH or P2WPKH transactions and follow the format below.

Case 1: OP_RETURN Transactions

The OP_RETURN field can be used to store 40 bytes in a given Bitcoin transaction. The transaction output that includes the OP_RETURN is provably unspendable. We require specific information in the OP_RETURN field to prevent replay attacks in interBTC.

Many Bitcoin wallets automatically order UTXOs. We require that the transaction contains at most three outputs, but we do not require any specific ordering of the Payment UTXO and Data UTXO. The reason behind checking for the first three outputs is that wallets like Electrum might insert the UTXOs returning part of the spent input at index 1. A merge transaction is allowed to contain any number of outputs, as long as all outputs are registered in the vault’s wallet.

Note

Please refer to the interBTC specification for more details on the Refund, Redeem and Replace protocols.

Inputs	Outputs
Arbitrary number of inputs	Index 0 to 2: Payment UTXO: P2PKH / P2WPKH output to btcAddress Bitcoin address. Data UTXO: OP_RETURN containing identifier Index 3-31: Any other UTXOs that will not be considered.

The value and recipient address (btcAddress) of the Payment UTXO and the identifier in the Data UTXO (OP_RETURN) depend on the executed interBTC protocol:

• In Refund btcAddress is the Bitcoin address of the user for the refunding process and identifier is the refundId of the RefundRequest in RefundRequests.

• In Redeem btcAddress is the Bitcoin address of the user who triggered the redeem process and identifier is the redeemId of the RedeemRequest in RedeemRequests.

• In Replace btcAddress is the Bitcoin address of the new vault, which has agreed to replace the vault which triggered the replace protocol and identifier is the replaceId of the ReplaceRequest in ReplaceRequests.

Case 2: Regular P2PKH / P2WPKH / P2SH / P2WSH Transactions

We accept regular P2PKH, P2WPKH, P2SH, and P2WSH transactions. We ensure that the recipient address is unique via the On-Chain Key Derivation Scheme.

Many Bitcoin wallets automatically order UTXOs. We require that the Payment UTXO is included within the first three indexes (index 0 - 2). We do not require any specific ordering of those outputs. The reason behind checking for the first three outputs is that wallets like Electrum might insert the UTXOs returning part of the spent input at index 1.

Note

Please refer to the interBTC specification for more details on the Issue protocol.

Inputs	Outputs
Arbitrary number of inputs	Index 0 to 2: Payment UTXO: Output to btcAddress Bitcoin address. Index 3-31: Any other UTXOs that will not be considered.

The recipient address (btcAddress) of the Payment UTXO is a address derived from the public key the vault submitted to the BTC-Parachain.