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Creating Bitcoin transactions

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Bitcoin

A Bitcoin transaction spends a number of unspent transaction outputs (UTXOs) and creates new UTXOs. In order to create a Bitcoin transaction, you need to:

  1. Get the available UTXOs corresponding to a Bitcoin address controlled by the canister using the bitcoin_get_utxos API endpoint.
  2. Calculate an appropriate transaction fee using the bitcoin_get_current_fee_percentiles API endpoint.
  3. Select a subset of the available UTXOs to spend that covers the transaction amount and fee.
  4. Create a transaction that spends the selected UTXOs and creates new UTXOs. At    least one for the recipient, and in most cases, one to collect the change.

Get available UTXOs​

The following snippet shows how to get the available UTXOs corresponding to own_address. Note that a canister can control many addresses, and each of them can have multiple UTXOs associated with it.

motoko/basic_bitcoin/src/basic_bitcoin/src/BitcoinWallet.mo
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An UTXO has the following structure:


/// An unspent transaction output.
public type Utxo = {
    outpoint : OutPoint;
    value : Satoshi;
    height : Nat32;
};

/// A reference to a transaction output.
public type OutPoint = {
    txid : Blob;
    vout : Nat32;
};


To create a transaction that sends X satoshis to a destination address, you need to select a subset of the available UTXOs that cover the amount X plus the transaction fee.

Calculate transaction fee per byte​

The transaction fee of a Bitcoin transaction is calculated based on the size of the transaction in bytes. An appropriate fee per byte can be determined by looking at the fees of recent transactions that were included in the blockchain. The following snippet shows how to estimate the fee per byte for a transaction using the bitcoin_get_current_fee_percentiles API endpoint and choosing the 50th percentile.

motoko/basic_bitcoin/src/basic_bitcoin/src/BitcoinWallet.mo
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Build the transaction​

Next, the transaction can be built. The following snippet shows a simplified version of how to build a transaction that sends amount satoshis to the dst_address, and returns the change to the own_address.

Since the fee of a transaction is based on its size, the transaction has to be built iteratively and signed with a mock signer that just adds the respective size of the signature. Each selected UTXO is used as an input for the transaction and requires a signature.

motoko/basic_bitcoin/src/basic_bitcoin/src/BitcoinWallet.mo
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