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TxBuilder example

Bitcoin-S features a transaction builder that constructs and signs Bitcoin transactions. Here's an example of how to use it

implicit val ec: ExecutionContext = ExecutionContext.Implicits.global
// ec: ExecutionContext = scala.concurrent.impl.ExecutionContextImpl$$anon$3@7fafdd66[Running, parallelism = 2, size = 2, active = 0, running = 0, steals = 4, tasks = 0, submissions = 0]

// generate a fresh private key that we are going to use in the scriptpubkey
val privKey = ECPrivateKey.freshPrivateKey
// privKey: ECPrivateKey = Masked(ECPrivateKeyImpl)
val pubKey = privKey.publicKey
// pubKey: ECPublicKey = ECPublicKey(026518dcf9c59495e3cdb201655e3d3c4b85b0a7ae66a7597340d1731fdf6412a9)

// this is the script that the TxBuilder is going to create a
// script signature that validly spends this scriptPubKey
val creditingSpk = P2PKHScriptPubKey(pubKey = privKey.publicKey)
// creditingSpk: P2PKHScriptPubKey = pkh(fe2e9873e70cae5803a85ee2e39ce0d597e56089)
val amount = 10000.satoshis
// amount: Satoshis = 10000 sats

// this is the UTXO we are going to be spending
val utxo =
  TransactionOutput(value = amount, scriptPubKey = creditingSpk)
// utxo: TransactionOutput = TransactionOutput(10000 sats,pkh(fe2e9873e70cae5803a85ee2e39ce0d597e56089))

// the private key that locks the funds for the script we are spending too
val destinationPrivKey = ECPrivateKey.freshPrivateKey
// destinationPrivKey: ECPrivateKey = Masked(ECPrivateKeyImpl)

// the amount we are sending -- 5000 satoshis -- to the destinationSPK
val destinationAmount = 5000.satoshis
// destinationAmount: Satoshis = 5000 sats

// the script that corresponds to destination private key, this is what is protecting the money
val destinationSPK =
  P2PKHScriptPubKey(pubKey = destinationPrivKey.publicKey)
// destinationSPK: P2PKHScriptPubKey = pkh(4a15356d7c1820e1b6bd1345eeb163c995d0a60e)

// this is where we are sending money too
// we could add more destinations here if we
// wanted to batch transactions
val destinations = {
    val destination1 = TransactionOutput(value = destinationAmount,
                                         scriptPubKey = destinationSPK)

    Vector(destination1)
}
// destinations: Vector[TransactionOutput] = Vector(TransactionOutput(5000 sats,pkh(4a15356d7c1820e1b6bd1345eeb163c995d0a60e)))

// we have to fabricate a transaction that contains the
// UTXO we are trying to spend. If this were a real blockchain
// we would need to reference the UTXO set
val creditingTx = BaseTransaction(version = Int32.one,
                                  inputs = Vector.empty,
                                  outputs = Vector(utxo),
                                  lockTime = UInt32.zero)
// creditingTx: BaseTransaction = BaseTransactionImpl(Int32Impl(1),Vector(),Vector(TransactionOutput(10000 sats,pkh(fe2e9873e70cae5803a85ee2e39ce0d597e56089))),UInt32Impl(0))

// this is the information we need from the crediting TX
// to properly "link" it in the transaction we are creating
val outPoint = TransactionOutPoint(creditingTx.txId, UInt32.zero)
// outPoint: TransactionOutPoint = TransactionOutPoint(1ca216421a31a7da897b9c1ae154b955bce3f11dbf558413f8357c5d5fc60799:0)

// this contains all the information we need to
// validly sign the UTXO above
val utxoSpendingInfo = BitcoinUTXOSpendingInfoFull(outPoint = outPoint,
                                                   output = utxo,
                                                   signers = Vector(privKey),
                                                   redeemScriptOpt = None,
                                                   scriptWitnessOpt = None,
                                                   hashType =
                                                       HashType.sigHashAll,
                                                   conditionalPath =
                                                       ConditionalPath.NoConditionsLeft)
// utxoSpendingInfo: BitcoinUTXOSpendingInfoFull = P2PKHSpendingInfo(TransactionOutPoint(1ca216421a31a7da897b9c1ae154b955bce3f11dbf558413f8357c5d5fc60799:0),10000 sats,pkh(fe2e9873e70cae5803a85ee2e39ce0d597e56089),Masked(ECPrivateKeyImpl),SIGHASH_ALL(Int32Impl(1)))

// all of the UTXO spending information, since we are only
//spending one UTXO, this is just one element
val utxos: Vector[BitcoinUTXOSpendingInfoFull] = Vector(utxoSpendingInfo)
// utxos: Vector[BitcoinUTXOSpendingInfoFull] = Vector(P2PKHSpendingInfo(TransactionOutPoint(1ca216421a31a7da897b9c1ae154b955bce3f11dbf558413f8357c5d5fc60799:0),10000 sats,pkh(fe2e9873e70cae5803a85ee2e39ce0d597e56089),Masked(ECPrivateKeyImpl),SIGHASH_ALL(Int32Impl(1))))

// this is how much we are going to pay as a fee to the network
// for this example, we are going to pay 1 satoshi per byte
val feeRate = SatoshisPerByte(1.satoshi)
// feeRate: SatoshisPerByte = SatoshisPerByte(1 sat)

val changePrivKey = ECPrivateKey.freshPrivateKey
// changePrivKey: ECPrivateKey = Masked(ECPrivateKeyImpl)
val changeSPK = P2PKHScriptPubKey(pubKey = changePrivKey.publicKey)
// changeSPK: P2PKHScriptPubKey = pkh(5cb373b8a7f857b2fe4278e63253c64e7f15ddf2)

// the network we are on, for this example we are using
// the regression test network. This is a network you control
// on your own machine
val networkParams = RegTest
// networkParams: RegTest.type = RegTest

// Yay! Now we have a TxBuilder object that we can use
// to sign the TX.
val txBuilder: BitcoinTxBuilder = {
  val builderF = BitcoinTxBuilder(
    destinations = destinations,
    utxos = utxos,
    feeRate = feeRate,
    changeSPK = changeSPK,
    network = networkParams)
  Await.result(builderF, 30.seconds)
}
// txBuilder: BitcoinTxBuilder = BitcoinTxBuilderImpl(Vector(TransactionOutput(5000 sats,pkh(4a15356d7c1820e1b6bd1345eeb163c995d0a60e))),Map(TransactionOutPoint(1ca216421a31a7da897b9c1ae154b955bce3f11dbf558413f8357c5d5fc60799:0) -> P2PKHSpendingInfo(TransactionOutPoint(1ca216421a31a7da897b9c1ae154b955bce3f11dbf558413f8357c5d5fc60799:0),10000 sats,pkh(fe2e9873e70cae5803a85ee2e39ce0d597e56089),Masked(ECPrivateKeyImpl),SIGHASH_ALL(Int32Impl(1)))),SatoshisPerByte(1 sat),pkh(5cb373b8a7f857b2fe4278e63253c64e7f15ddf2),RegTest)

// Let's finally produce a validly signed tx!
// The 'sign' method is going produce a validly signed transaction
// This is going to iterate through each of the UTXOs and use
// the corresponding UTXOSpendingInfo to produce a validly
// signed input. This UTXO has:
//   1: one input
//   2: outputs (destination and change outputs)
//   3: a fee rate of 1 satoshi/byte
val signedTx: Transaction = {
  val signF = txBuilder.sign
  Await.result(signF, 30.seconds)
}
// signedTx: Transaction = BaseTransactionImpl(Int32Impl(2),List(TransactionInputImpl(TransactionOutPoint(1ca216421a31a7da897b9c1ae154b955bce3f11dbf558413f8357c5d5fc60799:0),P2PKHScriptSignature(ECPublicKey(026518dcf9c59495e3cdb201655e3d3c4b85b0a7ae66a7597340d1731fdf6412a9), ECDigitalSignature(304402202de4b511e91dcfff7300729a56139f5a686bcac8ad0aa604613b1d969bdd5cb30220470413a053f9daae214c3408784bef86476328326a048e7d0b85a55639be3fdc01)),UInt32Impl(0))),Vector(TransactionOutput(5000 sats,pkh(4a15356d7c1820e1b6bd1345eeb163c995d0a60e)), TransactionOutput(4774 sats,pkh(5cb373b8a7f857b2fe4278e63253c64e7f15ddf2))),UInt32Impl(0))

signedTx.inputs.length
// res0: Int = 1

signedTx.outputs.length
// res1: Int = 2

//remember, you can call .hex on any bitcoin-s data structure to get the hex representation!
signedTx.hex
// res2: String = 02000000019907c65f5d7c35f8138455bf1df1e3bc55b954e11a9c7b89daa7311a4216a21c000000006a47304402202de4b511e91dcfff7300729a56139f5a686bcac8ad0aa604613b1d969bdd5cb30220470413a053f9daae214c3408784bef86476328326a048e7d0b85a55639be3fdc0121026518dcf9c59495e3cdb201655e3d3c4b85b0a7ae66a7597340d1731fdf6412a9000000000288130000000000001976a9144a15356d7c1820e1b6bd1345eeb163c995d0a60e88aca6120000000000001976a9145cb373b8a7f857b2fe4278e63253c64e7f15ddf288ac00000000