Using the TransactionBuilder

You have seen how the TransactionBuilder works when you have signed your first P2SH and multi-sig transaction.

We will see how you can harness its full power, for signing more complicated transactions.

With the TransactionBuilder you can:

  • Spend any

    • P2PK, P2PKH,

    • multi-sig,

    • P2WPK, P2WSH.

  • Spend any P2SH on the previous redeem script.

  • Spend Stealth Coin (DarkWallet).

  • Issue and transfer Colored Coins (open asset, following chapter).

  • Combine partially signed transactions.

  • Estimate the final size of an unsigned transaction and its fees.

  • Verify if a transaction is fully signed.

The goal of the TransactionBuilder is to take Coins and Keys as input, and return back a signed or partially signed transaction.

The TransactionBuilder will figure out what Coin to use and what to sign by itself.

The usage of the builder is done in four steps:

  • You gather the Coins that will be spent,

  • You gather the Keys that you own,

  • You enumerate how much Money you want to send to what scriptPubKey,

  • You build and sign the transaction,

  • Optional: you give the transaction to somebody else, then he will sign or continue to build it.

Now let’s gather some Coins. For that, let us create a fake transaction with some funds on it. Let’s say that the transaction has a P2PKH, P2PK, and multi-sig coin of Bob and Alice.

// Create a fake transaction
var bob = new Key();
var alice = new Key();
Script bobAlice =
PayToMultiSigTemplate.Instance.GenerateScriptPubKey(
2,
bob.PubKey, alice.PubKey);
var init = Network.Main.CreateTransaction();
init.Outputs.Add(Money.Coins(1m), bob.PubKey); // P2PK
init.Outputs.Add(Money.Coins(1m), alice.PubKey.Hash); // P2PKH
init.Outputs.Add(Money.Coins(1m), bobAlice);

Now let’s say they want to use the coins of this transaction to pay Satoshi.

var satoshi = new Key();

First they have to get their Coins.

Coin[] coins = init.Outputs.AsCoins().ToArray();
Coin bobCoin = coins[0];
Coin aliceCoin = coins[1];
Coin bobAliceCoin = coins[2];

Now let’s say bob wants to send 0.2 BTC, alice 0.3 BTC, and they agree to use bobAlice to send 0.5 BTC.

var builder = Network.Main.CreateTransactionBuilder();
Transaction tx = builder
.AddCoins(bobCoin)
.AddKeys(bob)
.Send(satoshi, Money.Coins(0.2m))
.SetChange(bob)
.Then()
.AddCoins(aliceCoin)
.AddKeys(alice)
.Send(satoshi, Money.Coins(0.3m))
.SetChange(alice)
.Then()
.AddCoins(bobAliceCoin)
.AddKeys(bob, alice)
.Send(satoshi, Money.Coins(0.5m))
.SetChange(bobAlice)
.SendFees(Money.Coins(0.0001m))
.BuildTransaction(sign: true);

Then you can verify it is fully signed and ready to send to the network.

Console.WriteLine(builder.Verify(tx)); // True

The nice thing about this model is that it works the same way for P2SH, P2WSH, P2SH(P2WSH), and P2SH(P2PKH) except you need to create ScriptCoin.

init = Network.Main.CreateTransaction();
init.Outputs.Add(Money.Coins(1.0m), bobAlice.Hash);
coins = init.Outputs.AsCoins().ToArray();
ScriptCoin bobAliceScriptCoin = coins[0].ToScriptCoin(bobAlice);

Then the signature:

builder = Network.Main.CreateTransactionBuilder();
tx = builder
.AddCoins(bobAliceScriptCoin)
.AddKeys(bob, alice)
.Send(satoshi, Money.Coins(0.9m))
.SetChange(bobAlice.Hash)
.SendFees(Money.Coins(0.0001m))
.BuildTransaction(true);
Console.WriteLine(builder.Verify(tx)); // True

For Stealth Coin, this is basically the same thing. Except that, if you remember our introduction on Dark Wallet, I said that you need a ScanKey to see the StealthCoin.

Let’s create darkAliceBob stealth address as in previous chapter:

Key scanKey = new Key();
BitcoinStealthAddress darkAliceBob =
new BitcoinStealthAddress
(
scanKey: scanKey.PubKey,
pubKeys: new[] { alice.PubKey, bob.PubKey },
signatureCount: 2,
bitfield: null,
network: Network.Main
);

Let’s say someone sent this transaction:

//Someone sent to darkAliceBob
init = Network.Main.CreateTransaction();
darkAliceBob
.SendTo(init, Money.Coins(1.0m));

The scanner will detect the StealthCoin:

//Get the stealth coin with the scanKey
StealthCoin stealthCoin
= StealthCoin.Find(init, darkAliceBob, scanKey);

And forward it to bob and alice, who will sign:

//Spend it
tx = builder
.AddCoins(stealthCoin)
.AddKeys(bob, alice, scanKey)
.Send(satoshi, Money.Coins(0.9m))
.SetChange(bobAlice.Hash)
.SendFees(Money.Coins(0.0001m))
.BuildTransaction(true);
Console.WriteLine(builder.Verify(tx)); // True

Note: You need the scanKey for spending a StealthCoin