P2SH (Pay To Script Hash)

As seen previously, Multi-Sig works easily in code, however, before p2sh, there was no way to ask a customer to pay to a multi-sig scriptPubKey as easily as we could hand him a BitcoinAddress.

P2SH, or Pay To Script Hash, is an easy way to represent any scriptPubKey as a simple BitcoinScriptAddress, no matter how complicated it is.

In the previous part we generated this multisig:

Key bob = new Key();
Key alice = new Key();
Key satoshi = new Key();

var scriptPubKey = PayToMultiSigTemplate
    .Instance
    .GenerateScriptPubKey(2, new[] { bob.PubKey, alice.PubKey, satoshi.PubKey });

Console.WriteLine(scriptPubKey);

2 0282213c7172e9dff8a852b436a957c1f55aa1a947f2571585870bfb12c0c15d61 036e9f73ca6929dec6926d8e319506cc4370914cd13d300e83fd9c3dfca3970efb 0324b9185ec3db2f209b620657ce0e9a792472d89911e0ac3fc1e5b5fc2ca7683d 3 OP_CHECKMULTISIG

Complicated isn’t it?

Instead, let’s see how such scriptPubKey would look like as a P2SH payment.

Key bob = new Key();
Key alice = new Key();
Key satoshi = new Key();

var paymentScript = PayToMultiSigTemplate
    .Instance
    .GenerateScriptPubKey(2, new[] { bob.PubKey, alice.PubKey, satoshi.PubKey }).PaymentScript;

Console.WriteLine(paymentScript);

OP_HASH160 57b4162e00341af0ffc5d5fab468d738b3234190 OP_EQUAL

Do you see the difference? This p2sh scriptPubKey represents the hash of my multi-sig script: redeemScript.Hash.ScriptPubKey

Since it is a hash, you can easily convert is as a base58 string BitcoinScriptAddress.

Key bob = new Key();
Key alice = new Key();
Key satoshi = new Key();

Script redeemScript =
    PayToMultiSigTemplate
    .Instance
    .GenerateScriptPubKey(2, new[] { bob.PubKey, alice.PubKey, satoshi.PubKey });
//Console.WriteLine(redeemScript.Hash.ScriptPubKey);
Console.WriteLine(redeemScript.Hash.GetAddress(Network.Main)); // 3E6RvwLNfkH6PyX3bqoVGKzrx2AqSJFhjo

Such address is understood by any client wallet. Even if such wallet does not understand what “multi sig” is.

In P2SH payment, we refer as the Redeem Script, the scriptPubKey that got hashed.

Since the payer only knows about the Hash of the RedeemScript, he does not know the Redeem Script, and so, in our case, don’t even have to know that he is sending money to a multi sig of Bob/Satoshi/Alice.

Signing such transaction is similar to what we have done before. The only difference is that you have to provide the Redeem Script when you build the Coin for the TransactionBuilder

Imagine that the multi sig P2SH receive a coin in a transaction called received.

Script redeemScript =
    PayToMultiSigTemplate
    .Instance
    .GenerateScriptPubKey(2, new[] { bob.PubKey, alice.PubKey, satoshi.PubKey });
////Console.WriteLine(redeemScript.Hash.ScriptPubKey);
//Console.WriteLine(redeemScript.Hash.GetAddress(Network.Main));

Transaction received = new Transaction();
//Pay to the script hash
received.Outputs.Add(new TxOut(Money.Coins(1.0m), redeemScript.Hash));

Warning: The payment is sent to redeemScript.Hash and not to redeemScript!

Then, once alice/bob/satoshi want to spend what they received, instead of creating a Coin they create a ScriptCoin.

//Give the redeemScript to the coin for Transaction construction
//and signing
ScriptCoin coin = received.Outputs.AsCoins().First()
                                    .ToScriptCoin(redeemScript);

The rest of the code concerning transaction generation and signing is exactly the same as in the previous part with native multi sig.