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10.6: Spending a P2SH Transaction

ℹ️ NOTE: This is a draft in progress, so that I can get some feedback from early reviewers. It is not yet ready for learning.

Before we close out this overview of P2SH transactions, we're going to touch upon how to spend them. This section is mainly an overview, referring back to a previous section where we already spent a P2SH transaction.

Use the Redeem Script

As we saw in §6.2: Spending a Transaction to a Multisig, spending a P2SH transaction is all about having that serialized version of the locking script, the so-called redeemScript. So, the first step in being able to spend a P2SH transaction is making sure that you save the redeemScript before you give out the P2SH address to everyone.

Collect Your Variables

Because P2SH addresses other than the special multisig and nested Segwit addresses aren't integrated into bitcoin-cli there will be no short-cuts for P2SH spending like you saw in §6.3: Sending an Automated Multisig. You're going to need to collect all the more complex variables on your own!

This means that you need to collect:

  • The hex of the scriptPubKey for the transaction you're spending
  • The serialized redeemScript
  • Any private keys, since you'll be signing by hand
  • All of the regular txids, vouts, and addresses that you'd need

Create the Transaction

As we saw in §6.2, the creation of a transaction is pretty standard:

$ rawtxhex=$(bitcoin-cli -named createrawtransaction inputs='''[ { "txid": "'$utxo_txid'", "vout": '$utxo_vout' } ]''' outputs='''{ "'$recipient'": 0.00005}''')
$ echo $rawtxhex
020000000121654fa95d5a268abf96427e3292baed6c9f6d16ed9e80511070f954883864b10000000000ffffffff0188130000000000001600142c48d3401f6abed74f52df3f795c644b4398844600000000

However, signing requires entering extra information for the (1) scriptPubKey; (2) the redeemScript; and (3) any required private keys.

Here's the example of doing so for that P2SH-embedded multisig in §6.2:

$ bitcoin-cli -named signrawtransactionwithkey hexstring=$rawtxhex prevtxs='''[ { "txid": "'$utxo_txid'", "vout": '$utxo_vout', "scriptPubKey": "'$utxo_spk'", "redeemScript": "'$redeem_script'" } ]''' privkeys='["cNPhhGjatADfhLD5gLfrR2JZKDE99Mn26NCbERsvnr24B3PcSbtR"]'

With any other sort of P2SH you're going to be including a different redeemscript, but otherwise the practice is exactly the same. The only difference is that after two chapters of work on Scripts you now understand what the scriptPubKey is and what the redeemScript is, so hopefully what were mysterious elements four chapters ago are now old hat.

Summary: Spending a Transaction with a Bitcoin Script

You already spent a P2SH back in Chapter 6, when you resent a multsig transaction the hard way, which required lining up the scriptPubKey and redeemScript information. Now you know that the scriptPubKey is a standardized P2SH locking script, while the redeemScript matches a hash in that locking script and that you need to be able to run it with the proper variables to receive a True result. But other than knowing more, there's nothing new in spending a P2SH transaction, because you already did it!

What's Next?

Advance through "Bitcoin Scripting" with Chapter Eleven: Empowering Timelock with Bitcoin Scripts.