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Charms

Cast a Spell

Now that we have implemented an app, we’re ready to cast a spell onto a real Bitcoin transaction. We’re going to use bitcoin-cli to interact with the Bitcoin network.

Quick note: Check if you have pre-requisites installed.

Important: we assume bitcoin-cli is aliased as b:

Terminal window
alias b=bitcoin-cli

Using an app

We’ve just tested the app with an NFT-minting spell. Let’s use it on Bitcoin testnet4 (we have a node set up in pre-requisites).

Prepare:

Terminal window
app_bins=$(charms app build)


# pick from the output of `bitcoin-cli listunspent`
# should NOT be the same as the one you used for minting the NFT
funding_utxo="2d6d1603f0738085f2035d496baf2b91a639d204b414ea180beb417a3e09f84e:1"


export RUST_LOG=info

Run:

Terminal window
cat ./spells/mint-nft.yaml | envsubst | charms spell prove --app-bins=${app_bins} --prev-txs=$prev_txs --funding-utxo=$funding_utxo --funding-utxo-value=$funding_utxo_value --change-address=$change_address

This will create (but not yet sign) two Bitcoin transactions:

  • commit transaction and
  • spell transaction.

The commit transaction creates exactly one output (committing to a spell and its proof) which is then spent by the spell transaction. The spell transaction contains the spell and proof (in the witness spending the output created by the commit tx), and it cannot exist without the commit transaction.

charms spell prove prints the 2 hex-encoded signed transactions at the end of its output, which looks like a JSON array (because it is a JSON array):

["020000000001015f...57505efa00000000", "020000000001025f...e14c656300000000"]

You can now sign these transactions. Then submit both to the Bitcoin network as a package:

Terminal window
b submitpackage '["020000000001015f...57505efa00000000", "020000000001025f...e14c656300000000"]'