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:

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:

app_bin=$(charms app build)

# pick from the output of `bitcoin-cli listunspent`
change_address=$(b getrawchangeaddress)

# set dest and amount env vars used by the spell template
# (dest is a hex-encoded destination derived from a Bitcoin address)
export dest_0=$(charms util dest --addr=$(b getnewaddress))
export amount_0=1000

export RUST_LOG=info

Run:

cat ./spells/mint-nft.yaml | envsubst | charms spell prove \
  --app-bins=${app_bin} \
  --prev-txs=$prev_txs \
  --private-inputs=<(cat ./spells/mint-nft-private.yaml | envsubst) \
  --change-address=$change_address

This will create (but not yet sign) a Bitcoin transaction containing the spell and its proof in an OP_RETURN output.

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

[{"bitcoin":"020000000001015f...57505efa00000000"}]

You can now sign and broadcast the transaction:

b signrawtransactionwithwallet "020000000001015f...57505efa00000000"
b sendrawtransaction "020000000001015f...57505efa00000000"