eBTC

eBTC (enchanted BTC) is Bitcoin wrapped as a charm.

eBTC tokens are minted by locking BTC in the eBTC vault and burned by spending BTC from it. The amount of eBTC minted or burned equals the BTC amount minus 300 sats (dust) per vault UTXO.

Vault Address

The vault address is a dedicated Scrolls contract address generated with nonce 1129595493 (eBTC encoded as UTF-8 bytes, interpreted as a little-endian 32-bit integer):

curl https://scrolls-v11.charms.dev/main/address/1129595493
# bc1qrn970793udj0ugc3pj0hyrptts4rw5n7qxeya2

This address is hardcoded in the contract as VAULT_DEST.

A CHIP-0420 reference NFT provides on-chain metadata for the token (ticker, name, description, etc.). See ref-nft.md for NFT minting instructions.

Building

Prerequisites

Install Wasm WASI P1 support:

rustup target add wasm32-wasip1

Build

cargo update
app_bin=$(charms app build)

The resulting Wasm binary will be at ./target/wasm32-wasip1/release/ebtc.wasm.

Get the verification key:

export app_vk=$(charms app vk $app_bin)

Minting eBTC

To mint eBTC tokens, create a transaction that sends BTC to the vault address and attaches the token and vault contract charms to the outputs.

# UTXO you're spending
export in_utxo_0="<txid>:<vout>"

# app_id: hash of the UTXO ID of the 1st input of the reference NFT minting transaction
export app_id="<app_id_hex>"

# vault output: BTC amount to lock (eBTC minted = amount - 300 dust)
export vault_amount=10000

# eBTC minted
export mint_amount=9700  # vault_amount - 300 (dust)

# change output
export change_dest=$(charms util dest --addr <your_change_address>)
export change_amount=<change_sats>

cat ./spells/mint-token.yaml | envsubst | charms spell check \
  --prev-txs=<prev_txs_hex> \
  --app-bins=${app_bin}

The spell in spells/mint-token.yaml creates:

• An output at the vault address carrying the vault contract charm (c)
• An output carrying the newly minted eBTC tokens (t)

The token contract verifies that the eBTC minted equals the BTC locked minus 300 sats dust per vault UTXO.

Burning eBTC

To burn eBTC and release the locked BTC, you need to spend a vault UTXO. Since the vault is a Scrolls contract address, spending from it requires signing via the Scrolls API.

1. Create a spell that spends the vault UTXO (destroying eBTC tokens) and sends the released BTC to your address. The transaction must include a fee output to the Scrolls fee address.

2. Query the Scrolls fee configuration:

curl https://scrolls-v11.charms.dev/config

The fee is calculated as: fixed_cost + fee_per_input * n_inputs + fee_basis_points / 10000 * total_input_sats. Add a fee output to the fee_address from the config.

3. Sign the vault input with Scrolls. The signing endpoint takes the unsigned transaction, the vault input indices with their derivation nonce, and the previous transactions:

curl -X POST https://scrolls-v11.charms.dev/main/sign \
  -H "Content-Type: application/json" \
  -d '{
    "sign_inputs": [{"index": 0, "nonce": 1129595493}],
    "prev_txs": ["<hex of the tx that created the vault UTXO>"],
    "tx_to_sign": "<unsigned_tx_hex>"
  }'

The response is the signed transaction hex.

4. The token contract verifies that the eBTC burned equals the BTC released from the vault minus 300 sats dust per vault input.