Further reading/ Minting native assets/

Unique minting

NFTs (Non-Fungible tokens) are tokens resulting from a unique minting event. To make an NFT the minting policy must make sure that that policy can only be used once and that only one token can ever be minted. The token name for the NFT in this example will be called example_nft. Usually the amount of each token is restricted to 1.

There are two approaches for these kind of minting policies:

• Deadline-based Approach
• UTxO-based Approach

Deadline-based Approach

NFTs were available on Cardano before smart contracts (Mary Hardfork) and they were implemented using deadlines. The main idea is that the token can only be minted before a deadline which already passed. This ensures no new tokens will ever be minted. This is very easy to implement:

minting deadline_nft

const DEADLINE: Time = Time::new(1661665196132) // milliseconds since 1970


func main(_, ctx: ScriptContext) -> Bool {
	tx: Tx = ctx.tx;

    nft_assetclass: AssetClass = AssetClass::new(
		ctx.get_current_minting_policy_hash(),
		"example-nft".encode_utf8()
	);

    value_minted: Value = tx.minted;

    value_minted == Value::new(nft_assetclass, 1) && tx.time_range.start < DEADLINE
}

UTxO-Based Approach

This method is based on an example in the Plutus Pioneer Program. This approach takes advantage of the fact that all UTxOs have a unique TxOutputId. A UTxO's TxOutputId is made up of the transaction hash of the transaction that made the UTxO and the index of the UTxO in the outputs of that transaction. It's a builtin type that is defined as:

struct TxOutputId {
    tx_id: TxId
    index: Int
}

So with this approach, we specify in the minting policy that the transaction minting the NFT must spend a UTxO with a specific output ID. Since a UTxO can only be spent once this means the token can only be minted once.

minting utxo_nft

const OUTPUT_ID: TxOutputId = TxOutputId::new(TxId::new(#1213), 1)

func main(_, ctx: ScriptContext) -> Bool {
	tx: Tx = ctx.tx;

    nft_assetclass: AssetClass = AssetClass::new(
		ctx.get_current_minting_policy_hash(),
		"example-nft".encode_utf8()
	);

    value_minted: Value = tx.minted;

    value_minted == Value::new(nft_assetclass, 1) &&
    tx.inputs
        .any((input: TxInput) -> Bool {input.output_id == OUTPUT_ID})

}