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Subject: [bitcoin-dev] BIP: Trust minimized swaps using PSBT,
	SINGLE|ANYONECANPAY and nostr
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Hi Bitcoin Developers,

I have written a BIP that describes the process to swap inscriptions howeve=
r there can be other use cases for it as well: https://gist.github.com/1440=
000bytes/a7deeb3f1740bc533a61fbcc1fe58d77

Feel free to share your opinion or feedback to improve the usage of PSBTs i=
n swaps.

    BIP: 2023-ordswap
    Layer: Applications
    Title: Trust minimized swaps using PSBTs
    Author: /dev/fd0
    Status: Draft
    Created: 2023-03-02
    License: Public Domain
 =20
 =20
### Introduction

This BIP describes a process for creating offers using PSBTs to swap inscri=
ptions. It was originally shared by [Casey](https://github.com/casey/ord/is=
sues/802). There are two other approaches (`joinpsbts` and coinswap) to swa=
p inscriptions however they degrade the UX and use of SINGLE|ANYONECANPAY w=
orks better.

### Specification

[SINGLE|ANYONECANPAY](https://en.bitcoin.it/wiki/Contract#SIGHASH_flags) is=
 used for creating a PSBT by the seller. It is signed and published as offe=
r. Buyer updates the PSBT with appropriate inputs and outputs. Order of inp=
uts and outputs in the PSBT is very important as wrong ordering can burn in=
scriptions. [Ordinal theory](https://docs.ordinals.com/faq.html?#how-does-o=
rdinal-theory-work) uses an algorithm to determine how satoshis hop from th=
e inputs of a transaction to its outputs.

### Protocol

Sequence diagram:

```mermaid
sequenceDiagram
    Note right of Seller: Create and Sign PSBT
    Seller->>+Nostr relays: Publish offer
    Buyer->>+Nostr relays: Accept offer
    Note left of Buyer: Add inputs, outputs, sign and broadcast PSBT
```

Seller:

- Create PSBT with inscription UTXO input and a new address with sell amoun=
t as output
- Sign PSBT
- Publish PSBT as defined in [NIP](https://github.com/orenyomtov/openordex/=
blob/main/NIP.md)

Buyer:

- Add new address as output in PSBT to receive inscription
- Create [dummy UTXO](https://i.imgur.com/8Rw3TFX.png) if not available in =
wallet (Less than 1000 sats)
- Add UTXO to pay seller and dummy UTXO as inputs in PSBT
- Sign and broadcast transaction.=20

Example tx: https://mempool.space/signet/tx/ee7032f08ed18113c16ab8759d294c0=
9f57492d8d255b5dbd16326df53bbdcac

This transaction has 3 inputs (dummy, inscription, UTXO used for paying sel=
ler) and 4 outputs (inscription, payment, new dummy for future, change)

Note: Openordex creates a dummy UTXO and reuses address if there is no dumm=
y UTXO found for the address entered by buyer. Example: https://mempool.spa=
ce/signet/tx/388942887f79358a1deba3aae86e97b982a923566b2ef2249eab42288efc5a=
bf

Pseudocode or Implementation (2 functions used by openordex for creating PS=
BTs)

```js
=09
async function generatePSBTListingInscriptionForSale(ordinalOutput, price, =
paymentAddress) {
    let psbt =3D new bitcoin.Psbt({ network });

    const [ordinalUtxoTxId, ordinalUtxoVout] =3D ordinalOutput.split(':')
    const tx =3D bitcoin.Transaction.fromHex(await getTxHexById(ordinalUtxo=
TxId))
    for (const output in tx.outs) {
        try { tx.setWitness(output, []) } catch { }
    }

    psbt.addInput({
        hash: ordinalUtxoTxId,
        index: parseInt(ordinalUtxoVout),
        nonWitnessUtxo: tx.toBuffer(),
        // witnessUtxo: tx.outs[ordinalUtxoVout],
        sighashType: bitcoin.Transaction.SIGHASH_SINGLE | bitcoin.Transacti=
on.SIGHASH_ANYONECANPAY,
    });

    psbt.addOutput({
        address: paymentAddress,
        value: price,
    });

    return psbt.toBase64();
}

```

```js

    generatePSBTBuyingInscription =3D async (payerAddress, receiverAddress,=
 price, paymentUtxos, dummyUtxo) =3D> {
        const psbt =3D new bitcoin.Psbt({ network });
        let totalValue =3D 0
        let totalPaymentValue =3D 0

        // Add dummy utxo input
        const tx =3D bitcoin.Transaction.fromHex(await getTxHexById(dummyUt=
xo.txid))
        for (const output in tx.outs) {
            try { tx.setWitness(output, []) } catch { }
        }
        psbt.addInput({
            hash: dummyUtxo.txid,
            index: dummyUtxo.vout,
            nonWitnessUtxo: tx.toBuffer(),
            // witnessUtxo: tx.outs[dummyUtxo.vout],
        });

        // Add inscription output
        psbt.addOutput({
            address: receiverAddress,
            value: dummyUtxo.value + Number(inscription['output value']),
        });

        // Add payer signed input
        psbt.addInput({
            ...sellerSignedPsbt.data.globalMap.unsignedTx.tx.ins[0],
            ...sellerSignedPsbt.data.inputs[0]
        })
        // Add payer output
        psbt.addOutput({
            ...sellerSignedPsbt.data.globalMap.unsignedTx.tx.outs[0],
        })

        // Add payment utxo inputs
        for (const utxo of paymentUtxos) {
            const tx =3D bitcoin.Transaction.fromHex(await getTxHexById(utx=
o.txid))
            for (const output in tx.outs) {
                try { tx.setWitness(output, []) } catch { }
            }

            psbt.addInput({
                hash: utxo.txid,
                index: utxo.vout,
                nonWitnessUtxo: tx.toBuffer(),
                // witnessUtxo: tx.outs[utxo.vout],
            });

            totalValue +=3D utxo.value
            totalPaymentValue +=3D utxo.value
        }

        // Create a new dummy utxo output for the next purchase
        psbt.addOutput({
            address: payerAddress,
            value: dummyUtxoValue,
        })

        const fee =3D calculateFee(psbt.txInputs.length, psbt.txOutputs.len=
gth, await recommendedFeeRate)

        const changeValue =3D totalValue - dummyUtxo.value - price - fee

        if (changeValue < 0) {
            throw `Your wallet address doesn't have enough funds to buy thi=
s inscription.
Price:          ${satToBtc(price)} BTC
Fees:       ${satToBtc(fee + dummyUtxoValue)} BTC
You have:   ${satToBtc(totalPaymentValue)} BTC
Required:   ${satToBtc(totalValue - changeValue)} BTC
Missing:     ${satToBtc(-changeValue)} BTC`
        }

        // Change utxo
        psbt.addOutput({
            address: payerAddress,
            value: changeValue,
        });

        return psbt.toBase64();
    }
=09
```

Note: Openordex reuses address for change, however this can be avoided.

### Acknowledgements

- Casey Rodarmor
- Oren Yomtov
- Rijndael

/dev/fd0
floppy disk guy


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