Dust Outputs

[Polybius93]

The selectUTXO function produces occasionally dust outputs.

This is how I use the selectUTXO function:

const selected = selectUTXO(userUTXOs, psbtOutputs, 'default', {
    changeAddress: depositAddress,
    feePerByte: feeRate,
    bip69: false,
    createTx: true,
    network: bitcoinNetwork,
  });
 
  const psbtOutputs = [
    { address: address1, amount: 99008600n },
    {
      address: address2,
      amount: 990086n,
    },
  ];

Here is the UTXO being used:

[
{
  type: 'wpkh',
  script: Uint8Array.from([
    0, 20, 111, 177, 23, 166, 58, 9, 93, 222, 94, 102, 35, 25, 4, 126, 27, 187, 50, 70, 252, 61,
  ]),
  address: 'bcrt1qd7c30f36p9wauhnxyvvsglsmhveydlpaqrz6md',
  txid: 'abb220f97c47289175af1ca1d199c19c9c37190bc6ea651e69e04cbd09f19501',
  index: 0,
  value: 100000000,
  witnessUtxo: {
    script: Uint8Array.from([
      0, 20, 111, 177, 23, 166, 58, 9, 93, 222, 94, 102, 35, 25, 4, 126, 27, 187, 50, 70, 252, 61,
    ]),
    amount: 100000000n,
  },
  redeemScript: undefined,
},
];

And after selectUTXO, this transaction is created:

Transaction {
        global: { txVersion: 2 },
        inputs: [
          {
            txid: [Uint8Array],
            index: 0,
            witnessUtxo: [Object],
            sequence: 4294967280,
            partialSig: [Array]
          }
        ],
        outputs: [
          { amount: 99008600n, script: [Uint8Array] },
          { amount: 990086n, script: [Uint8Array] },
          { amount: 210n, script: [Uint8Array] }
        ],
        opts: {
          createTx: true,
          bip69: false,
          changeAddress: 'bcrt1qd7c30f36p9wauhnxyvvsglsmhveydlpaqrz6md',
          feePerByte: 6n,
          network: {
            messagePrefix: '\x18Bitcoin Signed Message:\n',
            bech32: 'bcrt',
            bip32: [Object],
            pubKeyHash: 111,
            scriptHash: 196,
            wif: 239
          },
          version: 2,
          lockTime: 0,
          PSBTVersion: 0
        }
      }

Is there a way to avoid this, or I have to check if any of the transaction's output is under the dust limit?

Thank you very much!
@paulmillr

[conduition]

@Polybius93 it looks like this library uses a dust limit of 148, and so that TXO of value 210 isn't considered dust by scure.

scure-btc-signer/src/utxo.ts

Line 333 in 9a29c88

private dust = 148n; // compat with coinselect

When adding outputs to the transaction, scure will add a change output if the value of the change exceeds 148.

scure-btc-signer/src/utxo.ts

Line 565 in 9a29c88

if (change > this.dust) needChange = true;

@paulmillr I believe this is a bug. The line in core referenced by the code comment (link) describes a byte size of 148, not a dust limit in sats.

A typical spendable non-segwit txout is 34 bytes big, and will need a CTxIn of at least 148 bytes to spend

The actual dust limit that a node uses for its mempool relaying policy is defined by the minimum fee needed to spend an output at a particular (configurable) fee rate called the dustRelayFee.

    // "Dust" is defined in terms of dustRelayFee,
    // which has units satoshis-per-kilobyte.
    // If you'd pay more in fees than the value of the output
    // to spend something, then we consider it dust.
    // A typical spendable non-segwit txout is 34 bytes big, and will
    // need a CTxIn of at least 148 bytes to spend:
    // so dust is a spendable txout less than
    // 182*dustRelayFee/1000 (in satoshis).
    // 546 satoshis at the default rate of 3000 sat/kvB.
    // A typical spendable segwit P2WPKH txout is 31 bytes big, and will
    // need a CTxIn of at least 67 bytes to spend:
    // so dust is a spendable txout less than
    // 98*dustRelayFee/1000 (in satoshis).
    // 294 satoshis at the default rate of 3000 sat/kvB.

Source: https://github.com/bitcoin/bitcoin/blob/27a770b34b8f1dbb84760f442edb3e23a0c2420b/src/policy/policy.cpp#L28-L41

If you're looking to hardcode a single number you can use to cover most use cases, i suppose 546 should suffice (see comment above for the math).

---

@Polybius93 Regarding your specific case, you should be able to fix your problem by passing the dust option to selectUTXO.

scure-btc-signer/src/utxo.ts

Lines 346 to 354 in 9a29c88

	if (opts.dust) {
	if (typeof opts.dust !== 'bigint')
	throw new Error(
	`Estimator: wrong dust=${
	opts.dust
	}, should be of type bigint but got ${typeof opts.dust}.`
	);
	this.dust = opts.dust;
	}

This allows you to override the dust threshold of 148 hardcoded into scure. Since you're spending a segwit input, you should be able to use the segwit dust threshold of 294. So add { dust: 294n } to your options object, and hopefully that should prevent selectUTXO from adding a dust output.

[paulmillr]

Let me know of main branch works @Polybius93 or if you need help with building of the main branch.

@conduition I think it's fixed now

[Polybius93]

Let me know of main branch works @Polybius93 or if you need help with building of the main branch.

@conduition I think it's fixed now

Hey @paulmillr , thank you for the quick fix, it did work, when can we expect it to be released?

Thank you!

[conduition]

@paulmillr Thanks for the quick response, but I think you may need to reconsider the fix: d1bc93d#r145217554

[conduition]

I opened a PR with some improved dust fixes here: #109