Bitcoin Settlement

Bitcoin settlement uses Taproot (P2TR) outputs and BIP 174 PSBTs. No smart contract is deployed on Bitcoin.

HTLC lock output

Every deposit locks funds to a unique Taproot output with three script leaves:

Leaf	Script
Resolver withdraw	`OP_HASH256 <secretHash> OP_EQUALVERIFY <resolverPubKey> OP_CHECKSIG`
Timelock refund	`<deadline> OP_CLTV OP_DROP <protocolPubKey> OP_CHECKSIG`
Fast refund	`<protocolPubKey> OP_CHECKSIG <resolverPubKey> OP_CHECKSIGADD 2 OP_EQUAL`

secretHash is double SHA-256 of the 32-byte secret.
The internal key may be a NUMS point or a resolver-selected key. RGB USDT swaps use a NUMS internal key; see RGB USDT in Utexo Swap.
The resolver returns the refund and fast-refund control blocks to Utexo and keeps the withdraw control block internally.

Deposit transaction

Intent tagging

Every deposit contains an OP_RETURN output that tags the intent:

OP_RETURN <protocolPrefix> <flag> <intentID>

PSBT inputs

Field	Value
`WITNESS_UTXO`	Required for every input.
Sighash	`SIGHASH_ALL` (`0x01`).
`TAP_INTERNAL_KEY`	User’s x-only public key.
Sequence	`0xFFFFFFFF` (no RBF).

Outputs

Index	Output
`0`	`OP_RETURN` with `0` sats.
`1`	P2TR HTLC lock. The amount is the deposit minus a proportional share of network fees.

The deposit transaction has two or three outputs. Confirm the purpose of the optional third output with Utexo before adding change outputs.

Spend paths

Key-spend path

Used for deposit input signing, fulfillment, and native transfers. Witness:

[schnorr_signature]

Resolver withdraw (script-path)

Witness:

[schnorr_signature, secret_preimage, tap_script, control_block]

Fast refund (script-path)

Cooperative script-path spend co-signed by the protocol and the resolver.

Timelock refund

Script-path refund after deadline.

Confirm the final witness stack for the timelock refund path with Utexo before constructing this witness.

Fee model

The resolver computes fees from a header plus per-input and per-output costs:

header         = 11 vBytes
size           = header
               + sum(input_size + witness_size) * count
               + sum(output_size) * count
fee            = size * fee_rate

Default fee rate: 3 sat/vB.

Size table

Type	Input	Witness	Output
P2TR	41	17	43
P2WPKH	41	27	31
P2SH (2-of-3)	43	254	32
P2PKH	41	107	34
P2WSH (2-of-3)	41	64	43

What Utexo Is

Getting Started

Product Suite

API Reference

HTLC lock output

Deposit transaction

Intent tagging

PSBT inputs

Outputs

Spend paths

Key-spend path

Resolver withdraw (script-path)

Fast refund (script-path)

Timelock refund

Fee model

Size table

​HTLC lock output

​Deposit transaction

​Intent tagging

​PSBT inputs

​Outputs

​Spend paths

​Key-spend path

​Resolver withdraw (script-path)

​Fast refund (script-path)

​Timelock refund

​Fee model

​Size table

HTLC lock output

Deposit transaction

Intent tagging

PSBT inputs

Outputs

Spend paths

Key-spend path

Resolver withdraw (script-path)

Fast refund (script-path)

Timelock refund

Fee model

Size table