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, per source.
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 source states there are exactly two or three outputs, but does not define the optional third output. Do not invent change behavior; see Validation Gaps.

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.

The source does not document the final witness stack for the timelock refund path. Confirm with the current implementation 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