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Update instructions on how to build Bitcoin HTLC #90

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thomaseizinger merged 1 commit into from
Jul 5, 2019
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Update instructions on how to build Bitcoin HTLC #90

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61 changes: 31 additions & 30 deletions RFC-005-SWAP-Basic-Bitcoin.md
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Original file line number Diff line number Diff line change
Expand Up @@ -62,13 +62,13 @@ This may be expanded in subsequent RFCs.

The parameters for the Bitcoin HTLC follow [RFC003](./RFC-003-SWAP-Basic.md#hash-time-lock-contract-htlc) and are described concretely in the following table:

| Variable | Description |
|:----------------|:----------------------------------------------------------------------------|
| asset | The quantity of satoshi |
| secret_hash | The hash of the `secret` (32 bytes for SHA-256) |
| redeem_identity | The `pubkeyhash` of the redeeming party |
| refund_identity | The `pubkeyhash` of the refunding party |
| expiry | The absolute UNIX timestamp in seconds after which the HTLC can be refunded |
| Variable | Description |
| :--------------- | :-------------------------------------------------------------------------- |
| asset | The quantity of satoshi |
| secret_hash | The hash of the `secret` (32 bytes for SHA-256) |
| redeem_identity | The `pubkeyhash` of the redeeming party |
| refund_identity | The `pubkeyhash` of the refunding party |
| refund_timestamp | The absolute UNIX timestamp in seconds after which the HTLC can be refunded |

### Contract

Expand All @@ -80,31 +80,32 @@ OP_IF
OP_SHA256 <secret_hash> OP_EQUALVERIFY
OP_DUP OP_HASH160 <redeem_identity>
OP_ELSE
<expiry> OP_CHECKLOCKTIMEVERIFY OP_DROP
<refund_timestamp> OP_CHECKLOCKTIMEVERIFY OP_DROP
OP_DUP OP_HASH160 <refund_identity>
OP_ENDIF
OP_EQUALVERIFY
OP_CHECKSIG
```

As required by [RFC003](./RFC-003-SWAP-Basic.md), this HTLC uses absolute time locks to check whether `expiry` has been reached.
Specifically, `OP_CHECKLOCKTIMEVERIFY` (see [BIP65](https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki)) is used to compare the time in the block's header to the `expiry` in the contract.
As required by [RFC003](./RFC-003-SWAP-Basic.md), this HTLC uses absolute time locks to check whether `refund_timestamp` has been reached.
Specifically, `OP_CHECKLOCKTIMEVERIFY` (see [BIP65](https://github.com/bitcoin/bips/blob/master/bip-0065.mediawiki)) is used to compare the time in the block's header to the `refund_timestamp` in the contract.

Implementations MUST consider an `expiry` value below `500000000` for a Bitcoin HTLC to be invalid due to the [`lock_time`](https://en.bitcoin.it/wiki/Protocol_documentation#tx) transaction field interpreting values below `500000000` as block heights.
Implementations MUST consider an `refund_timestamp` value below `500000000` for a Bitcoin HTLC to be invalid due to the [`lock_time`](https://en.bitcoin.it/wiki/Protocol_documentation#tx) transaction field interpreting values below `500000000` as block heights.

To compute the exact Bitcoin script bytes of the contract, implementations should use the following offset table:
The following code snippet contains the above script compiled into hex with the parameters filled with placeholders.

| Data | Position of first byte | Position of last byte | Length (bytes) | Description |
|:------------------|:-----------------------|:----------------------|:---------------|:---------------------------------------------------------------------------|
| `6382012088a820` | 0 | 6 | 7 | `OP_IF OP_SIZE 32 OP_EQUALVERIFY OP_SHA256` + `PUSH32` for the secret_hash |
| `secret_hash` | 7 | 39 | 32 | See [Parameters](#parameters) |
| `8876a9` | 40 | 42 | 3 | `OP_EQUALVERIFY OP_DUP OP_HASH160` |
| `redeem_identity` | 43 | 64 | 20 | See [Parameters](#parameters) |
| `67` | 65 | 65 | 1 | `OP_ELSE` |
| `expiry` | 66 | 69 | 4 | See [Parameters](#parameters) |
| `b17576a9` | 70 | 73 | 4 | `OP_CHECKLOCKTIMEVERIFY OP_DROP OP_DUP OP_HASH160` |
| `refund_identity` | 74 | 93 | 20 | See [Parameters](#parameters) |
| `6888ac` | 94 | 96 | 3 | `OP_ENDIF OP_EQUALVERIFY OP_CHECKSIG` |
```
6382012088a82010000000000000000000000000000000000000000000000000000000000000018876a9143000000000000000000000000000000000000003670420000002b17576a91440000000000000000000000000000000000000046888ac
```

Use the following table to fill in the correct parameters and build the actual HTLC:

| Name | Byte Range | Length (bytes) |
| :----------------- | :--------- | :------------- |
| `secret_hash` | 7..39 | 32 |
| `redeem_identity` | 43..63 | 20 |
| `refund_timestamp` | 65..69 | 4 |
| `refund_identity` | 74..94 | 20 |

## Execution Phase

Expand Down Expand Up @@ -146,7 +147,7 @@ If Bitcoin is the `beta_ledger` (see [RFC003](./RFC-003-SWAP-Basic.md)), then th
### Refund

To refund the HTLC, the funder MUST submit a transaction to the blockchain which spends the P2WSH output.
The funder can use the following witness data to spend the output after the `expiry`:
The funder can use the following witness data to spend the output after the `refund_timestamp`:

| Data | Description |
|:-----------------|:--------------------------------------------------------------------------------------------------------|
Expand Down Expand Up @@ -219,12 +220,12 @@ A valid `RESPONSE` to the above `REQUEST` could look like:

The above `REQUEST` and `RESPONSE` results in the following parameters to the HTLC:

| Parameter | value |
|:----------------|--------------------------------------------------------------------|
| redeem_identity | `c021f17be99c6adfbcba5d38ee0d292c0399d2f5` |
| refund_identity | `1925a274ac004373bb5429553bdb55c40e57b124` |
| secret_hash | `1f69c8745f712da03fdd43486ef705fc24f3e34d54cf44d967cf5cd4204c835e` |
| expiry | 1552263040 |
| Parameter | value |
| :--------------- | ------------------------------------------------------------------ |
| redeem_identity | `c021f17be99c6adfbcba5d38ee0d292c0399d2f5` |
| refund_identity | `1925a274ac004373bb5429553bdb55c40e57b124` |
| secret_hash | `1f69c8745f712da03fdd43486ef705fc24f3e34d54cf44d967cf5cd4204c835e` |
| refund_timestamp | 1552263040 |


Which compiles into the following Bitcoin script bytes:
Expand Down