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Omit previous transaction for taproot inputs #18

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Omit previous transaction for taproot inputs #18

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22 changes: 18 additions & 4 deletions 02-peer-protocol.md
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Original file line number Diff line number Diff line change
Expand Up @@ -199,13 +199,23 @@ This message contains a transaction input.
* [`u32`:`prevtx_vout`]
* [`u32`:`sequence`]

1. `tlv_stream`: `tx_add_input_tlvs`
2. types:
1. type: 0 (`prevtxout`)
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@t-bast t-bast Jan 3, 2024 •
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When setting this, we will set prevtx_len to 0 since it is part of the main fields of the message, which feels a bit hackish. An alternative design would be to remove prevtx from the main message fields and make it a TLV, which would make it more similar to prevtxout. I find this cleaner and more future-proof since other types of inputs won't include the whole prevtx (e.g. the previous funding output when using splicing), but it's more backwards-incompatible. But hey, it's now or never that we have the opportunity to make this cleaner, so it's worth studying that option!

2. data:
* [`sha256`:`txid`]
* [`u64`:`amount_satoshis`]
* [`...*byte`:`scriptpubkey`]

#### Requirements

The sending node:
- MUST add all sent inputs to the transaction
- MUST use a unique `serial_id` for each input currently added to the
transaction
- MUST set `sequence` to be less than or equal to 4294967293 (`0xFFFFFFFD`)
- If the input is using segwit version 1 or greater:
- MAY omit `prevtx` and send `prevtxout` instead
- MUST NOT re-transmit inputs it has received from the peer
- if is the *initiator*:
- MUST send even `serial_id`s
Expand All @@ -216,11 +226,12 @@ The receiving node:
- MUST add all received inputs to the transaction
- MUST fail the negotiation if:
- `sequence` is set to `0xFFFFFFFE` or `0xFFFFFFFF`
- the `prevtx` and `prevtx_vout` are identical to a previously added
(and not removed) input's
- `prevtx` is not a valid transaction
- `prevtx` and `prevtxout` are both missing
- `prevtx` is provided and is not a valid transaction
- `prevtx_vout` is greater or equal to the number of outputs on `prevtx`
- the `scriptPubKey` of the `prevtx_vout` output of `prevtx` is not exactly a 1-byte push opcode (for the numeric values `0` to `16`) followed by a data push between 2 and 40 bytes
- the spent outpoint is identical to a previously added (and not removed) input's
- the `scriptpubkey` of the `prevtx_vout` output of `prevtx` is not exactly a 1-byte push opcode (for the numeric values `0` to `16`) followed by a data push between 2 and 40 bytes
- the `scriptpubkey` of `prevtxout` is not exactly a 1-byte push opcode (for the numeric values `1` to `16`) followed by a data push between 2 and 40 bytes
- the `serial_id` is already included in the transaction
- the `serial_id` has the wrong parity
- if has received 4096 `tx_add_input` messages during this negotiation
Expand All @@ -235,6 +246,9 @@ Inputs in the constructed transaction MUST be sorted by `serial_id`.

`prevtx` is the serialized transaction that contains the output
this input spends. Used to verify that the input is non-malleable.
When using segwit version 1 or higher, we only need to provide the
`prevtxout` instead: this is particularly useful when the previous
transaction is too large to be included in a lightning message.

`prevtx_vout` is the index of the output being spent.

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