Date: Mon, 23 Oct 2023 11:10:56 +0000
To: Peter Todd <pete@petertodd.org>
From: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
 security@ariard.me, "lightning-dev\\\\\\\\@lists.linuxfoundation.org"
 <lightning-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] [Lightning-dev] OP_Expire and Coinbase-Like
	Behavior: Making HTLCs Safer by Letting Transactions Expire Safely
Precedence: list

Hi all,

This was discussed partially on the platform formerly known as twitter, but=
 an alternate design goes like this:

* Add an `nExpiryTime` field in taproot annex.
  * This indicates that the transaction MUST NOT exist in a block at or abo=
ve the height specified.
  * Mempool should put txes buckets based on `nExpiryTime`, and if the bloc=
k is reached, drop all the buckets with `nExpiryTime` less than that block =
height.
* Add an `OP_CHECKEXPIRYTIMEVERIFY` opcode, mostly similar in behavior to `=
OP_EXPIRE` proposed by Peter Todd:
  * Check if `nExpiryTime` exists and has value equal or less than the stac=
k top.

The primary difference is simply that while Peter proposes an implicit fiel=
d for the value that `OP_EXPIRE` will put in `CTransaction`, I propose an e=
xplicit field for it in the taproot annex.

The hope is that "explicit is better than implicit" and that the design wil=
l be better implemented as well by non-Bitcoin-core implementations, as the=
 use of tx buckets is now explicit in treating the `nExpiryTime` field.

Regards,
ZmnSCPxj