Re: [bitcoin-dev] OP_BEFOREBLOCKVERIFY - discussing and opcode that invalidates a spend path after a certain block

From: "Russell O'Connor" <roconnor@blockstream•com>
To: Billy Tetrud <billy.tetrud@gmail•com>
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] OP_BEFOREBLOCKVERIFY - discussing and opcode that invalidates a spend path after a certain block
Date: Thu, 10 Jun 2021 19:20:19 -0400	[thread overview]
Message-ID: <CAMZUoK=EM6cN2YYrZ=YxtrAi5GfxTuY5_6nb5HGWD-WL4RNOCg@mail.gmail.com> (raw)
In-Reply-To: <CAGpPWDb4sp4XoQjb7qOfNK3BQTS3zNrx6SQ3s7N=HM+ZaiLPLw@mail.gmail.com>

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As it stands today, in order to double spend a transaction during a reorg,
one must take an active role of recognizing that a reorg has happened, hope
that the new branch has completely omitted your spending transaction, and
then quickly broadcast a replacement transaction with a higher fee to
outbid your previous transaction.

However with, pretty much any change to Bitcoin that leads to non-monotonic
validity rules, that is any rule where transactions that are valid at one
tip, can become invalid at a latter tip through some other means than their
inputs being spent, such as OP_BBV, one can design a wallet to passively
take advantage of reorgs by always spending through an OP_BBV that is on
the verge of becoming invalid.  Then you just have to sit back and wait for
a suitable reorg to take back your UTXO for you without any work.  I would
probably attempt to build such a wallet for myself should any OP_BBV-like
proposal be implemented.  Think of it as an auto-double spend wallet.

Some people hold the opinion that there is no meaningful distinction
between the active and passive roles in these two scenarios.  I'm not
convinced.  I see a material difference between needing to actively
broadcast a replacement transaction and passively waiting for your
transaction to fall out of validity.  I also see a material difference
between needing the transaction to be completely omitted from the reorging
chain versus just having the transaction fail a height qualification in the
reorging chain.

(There are a few other lesser problems with an OP_BBV proposal, including
the fact that Bitcoin software tends to cache script validity so you'd want
to use the taproot annex instead of pure script; and a possible issue that
the proposal defeats limits on transaction replacement because now instead
of meeting minimum thresholds for fee bumping you can just let the previous
transaction expire and bump the fee by a fraction (though you are
effectively rate limited so maybe that is considered sufficiently
mitigated?).  But there is little point in addressing these lesser concerns
if the main concern is outstanding.)

On Thu, Jun 10, 2021 at 6:20 PM Billy Tetrud <billy.tetrud@gmail•com> wrote:

> @Russell In that thread, you quoted Satoshi there, but neither he nor you
> really deeply explained the concern. Would you mind elaborating on a
> situation that calls for concern here? Some deeper explanation of the
> "reorg safety" property would also be helpful. I'd very much like to know
> what your thoughts are on the specific points I brought up in the BIP as
> well.
>
> On Thu, Jun 10, 2021 at 11:35 AM Russell O'Connor <
> roconnor@blockstream•com> wrote:
>
>> This is a continuation of the thread at
>> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2021-April/018760.html
>> on this topic.
>>
>> I still remain unconvinced that we ought to give up on the "reorg safety"
>> property that is explicitly part of Bitcoin's design.
>>
>> On Thu, Jun 10, 2021 at 1:56 PM Billy Tetrud via bitcoin-dev <
>> bitcoin-dev@lists•linuxfoundation.org> wrote:
>>
>>> Hi Everyone,
>>>
>>> I'd like to open a discussion of an opcode I call OP_BEFOREBLOCKVERIFY
>>> (OP_BBV) which is similar to ones that have been discussed before (eg
>>> OP_BLOCKNUMBER). The opcode is very simple: the it takes as a parameter
>>> a number representing a block height, and marks the transaction invalid if
>>> the current block the transaction is being evaluated for is greater than or
>>> equal to that block height, the transaction is invalid. I wrote up a bip
>>> for OP_BBV here
>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/bbv/bip-beforeblockverify.md>
>>> .
>>>
>>> The motivation for this opcode is primarily to do switch-off kinds of
>>> transactions. Eg, an output that contains both a spend path that uses
>>> OP_BBV and a spend path that uses OP_CHECKSEQUENCEVERIFY so that before a
>>> particular block one person can spend, and after that block a different
>>> person can spend. This can allow doing things like expiring payments or
>>> reversible payments in a cheaper way. Currently, things like that require a
>>> sequence of multiple transactions, however OP_BBV can do it in a single
>>> transaction, making these applications a lot more economically feasible.
>>>
>>> The particular application I'm most interested in is more efficient
>>> wallet vaults. However, wallet vaults requires other new opcodes, and I've
>>> been given the (good, I think) advice to start off this discussion with
>>> something a bit more bite sized and manageable. So I want to keep this
>>> discussion to OP_BBV and steer away from the specifics of the wallet vaults
>>> I'm thinking of (which are more involved, requiring other new opcodes that
>>> I think makes more sense to discuss in a different thread).
>>>
>>> The main thing I'd like to discuss is the historical avoidance of and
>>> stigma toward opcodes that can cause a valid transaction to become invalid.
>>>
>>> It seems there are two concerns:
>>>
>>> 1. that an opcode like might create a DOS vector where a malicious actor
>>> might be able to spam the mempool with transactions containing this opcode.
>>> 2. that an opcode like this could cause "bad" reorg behavior, where in a
>>> reorg, transactions that were spent become not spend and not spendable
>>> because they were mined too near their expiry point.
>>>
>>> While I don't want to claim anything about opcodes that can cause spend
>>> paths to expire in general, I do want to claim that *some* opcodes like
>>> that are safe - in particular OP_BBV. In the context of OP_BBV
>>> specifically, it seems to me like item 1 (mempool handling) is a solvable
>>> problem and that point 2 (reorg issues) is not really a problem since
>>> people should generally be waiting for 6 confirmations and software can
>>> warn the user to wait for 6 confirmations in relevant scenarios where a
>>> 6-block reorg might reverse the transaction. I discuss this in detail in
>>> the Design Tradeoffs and Risks
>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/bbv/bip-beforeblockverify.md#transaction-expiry> section
>>> of the document I wrote for OP_BBV. I'd love to hear thoughts from others
>>> on here about these things and especially the discussion of these issues in
>>> the document I linked to.
>>>
>>> Thanks,
>>> BT
>>>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists•linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>

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