From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from smtp3.osuosl.org (smtp3.osuosl.org [140.211.166.136]) by lists.linuxfoundation.org (Postfix) with ESMTP id 135B5C002F; Wed, 19 Jan 2022 16:52:10 +0000 (UTC) Received: from localhost (localhost [127.0.0.1]) by smtp3.osuosl.org (Postfix) with ESMTP id EE8B8607F0; Wed, 19 Jan 2022 16:52:09 +0000 (UTC) X-Virus-Scanned: amavisd-new at osuosl.org X-Spam-Flag: NO X-Spam-Score: -2.098 X-Spam-Level: X-Spam-Status: No, score=-2.098 tagged_above=-999 required=5 tests=[BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, FREEMAIL_FROM=0.001, HTML_MESSAGE=0.001, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001] autolearn=ham autolearn_force=no Authentication-Results: smtp3.osuosl.org (amavisd-new); dkim=pass (2048-bit key) header.d=gmail.com Received: from smtp3.osuosl.org ([127.0.0.1]) by localhost (smtp3.osuosl.org [127.0.0.1]) (amavisd-new, port 10024) with ESMTP id 9ig1Jsz4Pu3w; Wed, 19 Jan 2022 16:52:07 +0000 (UTC) X-Greylist: whitelisted by SQLgrey-1.8.0 X-Greylist: whitelisted by SQLgrey-1.8.0 Received: from mail-ed1-x531.google.com (mail-ed1-x531.google.com [IPv6:2a00:1450:4864:20::531]) by smtp3.osuosl.org (Postfix) with ESMTPS id 82A7A607EC; Wed, 19 Jan 2022 16:52:07 +0000 (UTC) Received: by mail-ed1-x531.google.com with SMTP id u18so891208edt.6; Wed, 19 Jan 2022 08:52:07 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20210112; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=IusVIW61tTGUB/DUCECozLE34mOnlyKAC0GrGZwwjwU=; b=NTwFidT6RYI3PQMD4P/OUYidwUiT6ktDefCRgrjWEW2RoV5zpvm1OSkxPY9ngjkVTv s+F69IiJKOj/lis2mluAWfjiTlppAnAiLtcM3Z2aPCxIskn+DvWNJWQYX4oANIOUY27M yyIiEG62vdi9+4wLza1PdUJIZet3di4m1bpO0THK+Q4rzfxPsf5ruEnh6K6xULApsyu/ YN5ljXo0O5cI74SNgq+CA0weBdhLrc/Lpd/9k3y1CNHIzYbiiTc7MXpUNKyEaM7apsfG pIZfwsFe5R5JXmGY1YPcH2iNWYxSGBwdrXQMa2FaziywikcfCL53E46S0FNMGFPeOZUW GhvA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=IusVIW61tTGUB/DUCECozLE34mOnlyKAC0GrGZwwjwU=; b=XYWJz1auIA1irgO2CPHrIwoMJKVS40JQoV7f5fWIHhE7s3Fl6fOq24dLf3AAw3q+4w nhcDYyj/UAIhCQBg0GzsRl3YZh6fW+i+4bGqFK+G84FH24qLsvDe9dnzDzazowAyuMRd gbqm2G6uEXIzaiES6jyVauIi2pLVAedxaTJLcQyaXkrm0JCy56kYMVOD22Syy0zyCypp 5zEhxrDDEReoqXDgK1moljLfmsszF9NWTy2HGPx2qZxiwepHgBK42DRlIY60G/wq9yv0 vsFyu9pRL9pXoy82PSeZSMPSe2ZySiVJ9aE335b1ZIZS2lHVTIxqOo3JYoHececFJe7y 7eyw== X-Gm-Message-State: AOAM5332nXJzwVjKMs4l6GspZKqKe9ujMINzv2JoTVpURqJC2fFWuxNT q3yhARUvj/9AAPzKVcL1da9fBEnC0bUr1168SMHuOVmcXeI= X-Google-Smtp-Source: ABdhPJyVLzEcG4ggiFRrzQJEqAzJwE+QJyE53CeZ50timc1o3JIEaH4n2FIj2h1VPSDD8q2BRexqs5vAn+w+PYdG6hc= X-Received: by 2002:a17:906:2bcf:: with SMTP id n15mr24201694ejg.184.1642611125426; Wed, 19 Jan 2022 08:52:05 -0800 (PST) MIME-Version: 1.0 References: In-Reply-To: From: Billy Tetrud Date: Wed, 19 Jan 2022 10:51:48 -0600 Message-ID: To: Jeremy Content-Type: multipart/alternative; boundary="000000000000a2a30405d5f23406" X-Mailman-Approved-At: Wed, 19 Jan 2022 17:04:01 +0000 Cc: Bitcoin Protocol Discussion , lightning-dev Subject: Re: [bitcoin-dev] [Pre-BIP] Fee Accounts X-BeenThere: bitcoin-dev@lists.linuxfoundation.org X-Mailman-Version: 2.1.15 Precedence: list List-Id: Bitcoin Protocol Discussion List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 19 Jan 2022 16:52:10 -0000 --000000000000a2a30405d5f23406 Content-Type: text/plain; charset="UTF-8" Hmm, I don't know anything about SIGHASH_BUNDLE. The only references online I can find are just mentions (mostly from you). What is SIGHASH_BUNDLE? > unless you're binding a WTXID That could work, but it would exclude cases where you have a transaction that has already been partially signed and someone wants to, say, only sign that transaction if some 3rd party signs a transaction paying part of the fee for it. Kind of a niche use case, but it would be nice to support it if possible. If the transaction hasn't been signed at all yet, a new transaction can just be created that includes the prospective fee-payer, and if the transaction is fully signed then it has a WTXID to use. > then you can have fee bumping cycles What kind of cycles do you mean? You're saying these cycles would make it less robust to reorgs? > OP_VER I assume you mean something other than pushing the version onto the stack ? Is that related to your fee account idea? On Wed, Jan 19, 2022 at 1:32 AM Jeremy wrote: > Ah my bad i misread what you were saying as being about SIGHASH_BUNDLE > like proposals. > > For what you're discussing, I previously proposed > https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2020-September/018168.html > which is similar. > > The benefit of the OP_VER output is that SIGHASH_EXTERNAL has the issue > that unless you're binding a WTXID (which is maybe too specific?) then you > can have fee bumping cycles. Doing OP_VER output w/ TXID guarantees that > you are acyclic. > > The difference between a fee account and this approach basically boils > down to the impact on e.g. reorg stability, where the deposit/withdraw > mechanism is a bit more "robust" for reorderings in reorgs than the in-band > transaction approach, although they are very similar. > > -- > @JeremyRubin > > > > On Tue, Jan 18, 2022 at 8:53 PM Billy Tetrud > wrote: > >> > because you make transactions third party malleable it becomes >> possible to bundle and unbundle transactions. >> >> What I was suggesting doesn't make it possible to malleate someone else's >> transaction. I guess maybe my proposal of using a sighash flag might >> have been unclear. Imagine it as a script opcode that just says "this >> transaction must be mined with this other transaction" - the only >> difference being that you can use any output with any encumberance as an >> input for fee bumping. It doesn't prevent the original transaction from >> being mined on its own. So adding junk inputs would be no more of a problem >> than dust attacks already are. It would be used exactly like cpfp, except >> it doesn't spend the parent. >> >> I don't think what I was suggesting is as different from your proposal. >> All the problems of fee revenue optimization and feerate rules that you >> mentioned seem like they'd also exist for your proposal, or for cpfp. Let >> me know if I should clarify further. >> >> On Tue, Jan 18, 2022 at 8:51 PM Jeremy wrote: >> >>> The issue with sighash flags is that because you make transactions third >>> party malleable it becomes possible to bundle and unbundle transactions. >>> >>> This means there are circumstances where an attacker could e.g. see your >>> txn, and then add a lot of junk change/inputs + 25 descendants and strongly >>> anchor your transaction to the bottom of the mempool. >>> >>> because of rbf rules requiring more fee and feerate, this means you have >>> to bump across the whole package and that can get really messy. >>> >>> more generally speaking, you could imagine a future where mempools track >>> many alternative things that might want to be in a transaction. >>> >>> suppose there are N inputs each with a weight and an amount of fee being >>> added and the sighash flags let me pick any subset of them. However, for a >>> txn to be standard it must be < 100k bytes and for it to be consensus < >>> 1mb. Now it is possible you have to solve a knapsack problem in order to >>> rationally bundle this transaction out of all possibilities. >>> >>> This problem can get even thornier, suppose that the inputs I'm adding >>> themselves are the outputs of another txn in the mempool, now i have to >>> track and propagate the feerates of that child back up to the parent txn >>> and track all these dependencies. >>> >>> perhaps with very careful engineering these issues can be tamed. however >>> it seems with sponsors or fee accounts, by separating the pays-for from the >>> participates-in concerns we can greatly simplify it to something like: >>> compute effective feerate for a txn, including all sponsors that pay more >>> than the feerate of the base txn. Mine that txn and it's subsidies using >>> the normal algo. If you run out of space, all subsidies are same-sized so >>> just take the ones that pay the highest amount up until the added marginal >>> feerate is less than the next eligible txn. >>> >>> >>> -- >>> @JeremyRubin >>> >>> >>> >>> On Tue, Jan 18, 2022 at 6:38 PM Billy Tetrud >>> wrote: >>> >>>> I see, its not primarily to make it cheaper to append fees, but also >>>> allows appending fees in cases that aren't possible now. Is that right? I >>>> can certainly see the benefit of a more general way to add a fee to any >>>> transaction, regardless of whether you're related to that transaction or >>>> not. >>>> >>>> How would you compare the pros and cons of your account-based approach >>>> to something like a new sighash flag? Eg a sighash flag that says "I'm >>>> signing this transaction, but the signature is only valid if mined in the >>>> same block as transaction X (or maybe transactions LIST)". This could be >>>> named SIGHASH_EXTERNAL. Doing this would be a lot more similar to other >>>> bitcoin transactions, and no special account would need to be created. Any >>>> transaction could specify this. At least that's the first thought I would >>>> have in designing a way to arbitrarily bump fees. Have you compared your >>>> solution to something more familiar like that? >>>> >>>> On Tue, Jan 18, 2022 at 11:43 AM Jeremy wrote: >>>> >>>>> Can you clarify what you mean by "improve the situation"? >>>>> >>>>> There's a potential mild bytes savings, but the bigger deal is that >>>>> the API should be much less vulnerable to pinning issues, fix dust leakage >>>>> for eltoo like protocols, and just generally allow protocol designs to be >>>>> fully abstracted from paying fees. You can't easily mathematically >>>>> quantify API improvements like that. >>>>> -- >>>>> @JeremyRubin >>>>> >>>>> >>>>> >>>>> On Tue, Jan 18, 2022 at 8:13 AM Billy Tetrud >>>>> wrote: >>>>> >>>>>> Do you have any back-of-the-napkin math on quantifying how much this >>>>>> would improve the situation vs existing methods (eg cpfp)? >>>>>> >>>>>> >>>>>> >>>>>> On Sat, Jan 1, 2022 at 2:04 PM Jeremy via bitcoin-dev < >>>>>> bitcoin-dev@lists.linuxfoundation.org> wrote: >>>>>> >>>>>>> Happy new years devs, >>>>>>> >>>>>>> I figured I would share some thoughts for conceptual review that >>>>>>> have been bouncing around my head as an opportunity to clean up the fee >>>>>>> paying semantics in bitcoin "for good". The design space is very wide on >>>>>>> the approach I'll share, so below is just a sketch of how it could work >>>>>>> which I'm sure could be improved greatly. >>>>>>> >>>>>>> Transaction fees are an integral part of bitcoin. >>>>>>> >>>>>>> However, due to quirks of Bitcoin's transaction design, fees are a >>>>>>> part of the transactions that they occur in. >>>>>>> >>>>>>> While this works in a "Bitcoin 1.0" world, where all transactions >>>>>>> are simple on-chain transfers, real world use of Bitcoin requires support >>>>>>> for things like Fee Bumping stuck transactions, DoS resistant Payment >>>>>>> Channels, and other long lived Smart Contracts that can't predict future >>>>>>> fee rates. Having the fees paid in band makes writing these contracts much >>>>>>> more difficult as you can't merely express the logic you want for the >>>>>>> transaction, but also the fees. >>>>>>> >>>>>>> Previously, I proposed a special type of transaction called a >>>>>>> "Sponsor" which has some special consensus + mempool rules to allow >>>>>>> arbitrarily appending fees to a transaction to bump it up in the mempool. >>>>>>> >>>>>>> As an alternative, we could establish an account system in Bitcoin >>>>>>> as an "extension block". >>>>>>> >>>>>>> *Here's how it might work:* >>>>>>> >>>>>>> 1. Define a special anyone can spend output type that is a "fee >>>>>>> account" (e.g. segwit V2). Such outputs have a redeeming key and an amount >>>>>>> associated with them, but are overall anyone can spend. >>>>>>> 2. All deposits to these outputs get stored in a separate UTXO >>>>>>> database for fee accounts >>>>>>> 3. Fee accounts can sign only two kinds of transaction: A: a fee >>>>>>> amount and a TXID (or Outpoint?); B: a withdraw amount, a fee, and >>>>>>> an address >>>>>>> 4. These transactions are committed in an extension block merkle >>>>>>> tree. While the actual signature must cover the TXID/Outpoint, the >>>>>>> committed data need only cover the index in the block of the transaction. >>>>>>> The public key for account lookup can be recovered from the message + >>>>>>> signature. >>>>>>> 5. In any block, any of the fee account deposits can be: released >>>>>>> into fees if there is a corresponding tx; consolidated together to reduce >>>>>>> the number of utxos (this can be just an OP_TRUE no metadata needed); or >>>>>>> released into fees *and paid back* into the requested withdrawal key >>>>>>> (encumbering a 100 block timeout). Signatures must be unique in a block. >>>>>>> 6. Mempool logic is updated to allow attaching of account fee spends >>>>>>> to transactions, the mempool can restrict that an account is not allowed >>>>>>> more spend more than it's balance. >>>>>>> >>>>>>> *But aren't accounts "bad"?* >>>>>>> >>>>>>> Yes, accounts are bad. But these accounts are not bad, because any >>>>>>> funds withdrawn from the fee extension are fundamentally locked for 100 >>>>>>> blocks as a coinbase output, so there should be no issues with any series >>>>>>> of reorgs. Further, since there is no "rich state" for these accounts, the >>>>>>> state updates can always be applied in a conflict-free way in any order. >>>>>>> >>>>>>> >>>>>>> *Improving the privacy of this design:* >>>>>>> >>>>>>> This design could likely be modified to implement something like >>>>>>> Tornado.cash or something else so that the fee account paying can be >>>>>>> unlinked from the transaction being paid for, improving privacy at the >>>>>>> expense of being a bit more expensive. >>>>>>> >>>>>>> Other operations could be added to allow a trustless mixing to be >>>>>>> done by miners automatically where groups of accounts with similar values >>>>>>> are trustlessly split into a common denominator and change, and keys are >>>>>>> derived via a verifiable stealth address like protocol (so fee balances can >>>>>>> be discovered by tracing the updates posted). These updates could also be >>>>>>> produced by individuals rather than miners, and miners could simply honor >>>>>>> them with better privacy. While a miner generating an update would be able >>>>>>> to deanonymize their mixes, if you have your account mixed several times by >>>>>>> independent miners that could potentially add sufficient privacy. >>>>>>> >>>>>>> The LN can also be used with PTLCs to, in theory, have another >>>>>>> individual paid to sponsor a transaction on your behalf only if they reveal >>>>>>> a valid sig from their fee paying account, although under this model it's >>>>>>> hard to ensure that the owner doesn't pay a fee and then 'cancel' by >>>>>>> withdrawing the rest. However, this could be partly solved by using >>>>>>> reputable fee accounts (reputation could be measured somewhat >>>>>>> decentralized-ly by longevity of the account and transactions paid for >>>>>>> historically). >>>>>>> >>>>>>> *Scalability* >>>>>>> >>>>>>> This design is fundamentally 'decent' for scalability because adding >>>>>>> fees to a transaction does not require adding inputs or outputs and does >>>>>>> not require tracking substantial amounts of new state. >>>>>>> >>>>>>> Paying someone else to pay for you via the LN also helps make this >>>>>>> more efficient if the withdrawal issues can be fixed. >>>>>>> >>>>>>> *Lightning:* >>>>>>> >>>>>>> This type of design works really well for channels because the >>>>>>> addition of fees to e.g. a channel state does not require any sort of >>>>>>> pre-planning (e.g. anchors) or transaction flexibility (SIGHASH flags). >>>>>>> This sort of design is naturally immune to pinning issues since you could >>>>>>> offer to pay a fee for any TXID and the number of fee adding offers does >>>>>>> not need to be restricted in the same way the descendant transactions would >>>>>>> need to be. >>>>>>> >>>>>>> *Without a fork?* >>>>>>> >>>>>>> This type of design could be done as a federated network that bribes >>>>>>> miners -- potentially even retroactively after a block is formed. That >>>>>>> might be sufficient to prove the concept works before a consensus upgrade >>>>>>> is deployed, but such an approach does mean there is a centralizing layer >>>>>>> interfering with normal mining. >>>>>>> >>>>>>> >>>>>>> Happy new year!! >>>>>>> >>>>>>> Jeremy >>>>>>> >>>>>>> -- >>>>>>> @JeremyRubin >>>>>>> >>>>>>> _______________________________________________ >>>>>>> bitcoin-dev mailing list >>>>>>> bitcoin-dev@lists.linuxfoundation.org >>>>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev >>>>>>> >>>>>> --000000000000a2a30405d5f23406 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
Hmm, I don't know anything about=C2=A0 SIGHASH_BUNDLE. The only references online I can find are just mentions (mo= stly from you). What is=C2=A0 SIGHASH_BUNDLE?

> unless you're binding a WTXID

That could work, but it would exclude cases where y= ou have a transaction that has already been partially signed and someone wa= nts to, say, only sign that transaction if some 3rd party signs a transacti= on paying part of the fee for it. Kind of a niche use case, but it would be= nice to support it if possible. If the transaction hasn't been signed = at all yet, a new transaction can just be created that includes the prospec= tive fee-payer, and if the transaction is fully signed then it has a WTXID = to use.

> then you can have fee bumping cycles<= /div>

What kind of cycles do you mean? You're saying= these cycles would make it less robust to reorgs?

> OP_VER

I assume you mean something other tha= n pushin= g the version onto the stack? Is that related to your fee account idea?=


On Wed, Jan 19, 2022 at 1:32 AM Jeremy <jlrubin@mit.edu> wrote:
Ah my bad i misread what you were saying as being about SI= GHASH_BUNDLE like proposals.

F= or what you're discussing, I previously proposed=C2=A0https://lists.linuxfoundation.org/pipermail/bitcoin-de= v/2020-September/018168.html which is similar.

The benefit of the OP_VER output is that SIGHASH_= EXTERNAL has the issue that unless you're binding a WTXID (which is may= be too specific?) then you can have fee bumping cycles. Doing OP_VER output= w/ TXID guarantees that you are acyclic.

The difference between a fee account and this approach bas= ically boils down to the impact on e.g. reorg stability, where the deposit/= withdraw mechanism is a bit more "robust" for reorderings in reor= gs than the in-band transaction approach, although they are very similar.

--

On Tue, Jan 18, 2022 at 8:53 PM Billy Tetrud <billy.tetrud@gmail.com= > wrote:
=
>=C2=A0 bec= ause you make transactions third party malleable it becomes possible to bun= dle and unbundle transactions.

What I was sugges= ting doesn't make it possible to malleate someone else's transactio= n.=C2=A0I guess maybe my proposal of using a sighash flag might have be= en unclear. Imagine it as a script opcode that just says "this transac= tion=C2=A0must be mined with this other transaction" - the only differ= ence being that you can use any output with any encumberance=C2=A0as an inp= ut for fee=C2=A0bumping. It doesn't prevent the original transaction fr= om being mined on its own. So adding junk inputs would be no more of a prob= lem than dust attacks already are. It would be used exactly like cpfp, exce= pt it doesn't spend the parent.=C2=A0

= I don't think what I was suggesting is as different from your proposal.= All the problems of fee revenue optimization and feerate rules that you me= ntioned seem like they'd also exist for your proposal, or for cpfp. Let= me know if I should clarify further.=C2=A0

On Tue, Jan 18, 202= 2 at 8:51 PM Jeremy <jlrubin@mit.edu> wrote:
The i= ssue with sighash flags is that because you make transactions third party m= alleable it becomes possible to bundle and unbundle transactions.

This means there are circumstances where an attacker could e.g. see your= txn, and then add a lot of junk change/inputs=C2=A0+ 25 descendants and st= rongly anchor your transaction to the bottom of the mempool.

be= cause of rbf rules requiring more fee and feerate, this means you have to b= ump across the whole package and that can get really messy.

mor= e generally speaking, you could imagine a future where mempools track many = alternative things that might want to be in a transaction.

sup= pose there are N inputs each with a weight and an amount of fee being added= and the sighash flags let me pick any subset of them. However, for a txn t= o be standard it must be < 100k bytes and for it to be consensus < 1m= b. Now it is possible you have to solve a knapsack problem in order to rati= onally bundle this transaction out of all possibilities.

This p= roblem can get even thornier, suppose that the inputs I'm adding themse= lves are the outputs of another txn in the mempool, now i have to track and= propagate the feerates of that child back up to the parent txn and track a= ll these dependencies.

<= div class=3D"gmail_default" style=3D"font-family:arial,helvetica,sans-serif= ;font-size:small;color:rgb(0,0,0)">perhaps with very careful engineering th= ese issues can be tamed. however it seems with sponsors or fee accounts, by= separating the pays-for from the participates-in concerns we can greatly s= implify it to something like: compute effective feerate for a txn, includin= g all sponsors that pay more than the feerate of the base txn. Mine that tx= n and it's subsidies using the normal algo. If you run out of space, al= l subsidies are same-sized so just take the ones that pay the highest amoun= t up until the added marginal feerate is less than the next eligible txn.


<= /div>

On Tue, Jan 18, 2022 at 6:38 PM Billy Tetrud <billy.tetrud@gmail.com> w= rote:
I see, its not primarily to make it cheaper to append fees, but al= so allows appending fees in cases that aren't possible now. Is that rig= ht? I can certainly see the benefit of a more general way to add a fee to a= ny transaction, regardless of whether you're related to that transactio= n or not.=C2=A0

How would you compare the pros and cons = of your account-based approach to something like a new sighash flag? Eg a s= ighash flag that says "I'm signing this transaction, but the signa= ture is only valid if mined in the same block as transaction=C2=A0X (or may= be transactions LIST)". This could be named SIGHASH_EXTERNAL. Doing th= is would be a lot more similar to other bitcoin transactions, and no specia= l account would need to be created. Any transaction could specify this. At = least that's the first thought I would have in designing a way to arbit= rarily bump fees. Have you compared your solution to something more familia= r like that?

On Tue, Jan 18, 2022 at 11:43 AM Jeremy <jlrubin@mit.edu> wrote:
<= /div>
Can you clarify what you mean by "imp= rove the situation"?

There's a potential mild bytes sa= vings, but the bigger deal is that the API should be much less vulnerable t= o pinning issues, fix dust leakage for eltoo like protocols, and just gener= ally allow protocol designs to be fully abstracted from paying fees. You ca= n't easily mathematically quantify=C2=A0API improvements=C2=A0like that= .

<= br>
On Tue,= Jan 18, 2022 at 8:13 AM Billy Tetrud <billy.tetrud@gmail.com> wrote:
<= blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-l= eft:1px solid rgb(204,204,204);padding-left:1ex">
Do y= ou have any back-of-the-napkin math on quantifying=C2=A0how much this would= improve the situation vs existing methods (eg cpfp)?

<= div>


On Sat, Jan 1, 2022 at 2:04 PM Jeremy via = bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
Happy new years devs,

I figured = I would share some thoughts for conceptual review that have been bouncing a= round my head as an opportunity to clean up the fee paying semantics in bit= coin "for good". The design space is very wide on the approach I&= #39;ll share, so below is just a sketch of how it could work which I'm = sure could be improved greatly.
Transaction fees are an integra= l part of bitcoin.

However, due to quirks of Bitcoin's tran= saction design, fees are a part of the transactions that they occur in.

While this works in a "Bitcoin 1.0" world, where all tra= nsactions are simple on-chain transfers, real world use of Bitcoin requires= support for things like Fee Bumping stuck transactions, DoS resistant Paym= ent Channels, and other long lived Smart Contracts that can't predict f= uture fee rates. Having the fees paid in band makes writing these contracts= much more difficult as you can't merely express the logic you want for= the transaction, but also the fees.

Previously, I proposed a s= pecial type of transaction called a "Sponsor" which has some spec= ial consensus=C2=A0+ mempool rules to allow arbitrarily appending fees to a= transaction to bump it up in the mempool.

As an alternative, w= e could establish an account system in Bitcoin as an "extension block&= quot;.

Here's how it might work:

1. Defi= ne a special anyone can spend output type that is a "fee account"= (e.g. segwit V2). Such outputs have a redeeming key and an amount associat= ed with them, but are overall anyone can spend.
2. All deposits to these outputs get stored in a separate UTX= O database for fee accounts
3. Fee= accounts can sign only two kinds of transaction: A: a fee amount and a TXI= D (or Outpoint?); B: a withdraw amount, a fee, and an=C2=A0address
4. These transactions are committed in an = extension block merkle tree. While the actual signature must cover the TXID= /Outpoint, the committed data need only cover the index in the block of the= transaction. The public key for account lookup can be recovered from the m= essage=C2=A0+ signature.
5. In any= block, any of the fee account deposits can be: released into fees if there= is a corresponding tx; consolidated together to reduce the number of utxos= (this can be just an OP_TRUE no metadata needed); or released into fees *a= nd paid back* into the requested withdrawal key (encumbering a 100 block ti= meout). Signatures must be unique in a block.
6. Mempool logic is updated to allow attaching of account fee s= pends to transactions, the mempool can restrict that an account is not allo= wed more spend more than it's balance.

But aren't ac= counts "bad"?

Yes, accounts are bad. But these ac= counts are not bad, because any funds withdrawn from the fee extension are = fundamentally locked for 100 blocks as a coinbase output, so there should b= e no issues with any series of reorgs. Further, since there is no "ric= h state" for these accounts, the state updates can always be applied i= n a conflict-free way in any order.


Improving the privacy of this design:

This design could likely be modified to implement something like Tornado.= cash or something else so that the fee account paying can be unlinked from = the transaction being paid for, improving privacy at the expense of being a= bit more expensive.

Other operations could be added to allow a= trustless mixing to be done by miners automatically where groups of accoun= ts with similar values are trustlessly =C2=A0split into a common denominato= r and change, and keys are derived via a verifiable stealth address like pr= otocol (so fee balances can be discovered by tracing the updates posted). T= hese updates could also be produced by individuals rather than miners, and = miners could simply honor them with better privacy. While a miner generatin= g an update would be able to deanonymize their mixes, if you have your acco= unt mixed several times by independent miners that could potentially add su= fficient privacy.

The LN can also be used with PTLCs to, in the= ory, have another individual paid to sponsor a transaction on your behalf o= nly if they reveal a valid sig from their fee paying account, although unde= r this model it's hard to ensure that the owner doesn't pay a fee a= nd then 'cancel' by withdrawing the rest. However, this could be pa= rtly solved by using reputable fee accounts (reputation could be measured s= omewhat decentralized-ly by longevity of the account and transactions paid = for historically).

Scalability

This d= esign is fundamentally 'decent' for scalability because adding fees= to a transaction does not require adding inputs or outputs and does not re= quire tracking substantial amounts of new state.

Paying someone= else to pay for you via the LN also helps make this more efficient if the = withdrawal issues can be fixed.
Lightning:

This type of design works really well for channels because the additi= on of fees to e.g. a channel state does not require any sort of pre-plannin= g (e.g. anchors) or transaction flexibility (SIGHASH flags). This sort of d= esign is naturally immune to pinning issues since you could offer to pay a = fee for any TXID and the number of fee adding offers does not need to be re= stricted in the same way the descendant transactions would need to be.

Without a fork?
This type of design could be don= e as a federated network that bribes miners -- potentially even retroactive= ly after a block is formed. That might be sufficient to prove the concept w= orks before a consensus upgrade is deployed, but such an approach does mean= there is a centralizing layer interfering with normal mining.


Happy new year!!

Jeremy

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