Re: [bitcoin-dev] bustapay BIP :: a practical sender/receiver coinjoin protocol

[Adam Gibson <ekaggata@gmail.com>]

On 27. 01. 19 8:36, rhavar@protonmail.com wrote:
> Thanks Adam,
> 
> I have fixed the mistakes you have pointed out: https://github.com/bitcoin/bips/pull/754
> 
> Thanks for the detailed look!
> 
>> but its virtue of steganographic hiding means only minimal uptake
>> is still enormously interesting and worth pursuing; that's my current feeling.
> 
> I very much agree =) I really think anything that (silently) breaks the assumption of common ownership of transaction inputs offers outsized benefits for the whole ecosystem.
> 
> One other idea I have  is (way) better support for moving utxo's between wallets. The least controversial use case is moving funds between wallets you own. Like I might want to move *specific* utxo's from/to my joinmarket wallet, but not create a (privacy losing / expensive) transaction. Both core and joinmarket fail at this at a practical point of view.

(tangential, but yes coin control in JM is an obviously necessary
feature and will be done, I just don't have time).

> 
> Like imho it'd be pretty cool having a standardized format for (txid:vout:privatekey) with wallets showing it as "External UTXO" and preferentially spending it (and wallet not automatically importing any other utxo from that address).
> 
> Taken a bit further (this is the part which everyone hates) you could send someone money (or withdraw it from a service) by giving a person. It's not generally useful (for obvious reasons), but there's a lot of cases I think it's super cool.

Is there a missing word. "by giving a person.."? Not actually sure what
you're getting at here but I suspect it's again tangential to this BIP
discussion.

> 
> ---
> 
> Getting back on topic, without trying to do a point-by-point reply, I agree with pretty much everything you said but I am reluctant to make any changes.
> 
> I don't meant to be obtuse or anything, but I strongly believe the limiting factor to adoption to all these protocols is actually just getting people to implement it. I made multiple implementations of bustapay from both the sending/receiving end, so I could try develop the easiest to implement system that is still practical.

You know, there's considerable evidence to the contrary, I'd argue: this
idea *has* been implemented already three times: by yourself, by myself
and by Samourai. And in fully incompatible ways :) So I think the
limiting factor is in fact creating a standard that a reasonable number
of people could agree with (and I like operational definitions, so
subjective as it is, I like the goal of "good/clear enough that it could
be incorporated into something like BtcPayServer")

> 
> For instance I like PSBT and it's nice in theory. I actually had an original implementation using it, which is how I found some bugs in the core and golang version of PSBT). But in practice it's hugely overkill and significantly increases the implementation complexity complexity and is poorly supported. Switching to just a raw transaction actually made everything easier. (And that's not to criticise PSBT, I would definitely want to use it in other contexts).

But this relates back to my first "generic" point that you haven't
addressed here - protocol versioning and the possibility of more than
one option. Perhaps more realistic (debatable): have the current version
be non-PSBT but with a plan to have a version bump with PSBT in future.
Stuff like that. It seems crazy to actually long term reject it.

> 
> Anyway, a big motivation for me even writing it as a BIP was to formalize my little anti-DOS trick of privately creating a "template transaction" which can just be dumped on the network as punishment. So if nothing else, hopefully I'll have demonstrated it's a pretty practical way of doing things.
> 

I don't want to be that guy, but this was a central part of the proposal
that came of the meetup last summer and is in Haywood's blogpost. I mean
if you came up it with separately, then cool :) But I was there, that
was established immediately as the right way of doing this to avoid a
trivial attack.
What might have confused you is all that stuff about multiple candidates
and even ZKP approaches - those were just extra ideas about making it
really secure at large scale; but those ideas don't quite meet the goal
(for various reasons); well, arguably. The basic anti-DOS of an initial
non-coinjoin was sorta central.
(Also I'm noting you didn't respond to my critique of your "always use
the same contributions" defence; I mean, probably that's fine, it was
only really saying it isn't perfect. Was just curious to hear
your/others thoughts on it).

> --
> 
> Also your analysis on "Unnecessary Input Heuristic" is pretty cool, but I also don't like telling people to "avoid the UIH2" without providing the actual algo they should use. But really I think it's better off in a sort of article "how to pick contributed inputs" or something, as while it's nice it's not a huge deal and there's a lot of debatable tradeoffs that can/should be used. For instance the implementation I wrote for bustabit.com currently just heavily biases tainted inputs (e.g. ones associated with address reuse).
> 

Good point about algo.
I wrote my best effort at a procedure here:
https://gist.github.com/AdamISZ/4551b947789d3216bacfcb7af25e029e#gistcomment-2799709

I asked for comments on it but got none back so far (gists are terrible
for this unfortunately, perhaps I'll have more luck on the list).

I would argue that this issue *should* be mentioned on the BIP. A *huge*
part of what makes PayJoin/BustaPay of interest is the steganographic
feature, if you don't pay attention to this then it doesn't look like a
payment (caveat.-->).
The counterargument is Laurent's statistics which I previously linked,
suggesting that maybe 30% of txs violate this anyway, today. I'm not
sure about that, will need more analysis; Core's SRD algo may be one
reason, but anyway ... better to make things look like payments.

It doesn't hurt to prompt an implementer to do this; whether it's
feasible in that specific wallet situation or not is up to them; whether
they want to go hog wild and control percentages of UIH1 and UIH2 and
whatnot is there business, or they can totally ignore it - but without
it being mentioned in the BIP, they may not even think of it.

A last point, you also don't see value in being more explicit about
simple things like transaction version and locktime? Even if you think
these things should *not* be controlled, e.g. the protocol should allow
either transaction version, then it'd be better to explicitly say so.
> 
> 
> -Ryan
> 
> ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐
> On Friday, January 25, 2019 6:47 AM, Adam Gibson via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
> 
>> Ryan and list,
>> I want to add some commentary to this (BIP79) to see if we can get
>> further in standardizing this idea.
>>
>> When I first mulled it over I thought it too impractical, but its virtue
>> of steganographic hiding means only minimal uptake is still enormously
>> interesting and worth pursuing; that's my current feeling. I've offered
>> more detailed thoughts in my blog post[1] (def not required reading here).
>>
>> Both Joinmarket and Samourai have started implementing this kind of
>> transaction. And while that's interesting experimentally, some kind of
>> cross-wallet standard would be helpful, albeit there some differences
>> between that and the merchant/centralized service use-case.
>>
>> We might imagine as a concrete goal for this BIP to create something
>> that would be acceptable for inclusion into a project like BTCPayServer,
>> so that it could be used in a realistic use case by smaller bitcoin
>> accepting merchants.
>>
>> Comments to the BIP[2] as follows, with generic comments first, and then
>> specific comments for existing points in the BIP:
>>
>> [1] https://joinmarket.me/blog/blog/payjoin
>> [2] https://github.com/bitcoin/bips/blob/master/bip-0079.mediawiki
>>
>> Generic comments
>>
>> ==================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================================
>>
>> -   Protocol versioning. Since inevitably (even if only merchants), this
>>     must be implemented by multiple wallets to be useful, the communication
>>     protocol will need versioning (for example i have in my
>>     simple/experimental Joinmarket PayJoin that sender sends min and max
>>     supported version and receiver responds with a chosen protocol version
>>     so we can update). I do understand that as a client-server model can
>>     apply here, we can ditch a lot of the complexities around network/p2p
>>     interaction, but this much at least seems necessary.
>>
>> -   Although it has its logic, I don't think "Bustapay" is a good name for
>>     this protocol. I prefer "PayJoin" which is neutral sounding and
>>     self-descriptive. Needless to say this is not a hill I intend to die on.
>>
>> -   PSBT/BIP174. I realise this has already been discussed, but this is a
>>     good example of what this standardisation was designed for, so I'd be
>>     against not including it, even given the reality that, as you correctly
>>     observe, it is not yet implemented in the majority of wallets and
>>     libraries. One way round that is to make it optional (possibly combined
>>     with above point about versioning). Note that for example you were
>>     observing the necessity to check the sequence number was unchanged; that
>>     would be encapsulated by checking equality of PSBT Input objects/fields.
>>     While one can make such software architecture arguments, the really
>>     fundamental point is the need for standards for x-wallet compatibility.
>>
>> -   Version, Locktime: Perhaps this is not needed; in a peer to peer
>>     wallet scenario I think there might be logic in trying to get cover
>>     traffic of (Core, Electrum, others), say, by using
>>     last-block-locktime-mostly, as they do. Version should be 2 and sequence
>>     is a function of your suggestion to use BIP125. Worth noting that BIP125
>>     is not currently widely used on the network, though (see
>>     https://p2sh.info/dashboard/db/replace-by-fee?orgId=1). For this reason
>>     it should perhaps be explicitly only optional.
>>
>> -   Avoidance of non-payment "Unnecessary Input Heuristic" (1, 2). For
>>     reference, see the definition here
>>     https://gist.github.com/AdamISZ/4551b947789d3216bacfcb7af25e029e#gistcomment-2796539
>>     and some data here
>>     https://gist.github.com/AdamISZ/4551b947789d3216bacfcb7af25e029e#gistcomment-2800791
>>     (whole comment thread may be of interest) - note this UIH name is afaik
>>     Chris Belcher's invention, it seems useful as a categorisation.
>>     So, it seems that UIH2 is more important to avoid; while some more
>>     sophisticated wallet coin selection algorithms may occasionally pick
>>     an input set where one input is larger than any output, most won't, and
>>     some in particular never will. So I think the text here should indicate
>>     that *the receiver's contributed input(s) SHOULD be chosen to avoid
>>     triggering the UIH2 heuristic where possible, so that the final payjoin
>>     transaction is maximally plausible as an ordinary payment" or similar.
>>     UIH1 is a nice-to-have (meaning the plausibility extends to two
>>     different (both wrong) payment amounts, but it may not be necessary to
>>     mention it in the BIP.
>>
>>     Specific comments
>>     =================
>>
>>
>>>> ====Step 4. Receiver validates, re-signs, and propagates on the
>>
>> bitcoin network====
>>
>> I believe this should say "Sender" not Receiver. Also for the next
>> sentence, s/receiver/sender/:
>>
>>>> The receiver MUST validate the ''partial transaction'' was changed
>>
>> correctly and non-maliciously (to allow using potentially untrusted
>> communication channels), re-sign its original inputs and propagate the
>> final transaction over the bitcoin network.
>>
>> Your very correct highlighting of the attack vector of "receiver gives
>> sender other inputs belonging to sender to unwittingly sign (described
>> below), should be highlighted here, perhaps with the phrase "re-sign its
>> ORIGINAL inputs" (only!)".
>>
>>>> When the sender is creating a "template transaction" it is done
>>
>> almost identically to creating a normal send, with the exception that
>> only segwit inputs may be used. The sender is also encouraged to use a
>> slightly more aggressive feerate than usual as well as BIP125 (Opt-in
>> Full Replace-by-Fee Signaling), but neither is strictly required.
>>
>> "slightly more aggressive feerate than usual" - this I understand is to
>> make up for receiver contributed utxo, OK.
>>
>> "only segwit inputs" - it certainly makes things simpler. One can work
>> with non-segwit inputs but especially considering (as mentioned below)
>> we really ought to "MUST" the part about matching input types, I tend to
>> agree that non-segwit should be disallowed.
>>
>>>> The receiver must add at least one input to the transaction (the
>>
>> "contributed inputs"). If the receiver has no inputs, it should use a
>> 500 internal server error, so the client can send the transaction as per
>> normal (or try again later).
>>
>> Would it not be much simpler for the server to return a different
>> (non-error) response indicating that it will broadcast the template tx
>> in this case?
>>
>>>> Its generally advised to only add a single contributed input, however
>>
>> they are circumstances where adding more than a single input can be useful.
>>
>> I don't see a good reason to advise the use of only 1 input? (but this
>> will also connect with the above generic comment about "UIH"). I guess
>> it's because of your approach to fees. I'd prefer not to create a
>> limitation here.
>>
>>>> To prevent an attack where a receiver is continually sent variations
>>
>> of the same transaction to enumerate the receivers utxo set, it is
>> essential that the receiver always returns the same contributed inputs
>> when it's seen the same inputs.
>>
>> This is an approach to avoiding this problem which has the virtue of
>> simplicity, but it seems a little problematic. (1) You must keep a
>> mapping of proposed payment utxos to one's proposed contributed input
>> utxos, but (2) how should this be updated if you need to spend the
>> contribution mentioned in (1)? Ironically use of payjoin exacerbates
>> this issue, because it results in a smaller number of utxos being held
>> by the receiver at any one time :) All this considered, I still see the
>> value in your approach, but it might end up with a re-attempted payment
>> being rejected. Certainly the more complex suggested solutions coming
>> out of the summer 2018 coinjoin workshop aren't as practical as this,
>> and may be overkill for small merchants/receivers.
>>
>>>> It is strongly preferable that the receiver makes an effort to pick a
>>
>> contributed input of the same type as the other transaction inputs if
>> possible.
>>
>> I have also thought about this and you could reasonably argue this
>> should be a MUST section in the BIP, that is, if the receiver cannot use
>> inputs of the same type, he should fall back to the template
>> transaction. A mixed-input payjoin/coinjoin is essentially
>> near-perfectly identifiable as such (there is almost zero other usage of
>> multi-type-input transactions), which is a very different thing than a
>> non-identifiable payjoin transaction. That may or may not be OK to the
>> sender. This is debatable though, for sure.
>>
>>>> After adding inputs to the transaction, the receiver generally will
>>
>> want to adjust the output that pays himself by increasing it by the sum
>> of the contributed input amounts (minus any fees he wants to
>> contribute). However the only strict requirement is that the receiver
>> must never add or remove inputs, and must not ever decrease any
>> output amount.
>>
>> "must never add or remove inputs" - did you mean "must never remove
>> inputs"? he surely has to add one! Or, perhaps you mean he must not
>> alter the list of inputs provided by the sender (in which case it should
>> be clarified).
>>
>> "must not decrease any output amount" - I initally disagreed with this
>> but it is a better option than the one I currently chose in Joinmarket
>> payjoin (sender pays all fee as long as receiver utxos are not too
>> much). So this means that the receiver either consciously chooses to not
>> increase the fee, meaning the fee rate may be a bit low (hence your
>> earlier comment about being generous, got it), or contributes via the
>> payout amount. I guess the latter might break merchant software
>> expecting to have amount output fixed and fees determined by change.
>>
>> Regards,
>> Adam Gibson/waxwing
>>
>> On 30. 08. 18 22:24, Ryan Havar via bitcoin-dev wrote:
>>
>>> I've just finished writing an implementing of this, and extremely happy
>>> with how it turned out. So I'd like to go and try go down the path of
>>> more formally describing it and getting some comments and ultimately
>>> encourage its wide-spread use.
>>> ==Abstract==
>>> The way bitcoin transactions are overwhelming used is known to leak more
>>> information than desirable. This has lead to fungibility concerns in bitcoin
>>> and a raise of unreasonably effective blockchain analysis.
>>> Bustapay proposes a simple, practical way to bust these assumptions to
>>> immediate
>>> benefit of the sender and recievers. Furthermore it does so in such a
>>> way that
>>> helps recievers avoid utxo bloat, a constant problem for bitcoin merchants.
>>> ==Copyright==
>>> This BIP is in the public domain.
>>> ==Motivation==
>>> One of the most powerful heuristic's employed by those whose goal is to
>>> undermine
>>> bitcoin's fungiblity has been to assume all inputs of a transaction are
>>> signed by
>>> a single party. In the few cases this assumption does not hold, it is
>>> generally
>>> readibly recognizable (e.g. traditional coinjoins have a very obvious
>>> structure,
>>> or multisig outputs are most frequently validated onchain).
>>> Bustapay requires no changes to bitcoin and creates bitcoin transactions
>>> that are
>>> indistinguishable from normal ones.
>>> It is worth noting that this specification has been intentionally kept
>>> as simple
>>> as possible to encourage adoption. There are almost an endless amount of
>>> extensions
>>> possible but the harder the implementation of clients/server the less
>>> likely it
>>> will ever be done. Should bustapay enjoy widespread adoption, a "v2"
>>> specification
>>> will be created with desired extensions.
>>> ==Specification==
>>> A bustapay payment is made from a sender to a receiver.
>>> Step 1. Sender creates a bitcoin transaction paying the receiver
>>> This transaction must be fully valid, signed and all inputs must use
>>> segwit. This transaction is known as the "template transaction". This
>>> transaction must not be propagated on the bitcoin network.
>>> Step 2. Sender gives the "template transaction" to the receiver
>>> This would generally be done as an HTTP POST. The exact URL to submit it
>>> to could be specified with a bip21 encoded address. Such as
>>> bitcoin:2NABbUr9yeRCp1oUCtVmgJF8HGRCo3ifpTT?bustapay=https://bp.bustabit.com/submit
>>> and the HTTP body should be the raw transaction hex encoded as text.
>>> Step 3. Receiver processes the transaction and returns a partially
>>> signed coinjoin
>>> The receiver validates the transaction is valid, pays himself and is
>>> eligible for propation. The receiver then adds one of his own inputs
>>> (known as the "contributed input") and increase the output that pays
>>> himself by the contributed input amount. Doing so will invalidate the
>>> "template transaction"'s original input signatures, so the sender needs
>>> to return this "partial transaction" back to the receiver to sign. This
>>> is returned as a hex-encoded raw transaction a response to the original
>>> HTTP POST request.
>>> Step 4. Receiver validates, re-signs, and propagates on the bitcoin network
>>> The receiver is responsible in making sure the "partial transaction"
>>> returned by the sender was changed correctly (it should assume the
>>> connection has been MITM'd and act accordingly), resign its original
>>> inputs and propagates this transaction over the bitcoin network. The
>>> client must be aware that the server can reorder inputs and outputs.
>>> Step 5. Receiver observes the finalized transaction on the bitcoin network
>>> Once the receiver has seen the finalized transactions on the network
>>> (and has enough confirmations) it can process it like a normal payment
>>> for the sent amount (as opposed to the amount that it looks like on the
>>> network). If the receiver does not see the finalized transaction after a
>>> timeout will propagate the original "template transaction" to ensure the
>>> payment happens and function a strong anti-DoS mechanism.
>>> === Implementation Notes ===
>>> For anyone wanting to implement bustapay payments, here are some notes
>>> for receivers:
>>>
>>> -   A transaction can easily be checked if it's suitable for the mempool
>>>     with testmempoolaccept in bitcoin core 0.17
>>>
>>> -   Tracking transactions by txid is precarious. To keep your sanity make
>>>     sure all inputs are segwit. But remember segwit does not prevent txid
>>>     malleability unless you validate the transaction. So really make sure
>>>     you're using testmempoolaccept at the very least
>>>
>>> -   Bustapay could be abused by a malicious party to query if you own a
>>>     deposit address or not. So never accept a bustapay transaction that pays
>>>     an already used deposit address
>>>
>>> -   You will need to keep a mapping of which utxos people have showed you
>>>     and which you revealed. So if you see them again, you can reveal the
>>>     same one of your own
>>>
>>> -   Check if the transaction was already sorted according to BIP69, if so
>>>     ensure the result stays that way. Otherwise probably just shuffle the
>>>     inputs/outpus
>>>
>>>
>>> Notes for sending applications:
>>>
>>> -   The HTTP response must not be trusted. It should be fully validated
>>>     that no unexpected changes have been made to the transaction.
>>>
>>> -   The sender should be aware the original "template transaction" may be
>>>     propagated at any time, and in fact can intentionally be
>>>       done so for the purpose of RBF as it should have a slightly higher fee
>>>     rate.
>>>
>>>
>>> == Credits ==
>>> The idea is obviously based upon Dr. Maxwell's seminal CoinJoin
>>> proposal, and reduced scope inspired by a simplification of the "pay 2
>>> endpoint" (now offline) blog post by blockstream.
>>> -Ryan
>>>
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
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> 
>