[bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

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Hi All,

I have been working on a proposal for an opcode I call
OP_CONSTRAINDESTINATION. The purpose of the opcode is to allow a spend-path
to restrict the destination address that an output's coins can be directed
to. When the destination address is something like a P2SH address, this
allows step-wise covenant scripts (where one script must lead to another).

This involves both specifying particular addresses the output is allowed to
send coins to, as well as constraining the amount of the fee that output is
allowed to contribute to. For example, if you had an output that contains
1000 satoshi, you could specify that a maximum of ~100 sats of that output
go to the miner fee and the other ~900 sats must go to one of a list of
specified addresses (~ meaning approximately, because the fee is specified
relative to recent median fee rates - details in the proposal).

This opcode has a few different applications, but my primary motivation for
creating this opcode is to create more flexible wallet vaults
<https://hackingdistributed.com/2016/02/26/how-to-implement-secure-bitcoin-vaults>
.

To compare this opcode to OP_CHECKTEMPLATEVERIFY, wallet vaults that can be
created with OP_CTV must be created in specified chunks: the address is
explicitly tied to a particular utxo sent to it. To retrieve coins from the
vault, the output must be spent by one of a specific set of transactions
(potentially one per spend path). Outputs cannot be arbitrarily combined
into a transaction, and there is no flexibility whatsoever in deciding
options at the time of spending from the vault - all options must be
premeditated and encoded into the address itself when sending money to the
vault. This has some related foot-gun scenarios, where the wallet vault has
addresses that if sent to would generally result in burning those coins,
unless done in a very specific way by the owner of the vault.

By contrast, OP_CD allows a lot more flexibility because it only constrains
the address to be sent to from the vault, but doesn't put additional
constraints on the transaction. This means that outputs can be combined
into a single transaction like you would expect in a normal transaction. It
also means that external users (people who don't own the vault) can safely
send money directly into the vault without coins being burned.

*I have the proposal for this opcode up here:
https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md>*.
I'd love to hear what people think about it, what problems it might have
that I've missed, or other issues or suggestions surrounding this. I'd also
appreciate any input that would help me improve the presentation of the
opcode.

Thanks!

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[David A. Harding]

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On Tue, Jul 20, 2021 at 10:56:10PM -0700, Billy Tetrud via bitcoin-dev wrote:
> This involves [...] constraining the amount of the fee that output is
> allowed to contribute to.  [...] fee is specified relative to recent
> median fee rates - details in the proposal).

Here are the relevant details:

> The medianFeeRate is defined as the median fee rate per vbyte for the
> most recent windowLength blocks. The maxFeeContribution is defined as
> medianFeeRate * 2^feeFactor of the fee. Note that this is a limitation
> on the fee, not on the fee-rate. If feeFactor is -1,
> maxFeeContribution is 0.

First, I don't think we want full nodes to have to store the feerate for
every transaction in a 3,000 block window (~2.5 million txes, assuming
all segwit).  I'm sure you could tweak this proposal to require a much
smaller dataset.

Second, I think this requires careful consideration of how it might
affect the incentives for miners.  Miners can include many small high-fee
pay-to-self transactions in their blocks to raise the median feerate,
but this puts them at increased risk of fee sniping from other miners,
which may incentivize fee-raisers to centralize their mining, which is
ultimately bad.  I'm not sure that's a huge concern with this proposal,
but I think it and other incentive problems require consideration.

Finally, I think this fee mechanism is redundant.  For any case where
this opcode will be used, you'll want to have two things:

    1. A mutual spend clause (e.g. a multisignature taproot keypath
       spend) where all parties agree on a spend of the output and so
       can set an appropriate feerate at that time.  You want this
       because it'll be the most efficient way to spend.

    2. A fee override that allows paying additional fees beyond what
       OP_CONSTRAINDESTINATION allows, either through attaching an
       additional input or through CPFP.  You want this because you
       will know more about feerate conditions at spend time than you
       did when you created the receiving script.

If you have the ability to choose feerates through the above mechanisms,
you don't need a constrained feerate mechanism that might be
manipulable by miners.

(I haven't looked closely at the rest of your proposal; the above just
caught my attention.)

-Dave

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

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Thanks for taking a look at the proposal David. I appreciate it.

> I don't think we want full nodes to have to store the feerate for every
transaction in a 3,000 block window

That's a good point. It would probably be just as good to find the median
fee-rate for each block and store that, then calculate the average of those
stored per-block median numbers. Do you think that would be sufficiently
cheap to store?

> Miners can include many small high-fee pay-to-self transactions in their
blocks to raise the median feerate

Definitely a reasonable thing to consider. One point I want to make about
that tho is that the opcode only limits how much of a particular output can
be put towards the transaction fee - for the vast majority of transactions
using this opcode, a lower fee would be used and the limit would be
irrelevant (and therefore raising the median fee rate would not affect
those transactions). The point of limiting the fee is to limit an
attacker's ability to grief a victim by sending all their funds as
transaction fee. So the only situations where miners would gain something
from raising the fee rate is for griefing situations, which should be so
rare as to be completely insignificant to miners. If griefing is not rare,
something else is pretty broken.

> I think this fee mechanism is redundant

See above, but to break down that situation a bit further, these are the
two situations I can think of:

   1. The opcode limits user/group A to send the output to user/group B
   2. The opcode limits user A to send from one address they own to another
   address they own.

In case 1, user/group A could be the attacker that attempts to direct as
much of the outputs as possible towards the fee (instead of the agreed upon
recipient user/group B). In case 2, the attacker would be someone that
steals a key from the user (eg in the case the attacker gets access to 1
key of the wallet vault keys) and attempts to grief them by making a
transaction with the highest possible fee. In both these scenarios, the fee
limit helps limit the amount of damage these attackers could do to their
victim. Without a fee limit, these attack vectors could spend up to the
full output amount as fee, which could be very damaging.

> A mutual spend clause

Have you considered the use case of wallet vaults? I designed this opcode
primarily with wallet vaults in mind. In such a case there is a "mutual
spend clause" of a kind - but all the keys may be owned by a single
individual. One of the keys would be kept close at hand, and other keys
would be kept in more secure and more difficult-to-access places (like a
safe in a remote location). While the key-spend-path would be cheapest on
chain, traveling to get the key itself might often be more expensive than
using the script spend-path (because it takes time and effort to travel to
those locations and access the keys). It might be informative to take a
look at these wallet vault scripts
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/op_cdWalletVault1.md>
that
could use this opcode or the larger vision
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/README.md>
I have for wallet vaults (which involves 2 other new opcodes). There are
certainly also multi-user multisig use cases for OP_CD that have similar
properties to this single-user use case.

> A fee override that allows paying additional fees .. through attaching an
additional input or through CPFP

I definitely agree those are desirable mechanisms. To reiterate what I said
above, the fee limitation is there to limit griefing attack vectors that
spend an unreasonable amount of a particular output towards the fee.
Spending *other* outputs (via either of those mechanisms) towards a
transaction's fee is perfectly acceptable and doesn't undermine the purpose
of the fee limitation.

At its core, the limitation is there because the miner is another
destination that the output's funds can be sent to, and while it wouldn't
be wise to prevent an output from being spent as fee at all (because then
the output is unspendable on its own, or with any other similarly
constrained outputs), if OP_CD allowed spending the entire output as a fee
then it wouldn't be successful in constraining the destination to the
listed addresses.

Do you see my points here, or do you still think the limitation is
redundant?

Thanks,
BT




On Sat, Jul 24, 2021 at 10:39 PM David A. Harding via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> On Tue, Jul 20, 2021 at 10:56:10PM -0700, Billy Tetrud via bitcoin-dev
> wrote:
> > This involves [...] constraining the amount of the fee that output is
> > allowed to contribute to.  [...] fee is specified relative to recent
> > median fee rates - details in the proposal).
>
> Here are the relevant details:
>
> > The medianFeeRate is defined as the median fee rate per vbyte for the
> > most recent windowLength blocks. The maxFeeContribution is defined as
> > medianFeeRate * 2^feeFactor of the fee. Note that this is a limitation
> > on the fee, not on the fee-rate. If feeFactor is -1,
> > maxFeeContribution is 0.
>
> First, I don't think we want full nodes to have to store the feerate for
> every transaction in a 3,000 block window (~2.5 million txes, assuming
> all segwit).  I'm sure you could tweak this proposal to require a much
> smaller dataset.
>
> Second, I think this requires careful consideration of how it might
> affect the incentives for miners.  Miners can include many small high-fee
> pay-to-self transactions in their blocks to raise the median feerate,
> but this puts them at increased risk of fee sniping from other miners,
> which may incentivize fee-raisers to centralize their mining, which is
> ultimately bad.  I'm not sure that's a huge concern with this proposal,
> but I think it and other incentive problems require consideration.
>
> Finally, I think this fee mechanism is redundant.  For any case where
> this opcode will be used, you'll want to have two things:
>
>     1. A mutual spend clause (e.g. a multisignature taproot keypath
>        spend) where all parties agree on a spend of the output and so
>        can set an appropriate feerate at that time.  You want this
>        because it'll be the most efficient way to spend.
>
>     2. A fee override that allows paying additional fees beyond what
>        OP_CONSTRAINDESTINATION allows, either through attaching an
>        additional input or through CPFP.  You want this because you
>        will know more about feerate conditions at spend time than you
>        did when you created the receiving script.
>
> If you have the ability to choose feerates through the above mechanisms,
> you don't need a constrained feerate mechanism that might be
> manipulable by miners.
>
> (I haven't looked closely at the rest of your proposal; the above just
> caught my attention.)
>
> -Dave
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[David A. Harding]

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On Sun, Jul 25, 2021 at 12:49:38PM -0700, Billy Tetrud wrote:
> find the median fee-rate for each block and store that, then calculate
> the average of those stored per-block median numbers. 

One datapoint per block seems fine to me and it works much nicer with
pruned nodes.

> So the only situations where miners would gain something
> from raising the fee rate is for griefing situations, which should be so
> rare as to be completely insignificant to miners. 

I don't believe the problem scope can be reduced this way.  Although we
we often look at miners as separate from users, it's important to
remember that every miner is also a user of Bitcoin and ever user of
Bitcoin may also someday be a miner.  Users may also employ miners
directly via out-of-band payments.

In your usecase of vaults, we can imagine Bob is attempting to store
100,000 BTC.  He designs his vault to allow spending on fees up to 10x
the 3,000 block median fee.  Mallory steals Bob's encumbered spending
key.  Mallory could immediately go to a miner and offer them a 50/50
split on the 10x fees over the median (~10,000 sat?), or Mallory could
take a bit more time and work with a cartel of miners to raise the
median over a period of three weeks (3k blocks) to 10,000
BTC/transaction, allowing them to take all of Bob's coins in fees.

> if OP_CD allowed spending the entire output as a fee then it wouldn't
> be successful in constraining the destination to the listed addresses.

The alternative is to never allow OP_CD to spend any of the UTXOs it
encumbers to fees, requiring all fees be paid via another mechanism.
Since satisfactory designs are going to provide those other mechanisms
anyway, it seems to me that there's no need for OP_CD to manage fees.
That said, I don't have a real strong opinion here.

-Dave

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

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>  it's important to remember that every miner is also a user of Bitcoin
and ever user of Bitcoin may also someday be a miner

That's certainly true. One good quantification for how much of a problem
this could be is to calculate the cost of the attack vs the damage done in
the attack. So let me try to estimate that:

Miners could collectively shift the fee rate up by sending payments to
themselves, like you said. However, each one represents an opportunity cost
of a low-value transaction. Given a bell curve distribution of transaction
fee-rates, filling 15% of each block with these self-pay transactions could
raise the median fee rate by about 1/4 of a standard deviation, or
something probably around a 5% increase in median fee rate. This would lose
miners approximately 5% of the fees for the blocks they did this for. So in
order to do 1-to-1 damage (which would have them break even on the attack),
there would have to be enough of these transactions to fill up maybe 1% of
3000 blocks (if we assume these transactions will generally have 10 times
the fee-rate of the displaced low-value transactions). That would be on the
order of hundreds of thousands of transactions. A shorter sample
window would be easier to abuse this way, but even still likely at least
hundreds of transactions would be needed to make up the difference.

Manipulating fees this way has diminishing returns, meaning that filling a
smaller percent of a block with high-fee self-payment transactions would
lead to a greater increase in the median fee rate per amount of fees lost.

Something interesting about this attack is that a successful attack would
make the next attack easier, because mining these transactions from stolen
keys would also help raise the median fee-rate a bit (tho only a fraction
of the self-pay transactions that would still be necessary for the next
round of attacks).

And the above is a situation with 100% dishonest miners. With fewer
dishonest miners, say 25%, the attack would have a much lower ROI.

For the wallet vault use case, this is still a security improvement over a
normal wallet, since in a normal wallet, a stolen key means all your funds
can be stolen, but in an OP_CD wallet vault the attackers are still limited
in how much can be stolen via the fee, stealing via the fee requires paying
miners a cut to receive back some of the fee, and stealing extra  (via
raising the median fee rate) has a real cost placed on the miners.

For multi-party scenarios, I think the fee limit might be slightly less
effective. Eg in contracts where some money is promised to be sent to
another person's address (e.g. congestion controlled payments), if a miner
controls the sending address that miner can simply send the maximum fee to
gain more money directly. The limit is still partially effective, but its
definitely worth noting that malicious miners can abuse the fee limit
mechanism. I would think manipulating the median fee rate is just as
difficult in this scenario tho.

> pay-to-self transactions .. puts them at increased risk of fee sniping
from other miners, which may incentivize fee-raisers to centralize their
mining

This is an interesting point I forgot to respond to from your first email.
I think even without the threat of fee-sniping, fee raisers would want to
cartelize because coordinating the timing of attacks would reduce their
collective costs. Tho fee-sniping would increase this pressure, I agree. It
seems like cartels like this would have to get near the range of being able
to 51% attack to really be effective tho.

> The alternative is to never allow OP_CD to spend any of the UTXOs it
encumbers to fees

I agree that functionally this would work ok. However, both other
mechanisms (gathering keys for a multisig spend or CPFP / adding other
inputs) are likely to often be more expensive than letting the UTXO
contribute to the fee directly. Also, it would complicate usability of
these outputs, sometimes even making them unspendable by the user directly
(in the case they don't have access to external outputs to contribute to
the fee).

In any case, I've updated my proposal with some of the things we've
discussed. Thanks!

@Randy What are you agreeing with?

On Mon, Jul 26, 2021 at 5:59 AM Randy Fox <mrkingfoxx@hotmail.com> wrote:

> Agree.
>
>
> Sent from Yahoo Mail for iPhone
> <https://overview.mail.yahoo.com/?.src=iOS>
>
> On Sunday, July 25, 2021, 7:07 PM, David A. Harding via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
> On Sun, Jul 25, 2021 at 12:49:38PM -0700, Billy Tetrud wrote:
> > find the median fee-rate for each block and store that, then calculate
> > the average of those stored per-block median numbers.
>
> One datapoint per block seems fine to me and it works much nicer with
> pruned nodes.
>
> > So the only situations where miners would gain something
> > from raising the fee rate is for griefing situations, which should be so
> > rare as to be completely insignificant to miners.
>
> I don't believe the problem scope can be reduced this way.  Although we
> we often look at miners as separate from users, it's important to
> remember that every miner is also a user of Bitcoin and ever user of
> Bitcoin may also someday be a miner.  Users may also employ miners
> directly via out-of-band payments.
>
> In your usecase of vaults, we can imagine Bob is attempting to store
> 100,000 BTC.  He designs his vault to allow spending on fees up to 10x
> the 3,000 block median fee.  Mallory steals Bob's encumbered spending
> key.  Mallory could immediately go to a miner and offer them a 50/50
> split on the 10x fees over the median (~10,000 sat?), or Mallory could
> take a bit more time and work with a cartel of miners to raise the
> median over a period of three weeks (3k blocks) to 10,000
> BTC/transaction, allowing them to take all of Bob's coins in fees.
>
> > if OP_CD allowed spending the entire output as a fee then it wouldn't
> > be successful in constraining the destination to the listed addresses.
>
> The alternative is to never allow OP_CD to spend any of the UTXOs it
> encumbers to fees, requiring all fees be paid via another mechanism.
> Since satisfactory designs are going to provide those other mechanisms
> anyway, it seems to me that there's no need for OP_CD to manage fees.
> That said, I don't have a real strong opinion here.
>
>
> -Dave
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
>

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[James MacWhyte]

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Hi Billy!

See above, but to break down that situation a bit further, these are the
> two situations I can think of:
>
>    1. The opcode limits user/group A to send the output to user/group B
>    2. The opcode limits user A to send from one address they own to
>    another address they own.
>
> I'm trying to think of a good use case for this type of opcode. In these
examples, an attacker who compromises the key for user A can't steal the
money because it can only be sent to user B. So if the attacker wants to
steal the funds, they would need to compromise the keys of both user A and
user B.

But how is that any better than a 2-of-2 multisig? Isn't the end result
exactly the same?

James

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

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Hey James,

In the examples you mentioned, what I was exploring was a mechanism of
attack by which the attacker could steal user A's key and use that key to
send a transaction with the maximum possible fee. User B would still
receive some funds (probably), but if the fee could be large, the attacker
would either do a lot of damage to user B (griefing) or could make an
agreement with a miner to give back some of the large fee (theft).

But as for use cases, the proposal mentions a number of use cases
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md#motivation>
and
most overlap with the use cases of op_ctv <https://utxos.org/uses/> (Jeremy
Rubin's website for op_ctv has a lot of good details, most of which are
also relevant to op_cd). The use case I'm most interested in is wallet
vaults. This opcode can be used to create a wallet vault where the user
only needs to use, for example, 1 key to spend funds, but the attacker must
steal 2 or more keys to spend funds. The benefits of a 2 key wallet vault
like this vs a normal 2-of-2 multisig wallet are that not only does an
attacker have to steal both keys (same level of security), but also the
user can lose one key and still recover their funds (better redundancy) and
also that generally the user doesn't need to access their second key - so
that can remain in a much more secure location (which would also probably
make that key harder to steal). The only time the second key only comes
into play if one key is stolen and the attacker attempts to send a
transaction. At that point, the user would go find and use his second key
(along with the first) to send a revoke transaction to prevent the attacker
from stealing their funds. This is somewhat akin to a lightning watchtower
scenario, where your wallet would watch the chain and alert you about an
unexpected transaction, at which point you'd manually do a revoke (vs a
watchtower's automated response). You might be interested in taking a look
at this wallet vault design
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/op_cdWalletVault1.md>
that uses OP_CD or even my full vision
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults> of the wallet
vault I want to be able to create.

With a covenant opcode like this, its possible to create very usable and
accessible but highly secure wallets that can allow normal people to hold
self custody of their keys without fear of loss or theft and without the
hassle of a lot of safe deposit boxes (or other secure seed storage
locations).

Cheers,
BT





On Mon, Jul 26, 2021 at 2:08 PM James MacWhyte <macwhyte@gmail.com> wrote:

> Hi Billy!
>
> See above, but to break down that situation a bit further, these are the
>> two situations I can think of:
>>
>>    1. The opcode limits user/group A to send the output to user/group B
>>    2. The opcode limits user A to send from one address they own to
>>    another address they own.
>>
>> I'm trying to think of a good use case for this type of opcode. In these
> examples, an attacker who compromises the key for user A can't steal the
> money because it can only be sent to user B. So if the attacker wants to
> steal the funds, they would need to compromise the keys of both user A and
> user B.
>
> But how is that any better than a 2-of-2 multisig? Isn't the end result
> exactly the same?
>
> James
>

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Zac Greenwood]

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Hi Billy,

On the topic of wallet vaults, are there any plans to implement a way to
limit the maximum amount to be sent from an address?

An example of such limit might be: the maximum amount allowed to send is
max(s, p) where s is a number of satoshi and p a percentage of the total
available (sendable) amount.

A minimum value may be imposed on the percentage to ensure that the address
can be emptied within a reasonable number of transactions. The second
parameter s allows a minimum permitted amount. (This is necessary because
with only the percentage parameter the minimum permitted amount converges
to zero, making it impossible to empty the address).

There may be other ways too. In my view, such kind of restriction would be
extremely effective in thwarting the most damaging type of theft being the
one where all funds are swept in a single transaction.

Zac


On Tue, 27 Jul 2021 at 03:26, Billy Tetrud via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hey James,
>
> In the examples you mentioned, what I was exploring was a mechanism of
> attack by which the attacker could steal user A's key and use that key to
> send a transaction with the maximum possible fee. User B would still
> receive some funds (probably), but if the fee could be large, the attacker
> would either do a lot of damage to user B (griefing) or could make an
> agreement with a miner to give back some of the large fee (theft).
>
> But as for use cases, the proposal mentions a number of use cases
> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md#motivation> and
> most overlap with the use cases of op_ctv <https://utxos.org/uses/> (Jeremy
> Rubin's website for op_ctv has a lot of good details, most of which are
> also relevant to op_cd). The use case I'm most interested in is wallet
> vaults. This opcode can be used to create a wallet vault where the user
> only needs to use, for example, 1 key to spend funds, but the attacker must
> steal 2 or more keys to spend funds. The benefits of a 2 key wallet vault
> like this vs a normal 2-of-2 multisig wallet are that not only does an
> attacker have to steal both keys (same level of security), but also the
> user can lose one key and still recover their funds (better redundancy) and
> also that generally the user doesn't need to access their second key - so
> that can remain in a much more secure location (which would also probably
> make that key harder to steal). The only time the second key only comes
> into play if one key is stolen and the attacker attempts to send a
> transaction. At that point, the user would go find and use his second key
> (along with the first) to send a revoke transaction to prevent the attacker
> from stealing their funds. This is somewhat akin to a lightning watchtower
> scenario, where your wallet would watch the chain and alert you about an
> unexpected transaction, at which point you'd manually do a revoke (vs a
> watchtower's automated response). You might be interested in taking a look
> at this wallet vault design
> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/op_cdWalletVault1.md>
> that uses OP_CD or even my full vision
> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults> of the
> wallet vault I want to be able to create.
>
> With a covenant opcode like this, its possible to create very usable and
> accessible but highly secure wallets that can allow normal people to hold
> self custody of their keys without fear of loss or theft and without the
> hassle of a lot of safe deposit boxes (or other secure seed storage
> locations).
>
> Cheers,
> BT
>
>
>
>
>
> On Mon, Jul 26, 2021 at 2:08 PM James MacWhyte <macwhyte@gmail.com> wrote:
>
>> Hi Billy!
>>
>> See above, but to break down that situation a bit further, these are the
>>> two situations I can think of:
>>>
>>>    1. The opcode limits user/group A to send the output to user/group B
>>>    2. The opcode limits user A to send from one address they own to
>>>    another address they own.
>>>
>>> I'm trying to think of a good use case for this type of opcode. In these
>> examples, an attacker who compromises the key for user A can't steal the
>> money because it can only be sent to user B. So if the attacker wants to
>> steal the funds, they would need to compromise the keys of both user A and
>> user B.
>>
>> But how is that any better than a 2-of-2 multisig? Isn't the end result
>> exactly the same?
>>
>> James
>>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

[-- Attachment #2: Type: text/html, Size: 6536 bytes --]

Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

[-- Attachment #1: Type: text/plain, Size: 8119 bytes --]

Hi Zac,

I haven't heard of any proposal for limiting the amount that can be sent
from an address. I assume you mean limiting the amount that can be sent in
a period of time - eg something that would encode that for address A, only
X bitcoin can be sent from the address in a given day/week/etc, is that
right? That would actually be a somewhat difficult thing to do in the
output-based system Bitcoin uses, and would be easier in an account based
system like Ethereum. The problem is that each output is separate, and
there's no concept in bitcoin of encumbering outputs together.

What you could do is design a system where coins would be combined in a
single output, and then encumber that output with a script that allows a
limited amount of coin be sent to a destination address and requires all
other bitcoins be returned to sender in a new change output that is also
timelocked. That way, the new change output can't be used again until the
timelock expires (eg a week). However, to ensure this wallet works
properly, any deposit into the wallet would have to also spend the wallet's
single output, so as to create a new single output at that address. So 3rd
parties wouldn't be able to arbitrarily send money in (or rather, they
could, but each output would have its own separate spending limit).

> such kind of restriction would be extremely effective in thwarting the
most damaging type of theft being the one where all funds are swept in a
single transaction

It would. However a normal wallet vault basically already has this property
- a thief can't simply sweep funds instantly, but instead the victim will
see an initiated transaction and will be able to reverse it within a delay
time-window. I don't think adding a spending limit would add meaningful
security to a delayed-send wallet vault like that. But it could be used to
increase the security of a wallet vault that can be instantly spent from -
ie if the attacker successfully steals funds, then the victim has time to
go gather their additional keys and move the remaining (unstolen) funds
into a new wallet.

OP_CD could potentially be augmented to allow specifying limit amounts for
each destination, which would allow you to create a wallet like this. It
would be a bit of an awkward wallet to use tho, since you couldn't receive
directly into it from a 3rd party and you also couldn't keep separate
outputs (which is bad for privacy).

An alternate way of doing this that you don't need any new opcodes for
would be to have a 3rd party service that signs multisig transactions from
a wallet only up to a limit. The end-user could have additional keys such
that the 3rd party can't prevent them from accessing that (if they turn
uncooperative), and the 3rd party would only have a single key so they
can't steal funds, but the user would sign a transaction with one key, and
the 3rd party with another as long as the spending limit hasn't been
reached. This wouldn't have much counterparty risk, but would be a less
awkward wallet than what I described above - meaning anyone could send
funds into the wallet without defeating the spending limit, and privacy
could be kept intact (minus the fact that the 3rd party would know what
your outputs are).

BT

On Tue, Jul 27, 2021 at 4:18 AM Zac Greenwood <zachgrw@gmail.com> wrote:

> Hi Billy,
>
> On the topic of wallet vaults, are there any plans to implement a way to
> limit the maximum amount to be sent from an address?
>
> An example of such limit might be: the maximum amount allowed to send is
> max(s, p) where s is a number of satoshi and p a percentage of the total
> available (sendable) amount.
>
> A minimum value may be imposed on the percentage to ensure that the
> address can be emptied within a reasonable number of transactions. The
> second parameter s allows a minimum permitted amount. (This is necessary
> because with only the percentage parameter the minimum permitted amount
> converges to zero, making it impossible to empty the address).
>
> There may be other ways too. In my view, such kind of restriction would be
> extremely effective in thwarting the most damaging type of theft being the
> one where all funds are swept in a single transaction.
>
> Zac
>
>
> On Tue, 27 Jul 2021 at 03:26, Billy Tetrud via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> Hey James,
>>
>> In the examples you mentioned, what I was exploring was a mechanism of
>> attack by which the attacker could steal user A's key and use that key to
>> send a transaction with the maximum possible fee. User B would still
>> receive some funds (probably), but if the fee could be large, the attacker
>> would either do a lot of damage to user B (griefing) or could make an
>> agreement with a miner to give back some of the large fee (theft).
>>
>> But as for use cases, the proposal mentions a number of use cases
>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md#motivation> and
>> most overlap with the use cases of op_ctv <https://utxos.org/uses/> (Jeremy
>> Rubin's website for op_ctv has a lot of good details, most of which are
>> also relevant to op_cd). The use case I'm most interested in is wallet
>> vaults. This opcode can be used to create a wallet vault where the user
>> only needs to use, for example, 1 key to spend funds, but the attacker must
>> steal 2 or more keys to spend funds. The benefits of a 2 key wallet vault
>> like this vs a normal 2-of-2 multisig wallet are that not only does an
>> attacker have to steal both keys (same level of security), but also the
>> user can lose one key and still recover their funds (better redundancy) and
>> also that generally the user doesn't need to access their second key - so
>> that can remain in a much more secure location (which would also probably
>> make that key harder to steal). The only time the second key only comes
>> into play if one key is stolen and the attacker attempts to send a
>> transaction. At that point, the user would go find and use his second key
>> (along with the first) to send a revoke transaction to prevent the attacker
>> from stealing their funds. This is somewhat akin to a lightning watchtower
>> scenario, where your wallet would watch the chain and alert you about an
>> unexpected transaction, at which point you'd manually do a revoke (vs a
>> watchtower's automated response). You might be interested in taking a look
>> at this wallet vault design
>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/op_cdWalletVault1.md>
>> that uses OP_CD or even my full vision
>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults> of the
>> wallet vault I want to be able to create.
>>
>> With a covenant opcode like this, its possible to create very usable and
>> accessible but highly secure wallets that can allow normal people to hold
>> self custody of their keys without fear of loss or theft and without the
>> hassle of a lot of safe deposit boxes (or other secure seed storage
>> locations).
>>
>> Cheers,
>> BT
>>
>>
>>
>>
>>
>> On Mon, Jul 26, 2021 at 2:08 PM James MacWhyte <macwhyte@gmail.com>
>> wrote:
>>
>>> Hi Billy!
>>>
>>> See above, but to break down that situation a bit further, these are the
>>>> two situations I can think of:
>>>>
>>>>    1. The opcode limits user/group A to send the output to user/group B
>>>>    2. The opcode limits user A to send from one address they own to
>>>>    another address they own.
>>>>
>>>> I'm trying to think of a good use case for this type of opcode. In
>>> these examples, an attacker who compromises the key for user A can't steal
>>> the money because it can only be sent to user B. So if the attacker wants
>>> to steal the funds, they would need to compromise the keys of both user A
>>> and user B.
>>>
>>> But how is that any better than a 2-of-2 multisig? Isn't the end result
>>> exactly the same?
>>>
>>> James
>>>
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>

[-- Attachment #2: Type: text/html, Size: 10248 bytes --]

Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Zac Greenwood]

[-- Attachment #1: Type: text/plain, Size: 11380 bytes --]

Hi Billy,

Thank you for your comprehensive reply. My purpose was to find out whether
a proposal to somehow limit the amount being sent from an address exists
and to further illustrate my thoughts by giving a concrete example of how
this might work functionally without getting to deep into the
technicalities.

As for your assumption: for an amount limit to have the desired effect, I
realize now that there must also exist some limit on the number of
transactions that will be allowed from the encumbered address.

Taking a step back, a typical use case would be a speculating user
intending to hodl bitcoin but who still wishes to be able to occasionally
transact minor amounts.

Ideally, such user should optionally still be able to bypass the rate limit
and spend the entire amount in a single transaction by signing with an
additional private key (multisig).

During the setup phase, a user sends all their to-be-rate-limited coin to a
single address. When spending from this rate limited address, any change
sent to the change address must be rate limited as well using identical
parameters. I believe that’s also what you’re suggesting.

I believe that a smart wallet should be able to set up and maintain
multiple rate-limited addresses in such a way that their aggregate
behaviour meets any rate-limiting parameters as desired by the user. This
ought to alleviate your privacy concerns because it means that the wallet
will be able to mix outputs.

The options for the to-be implemented rate-limiting parameters vary from
completely arbitrary to more restrictive.

Completely arbitrary parameters would allow users to set up a rate limit
that basically destroys their funds, for instance rate-limiting an address
to an amount of 1 satoshi per 100 blocks.

More restrictive rate limits would remove such footgun and may require that
only a combination of parameters are allowed such that all funds will be
spendable within a set number of blocks (for instance 210,000).

As for the rate-limiting parameters, in addition to a per-transaction
maximum of (minimum amount in satoshi or a percentage of the total amount
stored at the address), also the transaction frequency must be limited. I
would propose this to be expressed as a number of blocks before a next
transaction can be sent from the encumbered address(es).

I believe such user-enabled rate-limiting is superior to one that requires
a third party.

As an aside, I am not sure how a vault solution would be able to prevent an
attacker who is in possession of the vaults’ private key from sabotaging
the user by replacing the user transaction with one having a higher fee
every time the user attempts to transact. I am probably missing something
here though.

Zac


On Tue, 27 Jul 2021 at 19:21, Billy Tetrud <billy.tetrud@gmail.com> wrote:

> Hi Zac,
>
> I haven't heard of any proposal for limiting the amount that can be sent
> from an address. I assume you mean limiting the amount that can be sent in
> a period of time - eg something that would encode that for address A, only
> X bitcoin can be sent from the address in a given day/week/etc, is that
> right? That would actually be a somewhat difficult thing to do in the
> output-based system Bitcoin uses, and would be easier in an account based
> system like Ethereum. The problem is that each output is separate, and
> there's no concept in bitcoin of encumbering outputs together.
>
> What you could do is design a system where coins would be combined in a
> single output, and then encumber that output with a script that allows a
> limited amount of coin be sent to a destination address and requires all
> other bitcoins be returned to sender in a new change output that is also
> timelocked. That way, the new change output can't be used again until the
> timelock expires (eg a week). However, to ensure this wallet works
> properly, any deposit into the wallet would have to also spend the wallet's
> single output, so as to create a new single output at that address. So 3rd
> parties wouldn't be able to arbitrarily send money in (or rather, they
> could, but each output would have its own separate spending limit).
>
> > such kind of restriction would be extremely effective in thwarting the
> most damaging type of theft being the one where all funds are swept in a
> single transaction
>
> It would. However a normal wallet vault basically already has this
> property - a thief can't simply sweep funds instantly, but instead the
> victim will see an initiated transaction and will be able to reverse it
> within a delay time-window. I don't think adding a spending limit would add
> meaningful security to a delayed-send wallet vault like that. But it could
> be used to increase the security of a wallet vault that can be instantly
> spent from - ie if the attacker successfully steals funds, then the victim
> has time to go gather their additional keys and move the remaining
> (unstolen) funds into a new wallet.
>
> OP_CD could potentially be augmented to allow specifying limit amounts for
> each destination, which would allow you to create a wallet like this. It
> would be a bit of an awkward wallet to use tho, since you couldn't receive
> directly into it from a 3rd party and you also couldn't keep separate
> outputs (which is bad for privacy).
>
> An alternate way of doing this that you don't need any new opcodes for
> would be to have a 3rd party service that signs multisig transactions from
> a wallet only up to a limit. The end-user could have additional keys such
> that the 3rd party can't prevent them from accessing that (if they turn
> uncooperative), and the 3rd party would only have a single key so they
> can't steal funds, but the user would sign a transaction with one key, and
> the 3rd party with another as long as the spending limit hasn't been
> reached. This wouldn't have much counterparty risk, but would be a less
> awkward wallet than what I described above - meaning anyone could send
> funds into the wallet without defeating the spending limit, and privacy
> could be kept intact (minus the fact that the 3rd party would know what
> your outputs are).
>
> BT
>
> On Tue, Jul 27, 2021 at 4:18 AM Zac Greenwood <zachgrw@gmail.com> wrote:
>
>> Hi Billy,
>>
>> On the topic of wallet vaults, are there any plans to implement a way to
>> limit the maximum amount to be sent from an address?
>>
>> An example of such limit might be: the maximum amount allowed to send is
>> max(s, p) where s is a number of satoshi and p a percentage of the total
>> available (sendable) amount.
>>
>> A minimum value may be imposed on the percentage to ensure that the
>> address can be emptied within a reasonable number of transactions. The
>> second parameter s allows a minimum permitted amount. (This is necessary
>> because with only the percentage parameter the minimum permitted amount
>> converges to zero, making it impossible to empty the address).
>>
>> There may be other ways too. In my view, such kind of restriction would
>> be extremely effective in thwarting the most damaging type of theft being
>> the one where all funds are swept in a single transaction.
>>
>> Zac
>>
>>
>> On Tue, 27 Jul 2021 at 03:26, Billy Tetrud via bitcoin-dev <
>> bitcoin-dev@lists.linuxfoundation.org> wrote:
>>
>>> Hey James,
>>>
>>> In the examples you mentioned, what I was exploring was a mechanism of
>>> attack by which the attacker could steal user A's key and use that key to
>>> send a transaction with the maximum possible fee. User B would still
>>> receive some funds (probably), but if the fee could be large, the attacker
>>> would either do a lot of damage to user B (griefing) or could make an
>>> agreement with a miner to give back some of the large fee (theft).
>>>
>>> But as for use cases, the proposal mentions a number of use cases
>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md#motivation> and
>>> most overlap with the use cases of op_ctv <https://utxos.org/uses/> (Jeremy
>>> Rubin's website for op_ctv has a lot of good details, most of which are
>>> also relevant to op_cd). The use case I'm most interested in is wallet
>>> vaults. This opcode can be used to create a wallet vault where the user
>>> only needs to use, for example, 1 key to spend funds, but the attacker must
>>> steal 2 or more keys to spend funds. The benefits of a 2 key wallet vault
>>> like this vs a normal 2-of-2 multisig wallet are that not only does an
>>> attacker have to steal both keys (same level of security), but also the
>>> user can lose one key and still recover their funds (better redundancy) and
>>> also that generally the user doesn't need to access their second key - so
>>> that can remain in a much more secure location (which would also probably
>>> make that key harder to steal). The only time the second key only comes
>>> into play if one key is stolen and the attacker attempts to send a
>>> transaction. At that point, the user would go find and use his second key
>>> (along with the first) to send a revoke transaction to prevent the attacker
>>> from stealing their funds. This is somewhat akin to a lightning watchtower
>>> scenario, where your wallet would watch the chain and alert you about an
>>> unexpected transaction, at which point you'd manually do a revoke (vs a
>>> watchtower's automated response). You might be interested in taking a look
>>> at this wallet vault design
>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/op_cdWalletVault1.md>
>>> that uses OP_CD or even my full vision
>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults> of the
>>> wallet vault I want to be able to create.
>>>
>>> With a covenant opcode like this, its possible to create very usable and
>>> accessible but highly secure wallets that can allow normal people to hold
>>> self custody of their keys without fear of loss or theft and without the
>>> hassle of a lot of safe deposit boxes (or other secure seed storage
>>> locations).
>>>
>>> Cheers,
>>> BT
>>>
>>>
>>>
>>>
>>>
>>> On Mon, Jul 26, 2021 at 2:08 PM James MacWhyte <macwhyte@gmail.com>
>>> wrote:
>>>
>>>> Hi Billy!
>>>>
>>>> See above, but to break down that situation a bit further, these are
>>>>> the two situations I can think of:
>>>>>
>>>>>    1. The opcode limits user/group A to send the output to user/group
>>>>>    B
>>>>>    2. The opcode limits user A to send from one address they own to
>>>>>    another address they own.
>>>>>
>>>>> I'm trying to think of a good use case for this type of opcode. In
>>>> these examples, an attacker who compromises the key for user A can't steal
>>>> the money because it can only be sent to user B. So if the attacker wants
>>>> to steal the funds, they would need to compromise the keys of both user A
>>>> and user B.
>>>>
>>>> But how is that any better than a 2-of-2 multisig? Isn't the end result
>>>> exactly the same?
>>>>
>>>> James
>>>>
>>> _______________________________________________
>>> bitcoin-dev mailing list
>>> bitcoin-dev@lists.linuxfoundation.org
>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>
>>

[-- Attachment #2: Type: text/html, Size: 16676 bytes --]

Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

[-- Attachment #1: Type: text/plain, Size: 14416 bytes --]

Hi Zac,

> a smart wallet should be able to set up and maintain multiple
rate-limited addresses in such a way that their aggregate behaviour meets
any rate-limiting parameters as desired by the user

I think that would be possible if there was a way to say "within the last B
blocks, this output can only spend to addresses other than X,Y,Z an amount
less than C coins minus however much coins have been spent by those
addresses in the last B blocks". This would require that full nodes keep
around information about which addresses have been spent from recently, so
that information is accessible during script execution. This could be made
a bit less heavy by requiring countable transactions to run some particular
opcode (so only opted-in transactions would need to be stored).

> This ought to alleviate your privacy concerns because it means that the
wallet will be able to mix outputs.

The ability to mix outputs isn't a privacy issue. The ability to keep
outputs separate is the privacy issue. For rate-limiting to work, the
outputs must be associated with eachother so that rate limiting can take
them all into account. It seems to me that its fundamentally impossible to
do this while keeping outputs uncorrelated.

> such user-enabled rate-limiting is superior to one that requires a third
party.

Removing a 3rd party certainly has upsides. However, using a 3rd party
would be able to solve the privacy issue by keeping outputs uncorrelated
(in different addresses) to the outside world. Trade offs.

In any case, if you want to continue talking about rate-limiting
transactions, it might be a good idea to start a new thread for that, since
its a bit off topic for this one.

> how a vault solution would be able to prevent an attacker who is in
possession of the vaults’ private key from sabotaging the user by replacing
the user transaction with one having a higher fee every time the user
attempts to transact

A wallet vault has multiple keys. If one key is stolen, the user can use
two keys to override the attacker's transaction. If two keys are stolen,
the user can use three keys. Etc. The attacker must have as many keys as
the user can use in order to successfully steal funds. This can happen in
one of these kinds of ways:

A. The attacker steals all keys.
B. The attacker steals half the keys and ensures that the victim doesn't
have access to those keys (eg the attacker steals the only copy of half the
keys).
C. The attacker steals any key and incapacitates the victim for the entire
cooldown period, so they can't use any of their keys.

In case C, it would be useful to have rate limiting actually.

On Tue, Jul 27, 2021 at 9:57 PM Zac Greenwood <zachgrw@gmail.com> wrote:

> Hi Billy,
>
> Thank you for your comprehensive reply. My purpose was to find out whether
> a proposal to somehow limit the amount being sent from an address exists
> and to further illustrate my thoughts by giving a concrete example of how
> this might work functionally without getting to deep into the
> technicalities.
>
> As for your assumption: for an amount limit to have the desired effect, I
> realize now that there must also exist some limit on the number of
> transactions that will be allowed from the encumbered address.
>
> Taking a step back, a typical use case would be a speculating user
> intending to hodl bitcoin but who still wishes to be able to occasionally
> transact minor amounts.
>
> Ideally, such user should optionally still be able to bypass the rate
> limit and spend the entire amount in a single transaction by signing with
> an additional private key (multisig).
>
> During the setup phase, a user sends all their to-be-rate-limited coin to
> a single address. When spending from this rate limited address, any change
> sent to the change address must be rate limited as well using identical
> parameters. I believe that’s also what you’re suggesting.
>
> I believe that a smart wallet should be able to set up and maintain
> multiple rate-limited addresses in such a way that their aggregate
> behaviour meets any rate-limiting parameters as desired by the user. This
> ought to alleviate your privacy concerns because it means that the wallet
> will be able to mix outputs.
>
> The options for the to-be implemented rate-limiting parameters vary from
> completely arbitrary to more restrictive.
>
> Completely arbitrary parameters would allow users to set up a rate limit
> that basically destroys their funds, for instance rate-limiting an address
> to an amount of 1 satoshi per 100 blocks.
>
> More restrictive rate limits would remove such footgun and may require
> that only a combination of parameters are allowed such that all funds will
> be spendable within a set number of blocks (for instance 210,000).
>
> As for the rate-limiting parameters, in addition to a per-transaction
> maximum of (minimum amount in satoshi or a percentage of the total amount
> stored at the address), also the transaction frequency must be limited. I
> would propose this to be expressed as a number of blocks before a next
> transaction can be sent from the encumbered address(es).
>
> I believe such user-enabled rate-limiting is superior to one that requires
> a third party.
>
> As an aside, I am not sure how a vault solution would be able to prevent
> an attacker who is in possession of the vaults’ private key from sabotaging
> the user by replacing the user transaction with one having a higher fee
> every time the user attempts to transact. I am probably missing something
> here though.
>
> Zac
>
>
> On Tue, 27 Jul 2021 at 19:21, Billy Tetrud <billy.tetrud@gmail.com> wrote:
>
>> Hi Zac,
>>
>> I haven't heard of any proposal for limiting the amount that can be sent
>> from an address. I assume you mean limiting the amount that can be sent in
>> a period of time - eg something that would encode that for address A, only
>> X bitcoin can be sent from the address in a given day/week/etc, is that
>> right? That would actually be a somewhat difficult thing to do in the
>> output-based system Bitcoin uses, and would be easier in an account based
>> system like Ethereum. The problem is that each output is separate, and
>> there's no concept in bitcoin of encumbering outputs together.
>>
>> What you could do is design a system where coins would be combined in a
>> single output, and then encumber that output with a script that allows a
>> limited amount of coin be sent to a destination address and requires all
>> other bitcoins be returned to sender in a new change output that is also
>> timelocked. That way, the new change output can't be used again until the
>> timelock expires (eg a week). However, to ensure this wallet works
>> properly, any deposit into the wallet would have to also spend the wallet's
>> single output, so as to create a new single output at that address. So 3rd
>> parties wouldn't be able to arbitrarily send money in (or rather, they
>> could, but each output would have its own separate spending limit).
>>
>> > such kind of restriction would be extremely effective in thwarting the
>> most damaging type of theft being the one where all funds are swept in a
>> single transaction
>>
>> It would. However a normal wallet vault basically already has this
>> property - a thief can't simply sweep funds instantly, but instead the
>> victim will see an initiated transaction and will be able to reverse it
>> within a delay time-window. I don't think adding a spending limit would add
>> meaningful security to a delayed-send wallet vault like that. But it could
>> be used to increase the security of a wallet vault that can be instantly
>> spent from - ie if the attacker successfully steals funds, then the victim
>> has time to go gather their additional keys and move the remaining
>> (unstolen) funds into a new wallet.
>>
>> OP_CD could potentially be augmented to allow specifying limit amounts
>> for each destination, which would allow you to create a wallet like this.
>> It would be a bit of an awkward wallet to use tho, since you couldn't
>> receive directly into it from a 3rd party and you also couldn't keep
>> separate outputs (which is bad for privacy).
>>
>> An alternate way of doing this that you don't need any new opcodes for
>> would be to have a 3rd party service that signs multisig transactions from
>> a wallet only up to a limit. The end-user could have additional keys such
>> that the 3rd party can't prevent them from accessing that (if they turn
>> uncooperative), and the 3rd party would only have a single key so they
>> can't steal funds, but the user would sign a transaction with one key, and
>> the 3rd party with another as long as the spending limit hasn't been
>> reached. This wouldn't have much counterparty risk, but would be a less
>> awkward wallet than what I described above - meaning anyone could send
>> funds into the wallet without defeating the spending limit, and privacy
>> could be kept intact (minus the fact that the 3rd party would know what
>> your outputs are).
>>
>> BT
>>
>> On Tue, Jul 27, 2021 at 4:18 AM Zac Greenwood <zachgrw@gmail.com> wrote:
>>
>>> Hi Billy,
>>>
>>> On the topic of wallet vaults, are there any plans to implement a way to
>>> limit the maximum amount to be sent from an address?
>>>
>>> An example of such limit might be: the maximum amount allowed to send is
>>> max(s, p) where s is a number of satoshi and p a percentage of the total
>>> available (sendable) amount.
>>>
>>> A minimum value may be imposed on the percentage to ensure that the
>>> address can be emptied within a reasonable number of transactions. The
>>> second parameter s allows a minimum permitted amount. (This is necessary
>>> because with only the percentage parameter the minimum permitted amount
>>> converges to zero, making it impossible to empty the address).
>>>
>>> There may be other ways too. In my view, such kind of restriction would
>>> be extremely effective in thwarting the most damaging type of theft being
>>> the one where all funds are swept in a single transaction.
>>>
>>> Zac
>>>
>>>
>>> On Tue, 27 Jul 2021 at 03:26, Billy Tetrud via bitcoin-dev <
>>> bitcoin-dev@lists.linuxfoundation.org> wrote:
>>>
>>>> Hey James,
>>>>
>>>> In the examples you mentioned, what I was exploring was a mechanism of
>>>> attack by which the attacker could steal user A's key and use that key to
>>>> send a transaction with the maximum possible fee. User B would still
>>>> receive some funds (probably), but if the fee could be large, the attacker
>>>> would either do a lot of damage to user B (griefing) or could make an
>>>> agreement with a miner to give back some of the large fee (theft).
>>>>
>>>> But as for use cases, the proposal mentions a number of use cases
>>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md#motivation> and
>>>> most overlap with the use cases of op_ctv <https://utxos.org/uses/> (Jeremy
>>>> Rubin's website for op_ctv has a lot of good details, most of which are
>>>> also relevant to op_cd). The use case I'm most interested in is wallet
>>>> vaults. This opcode can be used to create a wallet vault where the user
>>>> only needs to use, for example, 1 key to spend funds, but the attacker must
>>>> steal 2 or more keys to spend funds. The benefits of a 2 key wallet vault
>>>> like this vs a normal 2-of-2 multisig wallet are that not only does an
>>>> attacker have to steal both keys (same level of security), but also the
>>>> user can lose one key and still recover their funds (better redundancy) and
>>>> also that generally the user doesn't need to access their second key - so
>>>> that can remain in a much more secure location (which would also probably
>>>> make that key harder to steal). The only time the second key only comes
>>>> into play if one key is stolen and the attacker attempts to send a
>>>> transaction. At that point, the user would go find and use his second key
>>>> (along with the first) to send a revoke transaction to prevent the attacker
>>>> from stealing their funds. This is somewhat akin to a lightning watchtower
>>>> scenario, where your wallet would watch the chain and alert you about an
>>>> unexpected transaction, at which point you'd manually do a revoke (vs a
>>>> watchtower's automated response). You might be interested in taking a look
>>>> at this wallet vault design
>>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/op_cdWalletVault1.md>
>>>> that uses OP_CD or even my full vision
>>>> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults> of the
>>>> wallet vault I want to be able to create.
>>>>
>>>> With a covenant opcode like this, its possible to create very usable
>>>> and accessible but highly secure wallets that can allow normal people to
>>>> hold self custody of their keys without fear of loss or theft and without
>>>> the hassle of a lot of safe deposit boxes (or other secure seed storage
>>>> locations).
>>>>
>>>> Cheers,
>>>> BT
>>>>
>>>>
>>>>
>>>>
>>>>
>>>> On Mon, Jul 26, 2021 at 2:08 PM James MacWhyte <macwhyte@gmail.com>
>>>> wrote:
>>>>
>>>>> Hi Billy!
>>>>>
>>>>> See above, but to break down that situation a bit further, these are
>>>>>> the two situations I can think of:
>>>>>>
>>>>>>    1. The opcode limits user/group A to send the output to
>>>>>>    user/group B
>>>>>>    2. The opcode limits user A to send from one address they own to
>>>>>>    another address they own.
>>>>>>
>>>>>> I'm trying to think of a good use case for this type of opcode. In
>>>>> these examples, an attacker who compromises the key for user A can't steal
>>>>> the money because it can only be sent to user B. So if the attacker wants
>>>>> to steal the funds, they would need to compromise the keys of both user A
>>>>> and user B.
>>>>>
>>>>> But how is that any better than a 2-of-2 multisig? Isn't the end
>>>>> result exactly the same?
>>>>>
>>>>> James
>>>>>
>>>> _______________________________________________
>>>> bitcoin-dev mailing list
>>>> bitcoin-dev@lists.linuxfoundation.org
>>>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>>>
>>>

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Jeremy]

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High level feedback:

you should spec out the opcodes as separate pieces of functionality as it
sounds like OP_CD is really 3 or 4 opcodes in one (e.g., amounts to
outputs, output addresses, something with fees).

One major drawback of your approach is that all transactions are twice as
large as they might otherwise need to be for simple things like congestion
control trees, since you have to repeat all of the output data twice.

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

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Thanks for taking another look Jeremy. That's an interesting idea to split
it up into simpler opcodes, however there are some
limitations/considerations there.

For example, with output addresses, I added specifying amounts to outputs
in order to make script evaluation simpler and eliminate a potential DOS
vector. I wrote about this in the section 'Specifying values sent to each
output
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md#specifying-values-sent-to-each-output>'.
Originally, I designed OP_CD without specifying what amounts an input
contributes to what outputs, but it seemed like this would require
calculating various combinations of inequalities, which could get expensive
in scenarios where many inputs had overlapping destinations. See the
examples under the OP_CD section in this commit
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/commit/9b2257410b5f0fc991f68e774c3faf601c02cc5d>
.

Maybe there's an elegant and cheap way of verifying that a number of inputs
that have destination address limitations is within limits, but if so I
don't know how to do that. If there was a good way to do that, then I
wouldn't want to propose the ability to validate that specific amounts go
to specific outputs. So unless there's a simple and dos-vector-free way of
evaluating what addresses an input goes to without knowing what amounts an
input contributes to each output, I don't think these functionalities
should be separated.

And about a fee-limit opcode, that could certainly be done on its own.
However, a version of OP_CD that doesn't specify fees would have to take
the fee-limit into account, and the calculation for the stand-alone
fee-limit operation would be moot for that output.

So I think it could make sense to split the fee limit off from the rest of
OP_CD. I'm curious to know what others think of that.

> all transactions are twice as large as they might otherwise need to be
for simple things like congestion control trees, since you have to repeat
all of the output data twice

Well, the transaction wouldn't be quite twice as large. Each output would
add 9 bytes to the transaction, and outputs already are a minimum of about
30 bytes I think? So for transactions with a lot of outputs, it could make
the transaction about 1/3 larger. I'll add a section on this into my
proposal.

Perhaps it would be a reasonable optimization to allow omitting an output
value in cases where the entire output amount is contributed by that input.
This would reduce the overhead of specifying output amounts to 2 bytes for
most outputs (1 byte for the index, another to indicate the full value),
meaning that it would only make the transaction about 7% larger. What do
you think about that idea?

On Wed, Jul 28, 2021 at 3:30 PM Jeremy via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> High level feedback:
>
> you should spec out the opcodes as separate pieces of functionality as it
> sounds like OP_CD is really 3 or 4 opcodes in one (e.g., amounts to
> outputs, output addresses, something with fees).
>
> One major drawback of your approach is that all transactions are twice as
> large as they might otherwise need to be for simple things like congestion
> control trees, since you have to repeat all of the output data twice.
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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[bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[Zac Greenwood]

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Hi list,

I'd like to explore whether it is feasible to implement new scripting
capabilities in Bitcoin that enable limiting the output amount of a
transaction based on the total value of its inputs. In other words, to
implement the ability to limit the maximum amount that can be sent from an
address.

Two use cases come to mind:

UC1: enable a user to add additional protection their funds by
rate-limiting the amount they are able to send during a certain period
(measured in blocks). A typical use case might be a user that intends to
hodl their bitcoin, but still wishes to occasionally send small amounts.
This avoids an attacker from sweeping all their funds in a single
transaction, allowing the user to become aware of the theft and intervene
to prevent further theft.

UC2: exchanges may wish to rate-limit addresses containing large amounts of
bitcoin, adding warm- or hot-wallet functionality to a cold-storage
address. This would enable an exchange to drastically reduce the number of
times a cold wallet must be accessed with private keys that enable access
to the full amount.

In a typical setup, I'd envision using multisig such that the user has two
sets of private keys to their encumbered address (with a "set" of keys
meaning "one or more" keys). One set of private keys allows only for
sending with rate-limiting restrictions in place, and a s second set of
private keys allowing for sending any amount without rate-limiting,
effectively overriding such restriction.

The parameters that define in what way an output is rate-limited might be
defined as follows:

Param 1: a block height "h0" indicating the first block height of an epoch;
Param 2: a block height "h1" indicating the last block height of an epoch;
Param 3: an amount "a" in satoshi indicating the maximum amount that is
allowed to be sent in any epoch;
Param 4: an amount "a_remaining" (in satoshi) indicating the maximum amount
that is allowed to be sent within the current epoch.

For example, consider an input containing 100m sats (1 BTC) which has been
rate-limited with parameters (h0, h1, a, a_remaning) of (800000, 800143,
500k, 500k). These parameters define that the address is rate-limited to
sending a maximum of 500k sats in the current epoch that starts at block
height 800000 and ends at height 800143 (or about one day ignoring block
time variance) and that the full amount of 500k is still sendable. These
rate-limiting parameters ensure that it takes at minimum 100m / 500k = 200
transactions and 200 x 144 blocks or about 200 days to spend the full 100m
sats. As noted earlier, in a typical setup a user should retain the option
to transact the entire amount using a second (set of) private key(s).

For rate-limiting to work, any change output created by a transaction from
a rate-limited address must itself be rate-limited as well. For instance,
expanding on the above example, assume that the user spends 200k sats from
a rate-limited address a1 containing 100m sats:

Start situation:
At block height 800000: rate-limited address a1 is created;
Value of a1: 100.0m sats;
Rate limiting params of a1: h0=800000, h1=800143, a=500k, a_remaining=500k;

Transaction t1:
Included at block height 800100;
Spend: 200k + fee;
Rate limiting params: h0=800000, h1=800143, a=500k, a_remaining=300k.

Result:
Value at destination address: 200k sats;
Rate limiting params at destination address: none;
Value at change address a2: 99.8m sats;
Rate limiting params at change address a2: h0=800000, h1=800143, a=500k,
a_remaining=300k.

In order to properly enforce rate limiting, the change address must be
rate-limited such that the original rate limit of 500k sats per 144 blocks
cannot be exceeded. In this example, the change address a2 were given the
same rate limiting parameters as the transaction that served as its input.
As a result, from block 800100 up until and including block 800143, a
maximum amount of 300k sats is allowed to be spent from the change address.

Example continued:
a2: 99.8 sats at height 800100;
Rate-limit params: h0=800000, h1=800143, a=500k, a_remaining=300k;

Transaction t2:
Included at block height 800200
Spend: 400k + fees.
Rate-limiting params: h0=800144, h1=800287, a=500k, a_remaining=100k.

Result:
Value at destination address: 400k sats;
Rate limiting params at destination address: none;
Value at change address a3: 99.4m sats;
Rate limiting params at change address a3: h0=800144, h1=800287, a=500k,
a_remaining=100k.

Transaction t2 is allowed because it falls within the next epoch (running
from 800144 to 800287) so a spend of 400k does not violate the constraint
of 500k per epoch.

As could be seen, the rate limiting parameters are part of the transaction
and chosen by the user (or their wallet). This means that the parameters
must be validated to ensure that they do not violate the intended
constraints.

For instance, this transaction should not be allowed:
a2: 99.8 sats at height 800100;
Rate-limit params of a2: h0=800000, h1=800143, a=500k, a_remaining=300k;

Transaction t2a:
Included at block height 800200;
Spend: 400k + fees;
Rate-limit params: h0=800124, h1=800267, a=500k, a_remaining=100k.

This transaction t2a attempts to shift the epoch forward by 20 blocks such
that it starts at 800124 instead of 800144. Shifting the epoch forward like
this must not be allowed because it enables spending more that the rate
limit allows, which is 500k in any epoch of 144 blocks. It would enable
overspending:

t1: spend 200k at 800100 (epoch 1: total: 200k);
t2a: spend 400k at 800200 (epoch 2: total: 400k);
t3a: spend 100k at 800201 (epoch 2: total: 500k);
t4a: spend 500k at 800268 (epoch 2: total: 1000k, overspending for epoch 2).

Specifying the rate-limiting parameters explicitly at every transaction
allows the user to tighten the spending limit by setting tighter limits or
for instance by setting a_remainder to 0 if they wish to enforce not
spending more during an epoch.

I will stop here because I would like to gauge interest in this idea first
before continuing work on other aspects. Two main pieces of work jump to
mind:

Define all validations;
Describe aggregate behaviour of multiple (rate-limited) inputs, proof that
two rate-limited addresses cannot spend more than the sum of their
individual limits.

Zac

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Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[Billy Tetrud]

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Hey Zac,

I think this could be a useful opcode. It kinda seems like UC1 and UC2 are
basically the same use case: using rate-limiting to reduce risk of theft or
mistake. I think this could be a helpful addition to a good wallet setup.

I don't quite understand why you'd want to define a specific span of blocks
for the rate limit. Why not just specify the size of the window (in blocks)
to rate limit within, and the limit?

You mentioned change addresses, however, with the parameters you defined,
there would be no way to connect together the change address with the
original address, meaning they would have completely separate rate limits,
which wouldn't work since the change output would ignore the previous rate
limit. I can think of the following options:

A. You could always send change back to the *same* address. This is the
simplest option, and the only downside I can think of is exposing the
public key of an address. I'm not quite sure what the consensus is on the
dangers of exposing the public key. It theoretically reduces quantum
resistance a bit, but I think I read that some of taproot's mechanisms
expose the bare public key, so maybe consensus has changed about that in
recent years?

B. Have some way to specify connected addresses in the output. This has the
edge case that one of the addresses wouldn't be able to specify all the
addresses that it should be connected with, because it would create a hash
loop (ie if you had address A and B that should be connected, you can
create address A and then specify that address B be connected to address A,
but address A cannot specify its connection to B because A was created
before B was created). You wouldn't want one address to be able to simply
define a connection to another address, because this would open up attack
vectors where people could encumber other people's addresses with rate
limits connected to theirs. You could define connections based on
signatures, which could be done without creating a hash loop, however it
would require exposing the public keys of other addresses when you do that,
at which point you might as well go with option A.

C. You could specify that rate limits follow a certain output. Eg, if you
create a transaction with destination output 1 and change output 2, your
rate limiting opcode could specify that output 2 should inherit the rate
limit. These inherited rate limits could all be connected together
automatically.

Another consideration is what to use for a receive-address. I would say the
simplest option here is to receive at an address that contains an existing
output already. If you allowed receiving at an address that contains no
coins, you'd have to specify at least one other address to connect it with.
This could work, but I don't see any advantage to it, since you don't gain
any privacy by creating a new address if you're going to immediately
programmatically tie it to the other addresses.

One thing to consider is the cost of carrying around and checking these
rate limits. Ideally it should be a very small amount of data carried
around in the UTXO set, and be very cheap to verify when the opcode comes
up. I think it would make sense for such an opcode to only be able to track
rate-limits over short spans, like a month or less. Allowing the user to
specify an arbitrary window over which to track a rate-limit seems like
something that would probably open up a dos vector or other node resource
usage abuse attacks. It might be useful enough to simply rate limit over
each epoch (two weeks), but having a small set of options could also be
useful (eg 1 day, 1 week, or 1 month).

In any case, I'd be interested in seeing you write a BIP for this. Of
course, don't take my word as community interest. I'm reasonably new to the
bitcoin dev community, so definitely don't jump the gun based on my
interest.

On Sat, Jul 31, 2021 at 2:51 PM Zac Greenwood via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi list,
>
> I'd like to explore whether it is feasible to implement new scripting
> capabilities in Bitcoin that enable limiting the output amount of a
> transaction based on the total value of its inputs. In other words, to
> implement the ability to limit the maximum amount that can be sent from an
> address.
>
> Two use cases come to mind:
>
> UC1: enable a user to add additional protection their funds by
> rate-limiting the amount they are able to send during a certain period
> (measured in blocks). A typical use case might be a user that intends to
> hodl their bitcoin, but still wishes to occasionally send small amounts.
> This avoids an attacker from sweeping all their funds in a single
> transaction, allowing the user to become aware of the theft and intervene
> to prevent further theft.
>
> UC2: exchanges may wish to rate-limit addresses containing large amounts
> of bitcoin, adding warm- or hot-wallet functionality to a cold-storage
> address. This would enable an exchange to drastically reduce the number of
> times a cold wallet must be accessed with private keys that enable access
> to the full amount.
>
> In a typical setup, I'd envision using multisig such that the user has two
> sets of private keys to their encumbered address (with a "set" of keys
> meaning "one or more" keys). One set of private keys allows only for
> sending with rate-limiting restrictions in place, and a s second set of
> private keys allowing for sending any amount without rate-limiting,
> effectively overriding such restriction.
>
> The parameters that define in what way an output is rate-limited might be
> defined as follows:
>
> Param 1: a block height "h0" indicating the first block height of an epoch;
> Param 2: a block height "h1" indicating the last block height of an epoch;
> Param 3: an amount "a" in satoshi indicating the maximum amount that is
> allowed to be sent in any epoch;
> Param 4: an amount "a_remaining" (in satoshi) indicating the maximum
> amount that is allowed to be sent within the current epoch.
>
> For example, consider an input containing 100m sats (1 BTC) which has been
> rate-limited with parameters (h0, h1, a, a_remaning) of (800000, 800143,
> 500k, 500k). These parameters define that the address is rate-limited to
> sending a maximum of 500k sats in the current epoch that starts at block
> height 800000 and ends at height 800143 (or about one day ignoring block
> time variance) and that the full amount of 500k is still sendable. These
> rate-limiting parameters ensure that it takes at minimum 100m / 500k = 200
> transactions and 200 x 144 blocks or about 200 days to spend the full 100m
> sats. As noted earlier, in a typical setup a user should retain the option
> to transact the entire amount using a second (set of) private key(s).
>
> For rate-limiting to work, any change output created by a transaction from
> a rate-limited address must itself be rate-limited as well. For instance,
> expanding on the above example, assume that the user spends 200k sats from
> a rate-limited address a1 containing 100m sats:
>
> Start situation:
> At block height 800000: rate-limited address a1 is created;
> Value of a1: 100.0m sats;
> Rate limiting params of a1: h0=800000, h1=800143, a=500k, a_remaining=500k;
>
> Transaction t1:
> Included at block height 800100;
> Spend: 200k + fee;
> Rate limiting params: h0=800000, h1=800143, a=500k, a_remaining=300k.
>
> Result:
> Value at destination address: 200k sats;
> Rate limiting params at destination address: none;
> Value at change address a2: 99.8m sats;
> Rate limiting params at change address a2: h0=800000, h1=800143, a=500k,
> a_remaining=300k.
>
> In order to properly enforce rate limiting, the change address must be
> rate-limited such that the original rate limit of 500k sats per 144 blocks
> cannot be exceeded. In this example, the change address a2 were given the
> same rate limiting parameters as the transaction that served as its input.
> As a result, from block 800100 up until and including block 800143, a
> maximum amount of 300k sats is allowed to be spent from the change address.
>
> Example continued:
> a2: 99.8 sats at height 800100;
> Rate-limit params: h0=800000, h1=800143, a=500k, a_remaining=300k;
>
> Transaction t2:
> Included at block height 800200
> Spend: 400k + fees.
> Rate-limiting params: h0=800144, h1=800287, a=500k, a_remaining=100k.
>
> Result:
> Value at destination address: 400k sats;
> Rate limiting params at destination address: none;
> Value at change address a3: 99.4m sats;
> Rate limiting params at change address a3: h0=800144, h1=800287, a=500k,
> a_remaining=100k.
>
> Transaction t2 is allowed because it falls within the next epoch (running
> from 800144 to 800287) so a spend of 400k does not violate the constraint
> of 500k per epoch.
>
> As could be seen, the rate limiting parameters are part of the transaction
> and chosen by the user (or their wallet). This means that the parameters
> must be validated to ensure that they do not violate the intended
> constraints.
>
> For instance, this transaction should not be allowed:
> a2: 99.8 sats at height 800100;
> Rate-limit params of a2: h0=800000, h1=800143, a=500k, a_remaining=300k;
>
> Transaction t2a:
> Included at block height 800200;
> Spend: 400k + fees;
> Rate-limit params: h0=800124, h1=800267, a=500k, a_remaining=100k.
>
> This transaction t2a attempts to shift the epoch forward by 20 blocks such
> that it starts at 800124 instead of 800144. Shifting the epoch forward like
> this must not be allowed because it enables spending more that the rate
> limit allows, which is 500k in any epoch of 144 blocks. It would enable
> overspending:
>
> t1: spend 200k at 800100 (epoch 1: total: 200k);
> t2a: spend 400k at 800200 (epoch 2: total: 400k);
> t3a: spend 100k at 800201 (epoch 2: total: 500k);
> t4a: spend 500k at 800268 (epoch 2: total: 1000k, overspending for epoch
> 2).
>
> Specifying the rate-limiting parameters explicitly at every transaction
> allows the user to tighten the spending limit by setting tighter limits or
> for instance by setting a_remainder to 0 if they wish to enforce not
> spending more during an epoch.
>
> I will stop here because I would like to gauge interest in this idea first
> before continuing work on other aspects. Two main pieces of work jump to
> mind:
>
> Define all validations;
> Describe aggregate behaviour of multiple (rate-limited) inputs, proof that
> two rate-limited addresses cannot spend more than the sum of their
> individual limits.
>
> Zac
>
>
>
>
>
>
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[ZmnSCPxj]

 fromGood morning Zac,


With some work, what you want can be implemented, to some extent, today, without changes to consensus.

The point you want, I believe, is to have two sets of keys:

* A long-term-storage keyset, in "cold" storage.
* A short-term-spending keyset, in "warm" storage, controlling only a small amount of funds.

What you can do would be:

* Put all your funds in a single UTXO, with an k-of-n of your cold keys (ideally P2TR, or some P2WSH k-of-n).
* Put your cold keys online, and sign a transaction spending the above UTXO, and spending most of it to a new address that is a tweaked k-of-n of your cold keys, and a smaller output (up to the limit you want) controlled by the k-of-n of your warm keys.
  * Keep this transaction offchain, in your warm storage.
* Put your cold keys back offline.
* When you need to spend using your warm keys, bring the above transaction onchain, then spend from the budget as needed.


If you need to have some estimated amount of usable funds for every future unit of time, just create a chain of transactions with future `nLockTime`.

                                  nLocktime +1day  nLockTime +2day
                  +------------+   +------------+   +------------+
     cold UTXO -->|    cold TXO|-->|    cold TXO|-->|    cold TXO|--> etc.
                  |            |   |            |   |            |
                  |    warm TXO|   |    warm TXO|   |    warm TXO|
                  +------------+   +------------+   +------------+

Pre-sign the above transactions, store the pre-signed transactions in warm storage together with your warm keys.
Then put the cold keys back offline.

Then from today to tomorrow, you can spend only the first warm TXO.
From tomorrow to the day after, you can spend only the first two warm TXOs.
And so on.

If tomorrow your warm keys are stolen, you can bring the cold keys online to claim the second cold TXO and limit your fund loss to only just the first two warm TXOs.

The above is bulky, but it has the advantage of not using any special opcodes or features (improving privacy, especially with P2TR which would in theory allow k-of-n/n-of-n to be indistinguishable from 1-of-1), and using just `nLockTime`, which is much easier to hide since most modern wallets will set `nLockTime` to recent block heights.

Regards,
ZmnSCPxj

Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[Zac Greenwood]

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Hi ZmnSCPxj,

Thank you for your insightful response.

Perhaps I should take a step back and take a strictly functional angle.
Perhaps the list could help me to establish whether the proposed
functionality is:

Desirable;
Not already possible;
Feasible to implement.

The proposed functionality is as follows:

The ability to control some coin with two private keys (or two sets of
private keys) such that spending is limited over time for one private key
(i.e., it is for instance not possible to spend all coin in a single
transaction) while spending is unrestricted for the other private key (no
limits apply). No limits must apply to coin transacted to a third party.

Also, it must be possible never having to bring the unrestricted private
key online unless more than the limit imposed on the restrictive private
key is desired to be spent.

Less generally, taking the perspective of a hodler: the user must be able
to keep one key offline and one key online. The offline key allows
unrestricted spending, the online key is limited in how much it is allowed
to spend over time.

Furthermore, the spending limit must be intuitive. Best candidate I believe
would be a maximum spend per some fixed number of blocks. For instance, the
restrictive key may allow a maximum of 100k sats per any window of 144
blocks. Ofcourse the user must be able to set these parameters freely.

I look forward to any feedback you may have.

Zac



On Tue, 10 Aug 2021 at 04:17, ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:

>  fromGood morning Zac,
>
>
> With some work, what you want can be implemented, to some extent, today,
> without changes to consensus.
>
> The point you want, I believe, is to have two sets of keys:
>
> * A long-term-storage keyset, in "cold" storage.
> * A short-term-spending keyset, in "warm" storage, controlling only a
> small amount of funds.
>
> What you can do would be:
>
> * Put all your funds in a single UTXO, with an k-of-n of your cold keys
> (ideally P2TR, or some P2WSH k-of-n).
> * Put your cold keys online, and sign a transaction spending the above
> UTXO, and spending most of it to a new address that is a tweaked k-of-n of
> your cold keys, and a smaller output (up to the limit you want) controlled
> by the k-of-n of your warm keys.
>   * Keep this transaction offchain, in your warm storage.
> * Put your cold keys back offline.
> * When you need to spend using your warm keys, bring the above transaction
> onchain, then spend from the budget as needed.
>
>
> If you need to have some estimated amount of usable funds for every future
> unit of time, just create a chain of transactions with future `nLockTime`.
>
>                                   nLocktime +1day  nLockTime +2day
>                   +------------+   +------------+   +------------+
>      cold UTXO -->|    cold TXO|-->|    cold TXO|-->|    cold TXO|--> etc.
>                   |            |   |            |   |            |
>                   |    warm TXO|   |    warm TXO|   |    warm TXO|
>                   +------------+   +------------+   +------------+
>
> Pre-sign the above transactions, store the pre-signed transactions in warm
> storage together with your warm keys.
> Then put the cold keys back offline.
>
> Then from today to tomorrow, you can spend only the first warm TXO.
> From tomorrow to the day after, you can spend only the first two warm TXOs.
> And so on.
>
> If tomorrow your warm keys are stolen, you can bring the cold keys online
> to claim the second cold TXO and limit your fund loss to only just the
> first two warm TXOs.
>
> The above is bulky, but it has the advantage of not using any special
> opcodes or features (improving privacy, especially with P2TR which would in
> theory allow k-of-n/n-of-n to be indistinguishable from 1-of-1), and using
> just `nLockTime`, which is much easier to hide since most modern wallets
> will set `nLockTime` to recent block heights.
>
> Regards,
> ZmnSCPxj
>
>

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Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[ZmnSCPxj]

Good morning Zac,


> Hi ZmnSCPxj,
>
> Thank you for your insightful response.
>
> Perhaps I should take a step back and take a strictly functional angle. Perhaps the list could help me to establish whether the proposed functionality is:
>
> Desirable;
> Not already possible;
> Feasible to implement.
>
> The proposed functionality is as follows:
>
> The ability to control some coin with two private keys (or two sets of private keys) such that spending is limited over time for one private key (i.e., it is for instance not possible to spend all coin in a single transaction) while spending is unrestricted for the other private key (no limits apply). No limits must apply to coin transacted to a third party.
>
> Also, it must be possible never having to bring the unrestricted private key online unless more than the limit imposed on the restrictive private key is desired to be spent.
>
> Less generally, taking the perspective of a hodler: the user must be able to keep one key offline and one key online. The offline key allows unrestricted spending, the online key is limited in how much it is allowed to spend over time.
>
> Furthermore, the spending limit must be intuitive. Best candidate I believe would be a maximum spend per some fixed number of blocks. For instance, the restrictive key may allow a maximum of 100k sats per any window of 144 blocks. Ofcourse the user must be able to set these parameters freely.

My proposal does not *quite* implement a window.
However, that is because it uses `nLockTime`.

With the use of `nSequence` in relative-locktime mode, however, it *does* implement a window, sort of.
More specifically, it implements a timeout on spending --- if you spend using a presigned transaction (which creates an unencumbered specific-valued TXO that can be arbitrarily spent with your online keyset) then you cannot get another "batch" of funds until the `nSequence` relative locktime passes.
However, this *does* implement a window that limits a maximum value spendable per any window of the relative timelock you select.

The disadvantage is that `nSequence` use is a lot more obvious and discernible than `nLockTime` use.
Many wallets today use non-zero `nLockTime` for anti-fee-sniping, and that is a good cover for `nLockTime` transactions.
I believe Dave Harding proposed that wallets should also use, at random, (say 50-50) `nSequence`-in-relative-locktime-mode as an alternate anti-fee-sniping mechanism.
This alternate anti-fee-sniping would help cover `nSequence` use.

Note that my proposal does impose a maximum limit on the number of windows.
With `nSequence`-in-relative-locktime-mode the limit is the number of times that the online keyset can spend.
After spending that many windows, the offline keyset has to be put back online to generate a new set of transactions.

It has the massive massive advantage that you can implement it today without any consensus change, and I think you can expect that consensus change will take a LONG time (xref SegWit, Taproot).

Certainly the functionality is desirable.
But it seems it can be implemented with Bitcoin today.

Regards,
ZmnSCPxj

Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[Zac Greenwood]

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Hi ZmnSCPxj,

Thank you for your counterproposal. I fully agree that as a first step we
must establish whether the proposed functionality can be implemented
without making any changes to consensus.

Your counterproposal is understandably more technical in nature because it
explores an implementation on top of Bitcoin as-is. However I feel that for
a fair comparison of the functionality of both proposals a purely
functional description of your proposal is essential.

If I understand your proposal correctly, then I believe there are some
major gaps between yours and mine:

Keys for unrestricted spending: in my proposal, they never have to come
online unless spending more than the limit is desired. In your proposal,
these keys are required to come online in several situations.

Presigning transactions: not required in my proposal. Wouldn’t such
presigning requirement be detrimental for the usability of your proposal?
Does it mean that for instance the amount and window in which the
transaction can be spent is determined at the time of signing? In my
proposal, there is no limit in the number of transactions per window.

Number of windows: limited in your proposal, unlimited in mine.

There are probably additional gaps that I am currently not technically able
to recognize.

I feel that the above gaps are significant enough to state that your
proposal does not meet the basic requirements of my proposal.

Next to consider is whether the gap is acceptable, weighing the effort to
implement the required consensus changes against the effort and feasibility
of implementing your counterproposal.

I feel that your counterproposal has little chance of being implemented
because of the still considerable effort required and the poor result in
functional terms. I also wonder if your proposal is feasible considering
wallet operability.

Considering all the above, I believe that implementing consensus changes in
order to support the proposed functionality would preferable  over your
counterproposal.

I acknowledge that a consensus change takes years and is difficult to
achieve, but that should not be any reason to stop exploring the appetite
for the proposed functionality and perhaps start looking at possible
technical solutions.

Zac


On Sat, 14 Aug 2021 at 03:50, ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:

> Good morning Zac,
>
>
> > Hi ZmnSCPxj,
> >
> > Thank you for your insightful response.
> >
> > Perhaps I should take a step back and take a strictly functional angle.
> Perhaps the list could help me to establish whether the proposed
> functionality is:
> >
> > Desirable;
> > Not already possible;
> > Feasible to implement.
> >
> > The proposed functionality is as follows:
> >
> > The ability to control some coin with two private keys (or two sets of
> private keys) such that spending is limited over time for one private key
> (i.e., it is for instance not possible to spend all coin in a single
> transaction) while spending is unrestricted for the other private key (no
> limits apply). No limits must apply to coin transacted to a third party.
> >
> > Also, it must be possible never having to bring the unrestricted private
> key online unless more than the limit imposed on the restrictive private
> key is desired to be spent.
> >
> > Less generally, taking the perspective of a hodler: the user must be
> able to keep one key offline and one key online. The offline key allows
> unrestricted spending, the online key is limited in how much it is allowed
> to spend over time.
> >
> > Furthermore, the spending limit must be intuitive. Best candidate I
> believe would be a maximum spend per some fixed number of blocks. For
> instance, the restrictive key may allow a maximum of 100k sats per any
> window of 144 blocks. Ofcourse the user must be able to set these
> parameters freely.
>
> My proposal does not *quite* implement a window.
> However, that is because it uses `nLockTime`.
>
> With the use of `nSequence` in relative-locktime mode, however, it *does*
> implement a window, sort of.
> More specifically, it implements a timeout on spending --- if you spend
> using a presigned transaction (which creates an unencumbered
> specific-valued TXO that can be arbitrarily spent with your online keyset)
> then you cannot get another "batch" of funds until the `nSequence` relative
> locktime passes.
> However, this *does* implement a window that limits a maximum value
> spendable per any window of the relative timelock you select.
>
> The disadvantage is that `nSequence` use is a lot more obvious and
> discernible than `nLockTime` use.
> Many wallets today use non-zero `nLockTime` for anti-fee-sniping, and that
> is a good cover for `nLockTime` transactions.
> I believe Dave Harding proposed that wallets should also use, at random,
> (say 50-50) `nSequence`-in-relative-locktime-mode as an alternate
> anti-fee-sniping mechanism.
> This alternate anti-fee-sniping would help cover `nSequence` use.
>
> Note that my proposal does impose a maximum limit on the number of windows.
> With `nSequence`-in-relative-locktime-mode the limit is the number of
> times that the online keyset can spend.
> After spending that many windows, the offline keyset has to be put back
> online to generate a new set of transactions.
>
> It has the massive massive advantage that you can implement it today
> without any consensus change, and I think you can expect that consensus
> change will take a LONG time (xref SegWit, Taproot).
>
> Certainly the functionality is desirable.
> But it seems it can be implemented with Bitcoin today.
>
> Regards,
> ZmnSCPxj
>
>

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Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[ZmnSCPxj]

Good morning Zac,

> Thank you for your counterproposal. I fully agree that as a first step we must establish whether the proposed functionality can be implemented without making any changes to consensus.
>
> Your counterproposal is understandably more technical in nature because it explores an implementation on top of Bitcoin as-is. However I feel that for a fair comparison of the functionality of both proposals a purely functional description of your proposal is essential.
>
> If I understand your proposal correctly, then I believe there are some major gaps between yours and mine:
>
> Keys for unrestricted spending: in my proposal, they never have to come online unless spending more than the limit is desired. In your proposal, these keys are required to come online in several situations.

Correct, that is indeed a weakness.

It is helpful to see https://zmnscpxj.github.io/bitcoin/unchained.html
Basically: any quorum of signers can impose any rules that are not implementable on the base layer, including the rules you desire.
That quorum is the "offline keyset" in my proposal.

>
> Presigning transactions: not required in my proposal. Wouldn’t such presigning requirement be detrimental for the usability of your proposal? Does it mean that for instance the amount and window in which the transaction can be spent is determined at the time of signing? In my proposal, there is no limit in the number of transactions per window.

No.
Remember, the output is a simple 1-of-1 or k-of-n of the online keyset.
The online keyset can spend that wherever and however, including paying it out to N parties, or paying part of the limit to 1 party and then paying the remainder back to the same onchain keyset so it can access the funds in the future.
Both cases are also available in your proposal, and the latter case (pay out part of the limit to a single output, then keep the rest back to the same onchain keyset) can be used to add an indefinite number of transactions per window.

>
> Number of windows: limited in your proposal, unlimited in mine.

Correct, though you can always have a fairly large number of windows ("640kB ought to be enough for anybody").

>
> There are probably additional gaps that I am currently not technically able to recognize.

It requires a fair amount of storage for the signatures at minimum, though that may be as small as 64 bytes per window.
1Mb of storage for signatures would allow 16,384 windows, assuming you use 1-day windows that is about 44.88 years, probably more than enough that a one-time onlining of the offline keys (or just print out the signatures on paper or display as a QR code, whatever) is acceptable.

> I feel that the above gaps are significant enough to state that your proposal does not meet the basic requirements of my proposal.
>
> Next to consider is whether the gap is acceptable, weighing the effort to implement the required consensus changes against the effort and feasibility of implementing your counterproposal.
>
> I feel that your counterproposal has little chance of being implemented because of the still considerable effort required and the poor result in functional terms. I also wonder if your proposal is feasible considering wallet operability.

See above, particularly the gap that does not, in fact, exist.

>
> Considering all the above, I believe that implementing consensus changes in order to support the proposed functionality would preferable  over your counterproposal.
>
> I acknowledge that a consensus change takes years and is difficult to achieve, but that should not be any reason to stop exploring the appetite for the proposed functionality and perhaps start looking at possible technical solutions.

You can also look into the "covenant" opcodes (`OP_CHECKSIGFROMSTACK`, `OP_CHECKTEMPLATEVERIFY`, etc.), I think JeremyRubin has a bunch of them listed somewhere, which may be used to implement something similar without requiring presigning.

Since the basic "just use `nSequence`" scheme already implements what you need, what the covenant opcodes buy you is that you do not need the offline keyset to be onlined and there is no need to keep signatures, removing the remaining gaps you identified.
With a proper looping covenant opcode, there is also no limit on the number of windows.

The issue with the covenant opcodes is that there are several proposals with overlapping abilities and different tradeoffs.
This is the sort of thing that invites bikeshed-painting.

I suggest looking into the covenant opcodes and supporting those instead of your own proposal, as your application is very close to one of the motivating examples for covenants in the first place.

Regards,
ZmnSCPxj

Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[Zac Greenwood]

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Hi ZmnSCPxj,

> I suggest looking into the covenant opcodes and supporting those instead
of your own proposal, as your application is very close to one of the
motivating examples for covenants in the first place.

I believe it is not the right approach to take a proposal, chop off key
aspects of its functionality, and rely to some future change in Bitcoin
that may perhaps enable implementing some watered down version of the
intended functionality. In my opinion the right order would be to first
discuss the unmodified proposal on a functional level and gauge community
interest, then move forward to discuss technical challenges for the
*unmodified* proposal instead of first knee-capping the proposal in order
to (presumably) reduce cost of implementation.

I believe that we both recognize that the proposed functionality would be
beneficial. I believe that your position is that functionality close to
what I have in mind can be implemented using covenants, albeit with some
gaps. For me personally however these gaps would not be acceptable because
they severely hurt the predictability and intuitiveness of the behavior of
the functionality for the end-user. But as noted, I believe at this point
it is premature to have this discussion.

Perhaps you could help me understand what would be required to implement
the *unmodified* proposal. That way, the community will be able to better
assess the cost (in terms of effort and risk) and weigh it against the
perceived benefits. Perhaps *then* we find that the cost could be
significantly reduced without any significant reduction of the benefits,
for instance by slightly compromising on the functionality such that no
changes to consensus would be required for its implementation. (I am
skeptical that this would be possible though). The cost reduction must be
carefully weighed against the functional gaps it creates.

I am aware that my proposal must be well-defined functionally before being
able to reason about its benefits and implementational aspects. I believe
that the proposed functionality is pretty straightforward, but I am happy
to come up with a more precise functional spec. However, such effort would
be wasted if there is no community interest for this functionality. So far
only few people have engaged with this thread, and I am not sure that this
is because there is no interest in the proposal or because most people just
lurk here and do not feel like giving their opinion on random proposals. It
would be great however to learn about more people's opinions.

As a reminder, the proposed functionality is to enable a user to limit the
amount that they able to spent from an address within a certain time-frame
or window (defined in number of blocks) while retaining the ability to
spend arbitrary amounts using a secondary private key (or set of private
keys). The general use case is to prevent theft of large amounts while
still allowing a user to spend small amounts over time. Hodlers as well as
exchanges dealing with cold, warm and hot wallets come to mind as users who
could materially benefit from this functionality.

Zac



On Mon, Aug 16, 2021 at 1:48 PM ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:

> Good morning Zac,
>
> > Thank you for your counterproposal. I fully agree that as a first step
> we must establish whether the proposed functionality can be implemented
> without making any changes to consensus.
> >
> > Your counterproposal is understandably more technical in nature because
> it explores an implementation on top of Bitcoin as-is. However I feel that
> for a fair comparison of the functionality of both proposals a purely
> functional description of your proposal is essential.
> >
> > If I understand your proposal correctly, then I believe there are some
> major gaps between yours and mine:
> >
> > Keys for unrestricted spending: in my proposal, they never have to come
> online unless spending more than the limit is desired. In your proposal,
> these keys are required to come online in several situations.
>
> Correct, that is indeed a weakness.
>
> It is helpful to see https://zmnscpxj.github.io/bitcoin/unchained.html
> Basically: any quorum of signers can impose any rules that are not
> implementable on the base layer, including the rules you desire.
> That quorum is the "offline keyset" in my proposal.
>
> >
> > Presigning transactions: not required in my proposal. Wouldn’t such
> presigning requirement be detrimental for the usability of your proposal?
> Does it mean that for instance the amount and window in which the
> transaction can be spent is determined at the time of signing? In my
> proposal, there is no limit in the number of transactions per window.
>
> No.
> Remember, the output is a simple 1-of-1 or k-of-n of the online keyset.
> The online keyset can spend that wherever and however, including paying it
> out to N parties, or paying part of the limit to 1 party and then paying
> the remainder back to the same onchain keyset so it can access the funds in
> the future.
> Both cases are also available in your proposal, and the latter case (pay
> out part of the limit to a single output, then keep the rest back to the
> same onchain keyset) can be used to add an indefinite number of
> transactions per window.
>
> >
> > Number of windows: limited in your proposal, unlimited in mine.
>
> Correct, though you can always have a fairly large number of windows
> ("640kB ought to be enough for anybody").
>
> >
> > There are probably additional gaps that I am currently not technically
> able to recognize.
>
> It requires a fair amount of storage for the signatures at minimum, though
> that may be as small as 64 bytes per window.
> 1Mb of storage for signatures would allow 16,384 windows, assuming you use
> 1-day windows that is about 44.88 years, probably more than enough that a
> one-time onlining of the offline keys (or just print out the signatures on
> paper or display as a QR code, whatever) is acceptable.
>
> > I feel that the above gaps are significant enough to state that your
> proposal does not meet the basic requirements of my proposal.
> >
> > Next to consider is whether the gap is acceptable, weighing the effort
> to implement the required consensus changes against the effort and
> feasibility of implementing your counterproposal.
> >
> > I feel that your counterproposal has little chance of being implemented
> because of the still considerable effort required and the poor result in
> functional terms. I also wonder if your proposal is feasible considering
> wallet operability.
>
> See above, particularly the gap that does not, in fact, exist.
>
> >
> > Considering all the above, I believe that implementing consensus changes
> in order to support the proposed functionality would preferable  over your
> counterproposal.
> >
> > I acknowledge that a consensus change takes years and is difficult to
> achieve, but that should not be any reason to stop exploring the appetite
> for the proposed functionality and perhaps start looking at possible
> technical solutions.
>
> You can also look into the "covenant" opcodes (`OP_CHECKSIGFROMSTACK`,
> `OP_CHECKTEMPLATEVERIFY`, etc.), I think JeremyRubin has a bunch of them
> listed somewhere, which may be used to implement something similar without
> requiring presigning.
>
> Since the basic "just use `nSequence`" scheme already implements what you
> need, what the covenant opcodes buy you is that you do not need the offline
> keyset to be onlined and there is no need to keep signatures, removing the
> remaining gaps you identified.
> With a proper looping covenant opcode, there is also no limit on the
> number of windows.
>
> The issue with the covenant opcodes is that there are several proposals
> with overlapping abilities and different tradeoffs.
> This is the sort of thing that invites bikeshed-painting.
>
> I suggest looking into the covenant opcodes and supporting those instead
> of your own proposal, as your application is very close to one of the
> motivating examples for covenants in the first place.
>
> Regards,
> ZmnSCPxj
>

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Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[ZmnSCPxj]

Good morning Zac,


> Perhaps you could help me understand what would be required to implement the *unmodified* proposal. That way, the community will be able to better assess the cost (in terms of effort and risk) and weigh it against the perceived benefits. Perhaps *then* we find that the cost could be significantly reduced without any significant reduction of the benefits, for instance by slightly compromising on the functionality such that no changes to consensus would be required for its implementation. (I am skeptical that this would be possible though). The cost reduction must be carefully weighed against the functional gaps it creates.

For one, such output need to be explicitly visible, to implement the "change outputs must also be rate-limited".
A tx spending a rate-limited output has to know that one of the outputs is also a rate-limited output.

This flagging needs to be done by either allocating a new SegWit version --- a resource that is not lightly allocated, there being only 30 versions left if my understanding is correct --- or blessing yet another anyone-can-spend `scriptPubKey` template, something we want to avoid which is why SegWit has versions (i.e. we want SegWit to be the last anyone-can-spend `scriptPubKey` template we bless for a **long** time).

Explicit flagging is bad as well for privacy, which is another mark against it.
Notice how Taproot improves privacy by making n-of-n indistinguishable from 1-of-1 (and with proper design or a setup ritual, k-of-n can be made indistinguishable from 1-of-1).
Notice as well that my first counterproposal is significantly more private than explicit flagging, and my second coutnerproposal is also more private if wallets change their anti-fee-sniping mitigation.
This privacy loss represented by explicit flagging will be resisted by some people, especially those that use a bunch of random letters as a pseudonym (because duh, privacy).

(Yes, people can just decide not to use the privacy-leaking explicitly-flagged outputs, but that reduces the anonymity set of people who *are* interested in privacy, so people who are interested in privacy will prefer that other people do not leak their privacy so they can hide among *those* people as well.)

You also probably need to keep some data with each output.
This can be done by explicitly storing that data in the output directly, rather than a commitment to that data --- again, the "change outputs must also be rate-limited" requirement needs to check those data.

The larger data stored with the output is undesirable, ideally we want each output to just be a commitment rather than contain any actual data, because often a 20-byte commitment is smaller than the data that needs to be stored.
For example, I imagine that your original proposal requires, for change outputs, to store:

* The actual rate limit.
* The time frame of the rate limit.
* The reduced rate limit, since we spent an amount within a specific time frame (i.e. residual limit) which is why this is a change output.
* How long that time frame lasts.
* A commitment to the keys that can spend this.

Basically, until the residual limit expires, we impose the residual limit, then after the expiry of the residual limit we go back to the original rate limit.

The commitment to the keys itself takes at least 20 bytes, and if you are planning a to support k-of-n then that takes at least 32 bytes.
If this was not explicitly tagged, then a 32 byte commitment to all the necessary data would have been enough, but you do need the explicit tagging for the "change outputs must be rate-limited too".

Note as well that the residual needs to be kept with the output.
Bitcoin Core does not store transactions in a lookup table, it stores individual *outputs*.
While the residual can be derived from the transaction, we do not have a transaction table.
Thus, we need to explicitly put it on the output itself, directly, since we only have a lookup table for the unspent outputs, not individual transactions.

(well there is `txindex` but that is an option for each node, not something consensus code can rely on)

So yes, that "change outputs must also be rate-limited" is the big sticking point, and a lot of the "gaps" you worry about occur when we drop this bit.
Drop this bit and you can implement it today without any consensus code change, and with privacy good enough to prevent people with random letters as pseudonym from trying to stop you.

Regards,
ZmnSCPxj

Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[Zac Greenwood]

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Hi ZmnSCPxj,

Thank you for your helpful response. We're on the same page concerning
privacy so I'll focus on that. I understand from your mail that privacy
would be reduced by this proposal because:

* It requires the introduction of a new type of transaction that is
different from a "standard" transaction (would that be P2TR in the
future?), reducing the anonymity set for everyone;
* The payment and change output will be identifiable because the change
output must be marked encumbered on-chain;
* The specifics of how the output is encumbered must be visible on-chain as
well reducing privacy even further.

I don't have the technical skills to judge whether these issues can somehow
be resolved. In functional terms, the output should be spendable in a way
that does not reveal that the output is encumbered, and produce a change
output that cannot be distinguished from a non-change output while still
being encumbered. Perhaps some clever MAST-fu could somehow help?

I imagine that the offered functionality does not justify the above
mentioned privacy reductions, so unless these can be addressed, without
functional modification this proposal sadly seems dead in the water.

Thanks again.

Zac


On Tue, Aug 31, 2021 at 11:00 AM ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:

> Good morning Zac,
>
>
> > Perhaps you could help me understand what would be required to implement
> the *unmodified* proposal. That way, the community will be able to better
> assess the cost (in terms of effort and risk) and weigh it against the
> perceived benefits. Perhaps *then* we find that the cost could be
> significantly reduced without any significant reduction of the benefits,
> for instance by slightly compromising on the functionality such that no
> changes to consensus would be required for its implementation. (I am
> skeptical that this would be possible though). The cost reduction must be
> carefully weighed against the functional gaps it creates.
>
> For one, such output need to be explicitly visible, to implement the
> "change outputs must also be rate-limited".
> A tx spending a rate-limited output has to know that one of the outputs is
> also a rate-limited output.
>
> This flagging needs to be done by either allocating a new SegWit version
> --- a resource that is not lightly allocated, there being only 30 versions
> left if my understanding is correct --- or blessing yet another
> anyone-can-spend `scriptPubKey` template, something we want to avoid which
> is why SegWit has versions (i.e. we want SegWit to be the last
> anyone-can-spend `scriptPubKey` template we bless for a **long** time).
>
> Explicit flagging is bad as well for privacy, which is another mark
> against it.
> Notice how Taproot improves privacy by making n-of-n indistinguishable
> from 1-of-1 (and with proper design or a setup ritual, k-of-n can be made
> indistinguishable from 1-of-1).
> Notice as well that my first counterproposal is significantly more private
> than explicit flagging, and my second coutnerproposal is also more private
> if wallets change their anti-fee-sniping mitigation.
> This privacy loss represented by explicit flagging will be resisted by
> some people, especially those that use a bunch of random letters as a
> pseudonym (because duh, privacy).
>
> (Yes, people can just decide not to use the privacy-leaking
> explicitly-flagged outputs, but that reduces the anonymity set of people
> who *are* interested in privacy, so people who are interested in privacy
> will prefer that other people do not leak their privacy so they can hide
> among *those* people as well.)
>
> You also probably need to keep some data with each output.
> This can be done by explicitly storing that data in the output directly,
> rather than a commitment to that data --- again, the "change outputs must
> also be rate-limited" requirement needs to check those data.
>
> The larger data stored with the output is undesirable, ideally we want
> each output to just be a commitment rather than contain any actual data,
> because often a 20-byte commitment is smaller than the data that needs to
> be stored.
> For example, I imagine that your original proposal requires, for change
> outputs, to store:
>
> * The actual rate limit.
> * The time frame of the rate limit.
> * The reduced rate limit, since we spent an amount within a specific time
> frame (i.e. residual limit) which is why this is a change output.
> * How long that time frame lasts.
> * A commitment to the keys that can spend this.
>
> Basically, until the residual limit expires, we impose the residual limit,
> then after the expiry of the residual limit we go back to the original rate
> limit.
>
> The commitment to the keys itself takes at least 20 bytes, and if you are
> planning a to support k-of-n then that takes at least 32 bytes.
> If this was not explicitly tagged, then a 32 byte commitment to all the
> necessary data would have been enough, but you do need the explicit tagging
> for the "change outputs must be rate-limited too".
>
> Note as well that the residual needs to be kept with the output.
> Bitcoin Core does not store transactions in a lookup table, it stores
> individual *outputs*.
> While the residual can be derived from the transaction, we do not have a
> transaction table.
> Thus, we need to explicitly put it on the output itself, directly, since
> we only have a lookup table for the unspent outputs, not individual
> transactions.
>
> (well there is `txindex` but that is an option for each node, not
> something consensus code can rely on)
>
> So yes, that "change outputs must also be rate-limited" is the big
> sticking point, and a lot of the "gaps" you worry about occur when we drop
> this bit.
> Drop this bit and you can implement it today without any consensus code
> change, and with privacy good enough to prevent people with random letters
> as pseudonym from trying to stop you.
>
> Regards,
> ZmnSCPxj
>
>

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Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[ZmnSCPxj]

Good morning Zac,

> Hi ZmnSCPxj,
>
> Thank you for your helpful response. We're on the same page concerning privacy so I'll focus on that. I understand from your mail that privacy would be reduced by this proposal because:
>
> * It requires the introduction of a new type of transaction that is different from a "standard" transaction (would that be P2TR in the future?), reducing the anonymity set for everyone;
> * The payment and change output will be identifiable because the change output must be marked encumbered on-chain;
> * The specifics of how the output is encumbered must be visible on-chain as well reducing privacy even further.
>
> I don't have the technical skills to judge whether these issues can somehow be resolved. In functional terms, the output should be spendable in a way that does not reveal that the output is encumbered, and produce a change output that cannot be distinguished from a non-change output while still being encumbered. Perhaps some clever MAST-fu could somehow help?

I believe some of the covenant efforts may indeed have such clever MAST-fu integrated into them, which is why I pointed you to them --- the people developing these (aj I think? RubenSomsen?) might be able to accommodate this or some subset of the desired feature in a sufficiently clever covenant scheme.

There are a number of such proposals, though, so I cannot really point you to one that seems likely to have a lot of traction.

Regards,
ZmnSCPxj

Re: [bitcoin-dev] Exploring: limiting transaction output amount as a function of total input value

[Zac Greenwood]

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Hi ZmnSCPxj,

The rate-limiting algorithm would be relatively straightforward. I
documented the rate-limiting part of the algorithm below, perhaps they can
evoke new ideas of how to make this MAST-able or otherwise implement this
in a privacy preserving way.

Something like the following:

=> Create an output at block height [h0] with the following properties:

Serving as input at any block height, the maximum amount is limited to
[limit] sats;  // This rule introduces [limit] and is permanent and always
copied over to a change output
Serving as input at a block height < [h0 + window], the maximum amount is
limited to [limit - 0] sats;  // [limit - 0] to emphasize that nothing was
spent yet and no window has started.

=> A transaction occurs at block height [h1], spending [h1_spent].
The payment output created at [h1] is not encumbered and of value
[h1_spent]; // Note, this is the first encumbered transaction so [h1] is
the first block of the first window

The change output created at block height [h1] must be encumbered as
follows:
Serving as input at any block height, the maximum amount is limited to
[limit] sats;  // Permanent rule repeats
Serving as input at a block height < [h1 + window], the maximum amount is
limited to [limit - h1_spent]  // Second permanent rule reduces spendable
amount until height [h1 + window] by [h1_spent]

=> A second transaction occurs at block height [h2], spending [h2_spent].
The payment output created at [h2] is not encumbered and of value
[h2_spent]; // Second transaction, so a second window starts at [h2]

The change output created at block height [h2] must be encumbered as
follows:
Serving as input at any block height, the maximum amount is limited to
[limit] sats;  // Permanent rule repeats
Serving as input at a block height < [h1 + window], the max amount is
limited to [limit - h1_spent - h2_spent] // Reduce spendable amount between
[h1] and [h1 + window] by an additional [h2_spent]
Serving as input in range [h1 + window] <= block height < [h2 + window],
the max amount is limited to [limit - h2_spent]  // First payment no longer
inside this window so [h1_spent] no longer subtracted

... and so on. A rule that pertains to a block height < the current block
height can be abandoned, keeping the number of rules equal to the number of
transactions that exist within the oldest still active window.

Zac


On Tue, Aug 31, 2021 at 4:22 PM ZmnSCPxj <ZmnSCPxj@protonmail.com> wrote:

> Good morning Zac,
>
> > Hi ZmnSCPxj,
> >
> > Thank you for your helpful response. We're on the same page concerning
> privacy so I'll focus on that. I understand from your mail that privacy
> would be reduced by this proposal because:
> >
> > * It requires the introduction of a new type of transaction that is
> different from a "standard" transaction (would that be P2TR in the
> future?), reducing the anonymity set for everyone;
> > * The payment and change output will be identifiable because the change
> output must be marked encumbered on-chain;
> > * The specifics of how the output is encumbered must be visible on-chain
> as well reducing privacy even further.
> >
> > I don't have the technical skills to judge whether these issues can
> somehow be resolved. In functional terms, the output should be spendable in
> a way that does not reveal that the output is encumbered, and produce a
> change output that cannot be distinguished from a non-change output while
> still being encumbered. Perhaps some clever MAST-fu could somehow help?
>
> I believe some of the covenant efforts may indeed have such clever MAST-fu
> integrated into them, which is why I pointed you to them --- the people
> developing these (aj I think? RubenSomsen?) might be able to accommodate
> this or some subset of the desired feature in a sufficiently clever
> covenant scheme.
>
> There are a number of such proposals, though, so I cannot really point you
> to one that seems likely to have a lot of traction.
>
> Regards,
> ZmnSCPxj
>

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Re: [bitcoin-dev] Covenant opcode proposal OP_CONSTRAINDESTINATION (an alternative to OP_CTV)

[Billy Tetrud]

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FYI I broke out the fee limiting functionality from OP_CD into an opcode
called OP_LIMITFEECONTRIBUTION
<https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/lfc/bip-limit-fee-contribution.md>
as
Jeremy suggested.

On Fri, Jul 30, 2021 at 1:42 PM Billy Tetrud <billy.tetrud@gmail.com> wrote:

> Thanks for taking another look Jeremy. That's an interesting idea to split
> it up into simpler opcodes, however there are some
> limitations/considerations there.
>
> For example, with output addresses, I added specifying amounts to outputs
> in order to make script evaluation simpler and eliminate a potential DOS
> vector. I wrote about this in the section 'Specifying values sent to each
> output
> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/blob/main/cd/bip-constraindestination.md#specifying-values-sent-to-each-output>'.
> Originally, I designed OP_CD without specifying what amounts an input
> contributes to what outputs, but it seemed like this would require
> calculating various combinations of inequalities, which could get expensive
> in scenarios where many inputs had overlapping destinations. See the
> examples under the OP_CD section in this commit
> <https://github.com/fresheneesz/bip-efficient-bitcoin-vaults/commit/9b2257410b5f0fc991f68e774c3faf601c02cc5d>
> .
>
> Maybe there's an elegant and cheap way of verifying that a number of
> inputs that have destination address limitations is within limits, but if
> so I don't know how to do that. If there was a good way to do that, then I
> wouldn't want to propose the ability to validate that specific amounts go
> to specific outputs. So unless there's a simple and dos-vector-free way of
> evaluating what addresses an input goes to without knowing what amounts an
> input contributes to each output, I don't think these functionalities
> should be separated.
>
> And about a fee-limit opcode, that could certainly be done on its own.
> However, a version of OP_CD that doesn't specify fees would have to take
> the fee-limit into account, and the calculation for the stand-alone
> fee-limit operation would be moot for that output.
>
> So I think it could make sense to split the fee limit off from the rest of
> OP_CD. I'm curious to know what others think of that.
>
> > all transactions are twice as large as they might otherwise need to be
> for simple things like congestion control trees, since you have to repeat
> all of the output data twice
>
> Well, the transaction wouldn't be quite twice as large. Each output would
> add 9 bytes to the transaction, and outputs already are a minimum of about
> 30 bytes I think? So for transactions with a lot of outputs, it could make
> the transaction about 1/3 larger. I'll add a section on this into my
> proposal.
>
> Perhaps it would be a reasonable optimization to allow omitting an output
> value in cases where the entire output amount is contributed by that input.
> This would reduce the overhead of specifying output amounts to 2 bytes for
> most outputs (1 byte for the index, another to indicate the full value),
> meaning that it would only make the transaction about 7% larger. What do
> you think about that idea?
>
> On Wed, Jul 28, 2021 at 3:30 PM Jeremy via bitcoin-dev <
> bitcoin-dev@lists.linuxfoundation.org> wrote:
>
>> High level feedback:
>>
>> you should spec out the opcodes as separate pieces of functionality as it
>> sounds like OP_CD is really 3 or 4 opcodes in one (e.g., amounts to
>> outputs, output addresses, something with fees).
>>
>> One major drawback of your approach is that all transactions are twice as
>> large as they might otherwise need to be for simple things like congestion
>> control trees, since you have to repeat all of the output data twice.
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists.linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>

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