[bitcoin-dev] [PROPOSAL] OP_TX: generalized covenants reduced to OP_CHECKTEMPLATEVERIFY

[bitcoin-dev] [PROPOSAL] OP_TX: generalized covenants reduced to OP_CHECKTEMPLATEVERIFY

[Rusty Russell]

Hi all,

	TL;DR: a v1 tapscript opcode for generic covenants, but
OP_SUCCESS unless it's used a-la OP_CHECKTEMPLATEVERIFY.  This gives an
obvious use case, with clean future expansion.  OP_NOP4 can be
repurposed in future as a shortcut, if experience shows that to be a
useful optimization.

(This proposal builds on Russell O'Connor's TXHASH[1], with Anthony
Towns' modification via extending the opcode[2]; I also notice on
re-reading that James Lu had a similar restriction idea[3]).

Details
-------

OP_TX, when inside v1 tapscript, is followed by 4 bytes of flags.
Unknown flag patterns are OP_SUCCESS, though for thoroughness some future
potential uses are documented here.  Note that pushing more than 1000
elements on the stack or an element more than 512 bytes will hit the
BIP-342 resource limits and fail.

Defined bits
------------

(Only those marked with * have to be defined for this soft fork; the
 others can have semantics later).

OPTX_SEPARATELY: treat fields separately (vs concatenating)
OPTX_UNHASHED: push on the stack without hashing (vs SHA256 before push)

- The first nicely sidesteps the lack of OP_CAT, and the latter allows
  OP_TXHASH semantics (and avoid stack element limits).

OPTX_SELECT_VERSION*: version
OPTX_SELECT_LOCKTIME*: nLocktime
OPTX_SELECT_INPUTNUM*: current input number
OPTX_SELECT_INPUTCOUNT*: number of inputs
OPTX_SELECT_OUTPUTCOUNT*: number of outputs

OPTX_INPUT_SINGLE: if set, pop input number off stack to apply to
		OPTX_SELECT_INPUT_*, otherwise iterate through all.
OPTX_SELECT_INPUT_TXID: txid
OPTX_SELECT_INPUT_OUTNUM: txout index
OPTX_SELECT_INPUT_NSEQUENCE*: sequence number
OPTX_SELECT_INPUT_AMOUNT32x2: sats in, as a high-low u31 pair
OPTX_SELECT_INPUT_SCRIPT*: input scriptsig
OPTX_SELECT_INPUT_TAPBRANCH: ?
OPTX_SELECT_INPUT_TAPLEAF: ?

OPTX_OUTPUT_SINGLE: if set, pop input number off stack to apply to
		OPTX_SELECT_OUTPUT_*, otherwise iterate through all.
OPTX_SELECT_OUTPUT_AMOUNT32x2*: sats out, as a high-low u31 pair
OPTX_SELECT_OUTPUT_SCRIPTPUBKEY*: output scriptpubkey

OPTX_SELECT_19...OPTX_SELECT_31: future expansion.

OP_CHECKTEMPLATEVERIFY is approximated by the following flags:
	OPTX_SELECT_VERSION
	OPTX_SELECT_LOCKTIME
	OPTX_SELECT_INPUTCOUNT
	OPTX_SELECT_INPUT_SCRIPT
	OPTX_SELECT_INPUT_NSEQUENCE
	OPTX_SELECT_OUTPUTCOUNT
	OPTX_SELECT_OUTPUT_AMOUNT32x2
	OPTX_SELECT_OUTPUT_SCRIPTPUBKEY
	OPTX_SELECT_INPUTNUM

All other flag combinations result in OP_SUCCESS.

Discussion
----------

By enumerating exactly what can be committed to, it's absolutely clear
what is and isn't committed (and what you need to think about!).

The bits which separate concatenation and hashing provide a simple
mechanism for template-style (i.e. CTV-style) commitments, or for
programatic treatment of individual elements (e.g. amounts, though the
two s31 style is awkward: a 64-bit push flag could be added in future).

The lack of double-hashing of scriptsigs and other fields means we
cannot simply re-use hashing done for SIGHASH_ALL.

The OP_SUCCESS semantic is only valid in tapscript v1, so this does not
allow covenants for v0 segwit or pre-segwit inputs.  If covenants prove
useful, dedicated opcodes can be provided for those cases (a-la
OP_CHECKTEMPLATEVERIFY).

Cheers,
Rusty.

[1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019813.html
[2] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019819.html
[3] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019816.html

Re: [bitcoin-dev] [PROPOSAL] OP_TX: generalized covenants reduced to OP_CHECKTEMPLATEVERIFY

[Brandon Black]

Hi Rusty,

Thanks for this. Seems like a productive direction to explore.

To me, one of the biggest limitations of CTV is that the script is
specific to the amount of the input being spent. OP_TX makes it
possible, although clumsy, to emulate OP_IN_OUT_AMOUNT, which could be
combined with CTV emulation less OPTX_SELECT_OUTPUT_AMOUNT32x2 to allow
a single script to be reused. Given that potential, I wonder if
OPTX_SELECT_IN_OUT_AMOUNT32x2 would be worth adding to the initial set
of flags.

With that addition, a small script can be constructed for a relocatable,
batchable construction (eg. as a building block for vaults):

OPTX_SEPARATELY|OPTX_UNHASHED|OPTX_INPUTNUM OP_TX OP_DUP

OPTX_SELECT_VERSION|
OPTX_SELECT_LOCKTIME|
OPTX_SELECT_INPUT_SINGLE|
OPTX_SELECT_INPUT_SCRIPT|
OPTX_SELECT_INPUT_NSEQUENCE|
OPTX_SELECT_OUTPUT_SINGLE|
OPTX_SELECT_OUTPUT_SCRIPTPUBKEY|
OPTX_SELECT_IN_OUT_AMOUNT32x2 OP_TX <expectedhash> OP_EQUAL

* Additional inputs and change outputs can be added
  * Could commit to 0 fee and still be useful
* Arbitrary amounts can be sent to the same script
* There is no txid predictability (unlike CTV)
  * Anyone could rearrange such input/output pairs after broadcast
	* Not suitable for some uses
	* Potentially helpful for others

Best,

--Brandon

On 2022-05-10 (Tue) at 20:05:54 +0930, Rusty Russell via bitcoin-dev wrote:
> Hi all,
> 
> 	TL;DR: a v1 tapscript opcode for generic covenants, but
> OP_SUCCESS unless it's used a-la OP_CHECKTEMPLATEVERIFY.  This gives an
> obvious use case, with clean future expansion.  OP_NOP4 can be
> repurposed in future as a shortcut, if experience shows that to be a
> useful optimization.
> 
> (This proposal builds on Russell O'Connor's TXHASH[1], with Anthony
> Towns' modification via extending the opcode[2]; I also notice on
> re-reading that James Lu had a similar restriction idea[3]).
> 
> Details
> -------
> 
> OP_TX, when inside v1 tapscript, is followed by 4 bytes of flags.
> Unknown flag patterns are OP_SUCCESS, though for thoroughness some future
> potential uses are documented here.  Note that pushing more than 1000
> elements on the stack or an element more than 512 bytes will hit the
> BIP-342 resource limits and fail.
> 
> Defined bits
> ------------
> 
> (Only those marked with * have to be defined for this soft fork; the
>  others can have semantics later).
> 
> OPTX_SEPARATELY: treat fields separately (vs concatenating)
> OPTX_UNHASHED: push on the stack without hashing (vs SHA256 before push)
> 
> - The first nicely sidesteps the lack of OP_CAT, and the latter allows
>   OP_TXHASH semantics (and avoid stack element limits).
> 
> OPTX_SELECT_VERSION*: version
> OPTX_SELECT_LOCKTIME*: nLocktime
> OPTX_SELECT_INPUTNUM*: current input number
> OPTX_SELECT_INPUTCOUNT*: number of inputs
> OPTX_SELECT_OUTPUTCOUNT*: number of outputs
> 
> OPTX_INPUT_SINGLE: if set, pop input number off stack to apply to
> 		OPTX_SELECT_INPUT_*, otherwise iterate through all.
> OPTX_SELECT_INPUT_TXID: txid
> OPTX_SELECT_INPUT_OUTNUM: txout index
> OPTX_SELECT_INPUT_NSEQUENCE*: sequence number
> OPTX_SELECT_INPUT_AMOUNT32x2: sats in, as a high-low u31 pair
> OPTX_SELECT_INPUT_SCRIPT*: input scriptsig
> OPTX_SELECT_INPUT_TAPBRANCH: ?
> OPTX_SELECT_INPUT_TAPLEAF: ?
> 
> OPTX_OUTPUT_SINGLE: if set, pop input number off stack to apply to
> 		OPTX_SELECT_OUTPUT_*, otherwise iterate through all.
> OPTX_SELECT_OUTPUT_AMOUNT32x2*: sats out, as a high-low u31 pair
> OPTX_SELECT_OUTPUT_SCRIPTPUBKEY*: output scriptpubkey
> 
> OPTX_SELECT_19...OPTX_SELECT_31: future expansion.
> 
> OP_CHECKTEMPLATEVERIFY is approximated by the following flags:
> 	OPTX_SELECT_VERSION
> 	OPTX_SELECT_LOCKTIME
> 	OPTX_SELECT_INPUTCOUNT
> 	OPTX_SELECT_INPUT_SCRIPT
> 	OPTX_SELECT_INPUT_NSEQUENCE
> 	OPTX_SELECT_OUTPUTCOUNT
> 	OPTX_SELECT_OUTPUT_AMOUNT32x2
> 	OPTX_SELECT_OUTPUT_SCRIPTPUBKEY
> 	OPTX_SELECT_INPUTNUM
> 
> All other flag combinations result in OP_SUCCESS.
> 
> Discussion
> ----------
> 
> By enumerating exactly what can be committed to, it's absolutely clear
> what is and isn't committed (and what you need to think about!).
> 
> The bits which separate concatenation and hashing provide a simple
> mechanism for template-style (i.e. CTV-style) commitments, or for
> programatic treatment of individual elements (e.g. amounts, though the
> two s31 style is awkward: a 64-bit push flag could be added in future).
> 
> The lack of double-hashing of scriptsigs and other fields means we
> cannot simply re-use hashing done for SIGHASH_ALL.
> 
> The OP_SUCCESS semantic is only valid in tapscript v1, so this does not
> allow covenants for v0 segwit or pre-segwit inputs.  If covenants prove
> useful, dedicated opcodes can be provided for those cases (a-la
> OP_CHECKTEMPLATEVERIFY).
> 
> Cheers,
> Rusty.
> 
> [1] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019813.html
> [2] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019819.html
> [3] https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019816.html
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev

-- 
--Brandon

Re: [bitcoin-dev] [PROPOSAL] OP_TX: generalized covenants reduced to OP_CHECKTEMPLATEVERIFY

[Alex Schoof]

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Hi Rusty,

One of the common sentiments thats been expressed over the last few months
is that more people want to see experimentation with different applications
using covenants. I really like this proposal because in addition to
offering a cleaner upgrade/extension path than adding “CTV++” as a new
opcode in a few years, it also seems like it would make it very easy to
create prototype applications to game out new ideas:
If the “only this combination of fields are valid, otherwise OP_SUCCESS”
check is just comparing with a list of bitmasks for permissible field
combinations (which right now is a list of length 1), it seems like it
would be *very* easy for people who want to play with other covenant field
sets to just add the relevant bitmasks and then go spin up a signet to
build applications.

Being able to make a very targeted change like that to enable
experimentation is super cool. Thanks for sharing!

Alex

On Tue, May 10, 2022 at 6:37 AM Rusty Russell via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Hi all,
>
>         TL;DR: a v1 tapscript opcode for generic covenants, but
> OP_SUCCESS unless it's used a-la OP_CHECKTEMPLATEVERIFY.  This gives an
> obvious use case, with clean future expansion.  OP_NOP4 can be
> repurposed in future as a shortcut, if experience shows that to be a
> useful optimization.
>
> (This proposal builds on Russell O'Connor's TXHASH[1], with Anthony
> Towns' modification via extending the opcode[2]; I also notice on
> re-reading that James Lu had a similar restriction idea[3]).
>
> Details
> -------
>
> OP_TX, when inside v1 tapscript, is followed by 4 bytes of flags.
> Unknown flag patterns are OP_SUCCESS, though for thoroughness some future
> potential uses are documented here.  Note that pushing more than 1000
> elements on the stack or an element more than 512 bytes will hit the
> BIP-342 resource limits and fail.
>
> Defined bits
> ------------
>
> (Only those marked with * have to be defined for this soft fork; the
>  others can have semantics later).
>
> OPTX_SEPARATELY: treat fields separately (vs concatenating)
> OPTX_UNHASHED: push on the stack without hashing (vs SHA256 before push)
>
> - The first nicely sidesteps the lack of OP_CAT, and the latter allows
>   OP_TXHASH semantics (and avoid stack element limits).
>
> OPTX_SELECT_VERSION*: version
> OPTX_SELECT_LOCKTIME*: nLocktime
> OPTX_SELECT_INPUTNUM*: current input number
> OPTX_SELECT_INPUTCOUNT*: number of inputs
> OPTX_SELECT_OUTPUTCOUNT*: number of outputs
>
> OPTX_INPUT_SINGLE: if set, pop input number off stack to apply to
>                 OPTX_SELECT_INPUT_*, otherwise iterate through all.
> OPTX_SELECT_INPUT_TXID: txid
> OPTX_SELECT_INPUT_OUTNUM: txout index
> OPTX_SELECT_INPUT_NSEQUENCE*: sequence number
> OPTX_SELECT_INPUT_AMOUNT32x2: sats in, as a high-low u31 pair
> OPTX_SELECT_INPUT_SCRIPT*: input scriptsig
> OPTX_SELECT_INPUT_TAPBRANCH: ?
> OPTX_SELECT_INPUT_TAPLEAF: ?
>
> OPTX_OUTPUT_SINGLE: if set, pop input number off stack to apply to
>                 OPTX_SELECT_OUTPUT_*, otherwise iterate through all.
> OPTX_SELECT_OUTPUT_AMOUNT32x2*: sats out, as a high-low u31 pair
> OPTX_SELECT_OUTPUT_SCRIPTPUBKEY*: output scriptpubkey
>
> OPTX_SELECT_19...OPTX_SELECT_31: future expansion.
>
> OP_CHECKTEMPLATEVERIFY is approximated by the following flags:
>         OPTX_SELECT_VERSION
>         OPTX_SELECT_LOCKTIME
>         OPTX_SELECT_INPUTCOUNT
>         OPTX_SELECT_INPUT_SCRIPT
>         OPTX_SELECT_INPUT_NSEQUENCE
>         OPTX_SELECT_OUTPUTCOUNT
>         OPTX_SELECT_OUTPUT_AMOUNT32x2
>         OPTX_SELECT_OUTPUT_SCRIPTPUBKEY
>         OPTX_SELECT_INPUTNUM
>
> All other flag combinations result in OP_SUCCESS.
>
> Discussion
> ----------
>
> By enumerating exactly what can be committed to, it's absolutely clear
> what is and isn't committed (and what you need to think about!).
>
> The bits which separate concatenation and hashing provide a simple
> mechanism for template-style (i.e. CTV-style) commitments, or for
> programatic treatment of individual elements (e.g. amounts, though the
> two s31 style is awkward: a 64-bit push flag could be added in future).
>
> The lack of double-hashing of scriptsigs and other fields means we
> cannot simply re-use hashing done for SIGHASH_ALL.
>
> The OP_SUCCESS semantic is only valid in tapscript v1, so this does not
> allow covenants for v0 segwit or pre-segwit inputs.  If covenants prove
> useful, dedicated opcodes can be provided for those cases (a-la
> OP_CHECKTEMPLATEVERIFY).
>
> Cheers,
> Rusty.
>
> [1]
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019813.html
> [2]
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019819.html
> [3]
> https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019816.html
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>
-- 


Alex Schoof

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Re: [bitcoin-dev] [PROPOSAL] OP_TX: generalized covenants reduced to OP_CHECKTEMPLATEVERIFY

[Anthony Towns]

On Tue, May 10, 2022 at 08:05:54PM +0930, Rusty Russell via bitcoin-dev wrote:

> OPTX_SEPARATELY: treat fields separately (vs concatenating)
> OPTX_UNHASHED: push on the stack without hashing (vs SHA256 before push)

> OPTX_SELECT_OUTPUT_AMOUNT32x2*: sats out, as a high-low u31 pair
> OPTX_SELECT_OUTPUT_SCRIPTPUBKEY*: output scriptpubkey

Doing random pie-in-the-sky contract design, I had a case where I
wanted to be able to say "update the CTV hash from commiting to outputs
[A,B,C,D,E] to outputs [A,B,X,D,E]". The approach above and the one CTV
takes are somewhat awkward for that:

 * you have to include all of A,B,D,E in order to generate both hashes,
   which seems less efficient than a merkle path

 * proving that you're taking an output in its entirety, rather than,
   say, the last 12 bytes of C and the first 30 bytes of D, seems hard.
   Again, it seems like a merkle path would be better?

This is more of an upgradability concern I think -- ie, only relevant if
additional features like CAT or TLUV or similar are added; but both OP_TX
and CTV seem to be trying to take upgradability into account in advance,
so I thought this was worth raising.

Cheers,
aj

Re: [bitcoin-dev] [PROPOSAL] OP_TX: generalized covenants reduced to OP_CHECKTEMPLATEVERIFY

[Jeremy Rubin]

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I can't find a link, but I've discussed this before somewhere a while
ago... perhaps one of the IRC meetings? I'll see if I can't turn something
up.

The main reason not to was validation performance -- we already usually
compute the flat hash, so the merkle tree would be extra work for just CTV.

However, from an API perspective, I agree that a merkle tree could be
superior for CTV. It does depend on use case. If you have just, say, 3
outputs, a merkle tree probably just 'gets in the way' compared to the
concatenation. It is only when you have many outputs and your need to do a
random-index insertion that it adds value. In many applications, you might
be biased to editing the last output (e.g., change outputs?) and then
SHASTREAM would allow you to O(1) edit the tail.

Best,

Jeremy

On Thu, Jun 23, 2022 at 11:06 PM Anthony Towns via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> On Tue, May 10, 2022 at 08:05:54PM +0930, Rusty Russell via bitcoin-dev
> wrote:
>
> > OPTX_SEPARATELY: treat fields separately (vs concatenating)
> > OPTX_UNHASHED: push on the stack without hashing (vs SHA256 before push)
>
> > OPTX_SELECT_OUTPUT_AMOUNT32x2*: sats out, as a high-low u31 pair
> > OPTX_SELECT_OUTPUT_SCRIPTPUBKEY*: output scriptpubkey
>
> Doing random pie-in-the-sky contract design, I had a case where I
> wanted to be able to say "update the CTV hash from commiting to outputs
> [A,B,C,D,E] to outputs [A,B,X,D,E]". The approach above and the one CTV
> takes are somewhat awkward for that:
>
>  * you have to include all of A,B,D,E in order to generate both hashes,
>    which seems less efficient than a merkle path
>
>  * proving that you're taking an output in its entirety, rather than,
>    say, the last 12 bytes of C and the first 30 bytes of D, seems hard.
>    Again, it seems like a merkle path would be better?
>
> This is more of an upgradability concern I think -- ie, only relevant if
> additional features like CAT or TLUV or similar are added; but both OP_TX
> and CTV seem to be trying to take upgradability into account in advance,
> so I thought this was worth raising.
>
> Cheers,
> aj
> _______________________________________________
> bitcoin-dev mailing list
> bitcoin-dev@lists.linuxfoundation.org
> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>

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