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
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>
-- 


Alex Schoof