This is insufficient: sequences must be committed to because they affect
TXID. As with scriptsigs (witness data fine to ignore). NUM_IN too.

Any malleability makes this much less useful.
--
@JeremyRubin <https://twitter.com/JeremyRubin>
<https://twitter.com/JeremyRubin>


On Fri, Jun 21, 2019 at 10:31 AM Anthony Towns via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> On Tue, Jun 18, 2019 at 04:57:34PM -0400, Russell O'Connor wrote:
> > So with regards to OP_SECURETHEBAG, I am also "not really seeing any
> reason to
> > complicate the spec to ensure the digest is precommitted as part of the
> > opcode."
>
> Also, I think you can simulate OP_SECURETHEBAG with an ANYPREVOUT
> (NOINPUT) sighash (Johnson Lau's mentioned this before, but not sure if
> it's been spelled out anywhere); ie instead of constructing
>
>   X = Hash_BagHash( version, locktime, [outputs], [sequences], num_in )
>
> and having the script be "<X> OP_SECURETHEBAG" you calculate an
> ANYPREVOUT sighash for SIGHASH_ANYPREVOUTANYSCRIPT | SIGHASH_ALL:
>
>   Y = Hash_TapSighash( 0, 0xc1, version, locktime, [outputs], 0,
>                        amount, sequence)
>
> and calculate a signature sig = Schnorr(P,m) for some pubkey P, and
> make your script be "<sig> <P> CHECKSIG".
>
> That loses the ability to commit to the number of inputs or restrict
> the nsequence of other inputs, and requires a bigger script (sig and P
> are ~96 bytes instead of X's 32 bytes), but is otherwise pretty much the
> same as far as I can tell. Both scripts are automatically satisfied when
> revealed (with the correct set of outputs), and don't need any additional
> witness data.
>
> If you wanted to construct "X" via script instead of hardcoding a value
> because it got you generalised covenants or whatever; I think you could
> get the same effect with CAT,LEFT, and RIGHT: you'd construct Y in much
> the same way you construct X, but you'd then need to turn that into a
> signature. You could do so by using pubkey P=G and nonce R=G, which
> means you need to calculate s=1+hash(G,G,Y)*1 -- calculating the hash
> part is easy, multiplying it by 1 is easy, and to add 1 you can probably
> do something along the lines of:
>
>     OP_DUP 4 OP_RIGHT 1 OP_ADD OP_SWAP 28 OP_LEFT OP_SWAP OP_CAT
>
> (ie, take the last 4 bytes, increment it using 4-byte arithmetic,
> then cat the first 28 bytes and the result. There's overflow issues,
> but I think they can be worked around either by allowing you to choose
> different locktimes, or by more complicated script)
>
> Cheers,
> aj
>
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