On Thu, May 31, 2018 at 05:25:04PM -0700, Pieter Wuille via bitcoin-dev wrote:
> The best argument for why Graftroot does not need to be optional I
> think was how Greg put it: "since the signer(s) could have signed an
> arbitrary transaction instead, being able to delegate is strictly less
> powerful.".
This seems persuasive to me. I think you could implement graftroot in
a way that makes this explicit:
* A graftroot input has >=2 items on the witness stack, a signature,
a script (S), and possibly witness elements for the script. The
signature has a SIGHASH_GRAFTROOT bit set.
* To validate the signature, a virtual transaction is constructed:
nVersion = 1
locktime = 0
inputs = [(txhash, txoutidx, 0, "", 0xffffffff)]
outputs = [(txvalue, len(S), S)]
locktime = 0
The signature is then checked against the virtual transaction.
* If the signature is valid, the virtual transaction is discarded, and
the script and witness elements are checked against the original tx.
I think this approach (or one like it) would make it clear that
graftroot is a simple optimisation, rather than changing the security
parameters. Some caveats:
* You'd presumably want to disallow signatures with SIGHASH_GRAFTROOT
from being used in signatures in scripts, so as not to end up having
to support recursive graftroot.
* Checking the script/witness against the original transaction instead
of the virtual one cheats a bit, but something like it is necessary
to ensure locktime/csv checks in the script S behave sanely. You
could have the virtual transaction be treated as being confirmed in
the same block as the original transaction instead though, I think.
* You would need to use SIGHASH_NOINPUT (or similar) in conjuction
to allow graftroot delegation prior to constructing the tx (otherwise
the signature would be committing to txhash/txoutidx). BIP118 would
still commit to txvalue, but would otherwise work fine, I think.
Cheers,
aj
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