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Date: Thu, 3 Oct 2019 11:47:58 +1000
From: Anthony Towns <aj@erisian.com.au>
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>
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Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>,
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Subject: Re: [bitcoin-dev] [Lightning-dev] Continuing the discussion about
 noinput / anyprevout
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On Wed, Oct 02, 2019 at 02:03:43AM +0000, ZmnSCPxj via Lightning-dev wrote:
> So let me propose the more radical excision, starting with SegWit v1:
> * Remove `SIGHASH` from signatures.
> * Put `SIGHASH` on public keys.
>     <sighash> <pubkey> OP_SETPUBKEYSIGHASH

I don't think you could reasonably do this for key path spends -- if
you included the sighash as part of the scriptpubkey explicitly, that
would lose some of the indistinguishability of taproot addresses, and be
more expensive than having the sighash be in witness data. So I think
that means sighashes would still be included in key path signatures,
which would make the behaviour a little confusingly different between
signing for key path and script path spends.

> This removes the problems with `SIGHASH_NONE` `SIGHASH_SINGLE`, as they are allowed only if the output specifically says they are allowed.

I don't think the problems with NONE and SINGLE are any worse than using
SIGHASH_ALL to pay to "1*G" -- someone may steal the money you send,
but that's as far as it goes. NOINPUT/ANYPREVOUT is worse in that if
you use it, someone may steal funds from other UTXOs too -- similar
to nonce-reuse. So I think having to commit to enabling NOINPUT for an
address may make sense; but I don't really see the need for doing the
same for other sighashes generally.

FWIW, one way of looking at a transaction spending UTXO "U" to address
"A" is something like:

 * "script" lets you enforce conditions on the transaction when you
   create "A" [0]
 * "sighash" lets you enforce conditions on the transaction when
   you sign the transaction
 * nlocktime, nsequence, taproot annex are ways you express conditions
   on the transaction

In that view, "sighash" is actually an *extremely* simple scripting
language itself (with a total of six possible scripts).

That doesn't seem like a bad design to me, fwiw.

Cheers,
aj

[0] "graftroot" lets you update those conditions for address "A" after
    the fact