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Date: Wed, 2 Oct 2019 00:45:48 +1000
From: Anthony Towns <aj@erisian.com.au>
To: ZmnSCPxj <ZmnSCPxj@protonmail.com>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
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Subject: Re: [bitcoin-dev] Continuing the discussion about noinput /
 anyprevout
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On Mon, Sep 30, 2019 at 11:28:43PM +0000, ZmnSCPxj via bitcoin-dev wrote:
> Suppose rather than `SIGHASH_NOINPUT`, we created a new opcode, `OP_CHECKSIG_WITHOUT_INPUT`.

I don't think there's any meaningful difference between making a new
opcode and making a new tapscript public key type; the difference is
just one of encoding:

   3301<key>AC   [CHECKSIG of public key type 0x01]
   32<key>B3     [CHECKSIG_WITHOUT_INPUT (replacing NOP4) of key]

> This new opcode ignores any `SIGHASH` flags, if present, on a signature,

(How sighash flags are treated can be redefined by new public key types;
if that's not obvious already)

Cheers,
aj