MIME-Version: 1.0
Sender: gmaxwell@gmail.com
In-Reply-To: <20180123064419.GA1296@erisian.com.au>
References: <CAAS2fgTXg5kk6TyUM9dS=tf5N0_Z-GKVmzMLwTW1HxUgrqdo+Q@mail.gmail.com>
	<20180123064419.GA1296@erisian.com.au>
From: Gregory Maxwell <greg@xiph.org>
Date: Tue, 23 Jan 2018 13:15:38 +0000
Message-ID: <CAAS2fgSy8qg71M6ZOr=xj=W6y2Jbz8hwygZOUYv-Brkt0JwVaQ@mail.gmail.com>
To: Anthony Towns <aj@erisian.com.au>
Content-Type: text/plain; charset="UTF-8"
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Subject: Re: [bitcoin-dev] Taproot: Privacy preserving switchable scripting
Precedence: list

On Tue, Jan 23, 2018 at 6:44 AM, Anthony Towns <aj@erisian.com.au> wrote:
> Is this really intended as paying directly to a pubkey, instead of a
> pubkey hash?
>
> If so, isn't that a step backwards with regard to resistance to quantum
> attacks against ECC?

You're reading too much into a description of the idea. It's not a BIP
or a spec; I tried to provide enough details to make the general idea
concrete. I didn't dive into details or optimizations (for example,
you can use this with a "no EC redemption path" by special casing
empty C as the point at infinity, and you'd have an output that was
indistinguishable until spend... yadda yadda).

Considering the considerable level of address reuse -- I recall prior
stats that a majority of circulating funds are on addresses that had
previously been used, on top of the general race limitations-- I am
now dubious to the idea that hashing provides any kind of meaningful
quantum resistance and somewhat regret introducing that meme to the
space in the first place. If we considered quantum resistance a
meaningful concern we should address that specifically.  --- so I
don't think that should be a factor that drives a decision here.

When collision resistance is needed (as I think it clearly is for
taproot) you don't get a space savings in the txout from hashing, so
there is an argument to use the public key directly at least... but
it's worth considering.  Direct SPK use is also adventitious for being
able to efficiently ZKP over the UTXO set, e.g. for private solvency
proofs, but it isn't absolutely mandatory for that (one can hash
inside the proof, but it's slower).