Date: Tue, 14 Nov 2017 05:07:28 -0500
From: Peter Todd <pete@petertodd.org>
To: Gregory Maxwell <greg@xiph.org>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20171114100728.GA29749@savin.petertodd.org>
References: <CAAS2fgQ0Cb2B=Ye2TnpfQqP4=kpZCxMWRXYB0CcFa71sQJaGuw@mail.gmail.com>
MIME-Version: 1.0
Content-Type: multipart/signed; micalg=pgp-sha256;
	protocol="application/pgp-signature"; boundary="BOKacYhQ+x31HxR3"
Content-Disposition: inline
In-Reply-To: <CAAS2fgQ0Cb2B=Ye2TnpfQqP4=kpZCxMWRXYB0CcFa71sQJaGuw@mail.gmail.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Subject: Re: [bitcoin-dev] Updates on Confidential Transactions efficiency
Precedence: list


--BOKacYhQ+x31HxR3
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
Content-Transfer-Encoding: quoted-printable

On Tue, Nov 14, 2017 at 01:21:14AM +0000, Gregory Maxwell via bitcoin-dev w=
rote:
> The primary advantage of this approach is that it can be constructed
> without any substantial new cryptographic assumptions (e.g., only
> discrete log security in our existing curve), that it can be high
> performance compared to alternatives, that it has no trusted setup,
> and that it doesn't involve the creation of any forever-growing
> unprunable accumulators.  All major alternative schemes fail multiple
> of these criteria (e.g., arguably Zcash's scheme fails every one of
> them).

Re: the unprunable accumulators, that doesn't need to be an inherent proper=
ty
of Zcash/Monero style systems.

It'd be quite feasible to use accumulator epochs and either make unspent co=
ins
in a previous epoch unspendable after some expiry time is reached - allowing
the spent coin accumulator data to be discarded - or make use of a merkeliz=
ed
key-value scheme with transaction provided proofs to shift the costs of
maintaining the accumulator to wallets.

The disadvantage of epoch schemes is of course a reduced k-anonymity set, b=
ut
if I understand the Confidential Transactions proposals correctly, they alr=
eady
have a significantly reduced k-anonymity set per transaction than Zcash
theoretically could (modulo it's in practice low anonymity set due to lack =
of
actual usage). In that respect, epoch size is simply a tradeoff between sta=
te
size and k-anonymity set size.

--=20
https://petertodd.org 'peter'[:-1]@petertodd.org

--BOKacYhQ+x31HxR3
Content-Type: application/pgp-signature; name="signature.asc"
Content-Description: Digital signature

-----BEGIN PGP SIGNATURE-----

iQEcBAEBCAAGBQJaCsBbAAoJECSBQD2l8JH7Fw0H/1z8v5SuFAjaVsCaBId6n26G
bFtLPv2loZnasgRHZUdGbyvD14Z7WXr3Mufjw3xwx7Tpv0xMgB+6wmz6ZtzZBz7L
1HZUa2yQCfyiH9EOuGy1xcr4l8/p7a5xQ4AYlA3iOPD5zGCgNJ7wt4cdkV8o5ZF1
2Q05jQaIJWETCoIbZ5xOygta9WRueUG5sdklhef02lx2kmtWUiRz+dbTHuBGrLpo
PZuO7AmORZZDVd/DWotEe7OaycKiHGKBDRRBd+WhvTr30XULiU2iklS7yfyzxsN0
kmqQymnNCxBhAruApHcva9Cmn++HYNJdz/pPgxqK4rQWMb0rrCH3wDBSENchaC0=
=B/VI
-----END PGP SIGNATURE-----

--BOKacYhQ+x31HxR3--