Date: Wed, 21 Mar 2018 12:45:21 +0000
From: Andrew Poelstra <apoelstra@wpsoftware.net>
To: Anthony Towns <aj@erisian.com.au>,
	Bitcoin Protocol Discussion <bitcoin-dev@lists.linuxfoundation.org>
Message-ID: <20180321124521.GI9082@boulet.lan>
References: <20180321040618.GA4494@erisian.com.au>
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In-Reply-To: <20180321040618.GA4494@erisian.com.au>
User-Agent: Mutt/1.7.1 (2016-10-04)
Subject: Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation
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On Wed, Mar 21, 2018 at 02:06:18PM +1000, Anthony Towns via bitcoin-dev wro=
te:
>=20
> That leads me to think that interactive signature aggregation is going to
> take a lot of time and work, and it would make sense to do a v1-upgrade
> that's "just" Schnorr (and taproot and MAST and re-enabling opcodes and
> ...) in the meantime. YMMV.
>

Unfortunately I agree. Another complication with aggregate signatures is
that they complicate blind signature protocols such as [1]. In particular
they break the assumption "one signature can spend at most one UTXO"
meaning that a blind signer cannot tell how many coins they're authorizing
with a given signature, even if they've ensured that the key they're using
only controls UTXOs of a fixed value.

This seems solvable with creative use of ZKPs, but the fact that it's even
a problem caught me off guard, and makes me think that signature aggregation
is much harder to think about than e.g. Taproot which does not change
signature semantics at all.


Andrew


[1] https://github.com/jonasnick/scriptless-scripts/blob/blind-swaps/md/par=
tially-blind-swap.md


--=20
Andrew Poelstra
Mathematics Department, Blockstream
Email: apoelstra at wpsoftware.net
Web:   https://www.wpsoftware.net/andrew

"A goose alone, I suppose, can know the loneliness of geese
 who can never find their peace,
 whether north or south or west or east"
       --Joanna Newsom


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