Subject: Re: [bitcoin-dev] Soft-forks and schnorr signature aggregation

From: Andrew Poelstra <[email-masked]>
Date: Wed, 21 Mar 2018 12:45:21 +0000

On Wed, Mar 21, 2018 at 02:06:18PM +1000, Anthony Towns via bitcoin-dev wrote:
> 
> 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/partially-blind-swap.md

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

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