Subject: Re: [bitcoin-dev] Updates on Confidential Transactions efficiency

From: Peter Todd <[email-masked]>
Date: Tue, 14 Nov 2017 05:07:28 -0500

On Tue, Nov 14, 2017 at 01:21:14AM +0000, Gregory Maxwell via bitcoin-dev wrote:
> 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 property
of Zcash/Monero style systems.

It'd be quite feasible to use accumulator epochs and either make unspent coins
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 merkelized
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, but
if I understand the Confidential Transactions proposals correctly, they already
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 state
size and k-anonymity set size.

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