Hi,

On Fri, 19 Jul 2019 at 17:49, Mike Brooks <m@ib.tc> wrote:

Users will adopt whatever the client accepts - this feature would be transparent.

My skepticism was based in an assumption on my part that most such data is produced by actors with a track record of neglecting transaction efficiency. I'd be happy to be proven wrong, but considering say usage rates of native witness outputs, or the fraction of transactions with more than 2 outputs so far I see little evidence in support of widespread adoption of cost saving measures. Assuming this is intended as a new script version, all fully validating nodes need to commit to keeping all data indefinitely before they can enforce the rules that make transactions benefiting from this opcode safe to broadcast.

That said, if successful, the main concern is still that of address reuse - currently there is no incentive in the protocol to do that, and with BIP32 wallets fewer reasons to do it as well, but this proposal introduces such an incentive for users which would otherwise generate new addresses (disregarding the ones that already reuse addresses anyway), and this is problematic for privacy and fungibility.

Since address reuse has privacy concerns, I think it's important to draw a distinction between clients accepting and producing such transactions, if the latter were done transparently that would be very concerning IMO, and even the former would not be transparent for users who run currently pruning nodes.

I'm not sure how an O(1) time complexity leads to DoS, that seems like a very creative jump.

For what it's worth, that was in reference to hypothetical deduplication only at the p2p layer, similar to compact blocks, but on further reflection I'd like to retract that, as since both scenarios which I had in mind seem easily mitigated.

Based on this response, it makes me want to calculate the savings, what if it was a staggering reduction in the tree size?

Assuming past address reuse rates are predictive this only places an upper bound on the potential size savings, so personally I would not find that very compelling. Even if the realized size savings would be substantial, I'm not convinced the benefits outweigh the downsides (increased validation costs, monotonically growing unprunable data, and direct incentive for address reuse), especially when compared to other methods/proposals that can reduce on chain footprint generally improve privacy while reducing validation costs for others (e.g. batching, lightning, MAST for sufficiently large scripts, Schnorr signatures (musig, adaptor signatures), {tap,graft}root, ).

Regards,

Yuval