phc-discussions - Re: [PHC] Memory-hard proof of work with fast verification (CPU Hash)

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Date: Wed, 8 Jul 2015 21:27:31 +0000
From: "Zooko Wilcox-O'Hearn" <zookog@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Memory-hard proof of work with fast verification (CPU Hash)

Dear Bill Cox and Solar Designer:

Thanks for the useful conversation.

On Tue, Jul 7, 2015 at 10:29 PM, Bill Cox <waywardgeek@...il.com> wrote:
>
> One more concern is a possible DDoS attack on the network.  Publishing
> 1,000's of fake solutions to blocks per second that require the entire
> network to debunk them with a very computationally expensive verification
> would be bad.  I read about that DDoS problem somewhere recently... can't
> remember if it was the Argon2 paper or somewhere else.

Hm, that's a good point. I haven't thought this one all the way
through. I don't know how to assign a budget to this one, other than
"the cheaper verification is the better".

Another solution that this issue suggests is "cheap rejection of
invalid proofs", i.e. it would be okay if it takes 1 second to verify
that a valid proof is valid, as long as it takes only 1/10 of a second
(usually) to verify that an invalid proof is invalid. The
Merkle-Tree-NIZK in https://eprint.iacr.org/2015/430 seems like it
would have that property.

> In any case, assuming the reason we want a memory-hard PoW is to discourage
> ASIC mining, I think we might want to consider memory-hard algorithms that
> max out L1 + L2 + L3 capacity and bandwidth (or some reasonable fraction of
> it, like 1/2).  I am not convinced we do better busting out of on-chip
> cache, since a GPU and probably an ASIC or GPU would win the total I/O
> bandwidth to external DRAM war.  However, it would be very hard for them to
> beat the performance of my L1+L2+L3 cache system on my CPU for random
> access.

Why are you saying "bandwidth" here instead of "latency"? My
(admittedly limited) understanding of Argon2d is that the memory
access patterns (for reads, not for writes) is unpredictable and is
determined by the some of the most recently computed state, and
therefore it should be latency-bound instead of bandwidth-bound. If
I'm right about that, does it change your argument above?

> I _think_ I saw the impact of botnets

I'm not interested in botnet-resistance as a design goal. The *intent*
is "resistance to resources which are accessed by theft", but we can't
implement that intent in our algorithm, we can only implement some
proxy for it, such as "resistance to resources which are cheaply spun
up or cheaply delegated". I don't think this is an effective way to
deter theft, and it might deter other legitimate uses, and it might
have other negative consequences. So don't talk to me about
botnet-resistance.

Regards,

Zooko