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Date: Fri, 3 Apr 2015 18:08:18 +0200
From: Dmitry Khovratovich <khovratovich@...il.com>
To: discussions@...sword-hashing.net
Subject: Re: [PHC] Panel: Please require the finalists to help with benchmarks

Thank you for the corrections, Gregory.

I will still try to defend the case. First, let us reduce the
verification time to 0.1 seconds. This would allow to verify 36000
blocks per hour. Bitcoin hardcodes every 20000-th bloch hash or so as
a checkpoint in the code. Using this feature, all blocks after the
checkpoint can be checked within 25 minutes, and this is done only
once. Mere downloading these 20000 blocks (1 GB now) would take
smaller but comparable time.

Of course, this setting excludes low-memory clients. Still, some
cryptocurrency designers may think it worth preventing ASIC mining.
Fastest PHC designs can fill 500 MB in 0.1 seconds.

The verification can still be asymmetric, as the difficulty test is
easy to add (just require certain number of trailing zeros). It will
not be memoryless, but finding a proof can be made arbitrarily harder.

Dmitry

On 4/3/15, Gregory Maxwell <gmaxwell@...il.com> wrote:
> On Fri, Apr 3, 2015 at 12:01 PM, Solar Designer <solar@...nwall.com> wrote:
>> I'm not an expert in cryptocurrency issues, but from what I heard so far
> [...]
>> around 2 MB currently.  Maybe an expert would chime in.
>
> If I must. I wasn't going to respond on Bitcoin things-- they're
> largely out of scope, for this list (for reasons I'll explain now
> below).
>
> On Fri, Apr 3, 2015 at 12:34 PM, Dmitry Khovratovich
> <khovratovich@...il.com> wrote:
>> The things are a bit different. Transactions are hashed and
>> ECC-signed, and indeed they must be verified fast. Still, an ECC
>> signature check takes about 1/1000-th of a second, and the current
>> rate of 700 transaction per block means that the total verification
>> takes almost a second already.
>
> This is pretty far off the mark.
>
> Little to no signature verification occurs when a new block is
> received from the network because virtually all signatures have been
> long since verified when the transactions were originally relayed.
> The only signature verification required is whatever small is required
> from transactions that hadn't (yet) made it there.
>
> Typical block processing time on the Bitcoin network is a a few tens
> of ms including things like disk I/O.
>
> [As an aside, state of the art verification of secp256k1 on a 3.2GHz
> desktop cpu is around 70,000 verifications per second]
>
> Additionally, a large fraction (probably a vast super-majority) of
> participants in the Bitcoin network use simplified payment
> verification (SPV; see section 8 of the Bitcoin whitepaper).
>
> SPV nodes have a reduced security model which does not autonomously
> validate the integrity of the data in the network. They do not
> transfer or verify signatures at all, excepting for transactions which
> are probably related to them.  They must, however, verify the proof of
> work in order to gauge the consensus of the computational majority via
> the longest (most work) chain rule.
>
> Right now a SPV node synchronizes with the Bitcoin network in a few
> tens of seconds. A POW which took 1 second to verify would take the
> initial startup from seconds to over four days. (Thats four days of
> computation on top of whatever other work the system is doing, e.g.
> basically on top of the tens of seconds for as SPV node; or on top of
> about 3 hours of computation for a full node.) This would make things
> like secure phone wallet apps completely unusable.
>
> SPV is also used for small embedded devices.  In theory, one could be
> constructed using only a few hundred bytes of dynamic ram (and the
> assistance of an untrusted network peer), and many interesting
> applications-- such as "smart property" the automatically transfers
> its ownership based on payments in the Bitcoin network-- require SPV
> functionality inside small embedded devices or the verification of SPV
> data inside a zero-knowledge proof.
>
>> However, mining is different, you do not hash transactions there, you
>> just hash a singe nonce. Spending 1 second for block hashing means
>> that the total block verification would take 2 seconds instead of
>> current 1. Not a big deal, blocks appear every 10 minutes (in Bitcoin)
>> anyway.
>
> The time between blocks is exponentially distributed with a mean value
> of 10 minutes. There is substantial variance between times and there
> _must_ be for the network to converge on a single state; the expected
> value must be many times larger than the propagation delay of the
> network to deliver convergence in a reasonable amount of time.  Adding
> a substantial delay also reduces the good-put of the network and would
> confer an advantage to a consolidated high power attacker,
> effectively, it makes the system not progress free (the attacker gains
> ground during the time everyone else is off working on their own forks
> because they've not yet learned and verified the best state).
>
> A one second POW verification delay would also not just result in a
> one second delay to block propagation, however-- it would result in a
> one second delay per-hop through the network as verification of the
> proof of work is the only real defense against denial of service
> flooding attacks using blocks. This would increase the network radius
> to several seconds; which would be enough to cause user visible
> network faults (large state reorganizations) even with a ten minute
> mean time and absent any actual attacker, given Bitcoin's current
> network topology.
>
> So I think this subject is mostly moot because the functions discussed
> here are largely unsuitable for this application space; and (less
> severely) not very suitable for hashcash in general. It's critically
> important for this application that proof creation and verification be
> highly asymmetric. Iterating a fast cryptographic hash function
> against a hard to meet test achieves that goal very well. There are
> additional considerations: The function chosen must be largely
> approximation free (it must not be possible to produce a faster
> defective implementation) a difficult consideration which is much less
> important for password hashing, and it must be possible to randomly
> generate instances with known, uniform, difficulty (e.g. requiring
> uniformity of the outputs), even in the face of approximation and
> optimization. etc. The decentralized property of the system also
> requires that as many participants as possible are running their own
> node software (SPV or, preferably, full) rather than trusting third
> parties-- which hardly seems likely if running a node results in
> gigabyte-scale memory spikes for verifiers.
>
> Some people, like John Tromp, have been exploring the space of other
> kinds of POW functions that achieve properties like using memory
> hardness while minimizing the harm against other considerations here;
> but this is specialized work and heads off in a very different
> direction than PHC. (I have doubts about how worthwhile those efforts
> are either, but at least they're done with a careful mind to the
> application requirements and tradeoffs by someone who understands
> them).
>
> I have no doubt there will be new "cryptocurrencies" that pick up
> anything output from here and stir it with a mishmash of half
> understood technobabble and some other transparently broken
> cryptography or outright backdoors, in an effort to generate another
> speculative asset pump-and-dump-- as we've seen in the past. But the
> security and functionality of these systems is largely irrelevant to
> their market success, so any technical considerations to serve them
> are likely a waste of time here.
>
> The space of trying to be a good password hash is for authentication
> hard enough, maybe being a good hardening KDF isn't too much to ask.
> But going into crypcurrency work functions is a big step in
> requirements in an area which is new, not well understood, and heavily
> misunderstood (including the kind of financially convenient
> misunderstanding which is especially hard to deal with).
>


-- 
Best regards,
Dmitry Khovratovich

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