Re: [bitcoin-dev] Does Bitcoin require or have an honest majority or a rational one? (re rbf)

From: Jeremy Rubin <jeremy.l.rubin@gmail•com>
To: "Russell O'Connor" <roconnor@blockstream•com>
Cc: Bitcoin Protocol Discussion <bitcoin-dev@lists•linuxfoundation.org>
Subject: Re: [bitcoin-dev] Does Bitcoin require or have an honest majority or a rational one? (re rbf)
Date: Mon, 17 Oct 2022 15:02:01 -0400	[thread overview]
Message-ID: <CAD5xwhjcC3tAf=QnEgQqyW1CdRB0eMRE-tRWXZWyTc5vTyZ2-g@mail.gmail.com> (raw)
In-Reply-To: <CAMZUoKkTVGDV6B3SiFr3x0wGNF3E0MBV60RdTeOeBAd_YjvwTQ@mail.gmail.com>

[-- Attachment #1: Type: text/plain, Size: 10456 bytes --]

Good points, Russell.

I think maybe for that particular property, one can partition the types of
rules one can put into the "honest rules" without compromising the system.

For example, your "keys deleted" property is one that is surely bad, but it
can be broken down into a many different buckets, such as:

1) Many Keys deleted one time at the start, all parties seem to have an
exogenous interest in not having the keys, as well as an endogenous one
(e.g., trusted setup ceremonies for ZCash, all parties seem to love
privacy, but also if anyone thinks you have your key maybe they rubber hose
you)
2) Keys should be deleted, but only "in play" for some amount of time
(Bitcoin NG maybe, statechains after the coin does a withdrawal, PoS with
checkpoints)
3) "Keys" should be deleted, but can only cause mild or local harms /
resolvable (Lightning, both eltoo and traditional, old transactions are
"Keys")
4) Keys must be deleted for all time (proof of work if done as leader
election

In particular, I think the honest behavior assumptions are OK as long as
they are reasonably time bounded and observable. For example, in
transaction selection, assuming "honest behavior" may be acceptable because
if the property is not true, it doesn't fundamentally brick the system or
cause mass outage, but it does cause an annoyance and is observable.
Further, agents may have a rationalization for following the honest policy
even above their pointwise interest in profit maximizing, if they think it
makes their overall participation more valuable. This is because it is an
infinite game and not finite, the most effective strategies aren't always
doing to be next-step profit maximizing (for those new to these concepts,
http://www.econ.uiuc.edu/~hrtdmrt2/Teaching/GT_2015_19/L12.pdf is a decent
primer). The example of deleting keys is interesting, because you don't
need to make your defection observable. But for transaction selection, it
absolutely is.

Ultimately, I think the reason why (some) systems do the cop-out of "honest
majority rules" --> "secure outcome" is because of a belief that there is
an "existential unknown proof" that there is an infinite game that can be
described where this should be the dominant strategy for all players,
whether defined or not. However, one must be incredibly careful with such
assumptions of an unknown existential game to which that is the dominant
strategy to not abuse them to ex-falso-quod-libet themselves into a corner
(Bertrand Russel is the Pope) if such a game does not actually exist. It's
obviously much better to actually prove the incentive compatibility against
an explicit game with explicitly stated assumptions for this reason (can
include exogenous details like "wanting number-go-up", "have a 5 year
hardware investment", or "belief that 0conf working required for adoption").

I (somewhat) suspect that things like the 0Conf safety assumptions are in
this category where one must be careful, because I think there might not be
a game where they are secure, so it leads to being able to prove false. But
I also understand why others might think such a game would exist, so
therein the debate.

Best,

Jeremy
--
@JeremyRubin <https://twitter.com/JeremyRubin>

On Mon, Oct 17, 2022 at 11:51 AM Russell O'Connor <roconnor@blockstream•com>
wrote:

> From my limited academic interactions, people generally take the "honest"
> to mean following the rules (regardless of how bad it is for you to follow
> those rules).  This has in turn led to some blockchain designs based on
> their own absurd set of rules, and simply waiving away their issues by
> stipulating their own honest majority or supermajority requirement.  For
> example, a proof of stake blockchain might require as a rule that users
> securely delete their signing keys after a period of time, and prove their
> blockchain secure under these rules.  They then argue that so long as the
> "honest" majority follows this rule, then there is no risk of
> reorganization.  If enough users don't delete their signing keys, well
> their honest majority assumption is violated, so anything goes.
>
> The thing is that it is most certainly in each user's interest to *not*
> delete their signing keys.   Each user has strictly more power and options
> available by keeping their keys and not deleting them.  This rule violation
> is undetectable, at least until it is too late and a coalition decides to
> try to collaborate for a reorg to their advantage.
>
> It is not reasonable to build a distributed pseudonymous system built on
> arbitrary rules and then simply define your system to be secure by fiat.
> Users need an incentive to follow the rules of the system or it just won't
> work.  In particular, the rules ought to form a Nash Equilibrium, and this
> is violated by, for example, a requirement that users delete their signing
> keys.  If Bitcoin relied on users acting against their own interest to
> function, I doubt Bitcoin would be in operation today.  Certainly I would
> have no interest in it.
>
> While it doesn't really matter, I do believe Satoshi was also aware that
> the rules cannot just be arbitrary, with no incentive to follow them.
> After all, he did note that it was designed to be in the miner's self
> interest to build upon the longest (most work) chain, even if that point
> ended up being rather involved.  That is to say, I don't think that an
> "honest" (i.e rule following) majority is meant to be taken as an
> assumption, rather it is something that ought to be a consequence of the
> design.
>
> Anyhow, the above is simply a comment on "honest majority", and I'm not
> trying to make a specific claim about RBF here, though I do have my
> opinions and I do see how it is related.
>
> On Sun, Oct 16, 2022 at 1:36 PM Jeremy Rubin via bitcoin-dev <
> bitcoin-dev@lists•linuxfoundation.org> wrote:
>
>> The Bitcoin white paper says:
>>
>> The proof-of-work also solves the problem of determining representation
>> in majority decision
>> making. If the majority were based on one-IP-address-one-vote, it could
>> be subverted by anyone
>> able to allocate many IPs. Proof-of-work is essentially one-CPU-one-vote.
>> The majority
>> decision is represented by the longest chain, which has the greatest
>> proof-of-work effort invested
>> in it. If a majority of CPU power is controlled by honest nodes, the
>> honest chain will grow the
>> fastest and outpace any competing chains. To modify a past block, an
>> attacker would have to
>> redo the proof-of-work of the block and all blocks after it and then
>> catch up with and surpass the
>> work of the honest nodes. We will show later that the probability of a
>> slower attacker catching up
>> diminishes exponentially as subsequent blocks are added.
>>
>>
>> This, Satoshi (who doesn't really matter anyways I guess?) claimed that
>> for Bitcoin to function properly you need a majority honest nodes.
>>
>> There are multiple behaviors one can describe as honest, and economically
>> rational or optimizing is not necessarily rational.
>>
>> For example, if I run a shop that takes rain checks, but I sell an item
>> to a higher bidder who didn't have a hold on the item, that is not honest,
>> but it may be selfish profit maximizing.
>>
>> Satoshi said an honest majority is required for the chain to be extended.
>> Honest is not really defined though. Honesty, in my definition, is that you
>> follow a pre specified rule, rational or not.
>>
>> It seems a lot of the RBF controversy is that Protocol developers have
>> aspired to make the honest behavior also be the rational behavior. This is
>> maybe a good idea because, in theory, if the honest behavior is rational
>> then we can make a weaker assumption of selfishness maximizing a parameter.
>>
>> However, Satoshi did not particularly bound what aspects of honesty are
>> important for the network, because there isn't a spec defining exactly what
>> is honest or not. And also as soon as people are honest, you can rely on
>> that assumption for good effect.
>>
>> And sometimes, defining an honest behavior can be creating a higher
>> utility system because most people are "law abiding citizens" who might not
>> be short term rational. For example, one might expect that miners would be
>> interested in making sure lightning closes are "accurate" because
>> increasing the utility of lightning is good for Bitcoin, even if it is
>> irrational.
>>
>> It seems that the NoRBF crowd want to rely on an honest majority
>> assumption where the honest behavior is not doing replacement if not
>> requested. This is really not much different than trying to close lightning
>> channels "the right way".
>>
>> However, where it may be different, is that even in the presence of
>> honest majority, the safety of 0conf isn't assured given the potential of
>> race conditions in the mempool. Therefore it's not clear to me that 0conf
>> working well is something you can drive from the Honest Majority Assumption
>> (where honest includes first seen).
>>
>>
>> Overall, it might be nice to more tightly document what bitcoins
>> assumptions are in practice and what those assumptions do in terms of
>> properties of Bitcoin, as well as pathways to weakening the assumptions
>> without compromising the behaviors users expect the network to have.  An
>> "extended white paper" if you will.
>>
>>
>>  It's somewhat clear to me that we shouldn't weaken assumptions that only
>> seem local to one subsystem of Bitcoin if they end up destabilizing another
>> system. In particular, things that decrease "transaction utility" for end
>> users decrease the demand for transactions which hurts the fee market's
>> longer term viability, even if we feel good about making an honest policy
>> assumption into a self interested policy assumption.
>>
>> A last reflection is that Bitcoin is specified with an honest majority
>> assumption, but also has a rational dishonest minority assumption over both
>> endogenous (rewards) and exogenous (electricity) costs. Satoshi did not
>> suggest, at least as I read it, that Bitcoin works with an rational
>> majority assumption. (If anyone thinks these three are similar properties
>> you can make some trivial counterexamples)
>>
>>
>> Cheers,
>>
>> Jeremy
>> _______________________________________________
>> bitcoin-dev mailing list
>> bitcoin-dev@lists•linuxfoundation.org
>> https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>>
>

[-- Attachment #2: Type: text/html, Size: 14997 bytes --]