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From: Paul Sztorc <truthcoin@gmail.com>
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Subject: Re: [bitcoin-dev] Two Drivechain BIPs
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Hi Other Chris,

Thanks for pointing this out. Here are my responses.

On 12/4/2017 3:11 PM, Chris Stewart wrote:
>There is another interesting analysis on BMM and drivechains from
/u/almkglor on reddit. I'm going to share here for visibility.
>> 3 and 7 will mean that non-verifying miners will be (inadvertently)
complicit in theft. Drivechains have 1008-block cycles ostensibly to
protect against theft, so that someone can "raise the alarm" and tell
miners to downvote a particular theft withdrawal, but that sounds too
much like centralized collusion to me.

Well, that is simply not what "centralized" means. "Centralized" means
that one person has special, irreplaceable influence. In contrast,
"decentralized" means that the process is not uniquely influenced by
what any *one* individual does or believes. Which is the case here: each
miner can independently make a decision about what to check, how to
check it, and what to do as a result. They could do undertake this
process, even if they ignored what everyone else was doing.

>> ...
>>
>> It seems Drivechain's security model is: miners always upvote by
default. If a theft withdrawal is done on the mainchain, some sidechain
nodes call up their miner friends (which makes me worry about miner
centralization) to downvote it instead.
>>
>> The problem is: what if after a theft withdrawal is defeated, another
theft withdrawal is done? And another, and another? This weakens the
peg: while a theft withdrawal is on-going, a genuine withdrawal can't be
posted (at least as I understand Sztorc's explanation). This chokes the
sidechain withdrawal.

This is a good question.

The answer is that there are mechanisms in place to address these
problems. Contrary to the reviewer's interpretation, multiple
withdrawal-attempts *are* allowed simultaneously. (This part of design
may have changed between now and Nov 2015, and I'm not sure if it was
ever documented anywhere until very recently). Second, only one
withdrawal can be upvoted at a time [ie, per sidechain per main:block].
Third, upvoting one withdrawal automatically downvotes all of the other
withdrawals (all in-progress withdrawals for that sidechain). Thus, we
have the asymmetry we desire. An "auditor class" can ignore all of the
withdrawals, until a significant amount of time has been invested in one
candidate. This makes the attempt more futile. Since they are unlikely
to be meaninglessly harassed, this opens the door to a "farmer class"
who can take it upon themselves to make sure the good withdrawals get
in, and get upvotes (especially early upvotes). Thus, the system can
tolerate a large "loafer class" who just lazily upvotes everything (or
nothing, or only the front-runner).

> TLDR: a miner is most profitable if he always accepts BMM bribes, but
downvotes withdrawal transactions (WT). This obviously isn't ideal
because a withdrawal will never occur from the drivechain if enough
miners employ this strategy -- which seems to be the most profitable
strategy.
>
>-Chris

I agree that miners should "always accept BMM bribes". In my recent
email to Matt Corallo, I described that this is basically the same as
saying that miners should "always mine on top of the heaviest chain".
(Of course, in mainchain Bitcoin miners are advised to mine atop the
heaviest *valid* chain, which is different from this case. It is
different because blind-merged-miners have no way of knowing if the
sidechain blocks they are mining are valid or not, which is kind of the
point. In practice I estimate that between 70% and 100% of today's
hashpower is already mining the mainchain "blind" -- through some
combination of pools, spv and spy mining.)

I don't agree with the conclusion (that the optimal policy is "always
downvoting", see above), but even if this analysis turns out to be
correct, it isn't a total disaster. The result (which is in the original
Nov 2015 specification) is that miners are the ones who perform the
atomic swaps. Then they walk the coins side-to-main (which, at this
point, are *their* coins). As long as there are a few large mining
groups, competition will drive the atomic swap fees down to negligible
levels. This slightly encourages mining to consolidate into two large
pools...but of course that is also true of the status quo!

Paul
=C2=A0
>
> On Mon, Dec 4, 2017 at 1:36 PM, Chris Pacia via bitcoin-dev
> <bitcoin-dev@lists.linuxfoundation.org
> <mailto:bitcoin-dev@lists.linuxfoundation.org>> wrote:
>
>
>         I think you are missing a few things.
>
>         First of all, I think the security model for sidechains is the
>         same as
>         that of every blockchain
>
>         People will say things, like "but with sidechains 51% hashrate
>         can steal
>         your coins!", but as I have repeated many times, this is also
>         true of
>         mainchain btc-tx. =C2=A0is something else?
>
>
>     There are substantial opportunity costs as well as a collective
>     action problem when it comes to re-writing the mainchain.=C2=A0
>
>     Is there anything similar for drivechains? As far as I can tell
>     there is no opportunity cost to casting a malicious vote, no
>     repercussions, and no collective action barrier that needs to be
>     overcome.=C2=A0
>
>     Unless I'm missing something I wouldn't liken the security of a
>     drivechain to that of the mainchain.
>
>     _______________________________________________
>     bitcoin-dev mailing list
>     bitcoin-dev@lists.linuxfoundation.org
>     <mailto:bitcoin-dev@lists.linuxfoundation.org>
>     https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev
>     <https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev>
>
>


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    <div dir="ltr">Hi Other Chris,<br>
      <br>
      Thanks for pointing this out. Here are my responses.<br>
      <br>
      On 12/4/2017 3:11 PM, Chris Stewart wrote:<br>
      &gt;There is another interesting analysis on BMM and drivechains
      from /u/almkglor on reddit. I'm going to share here for
      visibility.<br>
      &gt;&gt; 3 and 7 will mean that non-verifying miners will be
      (inadvertently) complicit in theft. Drivechains have 1008-block
      cycles ostensibly to protect against theft, so that someone can
      "raise the alarm" and tell miners to downvote a particular theft
      withdrawal, but that sounds too much like centralized collusion to
      me.<br>
      <br>
      Well, that is simply not what "centralized" means. "Centralized"
      means that one person has special, irreplaceable influence. In
      contrast, "decentralized" means that the process is not uniquely
      influenced by what any *one* individual does or believes. Which is
      the case here: each miner can independently make a decision about
      what to check, how to check it, and what to do as a result. They
      could do undertake this process, even if they ignored what
      everyone else was doing.<br>
      <br>
      &gt;&gt; ...<br>
      &gt;&gt;<br>
      &gt;&gt; It seems Drivechain's security model is: miners always
      upvote by default. If a theft withdrawal is done on the mainchain,
      some sidechain nodes call up their miner friends (which makes me
      worry about miner centralization) to downvote it instead.<br>
      &gt;&gt; <br>
      &gt;&gt; The problem is: what if after a theft withdrawal is
      defeated, another theft withdrawal is done? And another, and
      another? This weakens the peg: while a theft withdrawal is
      on-going, a genuine withdrawal can't be posted (at least as I
      understand Sztorc's explanation). This chokes the sidechain
      withdrawal.<br>
      <br>
      This is a good question.<br>
      <br>
      The answer is that there are mechanisms in place to address these
      problems. Contrary to the reviewer's interpretation, multiple
      withdrawal-attempts *are* allowed simultaneously. (This part of
      design may have changed between now and Nov 2015, and I'm not sure
      if it was ever documented anywhere until very recently). Second,
      only one withdrawal can be upvoted at a time [ie, per sidechain
      per main:block]. Third, upvoting one withdrawal automatically
      downvotes all of the other withdrawals (all in-progress
      withdrawals for that sidechain). Thus, we have the asymmetry we
      desire. An "auditor class" can ignore all of the withdrawals,
      until a significant amount of time has been invested in one
      candidate. This makes the attempt more futile. Since they are
      unlikely to be meaninglessly harassed, this opens the door to a
      "farmer class" who can take it upon themselves to make sure the
      good withdrawals get in, and get upvotes (especially early
      upvotes). Thus, the system can tolerate a large "loafer class" who
      just lazily upvotes everything (or nothing, or only the
      front-runner).<br>
      <br>
      &gt; TLDR: a miner is most profitable if he always accepts BMM
      bribes, but downvotes withdrawal transactions (WT). This obviously
      isn't ideal because a withdrawal will never occur from the
      drivechain if enough miners employ this strategy -- which seems to
      be the most profitable strategy.<br>
      &gt;<br>
      &gt;-Chris<br>
      <br>
      I agree that miners should "always accept BMM bribes". In my
      recent email to Matt Corallo, I described that this is basically
      the same as saying that miners should "always mine on top of the
      heaviest chain". (Of course, in mainchain Bitcoin miners are
      advised to mine atop the heaviest *valid* chain, which is
      different from this case. It is different because
      blind-merged-miners have no way of knowing if the sidechain blocks
      they are mining are valid or not, which is kind of the point. In
      practice I estimate that between 70% and 100% of today's hashpower
      is already mining the mainchain "blind" -- through some
      combination of pools, spv and spy mining.)<br>
      <br>
      I don't agree with the conclusion (that the optimal policy is
      "always downvoting", see above), but even if this analysis turns
      out to be correct, it isn't a total disaster. The result (which is
      in the original Nov 2015 specification) is that miners are the
      ones who perform the atomic swaps. Then they walk the coins
      side-to-main (which, at this point, are *their* coins). As long as
      there are a few large mining groups, competition will drive the
      atomic swap fees down to negligible levels. This slightly
      encourages mining to consolidate into two large pools...but of
      course that is also true of the status quo!<br>
      <br>
      Paul
      <div> </div>
    </div>
    <blockquote type="cite"
cite="mid:CAGL6+mF1YbjZ28MtvPxTye-HndEqmd6LkaFVr9BWvPiK-kfVTA@mail.gmail.com">
      <div class="gmail_extra"><br>
        <div class="gmail_quote">On Mon, Dec 4, 2017 at 1:36 PM, Chris
          Pacia via bitcoin-dev <span dir="ltr">&lt;<a
              href="mailto:bitcoin-dev@lists.linuxfoundation.org"
              target="_blank" moz-do-not-send="true">bitcoin-dev@lists.linuxfoundation.org</a>&gt;</span>
          wrote:<br>
          <blockquote class="gmail_quote" style="margin:0 0 0
            .8ex;border-left:1px #ccc solid;padding-left:1ex">
            <div dir="auto">
              <div><br>
                <div class="gmail_extra">
                  <div class="gmail_quote">
                    <blockquote class="m_4849272993808656596quote"
                      style="margin:0 0 0 .8ex;border-left:1px #ccc
                      solid;padding-left:1ex"><span class="">
                        <div class="m_4849272993808656596quoted-text">I
                          think you are missing a few things.<br>
                        </div>
                        <br>
                        First of all, I think the security model for
                        sidechains is the same as<br>
                        that of every blockchain<br>
                        <br>
                        People will say things, like "but with
                        sidechains 51% hashrate can steal<br>
                        your coins!", but as I have repeated many times,
                        this is also true of<br>
                      </span>
                      mainchain btc-tx.  is something else?<br>
                    </blockquote>
                  </div>
                </div>
              </div>
              <div dir="auto"><br>
              </div>
              <div dir="auto">There are substantial opportunity costs as
                well as a collective action problem when it comes to
                re-writing the mainchain. </div>
              <div dir="auto"><br>
              </div>
              <div dir="auto">Is there anything similar for drivechains?
                As far as I can tell there is no opportunity cost to
                casting a malicious vote, no repercussions, and no
                collective action barrier that needs to be overcome. </div>
              <div dir="auto"><br>
              </div>
              <div dir="auto">Unless I'm missing something I wouldn't
                liken the security of a drivechain to that of the
                mainchain.</div>
            </div>
            <br>
            ______________________________<wbr>_________________<br>
            bitcoin-dev mailing list<br>
            <a href="mailto:bitcoin-dev@lists.linuxfoundation.org"
              moz-do-not-send="true">bitcoin-dev@lists.<wbr>linuxfoundation.org</a><br>
            <a
              href="https://lists.linuxfoundation.org/mailman/listinfo/bitcoin-dev"
              rel="noreferrer" target="_blank" moz-do-not-send="true">https://lists.linuxfoundation.<wbr>org/mailman/listinfo/bitcoin-<wbr>dev</a><br>
            <br>
          </blockquote>
        </div>
        <br>
      </div>
    </blockquote>
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