Bitcoin Classic is an alternative implementation of the Bitcoin protocol that would cause a hard fork of the blockchain in order to increase the block size limit to 2 MB. The hard fork would be activated after 750 of the last 1000 mined blocks announced their support for the change via coinbase transactions.

Bitcoin Classic Developer Gavin Andresen recently wrote a blog post regarding the safety of a hard fork with 75 percent support from the network hashrate. In his view, there are major incentives for all miners and economic participants to join the larger fork after (or even before) a split happens. In another blog post, Andresen stated his support for the 75 percent threshold is due to his unwillingness to allow a large mining pool to veto a potential change to Bitcoin’s consensus rules.

Bitcoin Core Contributor Peter Todd does not agree with Andresen’s views. In a recent interview on The Bitcoin Game, Todd discussed a variety of possible issues with a hard fork activated via support from 75 percent of network hashing power.

No One Knows What Will Happen

In Peter Todd’s view, the first problem with a 75 percent hard fork activation threshold is that no one really knows what would happen once the fork was initiated. He stated, “I think the problem is we don’t know.”

Todd then mentioned a thread on the Bitcoin development mailing list in which Chaincode Labs Co-Founder and Bitcoin Core Developer Alex Morcos called out Gavin Andresen for being overly optimistic about the chances of success for the Bitcoin Classic fork. Morcos wrote:

“In truth, no one has any idea what would happen if the proposed Classic hard fork activated with 75% right now. There is some chance you are right, but there is a very legitimate possibility that a concerted effort would arise to maintain a minority fork or perhaps if miners don’t see nearly a complete switch over, many of them might themselves reverse the fork if they think it would be easier to achieve consensus that way. We as a community have never been in such a situation before and it behooves us to speak honestly and directly about the uncertainty of the situation.”

Peter Todd is uncomfortable with unknown risks when there is so much of other people’s money on the line. He noted, “When you’re working with billions of dollars, you don’t take risks lightly.”

Todd continued, “I think Gavin is willing to take much bigger risks than most people.”

One of the key arguments often made by supporters of the Bitcoin Core roadmap in regards to not knowing what will happen when a contentious hard fork activates is that the economic majority controls the Bitcoin network, not miners. Additionally, 75 percent support from the network’s total hashing power does not necessarily equate to 75 percent support from bitcoin holders, which means the economic participants have the power to effectively reject a hard fork after it has been activated by the miners.

Hard Fork Attacks from Miners

Peter Todd also pointed out a potential flaw in Bitcoin Classic’s specific implementation of a hard fork activation. He described a situation where a miner or mining pool who is against the hard fork may broadcast their support for the change and then remove their support after the 28-day countdown to activation begins. This means the hard fork would essentially be put in motion with a lower percentage of support from the hashing power on the network. Todd claimed other miners may get cold feet and revert back to Bitcoin Core as a result of the confusion.

Another issue Todd has brought up on the Bitcoin development mailing list in the past is “Not Bitcoin Classic” nodes. This controversial strategy of fighting against a potential hard fork first came about in the summer of 2015 in response to the release of Bitcoin XT. The idea is to essentially fake support for Bitcoin Classic without having any intention of supporting the possible hard fork once it has been activated by the 75 percent threshold. This has the potential to cause turmoil, a premature hard fork, or both.

Disenfranchising Bitcoin Users

In response to Gavin Andresen’s claim that a 95 percent or higher activation threshold would allow a single miner or mining pool to veto a change to Bitcoin’s consensus rules, Peter Todd pointed out that a 75 percent activation threshold disenfranchises 25 percent of the network. He explained:

“If you’re willing to go through with something like this, even though we have a quarter of the hashing power disagreeing with you, you’re disenfranchising a lot of people. You’re telling a lot of people, ‘We don’t care about your thoughts. We’re not going to try to compromise with you. We’re just going to go ahead with this.’ In a system that needs consensus, that’s a pretty dangerous thing to be doing.”

Todd pointed to the recent letter from the Bitcoin Roundtable as evidence that there are plenty of people out there who do not wish to see a move to Bitcoin Classic as the reference implementation of the Bitcoin protocol. He added, “Right there, that’s enough to say this Bitcoin Classic plan is probably not going to happen that way.”

Peter Todd also mentioned a proof-of-stake vote on where holders of just under $12 million worth of bitcoin have signed a statement claiming a belief that large holders of bitcoin will crash the price if a non-Bitcoin Core implementation of the Bitcoin protocol activates a hard fork. More liquidity is likely needed on the shareholder-voting-esque website before any serious conclusions can be made from it.

A 95 Percent Threshold

Throughout his interview, Peter Todd made it clear that one of his main issues with Bitcoin Classic is the 75 percent threshold rather than the increase to a 2 MB block size limit. Todd explained his preference for a much higher activation threshold for a hard fork:

“Why don’t you just release it with a higher threshold? I may not necessarily agree with what the change is, but if we, for instance, had 99 percent of hashing power agreeing to a hard fork, what happens to the 1 percent chain? 1 percent hashing power basically means you get a confirmation every sixteen hours or so. That’s not a viable chain.”

If there is to be a hard fork of the Bitcoin blockchain, Todd would like to conduct the fork in the safest manner possible. He concluded:

“You’re never going to get 100 percent, but 95 percent or 99 percent — that’s a heck of a lot safer than having a quarter of all hashing power potentially disagreeing with you.”


  1. That opening sentence though:

    > Bitcoin Classic is an alternative implementation of the Bitcoin protocol

    No. If it was nothing but an alternative implementation of the Bitcoin protocol it wouldn't be threatening to cause a hard fork.

    The problem is that it is an implementation of a different protocol, without the 1MB blocksize limit. It is implementing something that looks a lot like Bitcoin but is subtly different, and which (if successful, which it won't be) would cause all existing Bitcoins to split into Bitcoins and Classiccoins, creating widespread confusion.

    • I've actually been thinking about what to call Classic a bit lately. I called it an alternative implementation of the protocol because the protocol would essentially change if it were successful in gaining 95 percent of the traditional activation threshold (which protocol is bitcoin would be more questionable at their intended 75 percent threshold).

      I'm open to alternative descriptions, but that was my thinking while writing that sentence. Something between alternative implementation of the Bitcoin protocol and altcoin is likely most accurate.

      • There's no better way to word that first sentence than you did. The only way you could write it to make dooglus happy would be to say "Bitcoin Classic is a bullshit altcoin that is attempting to ruin Bitcoin."

        • It's an implementation of an alternative protocol, not an alternative implementation of the Bitcoin protocol.

          But even that's being kind to Classic, since Classic is 99% the same as Core. So it's mostly the same implementation, but of an incompatible protocol.

          I have no strong evidence that the people behind Classic are attempting to ruin Bitcoin. They may be idiots with good intentions who know no better.

          • So Bitcoin doesn't evolve? The protocol hasn't changed? It won't change? Bitcoin Core isn't planning to fork Bitcoin, to change the protocol again? Bitcoin is the thriving branch after a fork. Are you saying if the network did overwhelmingly follow the branch of Classic, it would not be Bitcoin? Hell, it's based on Core, and as you say, it 99% the same as Core. When protocols are changed 1% in the evolutionary process that systems go through, do reasonable people really take issue with calling the change an alternative implementation? That just seems like political propaganda, don't let the other side even get a finger hold.

            Oh crap, I just realized Kyle was considering calling Classic an altcoin (guess I skimmed that response too quickly before). Really, you too Kyle? If Core creates a hard fork, it's Bitcoin. If another developer creates a hard fork, it's an altcoin? Can you explain that logic? The need to avoid a continuous fork doesn't mean only one dev team can create "real" Bitcoin.

          • > Are you saying if the network did overwhelmingly follow the branch of Classic, it would not be Bitcoin?

            No. If a hardfork isn't contentious then it's still Bitcoin. But when a group attempts to damage Bitcoin by causing a contentious hard fork which is strong opposed by almost everyone with a clue, the contentious fork clearly isn't Bitcoin.

    • The condescension about the wording of the first sentence is a joke. Bullshit that Classic is not an alternative implementation of Bitcoin! I'm not even a classic guy, but I hate BS from either side, and that is some BS. Bitcoin's open nature should allow another development team to build an alternate implementation of Bitcoin, and also allow the community to fork to it if the community so chooses. Hard forks have happened in the past, and Core has one scheduled for the future. If consensus is to go with another dev team's implementation, then that's Bitcoin. If consensus is to stick with the Core team, than that's Bitcoin. This isn't supposed to be a fucking monopoly where only one dev team is allowed to define Bitcoin! But that seems to be what your resting your argument on. And here I thought Bitcoiners gave a shit about the free market.

      If you want to warn people that a contentious fork could split Bitcoin, then at least be straight forward and make that argument. It's not impossible that clear consensus might someday go to a team other than Core. As Peter Todd from Core says himself, if 99% goes to one side of a fork (yep, even if it's Classic), then it's pretty clear that the 1% chain won't be economically viable.

        • Alternative: available as another possibility
          synonyms: different, other, another, second, possible, substitute

          I understand that BTCD and libbitcoin are commonly referred to as alternative implementations of Bitcoin. But that doesn't mean another use of the phrase is wrong. Based on the English language, your first paragraph is 100% accurate. Doog is only complaining because he clearly dislikes classic, and this was a way for him to knock it. Had he not tried to knock classic, I could see giving him the benefit of the doubt. But clearly he has an agenda.

  2. I'm no expert here, but if you break the "1mb rule" wouldn't that cause the coin to verge off resulting in say, an old and new type of bitcoin? Once, again, not an expert, just throwing my two cents in there.


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