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 Bitcoinocracy.com 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: