What is Ethereum’s difficulty bomb?

With an alarming name for a relatively straightforward workaround (although in blockchain land, even straightforward things aren’t really), the “difficulty bomb” is an ingenious way to get all Ethereum miners to switch to another consensus system.

by Blake Richard Verdoorn for Unsplash - Ethereum
by Blake Richard Verdoorn for Unsplash

A bit of background: Ethereum currently uses the Proof of Work consensus algorithm, but has always planned to switch to a Proof of Stake system at some point in the future. Casper, as its Proof of Stake system will be called, is in development, and will be rolled out sometime in early 2017, according to the current plan.

One problem has been, as with every decentralized permissionless system, how to get everyone to switch over to the new system, to avoid split chains, replay attacks, etc. As we saw with the recent hard fork, there is always a strong possibility that some will stick with the old system, and there’s not much that the Foundation can do to coerce them into following everyone else.

Unless it’s in their interest to do so. Here’s the ingenious part: the difficulty of mining Ethereum blocks (= the time it takes to find the nonce that will, when combined with the block data, give a hash within a certain parameter) has been gradually increasing since August 2015, and will continue to do so with exponential increments.

What does that mean? That Ethereum blocks will gradually take longer and longer to mine. The time between bitcoin blocks is in theory about 10 minutes (although recently it’s more like 20 minutes). The time between Ethereum blocks is about 17 seconds. This is one of several aspects that makes Ethereum more attractive to some.

But that is likely to change when Ethereum blocks take longer to process than bitcoin blocks. And it will certainly change, for the miners especially, when Ethereum blocks are so “difficult” to process that it no longer is profitable to mine them, since the electricity consumed in the calculations costs more than the potential ether reward.

The “difficulty bomb” is a clever way to force miners to stop using one system and move to another. Rather than pressure through centralized control (“do this or else…”), it does so through decentralized incentives. No-one is told what to do. But the current Proof of Work algorithm has a built-in self-destruct function that, since it is part of the code, no-one can do anything about. You either move to the new system, or you go out of business. Your choice. No coercion.

Since the increase occurs very gradually, “bomb” is perhaps not the best word for the concept, since no explosions or sudden changes disrupt operations. But it does successfully imply the destructive intent of the code, and subliminally encourages everyone to jump over as soon as Casper becomes available. Ethereum developers call it the “difficulty adjustment algorithm” or the “difficulty adjustment scheme”, which are not nearly as dramatic.

It’s worth noting that the difficulty bomb was conceived as a way to get everyone to move over to Casper when the time came. But it could be used to get everyone to move over to a different hard fork. Ethereum has committed to moving to a version of Proof of Stake. But who knows? Change happens. There’s also no guarantee that Casper will be ready in time. What everyone jumps over to, is not yet “written in stone”, as they say. But they will be jumping over to something different.

It’s also worth noting that the difficulty increase could be modified in the upcoming hard fork that will introduce Metropolis, the next planned iteration of the Ethereum platform (expected in the fall of 2016). This type of modification has already happened once. With the Homestead release in mid-March, the difficulty adjustment algorithm was relaxed a bit. Could this indicate a delay in the release of Casper? With the adjusted system, blocks will become un-mineable by 2021, but the slowness will, according to the founder of Ethereum, become “very annoying by the second half of 2017”. This may change.

Ethereum Classic, the alternative result of the latest fork, also has this ticking difficulty bomb, obviously. But what it doesn’t have is the obligation to migrate to a Proof of Stake consensus system, it doesn’t have the commitment to move to Casper. Its miners do have to move to a different algorithm, though. Or, Ethereum Classic could hard fork to remove the difficulty bomb. It will probably let Ethereum launch Casper and see how it goes, before deciding or not to adopt it. Only by then it will have had to do something about its difficulty levels. Assuming, of course, that it’s still around.

The genius here was in knowing that a possibly contentious hard fork was coming, and devising a way to pre-empt resistance. That doesn’t mean that the process won’t be without drama, though. It’s possible that disagreement emerges as to whether Casper is the right consensus algorithm to be using. Some may believe that another alternative is preferable, and independently fork to that. As we’ve seen with Ethereum Classic, it is possible for more than one Ethereum chain to exist (for now, anyway). The difficulty bomb does not solve the problem of trying to get intelligent and strong-willed people to agree on an optimum process, to facilitate the communication as to why the Casper version is the best, and to demonstrate that the entire community is buying into the Ethereum mainstream creed. Clever as it may be, what the difficulty bomb fails to do is to achieve consensus about consensus.

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