A 51% Attack on Bitcoin Means Mutually Assured Destruction

What would happen if bitcoin were to suffer a 51% attack? It’s a hypothetical question, but one that has troubled some of the community’s brightest minds. Just as army generals play out countless war games, enacting doomsday scenarios, bitcoin defenders like to ponder ways in which the decentralized cryptocurrency could be attacked and brought to its knees.

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Contingency Planning for a Worst Case Scenario

A 51% attack, also known as a majority attack, refers to a situation in which a single miner or group of miners control the majority of the network hashrate. If attained, this would enable a bad actor to censor and reverse transactions, allowing them to double spend coins. One of bitcoin’s greatest attributes is its immunity to attacks, be they governmental or technological. With over 31 exahash now concentrated on the bitcoin network, launching a 51% attack would be virtually impossible. And yet the very act of contemplating such an event is critical in mitigating the likelihood of it ever occurring.

Bitcoin war games aren’t just larping: they’re strategic defense.

51% Is Probably Not Enough

In a widely read article last month, Jimmy Song pondered various hostile mining scenarios, including those presented by chip manufacturers, ASIC manufacturers, and mining pools. He ran through the ways in which a 51% attack could play out, but observed that owning 51% of the harshrate may not be enough to take over the bitcoin network. According to Song, an attacker armed with 60% of the hashrate would still be expected to take 100 minutes to overtake the rest of the network in confirming blocks. Meanwhile, the rest of the network would have caught on to what was happening, and begun invalidating the attacker’s blocks. (Conversely, it is theoretically possible to attack the bitcoin network with less than 51% of the hashrate). Song notes:

No rational merchant or exchange would ever take less than 30 confirmations in a scenario like this (at least without some knowledge about what’s going on)…Furthermore, a large reorg signals to the rest of the network that something nefarious is going on and nodes will likely view these new blocks with suspicion. It’s entirely possible that full node operators on the network will simply invalidate these blocks.

Who Wins by Attacking Bitcoin?

Due to bitcoin’s enormous hashrate, it would be impossible for anyone without any skin in the game – or rather ASICs in the game – to launch a 51% attack. The only players who could conceivably orchestrate such an attack are existing mining pools, or ASIC manufacturers if they were to backdoor their miners, for example, and later commandeer them. All of these entities are heavily invested in bitcoin, having spent hundreds of millions of dollars on the infrastructure required to compete in the mining sector. For their operations to remain profitable, bitcoin needs to maintain a certain price. If a bad actor (or pool of bad actors) were to start attacking bitcoin, they’d only be cannibalizing themselves.

There are scenarios – far-fetched admittedly – in which a 51% attack on the bitcoin network could be attempted. A hostile state could start accumulating ASIC miners, spending billions of dollars in readiness for the moment they had enough hashrate to greenlight an attack. Even Bitmain themselves would struggle to assemble enough ASICs to make such a feat possible however. An alternative scenario would be for a chip or ASIC manufacturer to make a breakthrough that provided a significant advantage over existing miners. A sort of Asicboost on steroids. Once again though, the best way to profit from this would be to honestly mine bitcoin with the souped-up units, or to sell them for a premium, rather than to launch a 51% attack.

Whatever way you slice it, a 51% attack on bitcoin isn’t just improbable – it makes zero sense for the attacker. Just because the cryptocurrency seems safe from mining attacks for now doesn’t mean it’s impervious to attack however. In a post entitled “Let’s destroy Bitcoin” published on MIT Technology Review, Morgan Peck proposes three ways in which bitcoin could be “brought down, co-opted, or made irrelevant”. None of them involve mining. A few altcoins, with a low hashrate, have been hit by a 51% attack in the past. In its nine-year history, bitcoin has never been attacked in such a manner. It didn’t happen in the past, even when one mining pool controlled a majority hashrate, and it’s probably not going to happen now.

In what other ways do you think bitcoin could be attacked? Let us know in the comments section below.

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