Timeline of 51% Attacks in May/June 2018

A 51% attack is where a malicious individual or group controls more than 51% of the hashpower for a proof-of-work coin (a similar scenario can also occur for proof-of-stake, but we focus on proof-of-work below).

In May & June 2018, there have been many 51% attacks mentioned on various news websites. This guide will go through what a 51% attack is, and a timeline of the May/June 2018 attacks.

What can you do in a 51% attack?

So assuming an individual/group controls more than 50% of hashpower on say Bitcoin, they can do some pretty nasty stuff with it. This is because at any point a miner can create a new chain of Bitcoin, where the longest chain at any point in time (that's been announced to the network) is the one that's considered valid. The theory is that if someone nasty tried creating their own chain, because more than 50% of hashpower is still mining the valid chain, this malicious person can't do anything.

This leads to a scenario where if someone controls more than 50% of the hashpower, they can for example:

  • Spend money on the valid chain while building a longer chain, where they haven't spent this money, without announcing it. Then after their transaction has been confirmed on the valid chain, they announce their new chain - which if they control 50%+ hashpower would now be longer than the valid one, and so replace it as the new valid chain. e.g. they just spent money, then they un-did that, so they can now spend it again. This is called a 'double spend', and is a form of 51% attack.
  • Block transactions from or to a particular address.
  • Be able to mine all future blocks & get all block rewards (preventing other miners from getting rewards, potentially causing them to stop mining and increasing the attackers' percentage from 51% to much higher).

Something to be aware of is that the cost of a 51% attack varies per coin, with Bitcoin being the most expensive to attack (as it has a very high hashrate). Many of the coins that have been attacked recently are on the cheaper end. For example:

  • Bitcoin 1h attack cost: ~$522k
  • Bitcoin Gold 1h attack cost: ~$3.7k
  • ZenCash 1h attack cost: ~$5.6k
  • Litecoin Cash 1h attack cost: ~$433
  • Monacoin 1h attack cost: ~$4.2k

See this website for the estimated cost of attacking many more coins. Alarmingly it's as cheap as $500 per hour to attack some coins.

What can be done during a 51% attack?

While a 51% attack is happening, there are a few things that can be done to deter it:

  • Any exchanges allowing deposits of the associated coin can temporarily increase required confirmations (where for each extra confirmation it would cost the attacker more money).
  • Miners on other coins running the same algorithm can temporarily mine the coin under attack to try and decrease the attacker's hashpower to less than 51% (although this may just offload the attack to another coin).
  • A coin can hard-fork to a new mining algorithm (if they use one that isn't shared with other coins or ASIC miners, this can be a good way to deter 51% attacks, although if the attacker is using CPUs/GPUs this may not help).

Who is affected by a 51% attack?

While a 51% attack is happening, if you hold the associated coin on a wallet you control, you don't really need to worry. Many sites make a big deal of 51% attacks, but unless they're sustained for a significant amount of time, only specific people & services are affected by them. A recent post by ZenCash explains this well.

The people affected by such an attack are:

  • Holders: if you hold your coins on a centralised exchange/wallet, if that exchange/wallet is affected by the 51% attack they may lose a lot of money, and potentially forward this loss to their users (similar to where when exchanges are hacked they sometimes absorb losses by taking away from user balances).
  • Exchanges & crypto payment processors: the attacker may try to deposit to an exchange or buy something during the attack. Any payments made during a 51% attack may be invalid (e.g. after the attack is over you might lose the received coins).
  • Miners: the attacker may receive all block rewards, causing any other miners to temporarily lose out on their mining profits.

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Timeline of 51% attacks in May/June 2018

April 2nd 2018

Electroneum (based on CryptoNight at time of the attack) was hit with a 51% attack.

May 13th 2018 - May 15th 2018

Monacoin (based on Lyra2REv2 at time of the attack) was hit with a 51% attack, estimated to have caused around $90k in damages.

May 16th 2018, 10:38pm UTC

Bitcoin Gold (based on Equihash at time of the attack) was hit with a 51% attack against block 528735.

May 19th 2018, 5:26am UTC

Bitcoin Gold (based on Equihash at time of the attack) was hit with a 51% attack that ended on block 529048.

May 22nd 2018

Verge (based on 5 mining algorithms at time of the attack) was hit with a 51% attack. It looks like the attacker took control of two of these algorithms, Scrypt and Lyra2RE, and the attack was between blocks 2155850 and 2206272 (with a loss of around 35 million XVG).

May 31st 2018

Litecoin Cash (based on SHA-256 at time of the attack) was hit with a 51% attack. See a Reddit thread confirming it here.

June 3rd 2018, 2:43am UTC

ZenCash (based on Equihash at time of the attack) was hit with a 51% attack. They experienced 3 double spends; the first for 3,317.4 ZEN, the second for 6,600 ZEN, and the third for 13,234.9 ZEN. The Zen Team recommended exchanges to increase required confirmations to 100 to deter the attack happening again (currently Binance & Cryptopia require 100 confirmations for ZEN deposits, and Bittrex requires 200).

DISCLAIMER: This site cannot substitute for professional investment or financial advice, or independent factual verification. This guide is provided for general informational purposes only. Bitcoin Kit is UK-based and not regulated by the FCA (Financial Conduct Authority). The group of individuals writing these guides are cryptocurrency enthusiasts and investors, not financial advisors. The ideas presented are our analysis, learning & opinions on a range of cryptocurrency topics. Trading or mining any form of cryptocurrency is very high risk, so never invest money you can't afford to lose - you should be prepared to sustain a total loss of all invested money.

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