Beware Cryptocurrencies – 51% Attacks Are Now for Rent

Beware Cryptocurrencies - 51% Attacks Are Now for Rent

In order to remain decentralized, cryptocurrencies using a proof of work system
must not allow a single party to control the majority of total hashing power.

But as the global pool of hashing power grows more liquid, cryptocurrencies need
to pass another important test. They must be able to resist an attack from the
total rentable global hashing power for their specific algorithm. Otherwise,
arbitrageurs may find it financially attractive to rent hashing power in order
to perform 51% attacks.

There are a few things preventing this from happening:

  • Algorithm-specific miners — Many rigs are optimized for a certain hashing
    algorithm, and switching to another, e.g. SHA-256 → X11, is unfeasible.
  • Illiquid mining market — Most of the global hash power is illiquid and not
    rentable. Therefore, a large upfront investment is required to build significant
    hashing power. The upfront cost for an attack is almost always not worth it.
  • Opportunity cost — Cryptocurrencies are usually designed to heavily favour good
    actors by providing them with greater rewards for acting in the benefit of the
    entire network. Any attack must outweigh the risk of failure including loss of
    mining rewards, loss of reputation and damage to the network. Long-term miners
    do not want to destroy their future earning potential by successfully attacking
    a network, shaking market confidence, and causing the price to fall.

But times are changing. The mining market is becoming more liquid.

Why is the Liquid Mining Market Growing?

Computer storage was once an illiquid market, now it is an extremely liquid
online commodity. The same thing is happening to hash power.

There are two major forces driving this.

  1. The long-run price increase of cryptocurrency will incentivize miners to invest
    in hashing power until any incremental gain is equal to the cost. In other
    words, if prices continue to go up, so will global hashing power.
  2. The total percentage of hashing power for rent will increase because buyers and
    sellers both benefit from the ability to rent and lend respectively. Separation
    of concern leads to higher degrees of specialization and increased operational
    efficiency. This is why hardware manufacturers sell their mining rigs and don’t
    mine themselves. If renters focus all of their time on finding opportunities
    with the highest amount of ROI, they are likely going to be the best at
    extracting value per unit of hashing power. Conversely, lenders can de-risk
    their business because their rental income is implicitly diversified across each
    entire hashing algorithm. In this world, lenders can simply focus on rental
    relations, asset utilization, and upkeep.

Rent-A-Miner Attacks Are Already Possible

Crypto51 calculates how much it would cost to rent
enough hashing power to match the given network hashing power for an hour.
NiceHash does not have enough hashing power for
most larger coins, so this figure is sometimes theoretically above 100%.

Hash rates are from Mine the Coin, coin prices are
from CoinMarketCap, and rental pricing is from
NiceHash.

A few caveats:

  • The quoted attack costs do not include the money you earn in the form of block
    rewards, so in many cases, the costs will actually be substantially lower.
  • Crypto51 is quoting the spot price for what is available on NiceHash. In real
    life, the more you rent, the more expensive it will be because of supply &
    demand.

Coins Vulnerable to Rent-A-Miner Attacks

Ranked by Market Cap

ETP is the #91 ranked coin on CMC. You can rent up to 21X the network’s hashing
power. The cost of an attack is only $162 per hour. ETP/BTC and ETP/USD pairs
are available on Bitfinex.

Vulnerable Coins Assuming 2x the Rental Capacity

Currently, these coins are out of reach since the total rental capacity
available on NiceHash is not enough to fully match the network’s hashing power.

Ranked by Market Cap

But let’s imagine the likely circumstance that NiceHash is able to 2x their
total rental capacity. Now coins like ETC (rank 18), BCN (rank 40), are easily
in reach.

Vulnerable Coins Assuming 5x the Rental Capacity

Ranked by Market Cap

A 5X increase in rental capacity puts coin like DASH (rank 15) and BTG (rank 28)
in danger.

**So what if 51% attacks are possible? How do attackers make money? **

Fortunately, it’s impossible to ever create a transaction for a wallet that you
do not own the private key to. But, controlling the majority hashing power means
you can execute a double spend attack by temporarily reverting certain
transactions on the ledger.

The Mechanics of a Double Spend Attack

When miners find a new block, they are supposed to broadcast this to all other
miners so that they can verify it, and add a new block to the blockchain.
However, a corrupt miner can create their own blockchain in stealth.

Graphic from Jimi S.’s Article on How 51% Attacks Work

To execute a double-spend, the attacker will spend his or her coins on the
truthful chain. But they will leave out these transactions on the stealth chain.

If the corrupted miner can build a longer chain faster than all the other miners
on the network, they can broadcast the stealth chain to the rest of the network.

Because the protocol adheres to the longest chain, the newly broadcasted corrupt
chain will become the de facto, truthful blockchain. The transaction history for
the attacker’s previous spend will be erased.

Note that just because a miner controls 51% of hashing power, does not mean they
will always have a longer chain. In long-run they will probably have a longer
chain. To guarantee this in the short-run, an attacker would likely want to
control closer to 80% of the network power.

Where to spend the coins? Exchanges are Likely the Target

For a double-spend to pay-off, you need to find a way to actually extract value
from the spent coins. If you can’t spend the coins in the first place, there’s
no point.

The most likely place an attacker would spend their coins on is an exchange
because they are the single biggest buyers of various cryptocurrencies.

Here’s what the attack would look like:

  1. Choose a target network that looks profitable
  2. Accumulate a significant amount of coins on the target network
  3. Rent NiceHash hashing power and silently grow the stealth chain
  4. Trade these coins on an exchange for another currency e.g. BTC
  5. Withdraw BTC to another wallet.
  6. Broadcast the stealth chain to the network
  7. Get the initial coins back
  8. Repeat with a different exchange

How Exchanges will Likely Respond

As you can probably imagine, exchanges do not enjoy being bamboozled. If this
kind of behaviour becomes too costly for them, they will likely respond by
increasing security surrounding withdrawal periods, deposit periods, and account
verification.

Waiting longer for withdrawal will make it more costly for attackers, as they
must then maintain the majority hashing power for longer. But this also draws
the ire of legitimate traders and exchange users who already complain about the
inordinate time it takes to get their cryptocurrencies out.

Another way exchanges may respond is by carefully screening coins that are so
easily compromised. However, delisting coins also mean a reduction in trading
volume and revenue. I hope this happens, because altcoins that are solely used
for speculation, are in dire need of an existential
threat
.

Ultimately, we’ll likely see a combination of both. The harder it becomes to
successfully get away with a double-spend attack, the less money an attacker can
justify spending. In the long-run, the balance of these two forces will converge
on some market equilibrium.

How Cryptocurrencies will Respond

Altcoins may find new ways to combat this threat by:

  • Using more obscure algorithms for which there are few miners. This is at best a
    bandaid solution. Fewer miners for your algorithm means it’s difficult to grow
    your hashing power. If your network grows, then the algorithm will no longer be
    obscure.
  • New projects may be to stake their security on the blockchains of larger
    networks. e.g. ERC20.
  • Pushing for new consensus algorithms that are more resilient to 51% attacks e.g.
    proof of stake. POS isn’t perfect though and has challenges of its own.

Big is Beautiful

How much larger is the rental market going to grow? It’s not inconceivable to
witness a 100x increase, so how many coins are really safe?

Coins with high market caps and low cost of attack are particularly fallible.
Given that this is true, will the market respond accordingly by discounting
insecure coins? Conversely, will the market place a premium on cryptocurrencies
with mammoth mining networks?

To quote a Hacker News comment, “Rent-a-miner attacks seems like another amusing
example of when the emergence of a market can break a system. Satoshi foresaw
people trying to mount a 51% attack by buying a ton of machines, and so he went
to great lengths to ensure this was unlikely using mining. I don’t think Satoshi
foresaw the liquid AWS-like market for instant hashing power. The ability to
mount a limited-time 51% attack makes the attack literally 1000x easier than a
buy-machine 51% attack.”

About the Author

I quit my job recently to start
HodlBot.

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Hayden P.

A blockchain and cryptocurrency enthusiast

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