What is a 51% attack?

A 51% attack is when malicious attackers attempt to take over a cryptocurrency’s network by gaining control of more than half—aka 51%—of a network’s mining hashrate. If attackers gain control of a network, they can falsify new transactions, spend coins they don’t have, prevent transactions from being validated, and more.

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What does hashrate have to do with it?

Some cryptocurrencies, including Bitcoin, rely on mining to secure the network and validate new transactions. Mining is the process of verifying and adding transactions to a blockchain network. In exchange for verifying and adding transactions, miners are rewarded with crypto.

Generally, the more miners participating in a network, the more difficult it is to successfully attack the network because there’s more computer power being used to process transactions. The amount of computing power being used by a network is referred to as its hashrate. You can learn more about hashrates in this article.

The fewer the miners, the lower the hashrate, and the more vulnerable the network.

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How does a 51% attack work?

One way to think about this is to picture miners as voters who can approve or invalidate a transaction. If you have 100 voters in a room, 51 or more of them would have to agree for a transaction to be validated. Even if 40 of the 100 voters were malicious attackers, they would still get outvoted by the 60 other voters.

But if there were only 10 people in the room, and 6 of them were malicious attackers, those attackers would have the majority—or over 51%—of the voting power.

When malicious attackers are able to take over a cryptocurrency’s network, they can falsify new transactions, prevent valid transactions from being validated, and more.

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Are 51% attacks common?

Generally, these types of attacks are uncommon because it can be prohibitively expensive to try and take over a cryptocurrency’s network.

For example, Bitcoin’s hashrate is measured in hundreds of exahashes per second. An exahash is a one followed by 18 zeros, or 1,000,000,000,000,000,000. If an attacker wanted to take over Bitcoin’s network, they would need to provide more than 51% of Bitcoin’s hashrate. The cost of the specialized mining equipment and electricity needed to do so makes it very prohibitive.

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Brokerage services are offered through Robinhood Financial LLC, (RHF) a registered broker dealer (member SIPC) and clearing services through Robinhood Securities, LLC, (RHS) a registered broker dealer (member SIPC). Cryptocurrency services are offered through Robinhood Crypto, LLC (RHC) (NMLS ID: 1702840). Robinhood Crypto is licensed to engage in virtual currency business activity by the New York State Department of Financial Services. The Robinhood spending account is offered through Robinhood Money, LLC (RHY) (NMLS ID: 1990968), a licensed money transmitter. A list of our licenses has more information. The Robinhood Cash Card is a prepaid card issued by Sutton Bank, Member FDIC, pursuant to a license from Mastercard®. Mastercard and the circles design are registered trademarks of Mastercard International Incorporated. RHF, RHY, RHC and RHS are affiliated entities and wholly owned subsidiaries of Robinhood Markets, Inc. RHF, RHY, RHC and RHS are not banks. Products offered by RHF are not FDIC insured and involve risk, including possible loss of principal. RHC is not a member of FINRA and accounts are not FDIC insured or protected by SIPC. RHY is not a member of FINRA, and products are not subject to SIPC protection, but funds held in the Robinhood spending account and Robinhood Cash Card account may be eligible for FDIC pass-through insurance (review the Robinhood Cash Card Agreement and the Robinhood Spending Account Agreement).

Options trading entails significant risk and is not appropriate for all customers. Customers must read and understand the Characteristics and Risks of Standardized Options before engaging in any options trading strategies. Options transactions are often complex and may involve the potential of losing the entire investment in a relatively short period of time. Certain complex options strategies carry additional risk, including the potential for losses that may exceed the original investment amount.

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