Double-spending

double spending51% attack51% attacks51% mining attacksdouble spenddouble-spenddouble-spending attack
Double-spending is a potential flaw in a digital cash scheme in which the same single digital token can be spent more than once.wikipedia
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Bitcoin

bitcoinsbitcoin walletBitcoin blockchain
While it hasn't happened against many of the largest cryptocurrencies, such as Bitcoin (with even the capability arising for it in 2014), it has happened to one of its forks, Bitcoin Gold, then 26th largest cryptocurrency. The cryptocurrency bitcoin implemented a solution in early 2009.
This allows bitcoin software to determine when a particular bitcoin was spent, which is needed to prevent double-spending.

Bitcoin Gold

While it hasn't happened against many of the largest cryptocurrencies, such as Bitcoin (with even the capability arising for it in 2014), it has happened to one of its forks, Bitcoin Gold, then 26th largest cryptocurrency.
Bitcoin Gold was hit by double-spending attack on May 18, 2018.

Cryptocurrency

cryptocurrenciescrypto-currencyaltcoin
The cryptocurrency bitcoin implemented a solution in early 2009.
Blockchains solve the double-spending problem without the need of a trusted authority or central server, assuming no 51% attack (that has worked against several cryptocurrencies).

Blockchain

Blockchainsblockchain technologyblock
Instead, transactions are recorded in a public ledger called a blockchain.
The invention of the blockchain for bitcoin made it the first digital currency to solve the double-spending problem without the need of a trusted authority or central server.

Trusted third party

trusted partyTrusted Third Partiesneutral third party
This is usually implemented using an online central trusted third party that can verify whether a token has been spent.
Double-spending

Digital currency

electronic moneydigital currencieselectronic purse
Double-spending is a potential flaw in a digital cash scheme in which the same single digital token can be spent more than once.

Counterfeit money

counterfeitingcounterfeitcounterfeiters
As with counterfeit money, such double-spending leads to inflation by creating a new amount of copied currency that did not previously exist.

Inflation

inflation rateprice inflationfood inflation
As with counterfeit money, such double-spending leads to inflation by creating a new amount of copied currency that did not previously exist.

Circulation (currency)

circulationcirculatedmoney circulation
This devalues the currency relative to other monetary units or goods and diminishes user trust as well as the circulation and retention of the currency.

Blind signature

RSA blinding
Fundamental cryptographic techniques to prevent double-spending, while preserving anonymity in a transaction, are blind signatures and, particularly in offline systems, secret splitting.

Secret sharing

secret splittingsharedBlakley-Shamir secret sharing
Fundamental cryptographic techniques to prevent double-spending, while preserving anonymity in a transaction, are blind signatures and, particularly in offline systems, secret splitting.

List of bitcoin forks

forks
While it hasn't happened against many of the largest cryptocurrencies, such as Bitcoin (with even the capability arising for it in 2014), it has happened to one of its forks, Bitcoin Gold, then 26th largest cryptocurrency.

Online and offline

onlineofflineon-line
This is usually implemented using an online central trusted third party that can verify whether a token has been spent.

Single point of failure

single points of failurecentral point of failurecauses the entire circuit to "open" or stop operating
This normally represents a single point of failure from both availability and trust viewpoints.

Distributed computing

distributeddistributed systemsdistributed system
By 2007, a number of distributed systems for the prevention of double-spending had been proposed.

Cryptographic protocol

protocolsecurity protocolprotocols
It uses a cryptographic protocol called a proof-of-work system to avoid the need for a trusted third party to validate transactions.

Proof-of-work system

proof-of-workproof of workreusable proof of work
It uses a cryptographic protocol called a proof-of-work system to avoid the need for a trusted third party to validate transactions.

Ledger

ledgersledger bookaccount book
Instead, transactions are recorded in a public ledger called a blockchain.

Satoshi Nakamoto

Dorian NakamotoSatoshiSatoshi Nakomoto
In the process, they were the first to solve the double-spending problem for digital currency using a peer-to-peer network.

Privacy and blockchain

In other words, nodes must check past transactions of the spender to ensure that he/she did not double spend, or spend more funds than they actually own.

Digital credential

anonymous credentialanonymous credentialsanonymous digital credential
Consequently, digital cash protocols have to make an extra effort to avoid the double spending of coins.

Bitcoin network

bitcoin miningminingBitcoin protocol
A specific problem that an internet payment system must solve is double-spending, whereby a user pays the same coin to two or more different recipients.

Unspent transaction output

unspent outputs of transactionsunspent transaction outputs
The requirement that only unspent outputs may be used in further transactions is necessary to prevent double spending and fraud.

Ethereum Classic

Ethereum Classic (ETC)
In January 2019, Ethereum Classic was subject to double-spending attacks.

Vertcoin

From October through December 2018, Vertcoin suffered from 51% attacks.