Double-spending is a major computing problem that has to be solved by every cryptocurrency. If not, the cryptocurrency in question is essentially worthless because anyone can duplicate a transaction with the currency at any time.

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• “Double-spending” means that the same units of a currency could be spent twice
• Double-spending would destroy the trust in a cryptocurrency
• Cryptocurrencies prevent double-spending by using a blockchain that combines an open ledger with cryptographic algorithms

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In this lesson, you will learn about double-spending.

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Double-spending means that the same units of a cryptocurrency could potentially be spent twice, thus it is crucial to technologically eliminate this possibility.

Double-spending would basically destroy the technological grounding on which a blockchain is founded - a database that is not only tamper-proof, but also records every transaction that has ever taken place within the network. Thus, the potential to execute double-spending would fundamentally undermine the trust in a cryptocurrency like Bitcoin or any other blockchain database.

A related analogy for explaining this conundrum is the “Byzantine Generals Problem”, which addresses the challenge multiple parties that do not trust each other face when they undertake a joint venture in which they need to cooperate to succeed.

The Byzantine Generals Problem is a thought experiment to illustrate the issue of disagreement between players in a decentralised system. In this comparison, only a coordinated attack by all generals each controlling his own army leads to victory. As soon as one general defects or attacks another general, the battle is lost.