Double-spending is a failure mode

Failure mode

Failure causes are defects in design, process, quality, or part application, which are the underlying cause of a failure or which initiate a process which leads to failure. Where failure depends on the user of the product or process, then human error must be considered.-Component failure:A part...

 of digital cash schemes, when it is possible to spend a single digital token twice. Since, unlike physical token money

Token money

Token money is money made from tokens of some form, as opposed to account money. Coins are token money, as are paper notes.Token money has a strong privacy feature in that it works as money without the intervention of any other party in each transaction between two parties. Privacy makes money...

 such as coin

Coin

A coin is a piece of hard material that is standardized in weight, is produced in large quantities in order to facilitate trade, and primarily can be used as a legal tender token for commerce in the designated country, region, or territory....

s, electronic files can be duplicated, and hence the act of spending a digital coin does not remove its data from the ownership of the original holder, some other means is needed to prevent double-spending.

This is usually implemented using an on-line central trusted third party

Trusted third party

In cryptography, a trusted third party is an entity which facilitates interactions between two parties who both trust the third party; The Third Party reviews all critical transaction communications between the parties, based on the ease of creating fraudulent digital content. In TTP models, the...

 that can verify whether a token has been spent. This normally represents a single point of failure

Single point of failure

A single point of failure is a part of a system that, if it fails, will stop the entire system from working. They are undesirable in any system with a goal of high availability or reliability, be it a business practice, software application, or other industrial system.-Overview:Systems can be made...

 from both the technical and trust viewpoints. However, a number of distributed systems for double spending prevention have been proposed. Bitcoin

Bitcoin

Bitcoin is a decentralized, peer-to-peer network over which users make transactions that are tracked and verified through this network. The word Bitcoin also refers to the digital currency implemented as the currency medium for user transactions over this network...

is an example of a digital cash system that uses a distributed system of this sort.