Banknote issuers have been using increasingly sophisticated paper and printing technology to curb counterfeiting. Physical money can easily solve the double payment problem because the same note cannot be in two different places at the same time. Of course, traditional money is often stored and transmitted digitally. In these cases, the problem of counterfeit currency and double payments is dealt with by the elimination of all electronic transaction records by the central authority, which holds a global view of currency in circulation. For digital currencies that can't make effective use of secret ink printing techniques or bar holograms, cryptography provides a foundation of trust for the legitimate value that users claim. Specifically, an encrypted digital signature enables a user to sign a digital asset or transaction that provides proof of ownership of the asset. With the right structure, digital signatures can also be used to solve the problem of double payments. 

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