Foundations of Blockchain

Double-spend attacks

A double-spend attack is when the same fund amount is spent twice or several times. A physical currency such as paper cash cannot be spent twice unless a duplicate copy is created, which is expensive. Digital money, on the other hand, can easily be duplicated without any cost because they are replicated and transmitted in bits. In Figure 10.3, Alice sends the same 10-dollars- worth of digital money to all her friends. If her friends are unaware of other transactions, they will believe that each of their 10 dollars is legitimate:

Figure 10.3: Double-spending of funds by Alice

Double-spending is a well-known problem that ...