[Introduction]

The author was focused on situations in which we use trusted third parties in these days of electronic payment.

In fact, It is useful and works well.
but It still has the inherent weaknesses of the trust-based model.

The third-party can cancel a transaction because they cannot avoid mediating disputes.

This reversible system has some unnecessary elements.

First, "cost of mediation increases transaction costs, limiting the minimum practical transaction size and cutting off the possibility for small casual transactions"
Second, "there is a broader cost in the loss of ability to make non-reversible payments for non- reversible services."

Usually, we use the third-party's help for reliability Merchants and customers.

So, "These costs and payment uncertainties can be avoided in person by using physical currency, but no mechanism exists to make payments over a communications channel without a trusted party."

The soltion is "electronic payment system based on cryptographic proof"

In this paper, "In this paper, we propose a solution to the double-spending problem using a peer-to-peer distributed timestamp server to generate computational proof of the chronological order of transactions."

A main point of security is "The system is secure as long as honest nodes collectively control more CPU power than any cooperating group of attacker nodes."