In December 2017, a South Korean Bitcoin Exchange center suffered from a serious cyber-attack and went bankrupt. This news raises us a question, if blockchain is a safe mechanism to store our money. However, after doing research about this cyber-attack, we know that the hacker did not break the blockchain it-self, the hackers only cracked the exchange center instead. We then use blockchain to compare the security with other online-payment such as PayPal. After analyzing their behavior under the injection attack and a DDoS attack, we get a conclusion that blockchain can effectively defend those two attacks because of its immutable and distributed characteristic. However, even if blockchain is safe currently, there is a risk in the future from quantum computing. According to Divesh Aggarwal, the computing speed will significantly rise within 10 years, this means blockchain's encrypted signature might not be safe. Therefore, we propose a technique that might improve the complexity of blockchain encryption by combining the Geoencryption and SHA- 256 approach.
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