With the development of cloud computing in the academia and industry

it is inevitable that many security problems arise.This paper summarizes the security requirements of cloud computing

which not only cover the traditional security requirements like confidentiality

data integrity

access control and identity authentication

but also introduce new security requirements in the credibility

configuration and virtual machinery.We make conclusions about the security situations on two typical cloud computing products:Amazon Web Services and Windows Azure and elaborate two attack mechanisms against cloud computing:Denial of service attack and Side channel attack.Based on the security requirements and attacks against cloud computing

we systematically summarize the current security protection mechanisms and further make a comparison among them.