## 3 Basic Definitions and System Framework
### The Public Key Grades  
There are a large amount of users with different identities in the system. Our approach relies on the voting response of legitimate users. In order to reasonably evaluate the credibility of each voting user, we classify the user public key into four grades. At the same time, the credibility of the user becomes higher in high grade of public key, because  
Figure 1: Relationship among user nodes, SNR nodes, and Audit Institutions.  
generating a high-grade public key will be required to complete a more rigorous user verification process during the certificate generation phase by Audit Institutions or a long-term accumulation of positive contributions to network security. It is avoided by malicious users. The public key application objects will be defined as the following. More notably, we will explain in detail the difference and correlation between the grade of the public key and the grade of the digital certificate. The setting rules of the initial security weight value will be introduced in the algorithm in Section 4.  
We divide public keys into the following four grades: _Top Grade_, _Professional Grade_, _Personal Grade_, and _Basic Grade_. In the public key and certificate generation phase, the user's public key is initially graded based on the scan type of his/her real identity--just like the grade of digital certificates as following:  
_Top Grade_: It is generally used by users with high security requirements such as finance, banking, and e-commerce. Top grade public key certificate generation has the most complex user verification process. Its initial security weight value is also maximum.  
_Professional Grade_: A general enterprise public key is suitable for administration, scientific research institutions, and universities, mailbox, forum, and other large and medium-sized websites. The complexity of the user verification process and initial security weight value are lower than _Top grade_ but higher than the _Personal grade_ and _Basic grade_.  
_Personal Grade_: It is mainly used for personal users with rich content resources such as bloggers and \"we media\". The user verification process and initial security weight value is higher than in _Basic grade_.  
_Basic Grade (quick public service)_: The public key certificate is issued quickly, with time efficiency and a low security level, aiming at other general users. The user verification process is the easiest, and the initial security weight value is at a minimum.  
The security weights will increase or decrease due to the user's contribution to the network security, that is, the user's contribution to the network security events can increase the user's security weight, which may raise the user's public key grade to a higher grade: for example, from _Basic grade_ to _Personal grade_. On the contrary, threatening behavior to network security can reduce the user's security weight, which may reduce the user's public key grade to a lower grade. It is important to note that the upgrade of the public key only represents the increase in users' credibility in the network and has a higher voting weight in the subsequent voting process. However, its own function cannot be changed. For example, a _Basic grade_ user has changed his public key into a _Top grade_ by actively participating in network security events for a long time, but he is still just an ordinary user and cannot carry out banking business, but his credibility in the network has become higher.