# A Blockchain-Based Decentralized Public Key Infrastructure Using the Web of Trust
## 5 System Overview
### System Architecture
#### 5.2.2 Validation
(B) **Ring-Member Authenticity:**  
The validation of a certificate relies on the introducer's signature, emphasizing the importance of introducers being trusted with valid keys when establishing a key ring. In the system, existing members can verify and sign new members' public keys to create bridges within the key-ring community. Analogous to certificate verification, a trust level is assigned during the validation process of new members. The trust level of an introducer can be categorized as _fully trusted_, _marginally trusted_, or _not trusted_. These trust levels are factored into the final calculation. For instance, a fully trusted introducer may assign a marginally trusted level to a new member if necessary, while a marginally trusted introducer can assign a fully trusted level during signing. Additionally, each introducer has a depth level along with their level of trust. Figure 4 shows an example of calculating the depth levels of introducers, whether they are directly or indirectly connected to the ring owner. For instance, introducers directly connected have a depth of one, while those indirectly connected have a depth greater than one, varying based on distance. Similar to the verification of public key certificates, a mathematical equation is employed to compute the trust level for the newly introduced. Figure 3: A diagram showcasing the participation of key-ring members with different trust levels in verifying a certificate. The equation for calculating the trust level during introducer validation is as follows:  
\[T=\frac{c}{C}+\frac{m}{M}+\frac{n}{N} \tag{2}\]  
\[c=\sum_{i=1}^{x}\frac{c_{c}}{d_{i}}+\sum_{i=1}^{y}\frac{c_{m}}{d_{i}}+\sum_{i=1} ^{z}\frac{c_{n}}{d_{i}} \tag{3}\]  
\[m=\sum_{i=1}^{x}\frac{m_{c}}{d_{i}}+\sum_{i=1}^{y}\frac{m_{m}}{d_{i}}+\sum_{i=1 }^{z}\frac{m_{n}}{d_{i}} \tag{4}\]  
\[n=\sum_{i=1}^{x}\frac{n_{c}}{d_{i}}+\sum_{i=1}^{y}\frac{n_{m}}{d_{i}}+\sum_{i=1 }^{z}\frac{n_{n}}{d_{i}} \tag{5}\]  
In Equation (2), the following variables are defined:  
* \(T\) represents the introducer's trust level. * \(c_{c}\), \(c_{m}\), and \(c_{n}\) denote the full, marginal, and not trusted trust levels given by fully trusted introducers. \(c_{c}\), \(c_{m}\), and \(c_{n}\) are the constant weights determined by the system authority, where, \(c_{c}>c_{m}>c_{n}\). Similarly, \(m_{c}\), \(m_{m}\), and \(m_{n}\) are the constant weights for the full, marginal, and not trusted trust levels given by marginally trusted introducers. The same goes for \(n_{c}\), \(n_{m}\), and \(n_{n}\) for not trusted introducers. * \(c\) denotes the sum of all trust level weights from the fully trusted introducers divided by their respective depth \(d\). Similarly, \(m\) represents the sum of all trust level weights from the marginally trusted introducers divided by their respective depth \(d\). The same goes for \(n\) not trusted introducers. * \(C\), \(M\), and \(N\) denote the number of signatures required from the fully, marginally, and not trusted introducers to validate new introducers. These values are determined by the system authority where \(C\geq 1\), \(M\geq 1\), and \(N\geq 1\). The introducer is classified as fully trusted if \(T\geq 1\), marginally trusted if \(0.5\leq T<1\), and not trusted if \(T<0.5\). Upon successful validation, the new introducer will be incorporated into the key-ring network, and an announcement will be broadcast over the network to inform existing key-ring members. This section outlines various scenarios to explain the trust evaluation mechanism. For illustration purposes, in all scenarios, the weights assigned to the constants are as follows: \(C=2\), \(M=3\), and \(N=4\); \(c_{c}=4\), \(c_{m}=3\), and \(c_{n}=2\); \(m_{c}=3\), \(m_{m}=2\), and \(m_{n}=1\); and \(n_{c}=2\), \(n_{m}=1\), and \(n_{n}=0\). **Scenario 1:**  
* Two endorsements are received from fully trusted introducers (one fully trusted, one marginally trusted). * Three endorsements are received from marginally trusted introducers (two fully trusted, one marginally trusted).