Imagine you are a system architect tasked with designing a distributed system for a financial institution. The system needs to process transactions securely and efficiently across multiple servers located in different geographical regions. The system should ensure fault tolerance in the event of node crashes, including the possibility of a critical master node failure. Additionally, the communication between servers should be encrypted to protect sensitive financial data. With these assumptions answer the following:

a$)$ Design a distributed system architecture for the financial institution, considering the use of a master$-$slave model for centralized control with the help of a pictorial diagram. Clearly define the roles of master and slave nodes in handling transaction processing.$$

b$)$ Introduce the concept of client$-$server communication using Remote Procedure Call $($RPC$)$ in the context of the financial system. Provide examples of specific transactions or operations that would benefit from using RPC for communication efficiency.