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Chapter 1: Introduction > Introduction - Pg. 2

2 Chapter1·Introduction Introduction The purpose of this book is to explore the needs and functions of Secure Shell (SSH). We will endeavor to explain the history of the networks we use today and how they developed and expanded to a point where tighter security became increasingly more important. We will look at how the OSI (Open Systems Interconnect) model and SSH relate to each other and also how to use the OSI model for troubleshooting network connectivity. Then we will look at the role of cryptography and the various methods of encryption from which we can draw. Once we understand the cryptography, we will then look at the actual SSH standards and how this protocol can aid in the secure transmission of controls and commands across the network. Then the various SSH platforms will be discussed and documented. The later chapters will round out the book with topics on port forwarding. So let us embark on our journey with a brief history and introduction to SSH; all aboard! Why Is There a Need To Use SSH? In the beginning there were main frame computers. These large computers allowed programmers to input large mathematical formulas that would take hours or days to solve by hand. These computers could take the same formula and datum and solve it in seconds or minutes. As these computers became more flexible and could handle not only mathematical datum but also text and numerical information, people began to use them to manage more and more business and research data. Computers became more than just a tool for college and government organizations, as they started to be able to manage business data. As they became smaller and more powerful, tools to input and store data came into being and costs became more reasonable. More customers were in the business world. These computers stored massive amounts of data and people could access these machines in a controlled environment. The topology of the network was called the Centralized Data Model; in this model all the data was stored on one central computer and access was through "dumb" terminals. The terminals themselves had no computer processing power or storage. This protected the data from loss, damage, theft, and spying. In this model encryption was not necessary as the data was never vulnerable to the outside world. People could see only what the administrators allowed through the "green screen," or dumb terminal. As computers became more powerful and a need to share data across diverse and distant locations became more prevalent, wide area connections were established. At first these connections were done over analog phone lines using modem (Modulator/Demodulator) technology. There were two types of modems, synchronous and asynchronous. Synchronous modems used a special timing bit in the stream to keep the communications channel operating smoothly. In asynchronous modems, instead of a constant timing bit, the technology used a start and stop bit for each part of the transmission, ensuring each piece of data was received consistently. These analog connections were point to point and it was not easy for people to "listen in" on these connections. As communications technology progressed and a shared, or interconnected, network of networks developed and more and more "private" data was being transmitted over these open links, the need for encrypted transmission become necessary. In addition, with the wide areas of transmission, personal computers also brought about internal or Local Area Networks (LANs). These internal networks allowed computers to transmit and receive data from other computers and servers within