Undoubtedly, you’ve come across the term ‘SSH‘ in your work. Perhaps you needed to log into a remote computer system from your work laptop. Or maybe you worked through a tutorial for setting up SSH keys with your github account so you could push and pull code to and from a remote repository.
SSH will surely be something you need to interact with. When it comes up, it’s often sufficient to find a tutorial that will walk you through the steps to accomplish a certain task related to SSH.
But what is SSH exactly? The three letters ‘S’-‘S’-‘H’ actually refer to more than one thing. SSH is both a protocol and a computer application that implements the protocol. In Part 1 of this series on SSH, we will discuss the protocol.
SSH the Protocol
As mentioned above, SSH refers to a network protocol. A protocol is just a system of rules for communication that two parties who need to talk to each other agree upon beforehand.
Imagine you are sitting in a coffee shop and you overhear two people seated next to you engaged in a lively conversation. They seem so animated in their discussion that you are curious to find out what they are talking about. You inconspicuously lean your chair a little bit closer to them and listen in carefully.
After a couple of moments, you realize they are speaking some sort of language that you do not understand at all. Even though you can hear everything they are saying, you still do not know what they are talking about. Our two conversationalists are using a linguistic protocol that allows them to communicate privately while being completely out in the open.
Similarly, SSH is a set of rules that two computer systems can use to talk to each other over an insecure line of communication without exposing what they are talking about. The protocol specifies a client-server architecture in which an SSH client application establishes a secure channel to communicate with an SSH server application. In other words, two computer systems can use the SSH protocol to privately communicate even though they have to use an insecure protocol like TCP to transport the messages back and forth.
The SSH protocol addresses the concern of secure communications by offering a way to:
- scramble the messages being communicated so that their meaning cannot be understood
- make sure that the identities of the two parties involved in communication are expected
- detect if any messages are tampered with or damaged as they move between the sender and the receiver
You can see the rules of the protocol specified in the following RFC documents on the Internet Engineering Task Force (IETF) tools website:
- The Secure Shell (SSH) Protocol Assigned Numbers
- The Secure Shell (SSH) Protocol Architecture
- The Secure Shell (SSH) Authentication Protocol
- The Secure Shell (SSH) Transport Layer Protocol
- The Secure Shell (SSH) Connection Protocol
- Using DNS to Securely Publish Secure Shell (SSH) Key Fingerprints
- Generic Message Exchange Authentication
Articles in the What Exactly is SSH?? (The Secure Shell) Series