Secure Shell (SSH) Protocol Explained

Transferring data safely from one point to another has always been a challenge. Aside from ensuring the information transmits correctly, we must also ensure it remains safe. That's where communication protocols like Secure Shell (SSH) and others come into play.

What is SSH?

SSH is a communications protocol that governs how devices send data over a network. It provides the framework and instructions that determine how information data gets packaged and moved. More importantly, it allows the data to be sent safely, even over insecure networks.

Because of its characteristics, SSH is suitable for a broad range of applications. One common use is by system or server administrators to communicate securely with remote servers. For example, website owners may use SSH to connect with and transfer data to virtual servers.

Tatu Ylönen and Linus Torvalds (of Linux fame) developed the first SSH (SSH-1) version in 1995. While some still use SSH-1, a more common version you'll encounter today is SSH-2.

How SSH Works

SSH allows secure communications with remote servers
SSH allows secure communications with remote servers (Source: Hostinger)

SSH works on the Client-Server model where one device (the Client) sends instructions to another (the Server). The server is responsible for responding to instructions from authorized Client devices.

It uses three main components to support this communication; The Transport Layer Protocol (TLP), the User Authentication Protocol, and Connection Protocol. Each protocol serves a distinct sequential purpose within SSH.

Example of SSH Use

Let's use the most typical SSH request, that of an administrator wishing to establish an SSH connection to a remote server.

The administrator will type the following command; ssh [email protected]

This command instructs the local device (the Client) that an SSH connection is wanted.  It comprises three parts. “ssh” is the command, “username” is the identity of the account you wish to access on the remote server, and “host” is the IP address or domain name of the remote server.

Once the command sends, a prompt will appear requesting a password to access the remote account. The remote server is ready to accept further commands if the password is correct.

Advantages of SSH

There are several key advantages to using SSH.

Strong Encryption

SSH is a secure protocol that encrypts communication between the client and server, making it much harder for hackers to intercept the information. Its encryption uses AES-256, one of the highest levels of encryption currently available. 

Controlled Authentication

When you connect over SSH, both sides verify each other's identity before allowing access to the system. You can configure SSH so that only certain users are allowed access or in a way that all users need a password to log in.


SSH is compatible with all major operating systems, including Windows, macOS, Linux, and Unix-based. The main difference is that SSH may sometimes not be a default option. For example, macOS machines must enable the protocol before use.

Disadvantages of SSH

SSH is a great tool, but it also has some disadvantages.

Some User Challenges

The default implementation of SSH requires users to learn command line instructions and establish connections. Without these skills, users will need third-party applications to use SSH for basic instructions.

Performance Issues

Compared to some protocols like FTP, SSH is slower. It's more suited for communicating short bursts of data like commands rather than file transfer. The issue is primarily caused by TCP/IP, which creates a more significant overhead for data communication.

Some Known Vulnerabilities

Even SSH-2 is currently known to have several vulnerabilities. Some are not native to the protocol but are caused by SSH inconveniences, such as the complexities of managing and tracking keys.

How SSH Compares With Other Communication Protocols

SSH vs. Telnet

You'll need SSH or Telnet if you need to connect to a remote server over the Internet and type commands into the terminal window. They both serve the same purpose. However, it's important to note that SSH is the secure version of Telnet. The latter sends data in plaintext and is also slower than SSH.


SSH and SSL/TLS are two protocols that encrypt data during transmission. They both provide authentication and integrity, but they use different methods. SSL/TLS supports authentication via certificates issued by a certificate authority (CA). 

SSL/TLS is more suitable than SSH for e-commerce transactions because it allows transactions to be validated using digital signatures.


The difference between SSH and VNC is that SSH is a full-featured remote shell protocol, whereas VNC is a more straightforward remote desktop protocol (RDP). Because of this difference, SSH provides more features than VNC. 

For example, SSH supports multiplexing, while VNC does not. Multiplexing means you can use one channel to send multiple streams of information at once — this can be useful if a lot of data is transferred between the client and server.


The need for secure, encrypted communication between computers has continued to increase in today's interconnected world. The Secure Shell protocol and its client, server, and related tools have become some of the most widely used network security solutions. 

SSH and its close relative, OpenSSH, can essentially be considered a secure replacement for unencrypted protocols such as Telnet. It is essential knowledge for any network or system administrator to understand the Secure Shell environment on their systems and networks.

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Article by Timothy Shim