Security

What is SSH and How it works?

Subhash Gottumukkala

• 5 min read
What is SSH and How it works?

Imagine you’re logging into a remote server, managing critical systems, or transferring sensitive files. Now, imagine doing all that while someone on the same network could intercept every keystroke, password, and command. Scary, right? This was the harsh reality of early networking.

Then came Secure Shell (SSH)—a protocol that revolutionized how we communicate securely over the internet. Since its creation in 1995, SSH has been a trusted companion for system administrators, developers, and cybersecurity experts worldwide. This post explores SSH’s rich history, technical workings, advanced features, and its enduring relevance in a world increasingly reliant on secure digital communication.

The Need for Secure Communication

The Fragile Foundations of Early Networking

Before SSH, network communication was dangerously exposed. Tools like Telnet, rlogin, and RSH (Remote Shell) were widely used to access remote systems. These protocols were simple, lightweight, and functional—but they had a critical flaw: no encryption. Everything sent between the client and server was transmitted in plain text, including sensitive credentials.

Imagine being in a crowded café, logging into a remote server using Telnet, and unknowingly broadcasting your username and password to anyone with the right tools. A simple packet sniffer could capture everything. For malicious actors, it was like fishing in a barrel.

A Growing Threat Landscape

As the internet exploded in the 1990s, so did cyber threats. The once-small internet community grew into a bustling network with businesses, universities, and individuals relying on it for everything from communication to financial transactions. Unfortunately, this growth also attracted malicious actors eager to exploit vulnerabilities.

In 1995, a now-famous incident at a Finnish university highlighted the severity of these risks. A password-sniffing attack exposed how easy it was to steal sensitive credentials on unencrypted networks. This wasn’t just a wake-up call—it was a full-blown alarm for the tech community. The need for secure communication methods had never been more urgent

The Birth of SSH

Tatu Ylönen: The Man Behind the Protocol

Amid the chaos of growing cyber threats, Finnish researcher Tatu Ylönen stepped forward. Working at Helsinki University of Technology, Ylönen decided to create a solution that would eliminate the vulnerabilities of protocols like Telnet. His goal was straightforward: to build a system that encrypted all communication between machines, making intercepted data useless to attackers.

In 1995, Ylönen unveiled the first version of SSH (Secure Shell). Unlike its insecure predecessors, SSH used encryption to protect data and prevent eavesdropping. It wasn’t just a new protocol—it was a revolution in how remote communication was handled.

Ylönen released SSH as open-source software, allowing developers and administrators worldwide to adopt it freely. The timing couldn’t have been better. By the end of 1995, 20,000 users across 50 countries had started using SSH. Its adoption wasn’t limited to universities and tech companies—SSH quickly became a favorite tool for anyone needing secure remote access.

Recognizing the growing demand, Ylönen founded SSH Communications Security Corp., which focused on further developing and commercializing the technology. SSH had arrived, and there was no turning back.

SSH: From Humble Beginnings to a Global Standard

The first version of SSH, now called SSH1, was groundbreaking for its time. It introduced features like encrypted communication and authentication, setting a new standard for secure remote access. However, as more people adopted SSH1, researchers discovered security flaws that needed addressing.

In 1996, just a year after SSH1’s debut, Ylönen and his team introduced SSH2. This updated version fixed vulnerabilities in SSH1, introduced stronger encryption algorithms, and improved the overall security architecture. SSH2 became the preferred standard for secure communication and was eventually formalized by the Internet Engineering Task Force (IETF).

The OpenSSH Revolution

The evolution of SSH reached another milestone in 1999 with the release of OpenSSH. Developed by the OpenBSD project, OpenSSH was an open-source implementation of SSH2. It quickly gained popularity for its:

  • Robust Security: OpenSSH added features like stronger encryption and better key management.
  • Cross-Platform Compatibility: It worked seamlessly on Unix, Linux, macOS, and even Windows.
  • Active Maintenance: The OpenBSD community ensured OpenSSH stayed ahead of emerging threats.

Today, OpenSSH is the most widely used implementation of SSH, powering millions of secure connections daily.

The Working

SSH’s magic lies in its ability to create a secure tunnel between two machines. Let’s break it down step by step:

1. Establishing a Connection

The process begins with a TCP connection between the client (your computer) and the server (the remote machine). This connection serves as the foundation for all communication.

2. Key Exchange

Once connected, the client and server perform a key exchange to agree on encryption algorithms and generate session keys. These keys are used to encrypt and decrypt data during the session.

3. Authentication

SSH verifies the client’s identity through one or more methods:

  • Password Authentication: The user enters a password, which is securely transmitted and verified.
  • Public Key Authentication: The client uses a pair of cryptographic keys (private and public) for authentication.
  • Multi-Factor Authentication: Additional layers, like one-time passwords (OTPs) or biometric scans, can be added for extra security.

4. Secure Communication

After authentication, all data sent between the client and server is encrypted. SSH packets contain:

  • Payloads: The actual data being transmitted.
  • Padding: Random bytes added to obscure packet size.
  • Message Authentication Codes (MACs): These ensure the integrity of the data.

Even if an attacker intercepts the packets, they can’t decipher the contents without the session keys.

SSH’s Advanced Features

SSH is more than just a secure alternative to Telnet. Its advanced features have made it indispensable for modern IT and cybersecurity:

1. File Transfers

SSH supports secure file transfer protocols like SFTP (Secure File Transfer Protocol) and SCP (Secure Copy Protocol). These tools allow users to transfer files between machines without compromising security.

2. Tunneling and Port Forwarding

SSH can redirect network traffic through secure tunnels, a process known as port forwarding. This is particularly useful for:

  • Bypassing firewalls.
  • Accessing remote databases securely.
  • Enabling encrypted connections for otherwise unencrypted services.

3. Multiplexing

SSH’s ability to handle multiple channels over a single connection is a game-changer. For example, you can run a command-line session, transfer files, and forward ports—all through one SSH connection.

4. Dynamic Port Forwarding

With dynamic port forwarding, SSH can act as a SOCKS proxy, allowing users to securely access services on different networks.

Real-World Applications of SSH

DevOps and Automation

Tools like Ansible, Terraform, and Jenkins rely on SSH for secure automation. Whether deploying applications or managing servers, SSH is a critical part of DevOps workflows.

IoT Device Management

As IoT devices become more prevalent, managing them securely has become a priority. SSH provides a reliable way to monitor and update these devices without exposing them to vulnerabilities.

Secure Remote Work

In a post-pandemic world, where remote work is the norm, SSH ensures that employees can securely access corporate servers and resources from anywhere.

The Future of SSH

Despite being over two decades old, SSH remains as relevant as ever. Its future will likely be shaped by emerging technologies and new security challenges:

1. Enhanced Security

The OpenSSH project continues to release updates, ensuring that SSH stays ahead of modern threats like quantum computing, which could potentially break current encryption methods.

2. Cloud Integration

As more companies migrate to the cloud, SSH is evolving to integrate seamlessly with cloud-native platforms. Many cloud providers now offer built-in SSH-based access for managing virtual machines and containers.

3. Expanding Use Cases

From securing blockchain transactions to enabling remote surgeries in telemedicine, SSH’s versatility ensures its continued relevance in diverse industries.

Conclusion

SSH has come a long way since its inception in 1995. What started as a simple solution to insecure protocols has become a cornerstone of modern cybersecurity. Its ability to adapt to new challenges while maintaining its core principles of security and simplicity is a testament to its enduring value.

Whether you’re a system administrator logging into a remote server or a developer automating deployments, SSH is quietly working in the background, ensuring your data stays safe. So the next time you type ssh into your terminal, take a moment to appreciate the history, innovation, and community effort behind this remarkable protocol.