SSH: what it is, how it works, and everything you need to know

SSH has become one of the key protocols for secure remote administration across all kinds of networks. From data centers to personal computers, this protocol lets you access other systems in encrypted form, making it an indispensable tool for system administrators and developers.

In this article we dive deep into SSH: what it is, what it’s for, how it works, the advantages it offers over alternatives such as Telnet or SSL, its most common uses, how to configure it, the tools that support it, the weaknesses it can show when poorly managed, and much more. Everything is explained clearly and thoroughly, drawing on the best sources in the field.

What is SSH and where did it come from?

SSH, short for Secure Shell (or Secure Socket Shell), is a network-communication protocol that enables secure connections between two devices over an untrusted medium—such as the Internet. Its main purpose is to provide a safe way to log in, control and manipulate remote machines without anyone else being able to intercept the data.

SSH was created in 1995 by Finnish researcher Tatu Ylönen after a password-theft attack on his university network. His goal was to replace the obsolete Telnet, which sent data in plain text. The first version, SSH-1, quickly gained popularity but had weaknesses that led to SSH-2 in 2006—the current, more secure version standardized by the IETF.

Over the years many SSH implementations have appeared; the most popular is OpenSSH, developed by the OpenBSD team and now built into most *nix systems and available for Windows as well.

How the SSH protocol works

SSH uses a multi-layer design to ensure confidentiality, authentication and integrity between client and server.

1. Transport layer: establishes the secure connection with encryption and algorithm negotiation. Key-exchange methods such as Diffie-Hellman or ECDH run here.

2. User authentication layer: lets the client prove its identity to the server using passwords, public keys or even two-factor authentication (2FA).

3. Connection layer: once identity is verified, a single encrypted tunnel is set up, inside which multiple sub-channels can run—interactive sessions, command execution, file transfers, tunnels, etc.

The default SSH port is 22, but you can change it to improve security.

Most common uses of SSH

SSH is far more than a remote terminal. Typical applications include:

• Secure remote access for servers, routers or IoT devices.
• File transfers via SCP or SFTP, replacing insecure protocols like FTP.
• Task automation with scripts that connect automatically using key-based logins.
• Tunneling or port forwarding to redirect traffic through an encrypted link.
• X11 forwarding to run remote GUI apps locally.

Thanks to this versatility, SSH is used everywhere from cloud infrastructure to mission-critical enterprise systems.

Authentication methods in SSH

SSH can verify user identity in several ways:

• Password: the traditional method, less secure because it depends on password strength.
• Public/private keys: widely used. You keep your private key locally and upload the public key to the server. They are matched to validate access without a password.
• Two-factor authentication (2FA): combines a key (or password) with a dynamic token from tools like Google Authenticator or a YubiKey.

You can protect keys with a passphrase and manage them with agents such as ssh-agent, so you don’t have to enter the passphrase every time.

Encryption types used by SSH

All SSH traffic is shielded by strong ciphers. The most common are:

• Symmetric encryption: after key exchange, a shared secret encrypts data during the session (e.g., AES or ChaCha20).
• Asymmetric encryption: used for authentication and initial key exchange with public/private pairs.
• Hashing: to guarantee data integrity with mechanisms like HMAC-SHA2.

The exact algorithms depend on server and client versions and can be configured to allow only strong methods.

What SSH tunneling is and why it matters

SSH tunneling (port forwarding) lets you redirect network traffic through an encrypted SSH connection, creating secure access to services that would normally be exposed.

There are three types:

• Local forwarding: redirects local connections to a remote service through SSH.
• Remote forwarding: exposes a local application through the remote SSH server.
• Dynamic forwarding: sets up a SOCKS proxy to route traffic to many destinations—handy for secure browsing.

You can even build simple VPNs with tunnels, though they need careful configuration.

Popular SSH implementations

There are many SSH clients and servers for each OS:

• OpenSSH: the most widespread option—includes ssh, sshd, scp, sftp, and more; preinstalled on most Linux, Unix and macOS systems.
• PuTTY: a well-known Windows client (also ported to other OSes).
• WinSCP: a graphical tool for secure file transfers (SCP/SFTP) on Windows.
• Tectia SSH: a commercial enterprise-grade suite from SSH Communications, Tatu Ylönen’s company.
• Termius: a cross-platform client for desktop and mobile.

Configuration-management tools like Ansible, backup scripts, and monitoring systems also rely on SSH for connectivity.

Key differences between SSH and Telnet

Both protocols provide remote access, but with critical contrasts:

• SSH encrypts all traffic; Telnet sends everything in plain text.
• SSH authenticates identities securely; Telnet does not.
• SSH supports multiple auth methods (keys, passwords, 2FA).
• SSH allows file transfers, tunneling, and remote command execution.

As a result, Telnet is obsolete in most modern environments, replaced by SSH’s security.

SSH vs. SSL/TLS: a quick comparison

Though both encrypt data, they serve different purposes:

• SSL/TLS is used in apps like HTTPS, protecting the transport layer between browser and web server.
• SSH provides an interactive terminal connection geared toward system administration and automation.
• In TLS you typically authenticate only the server; in SSH both server and client can authenticate with keys.

In short, TLS secures services such as websites or email, while SSH focuses on direct user- or admin-level interactions with other systems.

Risks and weaknesses to watch for with SSH

Even robust SSH can be dangerous if mismanaged:

• Poor key management: without rotation or revocation, keys can grant unlimited access to critical systems.
• Exposed ports: especially on IoT gadgets, can invite unauthorized access.
• Blindly accepting host keys: skipping verification may allow MITM attacks.
• Allowing direct root logins: poses high risk if not combined with extra controls like 2FA.

Therefore it’s vital to set clear policies, regular audits and centralized key-pair management in organizations—an overlooked key can become a back door.

History of SSH vulnerabilities

Like any protocol, SSH has seen security flaws through the years. Notable examples include:

• Vulnerabilities in SSH-1, which is now strongly discouraged.
• A CRC-32 packet-insertion bug in early versions.
• In 2023, the Terrapin attack exploited weaknesses in CBC ciphers in OpenSSH, later patched.

Nevertheless, a properly configured SSH-2 setup is still considered highly secure by the community.

Proper management, constant updates and strict access control are essential for safe SSH use. Done right, SSH can mean the difference between a hardened infrastructure and one vulnerable to attacks or unauthorized entry.