how does ssh encryption work

This article explores Telnet vs. SSH, two protocols for remote access to devices and systems, comparing their security, functionality, and modern relevance. Whether you’re managing network devices, debugging services, or securing remote connections, understanding these protocols is essential. Use the table of contents below to navigate to specific sections.

Telnet Protocol Overview

Telnet is a foundational protocol for remote communication, officially defined in 1973, though its origins trace back to 1969 (RFC 15). Its age is evident in its design: Telnet transmits all information, including usernames and passwords, in plain text over the network. In the 1970s, this was acceptable due to limited network threats, but today, it’s a significant security risk. Telnet predates TCP, UDP, and IP, making it a pioneer in network communication, but outdated for modern security needs.

Telnet as a Client-Server Protocol

Telnet is a client-server protocol where the server listens on port 23, and clients connect to control remote systems as if physically present. Standardized in RFC 206, Telnet was one of the first protocols for remote login, influencing modern protocols like SSH. It was commonly used to log into remote computers via a virtual terminal. Today, SSH has largely replaced Telnet for secure remote access.

Technical Details of Telnet

Telnet uses 8-bit chunks to transmit 7-bit ASCII data, reserving one bit for control characters. This supports basic ASCII characters but fails with extended 8-bit ASCII or Unicode (e.g., emojis), as the eighth bit is interpreted as a command. Telnet’s negotiation process involves commands like IAC (Interpret as Command, hex code FF) to initiate sequences, followed by commands such as WILL (FB), WON’T (FC), DO (FD), and DON’T (FE) to negotiate options like terminal type or window size. These commands enable the client and server to agree on session parameters.

Telnet Extensions

Telnet supports option negotiation, allowing extensions like Transport Layer Security (TLS) for encryption and Simple Authentication and Security Layer (SASL) for authentication. However, these extensions have seen poor adoption due to Telnet’s widespread use in legacy systems without updates.

Modern Use Cases for Telnet

Telnet’s primary modern use is manual debugging of text-based protocols like HTTP or SMTP. For example, you can debug an HTTP server by running telnet shotecamera.com 80 and typing GET / HTTP/1.1 followed by two Enter key presses to retrieve a webpage’s headers. Other tools like Netcat are often preferred for debugging due to greater flexibility. Telnet persists in legacy systems, IoT devices, and educational settings for teaching network fundamentals.

Security Concerns with Telnet

Telnet’s lack of encryption makes it insecure. You should generally avoid enabling Telnet servers, as they transmit credentials in plain text, leaving them vulnerable to eavesdropping and man-in-the-middle attacks. The 2016 Mirai botnet exploited Telnet’s vulnerabilities, targeting IoT devices and routers with open Telnet ports and weak or default credentials, highlighting its risks. SSH offers a secure alternative, as discussed below.

SSH Protocol Overview {#ssh-overview}

The SSH (Secure Shell) protocol, developed in 1995 by Tatu Ylönen, was created in response to security flaws in protocols like Telnet and rlogin. After discovering a password sniffer on his university’s network, Ylönen designed SSH to encrypt data, ensuring secure remote access over unsecured networks. Unlike Telnet, SSH protects usernames, passwords, and data, making it the standard for remote administration.

Importance of Secure Connections

Telnet and rlogin send packets in plain text, which was less concerning in the early 1990s when networks were smaller and often local. As internet usage grew, plaintext connections became a liability, especially for critical systems like web servers. SSH encrypts data, preventing sniffers from reading credentials or commands, even on insecure networks. This is critical for administrators managing servers with sensitive data, where compromised credentials could cause significant damage.

Technical Details of SSH Connections

SSH operates over TCP, typically on port 22, though it can use WebSockets in some cases. An SSH connection begins with a TCP handshake, followed by a key exchange (e.g., Diffie-Hellman) to generate session keys for encryption. The packet structure includes:

• Packet Length (4 bytes): Indicates the packet size.
• Padding Length (1 byte): Specifies the size of random padding.
• Payload: The encrypted data (e.g., commands or file transfers).
• Padding: Random bytes to obscure data patterns.
• Message Authentication Code (MAC): Ensures data integrity and authenticity.

The encryption algorithm (e.g., AES) and key exchange method are negotiated between client and server, with weaker algorithms like RC4 deprecated (RFC 8758). Padding makes traffic analysis harder, similar to mixing decoy packages with real ones.

SSH Channels and Multiplexing

SSH supports multiple channels, enabling simultaneous connections (e.g., uploading data, editing files, and tunneling). For example, you can open multiple terminal tabs to the same server, each performing different tasks. SSH also supports X11 forwarding for graphical applications and tunneling for secure access to services behind firewalls.

Practical SSH Example

To illustrate SSH’s capabilities, consider connecting to a remote Linux server to check disk usage and set up a local port forwarding tunnel:

Connect to the Server:
ssh user@192.168.1.100

1.  Enter the password or use public key authentication to access the server’s command-line interface.

Check Disk Usage:
df -h

2.  This displays disk usage in a human-readable format, e.g., Filesystem Size Used Avail Use% Mounted on /dev/sda1 50G 15G 35G 30% /.

Set Up Local Port Forwarding:
ssh -L 8080:localhost:80 user@192.168.1.100

3.  Access the remote web server at http://localhost:8080 on your local machine. The connection remains active until terminated (Ctrl+C).

Optional: Public Key Authentication:
Generate a key pair:
ssh-keygen -t rsa -b 4096

 Copy the public key to the server:
ssh-copy-id user@192.168.1.100

 Test passwordless login:
ssh user@192.168.1.100

This example demonstrates SSH’s secure remote access and tunneling features, ideal for system administration.

Telnet vs. SSH: Key Differences

The following table summarizes the key differences between Telnet and SSH, highlighting why SSH is the preferred choice for modern remote access:

Unlike Telnet’s plaintext transmission, SSH’s encryption and robust authentication make it the go-to choice for secure remote access, as seen in its support for secure file transfers (SCP, SFTP) and tunneling.

When to Use Telnet vs. SSH

Telnet is acceptable only in specific scenarios, such as debugging text-based protocols (e.g., telnet buzzvista.com 80 to test an HTTP server) or managing isolated legacy systems within a secure network, ideally protected by a VPN. However, its lack of encryption makes it unsuitable for public networks or sensitive systems. 

SSH is the preferred choice for nearly all remote access needs due to its security features, supporting tasks like system administration, secure file transfers, and tunneling. For example, use SSH for managing cloud servers, automating DevOps workflows, or accessing services behind firewalls via commands like ssh -L 8080:localhost:80 user@remote-server.

Conclusion

This article has explored Telnet and SSH, two protocols for remote access. Telnet, despite its historical significance, is insecure due to plaintext transmission, making it suitable only for niche debugging or legacy systems with additional security measures. SSH, with its encryption and versatile features, is the standard for secure remote administration, file transfers, and tunneling. For further learning, explore our guide on setting up SSH keys or share your experiences with Telnet and SSH in the comments below. Stay secure and keep your network connections encrypted! If you found this article useful, please share it with your network.