SSH Demystified: Building a Minimal SSH Implementation in Go
Introduction to SSH
Secure Shell (SSH) is one of those technologies that developers use almost daily, yet many of us don't understand how it actually works. If you've ever run ssh user@server to access a remote machine or used Git with SSH authentication, you're using this remarkable protocol that securely connects computers over an unsecured network.
But what's happening under the hood? I decided to find out by building my own simplified SSH implementation in Go, called TinySSH-Go.
How SSH Works (The Simple Version)
At its core, SSH establishes a secure channel between two computers through several key steps:
TCP Connection: First, a basic TCP connection is established on port 22 (or another configured port).
Version Exchange: The client and server share their SSH protocol versions.
Key Exchange: Both sides negotiate encryption parameters and exchange keys using algorithms like Diffie-Hellman, establishing a shared secret without ever transmitting the actual key.
Authentication: The client proves its identity, typically using passwords or cryptographic keys.
Channel Setup: After authentication, secure channels are created for transmitting commands and data.
Command Execution: Finally, commands are exchanged through these encrypted channels.
The Journey: Building TinySSH-Go
Starting with the fundamentals, I set up the project structure and implemented basic TCP connectivity between client and server. This laid the foundation for everything else to come.
Next came the SSH protocol version exchange ("SSH-2.0-TinySSH_Go") and binary packet encoding/decoding. I also implemented a keep-alive mechanism to maintain connections. The binary packet protocol was particularly interesting – each message is wrapped in a specific format with length fields, padding, and message types.
The security magic happens in the key exchange phase. I implemented a simplified Diffie-Hellman key exchange, allowing two parties to establish a shared secret over an unsecured channel. This was probably the most mathematically complex part of the project!
With secure communication established, I implemented password authentication along with a basic credential store. For security, I added brute-force protection to prevent password-guessing attacks.
SSH doesn't just create one connection – it multiplexes multiple logical channels over a single encrypted connection. I implemented channel creation, management, and flow control with window size adjustments.
Finally, I added the ability to execute commands remotely and handle their input/output streams. This is where SSH becomes truly useful – enabling secure remote command execution.
Learnings.....
Building TinySSH-Go taught me several valuable lessons:
Protocol Complexity: SSH is deceptively complex, with many moving parts and edge cases to handle.
Security First: Every design decision in SSH prioritizes security, which requires careful implementation.
Go's Network Handling: Go's standard library made networking tasks surprisingly straightforward.
Binary Protocols: Working directly with binary protocols requires careful byte manipulation and attention to detail.
Try It Yourself
If you're curious about SSH internals or want to see a simplified implementation for educational purposes, check out the TinySSH-Go project on GitHub:
Remember, this is an educational implementation – don't use it for production systems! But it's perfect for learning how SSH actually works behind the scenes.