What is SSH?

SSH, or Secure Shell, is a cryptographic network protocol used for secure communication over an unsecured network. It provides a secure channel over an unsecured network by using encryption techniques to ensure that communication between two computers is protected from eavesdropping and manipulation. With this we are able to log into a server, execute commands, and transfer files securely.

Here’s how it works:

• Establishing Connection: When you initiate an SSH connection from your computer to a remote server, your SSH client (such as OpenSSH) contacts the server’s SSH daemon (sshd).
• Key Exchange: The client and server perform a key exchange process to establish a secure connection. This involves generating cryptographic keys that will be used for encrypting the data exchanged during the session. This typically involves a two-step process:

Diffie-Hellman key exchange: This establishes a shared secret key that only the client and server can calculate, even if someone is eavesdropping on the communication.

Public-key authentication (optional): The server might send its public key, which the client uses to encrypt messages and verify the server’s identity. The client might also offer its public key for the server to verify its identity (two-factor authentication).

• Encryption: Once the key exchange is complete, the client and server use symmetric encryption to encrypt the data transmitted over the connection. This encryption ensures that even if someone intercepts the data, they cannot read it without the encryption key.
• Authentication: SSH supports various methods for user authentication, including password-based authentication, public key authentication, and more advanced methods like multi-factor authentication. This step ensures that only authorized users can access the server.
• Data Transfer: After the connection is established and authenticated, data can be transmitted securely between the client and server. This can include commands executed on the server, files transferred between the client and server, or even tunnelling other network protocols securely over the SSH connection.
• Session Management: Throughout the SSH session, the client and server maintain a secure connection, allowing the user to interact with the remote system securely.

Benefits of SSH:

• Security: Encrypts communication, protecting usernames, passwords, commands, and transferred data from unauthorized access.
• Versatility: Allows remote access to various operating systems and supports different functionalities (command execution, file transfer).
• Efficiency: Provides a reliable and efficient way to manage remote systems.

History of SSH:

• 1995: Developed by Tatu Ylonen in Finland after his university network was compromised due to a password-sniffing attack.
• SSH-1: The initial version of SSH used a proprietary encryption algorithm.
• Late 1990s: SSH gained popularity due to its focus on security compared to insecure protocols like Telnet.
• 1998: The IETF (Internet Engineering Task Force) began standardizing SSH, leading to the creation of SSH-2.
• SSH-2: Adopted stronger encryption algorithms (AES, 3DES) and improved authentication methods (public-key authentication).
• Present Day: SSH-2 is the widely used standard for secure remote access. Secure Shell continues to evolve with new features and refinements for enhanced security.

co-authored this article with Risinu_Wijesinghe