Written by Christian Ahmer | 11/17/2023

SSH (Secure Shell)

SSH, short for Secure Shell, is a protocol for secure remote login and other secure network services over an insecure network. Developed by Tatu Ylonen in 1995, SSH has become a standard for secure system administration and file transfers via unsecured networks like the internet.

SSH operates on a client-server model, where the 'SSH client' uses the 'SSH protocol' to connect to an 'SSH server' installed on a remote machine. The key feature of SSH is its use of encryption to ensure the confidentiality and integrity of data over an insecure network. By encrypting the data, SSH provides a secure channel over an unsecured network in a client-server architecture.

To initiate a secure session, the SSH client starts by contacting the server. The two then perform a version exchange to determine the most compatible version of the SSH protocol to use. Following this, they engage in a process known as the SSH handshake, which establishes the encrypted session. The handshake involves several steps:

  1. Algorithm Negotiation: The client and server exchange lists of supported algorithms for various tasks and agree on the algorithms that they will use.

  2. Key Exchange: They then use these algorithms to securely exchange keys and establish a shared secret through a process known as a key exchange.

  3. Host Authentication: The client verifies the server's identity. The server can present a digital certificate or use a host key that the client has already accepted in previous sessions.

  4. User Authentication: After verifying the server, the client must authenticate itself. This can be done using passwords, but more secure methods like public-key authentication are common.

  5. Channel Setup: Finally, the client and server establish a secure channel. All data passed through this channel is encrypted according to the agreed-upon algorithms.

The SSH protocol also supports various methods of user authentication. The most common methods are:

  • Password Authentication: The simplest form, where the user provides a username and password to log in.

  • Public Key Authentication: A more secure method where the SSH client proves its identity by using a cryptographic key rather than a password.

  • Host-based Authentication: This method uses the identity of the client host to authenticate a user.

  • Keyboard-interactive Authentication: An extensible method that can support different types of authentication mechanisms, such as one-time passwords or challenge-response systems.

Once the session is established, SSH allows for several functionalities including the execution of commands on the remote server, port forwarding, and file transfers. Port forwarding allows users to secure the transfer of data for other applications, while file transfers are usually handled by SFTP (SSH File Transfer Protocol) or SCP (Secure Copy Protocol).

SSH also supports dynamic port forwarding, creating a secure tunnel for other protocol traffic. This feature can secure otherwise insecure protocols like SMTP (email) or HTTP and is often used to bypass firewalls and create a virtual private network (VPN)-like experience.

SSH's robust security features, ease of use, and broad support across operating systems have made it the de facto standard for secure remote management. It is widely used in nearly all corporate environments, academic institutions, and by individual users to securely manage systems and transfer files across the internet.