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SSH in Linux: A Comprehensive Overview

SSH, or Secure Shell, is a cryptographic network protocol predominantly used for securely executing commands and managing files on remote servers. However, the journey of SSH starts much before its prevalent use today. Let's explore its history, what it is, its relation to FTP, and its underlying theoretical mechanisms.

History of SSH

SSH was invented by Tatu Ylönen in 1995 after a password-sniffing attack at his university network in Finland. Recognizing the need for a secure way to log into and administer remote servers, Ylönen created the first version of SSH (now known as SSH-1) and freely released it to the public. As its adoption grew, further versions were developed, leading to the creation of SSH-2, which addressed various vulnerabilities and shortcomings in the initial version.

What is SSH?

SSH allows for secure remote login from one computer to another. It provides encrypted communication over an insecure network, ensuring that transmitted data, including passwords and other sensitive information, remains confidential. SSH facilitates various other services as well, including secure file transfers and port forwarding.

Why SSH when there's FTP?

While FTP (File Transfer Protocol) has been a standard for file transfers, it had notable security vulnerabilities:

  • Unencrypted Communication: FTP sends data, including passwords, in plain text, making it susceptible to eavesdropping.
  • Multiple Ports: FTP requires multiple ports (control and data), which can be problematic for firewall configurations.

SSH, on the other hand, provides encrypted communication, thereby addressing these vulnerabilities. SFTP (Secure FTP), which runs over the SSH protocol, is a direct replacement for FTP, offering both security and file transfer capabilities.

SSH Data Flow

The process of establishing an SSH connection involves several steps, ensuring both authentication and encryption:

Step-by-Step Explanation of SSH Data Flow

  1. Initiate Connection Request:

    • Client → Server: The SSH client initiates the connection by sending a request to the SSH server. This is the first step to establish an SSH session.
  2. Server's Public Key Transmission:

    • Server → Client: In response, the SSH server sends its public key to the client. This key will be used by the client to encrypt data that only the server can decrypt using its private key.
  3. Verification of Server's Public Key:

    • Client: The client checks the received public key. If the key is already in its list of known hosts, the connection proceeds. If not, the client will either prompt the user for confirmation (to avoid man-in-the-middle attacks) or may terminate the connection, based on the configuration.
  4. Encrypt Random Number:

    • Client → Server: The client generates a random number, encrypts it using the server's public key, and sends it to the server.
  5. Decrypt Received Number:

    • Server: The server uses its private key to decrypt the received number. This proves to the client that it's communicating with the legitimate server since only the true server with the corresponding private key can correctly decrypt the message.
  6. Send a Challenge:

    • Server → Client: The server sends a challenge based on the decrypted number to the client. This challenge ensures mutual authentication.
  7. Compute and Verify Challenge:

    • Client: The client computes and verifies the challenge. If successful, the client knows it's safe to send authentication data to the server.
  8. Send Authentication:

    • Client → Server: The client sends its authentication data to the server. This could be in the form of a password or an RSA key for key-based authentication.
  9. Establish Encrypted Session:

    • Server → Client: Once the server successfully authenticates the client, an encrypted SSH session is established between them.
  10. Encrypted Communication:

  • The communication between the client and server is now encrypted, ensuring data confidentiality and integrity. Both parties can now securely exchange messages, commands, or files over this encrypted channel.

This sequence of steps ensures a secure and authenticated connection establishment, leveraging public key cryptography and challenge-response mechanisms, which are central to the SSH protocol.

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