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SSH

SSH (Secure Shell) is a cryptographic network protocol used to securely access remote systems over an unsecured network. It ensures confidentiality, integrity, and authentication for data communication between a client and a server. SSH is widely used for remote login, file transfers, and secure command execution.

Key Features of SSH​

  1. Encryption: Ensures that the communication between the client and server is encrypted, preventing eavesdropping.
  2. Authentication: Verifies the identity of the client and the server.
  3. Data Integrity: Ensures that the data transmitted is not tampered with during transit.
  4. Port Forwarding: Supports tunneling other protocols through SSH.
  5. File Transfer: Provides secure file transfer mechanisms (e.g., SCP, SFTP).

How SSH Works​

Key Components of SSH​

  • Client: The machine initiating the SSH connection.
  • Server: The machine hosting the service and listening for incoming SSH connections.
  • Port: SSH uses port 22 by default.
  • Authentication Keys:
    • Public Key: Shared with the server for authentication.
    • Private Key: Kept secret by the client for decryption.

Steps in SSH Communication​

Step 1: Establishing the TCP Connection​

  • The client sends a request to the SSH server over TCP (default port 22).
  • The server responds and establishes a connection.

Step 2: Negotiating Protocol Version​

  • The client and server exchange their SSH protocol versions.
  • The server sends its supported cryptographic algorithms (e.g., encryption, hashing).

Step 3: Key Exchange​

The Diffie-Hellman Key Exchange (or similar algorithm) is used to securely agree on a shared symmetric key for encryption.

  1. The client and server exchange public parameters required for the Diffie-Hellman algorithm.
  2. Both sides compute a shared secret key using their private parameters.
  3. The resulting shared secret key is used for symmetric encryption.

Here:

  • p is a prime number.
  • g is a generator.
  • a and b are private keys of the client and server, respectively.

Step 4: Server Authentication​

  • The server sends its public key to the client.
  • The client verifies the server's identity by comparing the server's public key with its known hosts.

Step 5: Client Authentication​

The client authenticates itself using one of the following methods:

  • Password-Based Authentication: The client provides a username and password.
  • Public Key Authentication: The client signs a challenge using its private key, and the server verifies it using the client's public key.

Step 6: Secure Communication​

  • Once authentication is successful, a secure, encrypted session is established.
  • All further communication between the client and server is encrypted using the shared symmetric key.

Types of Authentication in SSH​

  1. Password Authentication:
    • The user enters a password, which is sent securely over the encrypted session.
    • Simple but less secure than public key authentication.
  2. Public Key Authentication:
    • The client and server exchange cryptographic keys for authentication.
    • More secure because it eliminates the need to send passwords over the network.

Encryption and Security​

  • Symmetric Encryption: After the handshake, symmetric encryption (e.g., AES, ChaCha20) is used for faster communication.
  • HMAC (Hash-based Message Authentication Code): Ensures data integrity.

Advantages of SSH​

  1. Strong encryption and authentication.
  2. Protects against man-in-the-middle attacks.
  3. Supports tunneling other protocols.

Common SSH Tools​

  1. OpenSSH: Widely used SSH implementation.
  2. PuTTY: A popular SSH client for Windows.
  3. WinSCP: SSH-based file transfer tool.