How SSH works?

 SSH (Secure Shell) is a cryptographic protocol used for securely accessing and managing remote systems over a network. It ensures confidentiality, integrity, and authentication of communication between two machines. Here's a breakdown of how SSH works:

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Key Concepts of SSH

1. Encryption: Ensures that all data exchanged between the client and server is encrypted, making it unreadable to third parties.
2. Authentication: Verifies the identities of both the client and server.
3. Integrity: Ensures that the transmitted data hasn't been tampered with during transit.

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Steps in an SSH Connection

1. Establishing a Connection:

   • The client initiates the SSH connection to the server on port 22 (default SSH port).
   • The client and server perform a handshake to agree on the encryption algorithms and exchange cryptographic keys.

2. Server Authentication:

   • The server proves its identity by sending its public key to the client.
   • The client verifies the server's identity using a known-hosts file (stored at ~/.ssh/known_hosts) or prompts the user to trust the server if it’s connecting for the first time.

3. Key Exchange and Session Encryption:

   • After verifying the server's identity, the client and server perform a key exchange (using protocols like Diffie-Hellman or ECDH) to agree on a shared secret key.
   • This shared secret is used to encrypt the session, ensuring confidentiality.

4. Client Authentication:

   • The client proves its identity to the server using one of several methods:
     • Password-based Authentication: The client sends a password securely over the encrypted channel.
     • Key-based Authentication: The client sends a public key, and the server checks if it matches an authorized key in ~/.ssh/authorized_keys. If it does, the server sends a challenge encrypted with the client’s private key, which the client must decrypt to prove ownership of the private key.

5. Session Establishment:

   • Once authentication is complete, a secure session is established.
   • The client can now execute commands on the remote server, transfer files, or use port forwarding, all over the encrypted connection.

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Key Components in SSH

1. SSH Protocol:

   • Built on a client-server model.
   • Uses protocols like RSA, ECDSA, or Ed25519 for authentication, and AES for encrypting the communication.

2. Public/Private Key Pair (Key-Based Authentication):

   • Public Key: Shared with the server and stored in ~/.ssh/authorized_keys.
   • Private Key: Stored securely on the client side (e.g., ~/.ssh/id_rsa or ~/.ssh/id_ed25519).

3. Known Hosts:

   • The file ~/.ssh/known_hosts stores the public keys of previously connected servers to verify their identity on subsequent connections.

4. Port Forwarding:

   • SSH can forward ports securely, allowing the client to access resources on the remote server’s network.

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Benefits of SSH

• Secure Communication: All data exchanged is encrypted, preventing eavesdropping.
• Authentication Options: Supports password-based or key-based authentication.
• Port Forwarding: Securely tunnels other network services.
• File Transfer: Can use scp or sftp for secure file transfers.

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Example SSH Command

ssh username@remote_server

• Connects to the remote_server as the specified username.
• If key-based authentication is set up, no password is needed.

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Secure Shell (SSH) creates an encrypted channel between client and server.

The process begins with a TCP connection, followed by version negotiation. Both parties then agree on encryption algorithms, key exchange methods, and message authentication codes.

The client and server perform a key exchange (typically using Diffie-Hellman) to securely generate a shared session key for encrypting the connection.

For authentication, SSH commonly uses public key authentication. The server verifies the client's identity through a challenge-response mechanism using the client's public key, without the private key ever being transmitted.

Once authenticated, the session key encrypts all further communication, providing a secure channel.

Summary of SSH Workflow:

1. Client initiates a connection to the SSH server on port 22.
2. The server sends its public key to the client.
3. The client and server exchange keys to establish a shared secret for encryption.
4. The client is authenticated using either a password or public key.
5. A secure, encrypted channel is established for data transmission.
6. The client interacts with the server securely over the encrypted channel.
7. The connection is closed after use.

SSH is widely used for remote server management, secure file transfer (via SFTP or SCP), and secure communications