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Question

What is OpenSSH and what are its main uses?

Answer

OpenSSH is the open-source implementation of SSH (Secure Shell): an encrypted channel for logging into and running commands on a remote machine.

It replaced the old cleartext tools (telnet, rsh, rlogin) that sent passwords and keystrokes in the open. Everything between client and server is encrypted, so an eavesdropper on the network sees only ciphertext. Default port is 22/TCP.

Main uses:

Use Case	Description
Remote shell	Interactive command-line access to remote systems
Remote commands	Execute single commands without interactive shell
File transfer	Secure copy with `scp` or `sftp`
Port forwarding	Tunnel other protocols through SSH
X11 forwarding	Run graphical apps remotely

Basic connection:

ssh username@hostname
ssh student@192.168.1.100

Key features:

Encrypted communication (replaces telnet, rsh)
Password or key-based authentication
Default port: 22

Tip: SSH is essential for Linux administration - servers typically have no GUI, only SSH access.

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Question

How does SSH public key authentication work?

Answer

The client proves its identity by signing a challenge with its private key; the server verifies that signature using the matching public key it already trusts — no password ever crosses the wire.

Key authentication rests on an asymmetric key pair: a private key that never leaves the client, and a public key that is freely copied to every server you want to reach. The two are mathematically linked — anything signed by the private key can be verified by the public key, but you cannot derive the private key from the public one.

What actually happens at login:

The server finds your public key in its ~/.ssh/authorized_keys and sends a random challenge.
The client signs that challenge with its private key (the key may itself be passphrase-protected).
The server verifies the signature against the stored public key. A valid signature proves the client holds the private key, so it grants access.

(This is separate from the host key step earlier in the handshake, where the server proves its identity to the client and gets recorded in known_hosts.)

Key locations:

Key	Location	Shared?
Private key	Client: `~/.ssh/id_rsa`	Never share!
Public key	Server: `~/.ssh/authorized_keys`	Safe to share

Why use keys over passwords?

More secure (can't be brute-forced)
Enables passwordless automation
Can be revoked per-device
Required by many production servers

Generate keys:

ssh-keygen -t rsa -b 4096
ssh-copy-id user@server

Go deeper:

sshd(8) — AUTHENTICATION & AUTHORIZED_KEYS — the public-key auth flow and the authorized_keys / known_hosts file formats.

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