Key-based authentication in Linux CLI - Mini Project: Build & Apply

Key-based authentication

📖 Scenario: You want to securely connect to a remote server without typing your password every time. Key-based authentication lets you do this by using a pair of keys: a private key on your computer and a public key on the server.This project will guide you through creating a key pair, copying the public key to the server, and testing the connection.

🎯 Goal: Set up key-based SSH authentication to a remote server so you can log in without a password.

📋 What You'll Learn

Create an SSH key pair with ssh-keygen

Copy the public key to the remote server using ssh-copy-id

Test the SSH connection without a password prompt

💡 Why This Matters

🌍 Real World

Key-based authentication is used daily by system administrators and developers to securely access servers without typing passwords.

💼 Career

Knowing how to set up SSH keys is essential for roles in IT support, DevOps, cloud engineering, and cybersecurity.

1

Generate SSH key pair

Use the command ssh-keygen -t rsa -b 2048 -f ~/.ssh/id_rsa -N "" to create an RSA key pair with no passphrase.

Linux CLI

# Run the ssh-keygen command to create the key pair
# Your code here

Hint

This command creates a 2048-bit RSA key pair without a password. The private key is saved as ~/.ssh/id_rsa and the public key as ~/.ssh/id_rsa.pub.

2

Copy public key to remote server

Use the command ssh-copy-id -i ~/.ssh/id_rsa.pub user@remote_host to copy your public key to the remote server. Replace user and remote_host with the actual username and server address.

Linux CLI

ssh-keygen -t rsa -b 2048 -f ~/.ssh/id_rsa -N ""
# Run ssh-copy-id to copy the public key to the remote server
# Your code here

Hint

This command copies your public key to the remote server's ~/.ssh/authorized_keys file, enabling key-based login.

3

Test SSH connection without password

Use the command ssh user@remote_host to connect to the remote server. You should not be asked for a password.

Linux CLI

ssh-keygen -t rsa -b 2048 -f ~/.ssh/id_rsa -N ""
ssh-copy-id -i ~/.ssh/id_rsa.pub user@remote_host
# Connect to the remote server using ssh
# Your code here

Hint

If set up correctly, this command logs you in without asking for a password.

4

Display success message

After connecting, run echo "Login successful without password" on the remote server to confirm key-based authentication works.

Linux CLI

ssh-keygen -t rsa -b 2048 -f ~/.ssh/id_rsa -N ""
ssh-copy-id -i ~/.ssh/id_rsa.pub user@remote_host
ssh user@remote_host
# Run echo command on remote server
# Your code here

Hint

This message confirms you logged in without typing a password.

Practice

1. What is the main purpose of key-based authentication in SSH?

easy

A. To encrypt files on the local computer

B. To allow secure login without typing a password every time

C. To speed up the internet connection

D. To create a backup of the server data

Solution

1. Step 1: Understand key-based authentication

   Key-based authentication uses a pair of keys to allow login without passwords.

2. Step 2: Identify the main benefit

   This method lets users connect securely without typing passwords each time.

3. Final Answer:

   To allow secure login without typing a password every time -> Option B

4. Quick Check:

   Key-based authentication = passwordless secure login [OK]

Hint: Key-based auth means no password typing needed [OK]

Common Mistakes:

• Confusing key-based auth with file encryption
• Thinking it speeds up internet
• Believing it backs up data

2. Which command correctly generates a new SSH key pair?

easy

A. ssh-keygen

B. ssh-copy-id

C. ssh-add

D. ssh-connect

Solution

1. Step 1: Identify the key generation command

   The command ssh-keygen creates a new SSH key pair.

2. Step 2: Differentiate from other commands

   ssh-copy-id copies keys, ssh-add adds keys to agent, ssh-connect is invalid.

3. Final Answer:

   ssh-keygen -> Option A

4. Quick Check:

   Generate keys = ssh-keygen [OK]

Hint: ssh-keygen always creates keys [OK]

Common Mistakes:

• Using ssh-copy-id to generate keys
• Confusing ssh-add with key creation
• Typing a non-existent ssh-connect command

3. What will happen if you run ssh-copy-id user@server after generating your SSH keys?

medium

A. The SSH service on the server restarts

B. Your private key is copied to the server

C. The server's keys are copied to your local machine

D. Your public key is copied to the server for passwordless login

Solution

1. Step 1: Understand ssh-copy-id purpose

   This command copies your public key to the server's authorized keys file.

2. Step 2: Identify what is copied

   Only the public key is copied, enabling passwordless login; private keys stay local.

3. Final Answer:

   Your public key is copied to the server for passwordless login -> Option D

4. Quick Check:

   ssh-copy-id copies public key to server [OK]

Hint: ssh-copy-id copies public key, not private [OK]

Common Mistakes:

• Thinking private key is copied
• Believing server keys copy locally
• Assuming ssh-copy-id restarts SSH

4. You tried to connect with SSH key authentication but still get a password prompt. Which is the most likely cause?

medium

A. The public key was not copied to the server

B. The private key file is missing on the server

C. The SSH server is turned off

D. The username is incorrect

Solution

1. Step 1: Check key placement

   If the public key is not on the server, SSH cannot authenticate without a password.

2. Step 2: Understand private key location

   The private key stays on the client, so missing it on the server is irrelevant.

3. Final Answer:

   The public key was not copied to the server -> Option A

4. Quick Check:

   Missing public key on server = password prompt [OK]

Hint: Public key must be on server for key login [OK]

Common Mistakes:

• Looking for private key on server
• Assuming SSH server off causes password prompt
• Blaming wrong username without checking keys

5. You want to automate SSH login to multiple servers without passwords. Which sequence of commands is correct?

hard

A. ssh user@server -> ssh-copy-id user@server -> ssh-keygen

B. ssh-copy-id user@server -> ssh-keygen -> ssh user@server

C. ssh-keygen -> ssh-copy-id user@server -> ssh user@server

D. ssh user@server -> ssh-keygen -> ssh-copy-id user@server

Solution

1. Step 1: Generate SSH keys first

   Use ssh-keygen to create your key pair locally.

2. Step 2: Copy public key to server

   Use ssh-copy-id user@server to add your public key to the server.

3. Step 3: Connect without password

   Now ssh user@server logs in using the key without asking for a password.

4. Final Answer:

   ssh-keygen -> ssh-copy-id user@server -> ssh user@server -> Option C

5. Quick Check:

   Generate keys, copy key, then connect [OK]

Hint: Generate keys first, then copy, then connect [OK]

Common Mistakes:

• Copying keys before generating them
• Trying to connect before copying keys
• Mixing command order