Key-based authentication in Linux CLI - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this ssh-keygen command?

You run the command ssh-keygen -t rsa -b 2048 -f ~/.ssh/id_rsa_test -N "". What is the expected output?

Linux CLI

ssh-keygen -t rsa -b 2048 -f ~/.ssh/id_rsa_test -N ""

A

Generating public/private rsa key pair.
Your identification has been saved in /home/user/.ssh/id_rsa_test.
Your public key has been saved in /home/user/.ssh/id_rsa_test.pub.
The key fingerprint is:
SHA256:... user@hostname
The key's randomart image is:
+---[RSA 2048]----+
|    .o+*o.       |
|   .+o=+o.       |
|  . +o+o.        |
|   . +o.         |
|    . S          |
|                 |
|                 |
|                 |
|                 |
+----[SHA256]-----+

B ssh: Could not resolve hostname ~/.ssh/id_rsa_test: Name or service not known

C Permission denied (publickey).

D Error: invalid option -- 't'

💡 Hint

Think about what ssh-keygen does when creating a key pair with no passphrase.

The ssh-keygen command generates a new RSA key pair with 2048 bits, saves it to the specified file, and outputs the fingerprint and randomart image. The other options are error messages unrelated to this command.

💻 Command Output

intermediate

What is the output of this ssh-copy-id command?

You run ssh-copy-id -i ~/.ssh/id_rsa_test.pub user@remotehost. What output do you expect if the key is successfully copied?

Linux CLI

ssh-copy-id -i ~/.ssh/id_rsa_test.pub user@remotehost

A

Number of key(s) added: 1
Now try logging into the machine, with:   "ssh 'user@remotehost'"
and check to make sure that only the key(s) you wanted were added.

B ssh-copy-id: command not found

C Permission denied (publickey,password).

D Error: Could not open file ~/.ssh/id_rsa_test.pub

💡 Hint

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

ssh-copy-id copies the public key to the remote user's authorized_keys file and confirms the number of keys added. Other options are errors or permission issues.

🔧 Debug

advanced

Why does this SSH login fail with 'Permission denied (publickey)'?

You try to ssh into a server using key-based authentication but get this error: Permission denied (publickey). The public key is correctly in ~/.ssh/authorized_keys. What is the most likely cause?

Linux CLI

ssh user@remotehost

A The private key file on the client is missing the execute permission.

B The permissions on ~/.ssh or ~/.ssh/authorized_keys on the server are too open (e.g., 777).

C The sshd service on the server is not running.

D The remote server's IP address is blocked by a firewall.

💡 Hint

Check file and directory permissions on the server for SSH keys.

SSH requires strict permissions on ~/.ssh and authorized_keys (usually 700 for ~/.ssh and 600 for authorized_keys). Too open permissions cause SSH to reject keys for security.

🚀 Application

advanced

Which command sets up an SSH agent and adds your private key for authentication?

You want to avoid typing your SSH key passphrase repeatedly. Which command sequence correctly starts the SSH agent and adds your private key?

A ssh-agent & ssh-add ~/.ssh/id_rsa

B

ssh-agent start
ssh-add ~/.ssh/id_rsa

C

start ssh-agent
ssh-add ~/.ssh/id_rsa

D

eval $(ssh-agent -s)
ssh-add ~/.ssh/id_rsa

💡 Hint

The ssh-agent must be started in the current shell environment.

The correct way is to evaluate the output of ssh-agent to set environment variables, then add the key with ssh-add. Other options either use invalid commands or do not set environment variables properly.

🧠 Conceptual

expert

What is the main security advantage of using SSH key-based authentication over password authentication?

Choose the best explanation for why SSH key-based authentication is more secure than password authentication.

A SSH keys are longer and more complex than passwords, making them impossible to guess or crack.

B Passwords can be reused on multiple sites, but SSH keys are unique to each server.

C SSH key authentication uses asymmetric cryptography, so the private key never leaves the client, preventing interception and replay attacks.

D SSH keys automatically expire after a short time, reducing risk of compromise.

💡 Hint

Think about how the private key is used during authentication.

SSH key authentication uses a private key stored only on the client and a public key on the server. The private key is never sent over the network, preventing interception. Passwords are sent (even if encrypted) and can be guessed or phished.

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