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.