SSH key generation (ssh-keygen) 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 mykey -N "" in a Linux terminal. What output will you see immediately after?

Linux CLI

ssh-keygen -t rsa -b 2048 -f mykey -N ""

A Error: Invalid option -b 2048

B ssh-keygen: command not found

C

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

D

Generating DSA key pair.
Your identification has been saved in mykey.
Your public key has been saved in mykey.pub.

💡 Hint

Think about what ssh-keygen outputs when generating an RSA key with no passphrase.

The command generates an RSA key pair with 2048 bits, saves it to 'mykey' and 'mykey.pub', and prints the fingerprint and randomart image. Option C matches this output exactly.

💻 Command Output

intermediate

What error does this ssh-keygen command produce?

You run ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519 -N without specifying a passphrase value. What error message appears?

Linux CLI

ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519 -N

A Permission denied: cannot write to ~/.ssh/id_ed25519

B

Generating public/private ed25519 key pair.
Your identification has been saved in ~/.ssh/id_ed25519.

C Error: Unknown key type ed25519

D

ssh-keygen: option requires an argument -- 'N'
Usage: ssh-keygen [options]

💡 Hint

The -N option requires a passphrase argument, even if empty.

The -N option must be followed by a passphrase string. Omitting it causes an error about missing argument.

📝 Syntax

advanced

Which ssh-keygen command correctly generates a 4096-bit RSA key with a comment?

Select the command that generates a 4096-bit RSA key with the comment '[e-mail]' and saves it to 'admin_rsa'.

A ssh-keygen -t rsa -b 4096 -C [e-mail] -f admin_rsa

B ssh-keygen -t rsa -b 4096 -comment [e-mail] -file admin_rsa

C ssh-keygen --type rsa --bits 4096 --comment '[e-mail]' --file admin_rsa

D ssh-keygen -type rsa -bits 4096 -C '[e-mail]' -f admin_rsa

💡 Hint

Check the correct ssh-keygen option names for comment and file.

Option A uses the correct short options: -t for type, -b for bits, -C for comment, and -f for file. Other options use invalid or incorrect flags.

🚀 Application

advanced

How to automate ssh-keygen to generate a key without prompts?

You want to write a script that generates an ed25519 SSH key named 'deploy_key' with no passphrase and no user interaction. Which command achieves this?

A ssh-keygen -t ed25519 -f deploy_key

B ssh-keygen -t ed25519 -f deploy_key -N "" -q

C ssh-keygen -t ed25519 -f deploy_key -P ""

D ssh-keygen -t ed25519 -f deploy_key -n

💡 Hint

Use options to suppress prompts and specify empty passphrase.

Option B uses -N "" to set empty passphrase and -q to suppress output, so it runs without prompts. Others either prompt or use invalid options.

🧠 Conceptual

expert

What is the effect of the -o option in ssh-keygen?

You run ssh-keygen -t rsa -b 2048 -o -f keyfile. What does the -o option do?

A It saves the private key using the new OpenSSH format instead of the older PEM format.

B It overwrites the existing keyfile without asking for confirmation.

C It outputs the public key to standard output instead of a file.

D It optimizes the key generation speed by using fewer random bytes.

💡 Hint

Think about key file formats and security improvements.

The -o option tells ssh-keygen to save the private key in the new OpenSSH format, which is more secure than the older PEM format.