SSH key generation (ssh-keygen) in Linux CLI - Mini Project: Build & Apply

SSH Key Generation with ssh-keygen

📖 Scenario: You want to securely connect to a remote server without typing your password every time. To do this, you will create an SSH key pair on your local machine.

🎯 Goal: Learn how to generate an SSH key pair using the ssh-keygen command and display the public key.

📋 What You'll Learn

Use the ssh-keygen command to create an SSH key pair

Specify the key type as RSA

Save the key with the filename my_ssh_key

Do not use a passphrase (empty passphrase)

Display the contents of the public key file

💡 Why This Matters

🌍 Real World

SSH keys are used to securely connect to servers without typing passwords every time, making remote work faster and safer.

💼 Career

System administrators, developers, and IT professionals use SSH keys daily to manage servers and deploy applications securely.

1

Create an SSH key pair

Run the ssh-keygen command with the options to generate an RSA key and save it as my_ssh_key. When prompted for a passphrase, press Enter twice to leave it empty.

Linux CLI

# Run ssh-keygen to create an RSA key named my_ssh_key
# Your command here

Hint

The -t option sets the key type, -f sets the filename, and -N sets the passphrase.

2

Verify the key files exist

Use the ls command to list the files starting with my_ssh_key in the current directory.

Linux CLI

ssh-keygen -t rsa -f my_ssh_key -N ""
# List the files named my_ssh_key and my_ssh_key.pub
# Your command here

Hint

The public key file will have the same name with .pub appended.

3

Display the public key content

Use the cat command to show the contents of the public key file my_ssh_key.pub.

Linux CLI

ssh-keygen -t rsa -f my_ssh_key -N ""
ls my_ssh_key*
# Display the contents of my_ssh_key.pub
# Your command here

Hint

The public key is a long string starting with ssh-rsa.

4

Show the public key output

Run the full script to generate the key, list the files, and display the public key content. The output should show the public key starting with ssh-rsa.

Linux CLI

ssh-keygen -t rsa -f my_ssh_key -N ""
ls my_ssh_key*
cat my_ssh_key.pub
# Run the commands above and observe the output

Hint

The output should include a line starting with ssh-rsa followed by a long string.

Practice

1. What is the primary purpose of the ssh-keygen command in Linux?

easy

A. To start an SSH server on the local machine

B. To generate a pair of cryptographic keys for SSH authentication

C. To connect to a remote server using SSH

D. To display the current SSH connections

Solution

1. Step 1: Understand the function of ssh-keygen

   The ssh-keygen command is used to create a pair of keys: a private key and a public key, which are used for secure SSH authentication.

2. Step 2: Differentiate from other SSH commands

   Starting an SSH server or connecting to a server uses different commands like sshd or ssh. Displaying connections is unrelated.

3. Final Answer:

   To generate a pair of cryptographic keys for SSH authentication -> Option B

4. Quick Check:

   ssh-keygen creates keys = B [OK]

Hint: ssh-keygen always creates keys, not connections or servers [OK]

Common Mistakes:

• Confusing ssh-keygen with ssh command
• Thinking ssh-keygen starts SSH server
• Assuming ssh-keygen shows active connections

2. Which of the following is the correct syntax to generate an RSA SSH key pair with ssh-keygen?

easy

A. ssh-keygen -t rsa

B. ssh-keygen -g rsa

C. ssh-keygen --generate rsa

D. ssh-keygen -type rsa

Solution

1. Step 1: Identify the correct option flag for key type

   The -t flag specifies the type of key to generate, such as rsa, dsa, or ed25519.

2. Step 2: Check the syntax correctness

   Only ssh-keygen -t rsa is valid syntax. Other options like -g, --generate, or -type are invalid or do not exist.

3. Final Answer:

   ssh-keygen -t rsa -> Option A

4. Quick Check:

   -t specifies type = D [OK]

Hint: Use -t to specify key type in ssh-keygen [OK]

Common Mistakes:

• Using -g instead of -t for key type
• Adding extra words like --generate
• Using -type instead of -t

3. What will be the output of the following command if the default key files already exist?

ssh-keygen -t ed25519

medium

A. It throws an error and exits immediately

B. It overwrites the existing key files without warning

C. It prompts to overwrite the existing files or choose a new name

D. It generates a new key with a random filename automatically

Solution

1. Step 1: Understand ssh-keygen behavior on existing files

   When key files already exist, ssh-keygen asks the user if they want to overwrite or specify a new filename.

2. Step 2: Confirm it does not overwrite silently or error out

   It does not overwrite without confirmation, nor does it throw an error or generate random filenames automatically.

3. Final Answer:

   It prompts to overwrite the existing files or choose a new name -> Option C

4. Quick Check:

   ssh-keygen asks before overwrite = A [OK]

Hint: ssh-keygen asks before overwriting existing keys [OK]

Common Mistakes:

• Assuming it overwrites files silently
• Expecting an error instead of prompt
• Thinking it auto-generates random filenames

4. You ran ssh-keygen -t rsa -b 4096 but got an error: unknown option: -b. What is the most likely cause?

medium

A. The command requires sudo privileges to use -b

B. The -b option is invalid for RSA keys

C. You must specify the key filename before the -b option

D. You used an outdated ssh-keygen version that does not support the -b option

Solution

1. Step 1: Understand the -b option usage

   The -b option sets the number of bits for the key size and is valid for RSA keys.

2. Step 2: Identify why the error occurs

   An unknown option error usually means the ssh-keygen version is old and does not recognize the -b flag.

3. Final Answer:

   You used an outdated ssh-keygen version that does not support the -b option -> Option D

4. Quick Check:

   Old ssh-keygen lacks -b support = A [OK]

Hint: Check ssh-keygen version if options cause unknown errors [OK]

Common Mistakes:

• Thinking -b is invalid for RSA
• Believing filename order matters for options
• Assuming sudo is needed for key size

5. You want to automate SSH key generation in a script without any prompts, saving the key as ~/.ssh/id_ed25519_custom. Which command correctly achieves this?

hard

A. ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_custom -N ''

B. ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_custom

C. ssh-keygen -t ed25519 -N ''

D. ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_custom -y

Solution

1. Step 1: Understand flags for silent key generation

   The -f flag sets the filename, and -N '' sets an empty passphrase to avoid prompts.

2. Step 2: Check each option for automation suitability

   ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_custom -N '' uses both -f and -N '' correctly. ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_custom misses -N so it prompts for passphrase. ssh-keygen -t ed25519 -N '' misses filename, so default is used. ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_custom -y uses -y which outputs public key but does not generate keys silently.

3. Final Answer:

   ssh-keygen -t ed25519 -f ~/.ssh/id_ed25519_custom -N '' -> Option A

4. Quick Check:

   Use -f for file and -N '' for no passphrase = C [OK]

Hint: Use -f for file and -N '' for no passphrase to avoid prompts [OK]

Common Mistakes:

• Omitting -N '' causes passphrase prompt
• Forgetting to specify filename with -f
• Using -y which only outputs public key