SSH key generation (ssh-keygen) in Linux CLI - Step-by-Step Execution

Concept Flow - SSH key generation (ssh-keygen)

Start ssh-keygen command

↓

Prompt for file location

↓

Prompt for passphrase

↓

Generate key pair files

↓

Show key fingerprint and location

↓

End

The ssh-keygen command runs, asks where to save the key, asks for a passphrase, then creates the key files and shows their info.

Execution Sample

Linux CLI

ssh-keygen -t rsa -b 2048
# User presses Enter to accept default file
# User enters passphrase or empty
# Key files created

Generates an RSA SSH key pair with 2048 bits, saving to default location, optionally secured by a passphrase.

Execution Table

Step	Action	User Input	System Output	Result
1	Run ssh-keygen command		Generating public/private rsa key pair.	Starts key generation
2	Prompt for file location	Press Enter	Enter file in which to save the key (/home/user/.ssh/id_rsa):	Default path chosen
3	Prompt for passphrase	Enter passphrase or empty	Enter passphrase (empty for no passphrase):	Passphrase recorded or empty
4	Prompt for passphrase confirmation	Repeat passphrase or empty	Enter same passphrase again:	Passphrase confirmed
5	Generate keys		Your identification has been saved in /home/user/.ssh/id_rsa Your public key has been saved in /home/user/.ssh/id_rsa.pub The key fingerprint is: SHA256:abc123... user@host	Key files created
6	Exit			ssh-keygen process ends

💡 Key files created and saved, process ends

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
file_path	none	/home/user/.ssh/id_rsa	/home/user/.ssh/id_rsa	/home/user/.ssh/id_rsa	/home/user/.ssh/id_rsa
passphrase	none	none	user input or empty	confirmed passphrase or empty	stored securely

Key Moments - 2 Insights

Why does ssh-keygen ask twice for the passphrase?

It asks twice to make sure you typed the passphrase correctly, as shown in steps 3 and 4 in the execution table.

What happens if I just press Enter at the file location prompt?

Pressing Enter chooses the default file path for the key, as shown in step 2 where the default /home/user/.ssh/id_rsa is selected.

Visual Quiz - 3 Questions

Look at the execution table, what is the file path chosen after step 2?

A /root/.ssh/id_rsa

B /etc/ssh/id_rsa

C /home/user/.ssh/id_rsa

D No file chosen yet

💡 Hint

Check the 'User Input' and 'Result' columns in step 2 of the execution table.

At which step does ssh-keygen confirm the passphrase?

A Step 4

B Step 3

C Step 2

D Step 5

💡 Hint

Look for the step where the system asks to 'Enter same passphrase again' in the execution table.

If you enter a passphrase at step 3, what happens at step 5?

A ssh-keygen asks for passphrase again

B Key files are created and passphrase protects the private key

C Key files are created without passphrase protection

D ssh-keygen exits without creating keys

💡 Hint

Refer to the 'Result' column at step 5 and the 'passphrase' variable in variable_tracker.

Concept Snapshot

ssh-keygen -t rsa -b 2048
- Runs key generation
- Prompts for file location (default if Enter)
- Prompts twice for passphrase
- Creates private and public key files
- Shows key fingerprint and location

Full Transcript

The ssh-keygen command starts and asks where to save the SSH key. If you press Enter, it uses the default location. Then it asks for a passphrase twice to confirm it. After that, it creates the private and public key files and shows their fingerprint and location. The process ends after keys are created. Variables like file path and passphrase change as you provide input. This helps secure your SSH connections.

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