Why SSH key generation (ssh-keygen) in Linux CLI? - Purpose & Use Cases

The Big Idea

What if you could unlock all your servers with a single, secure key instead of dozens of passwords?

The Scenario

Imagine you need to log into multiple remote servers every day. You type your password each time, hoping you don't mistype it or get locked out.

The Problem

Typing passwords repeatedly is slow and tiring. Mistakes happen, and passwords can be stolen or guessed. Managing many passwords is confusing and risky.

The Solution

SSH key generation creates a pair of keys: one public, one private. You keep the private key safe and share the public key with servers. This lets you log in securely without typing passwords every time.

Before vs After

✗ Before

ssh user@server
# then type password every time

✓ After

ssh-keygen
ssh-copy-id user@server
ssh user@server
# logs in without password prompt

What It Enables

You can securely and quickly access servers without typing passwords, making automation and daily work smoother and safer.

Real Life Example

A system admin managing 20 servers can log in instantly to each one without entering passwords, saving hours and avoiding lockouts.

Key Takeaways

Typing passwords manually is slow and risky.

SSH keys let you log in securely without passwords.

This saves time and improves security for remote access.

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