🔐 Linux ssh-keygen Command Tutorial

The ssh-keygen command is a powerful utility used to generate, manage, and convert SSH key pairs for secure authentication. SSH keys are a more secure alternative to passwords for remote logins and automated access.

---

📦 Installation

Most Linux distributions include ssh-keygen as part of the OpenSSH package. To ensure it's installed:

Debian/Ubuntu:

sudo apt install openssh-client

Red Hat/CentOS/Fedora:

sudo dnf install openssh

---

🧰 Basic Syntax

ssh-keygen [options]

By default, it generates a 2048-bit RSA key pair and stores it in ~/.ssh/id_rsa and ~/.ssh/id_rsa.pub.

---

🔧 Common Options

Option	Description
-t	Specify key type (rsa, ed25519, ecdsa, etc.)
-b	Set key size in bits (e.g., 4096 for RSA)
-C	Add a comment (e.g., email or purpose)
-f	Specify output file name
-N	Set a passphrase (empty for no passphrase)
-q	Quiet mode (suppress output)
-y	Extract public key from a private key
-p	Change passphrase of an existing key
-R	Remove a host from known_hosts

---

🧪 Examples

1. Generate a Default RSA Key Pair

ssh-keygen

2. Generate a 4096-bit RSA Key with a Comment

ssh-keygen -t rsa -b 4096 -C "[email protected]"

3. Generate an Ed25519 Key with a Custom Filename

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

4. Generate a Key Without a Passphrase

ssh-keygen -N "" -f ~/.ssh/id_rsa_nopass

5. Change the Passphrase of an Existing Key

ssh-keygen -p -f ~/.ssh/id_rsa

6. Extract the Public Key from a Private Key

ssh-keygen -y -f ~/.ssh/id_rsa > ~/.ssh/id_rsa.pub

7. Remove a Host from known_hosts

ssh-keygen -R example.com

---

🧠 Tips

• Use ssh-copy-id to install your public key on a remote server:

  ssh-copy-id user@remote_host
  

• Use ssh-agent and ssh-add to manage keys with passphrases.
• Store keys in ~/.ssh/ and set permissions:

  chmod 700 ~/.ssh
  chmod 600 ~/.ssh/id_rsa
  chmod 644 ~/.ssh/id_rsa.pub
  

---

📚 References

• SSH.com: How to Use ssh-keygen
• GeeksforGeeks: ssh-keygen in Linux
• CommandMasters: ssh-keygen Examples
• man7.org: ssh-keygen Manual Page

---

Happy key crafting! 🔑

Let me know if you'd like a version that includes key-based login setup or integrates with GitHub or remote servers!