An Introduction to SSH and Key Authentication

> 💡 Learning Guide: Tired of entering your password every time you git push? Getting "Permission denied" when connecting to a server? This chapter takes 5 minutes to explain how SSH key authentication works and how to set up password-free login to GitHub and servers with a single command.> 💡 Learning Guide: Tired of entering your password every time you git push? Getting "Permission denied" when connecting to a server? This chapter takes 5 minutes to explain how SSH key authentication works and how to set up password-free login to GitHub and servers with a single command.

1. Password vs. Key: Why Keys Are Better1. Password vs. Key: Why Keys Are Better

👇 Try it out: Compare the difference between password login and key login👇 Try it out: Compare the difference between password login and key login

💡 Tips Praktis💡 Pro Tip

Password login = sending the password each time for the other side to verify (the password could be intercepted); Key login = proving "I have the key" without showing the key to anyone (the private key is never transmitted).Password login = sending the password each time for the other side to verify (the password could be intercepted); Key login = proving "I have the key" without showing the key to anyone (the private key is never transmitted).

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2. Asymmetric Encryption: Public Key and Private Key2. Asymmetric Encryption: Public Key and Private Key

SSH keys are based on asymmetric encryption, generating two keys at once:SSH keys are based on asymmetric encryption, generating two keys at once:

	Private Key	Public Key
Stored on	Your computer `~/.ssh/id_ed25519`	Server / GitHub
Can you share it?	❌ Never	✅ Share freely
Function	Sign (prove identity)	Verify signature (confirm identity)
Analogy	Key	Lock

Common Key TypesCommon Key Types

Type	Command	Recommendation	Notes
Ed25519	`ssh-keygen -t ed25519`	⭐⭐⭐	Newest, fastest, most secure
RSA	`ssh-keygen -t rsa -b 4096`	⭐⭐	Good compatibility, but slower
ECDSA	`ssh-keygen -t ecdsa`	⭐	Generally not recommended

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3. Hands-on: Generating and Configuring SSH Keys3. Hands-on: Generating and Configuring SSH Keys

3.1 Generate a Key Pair3.1 Generate a Key Pair

bash

ssh-keygen -t ed25519 -C "your@email.com"

After running, you'll be prompted for:After running, you'll be prompted for:

File path: Press Enter to use the default path ~/.ssh/id_ed25519File path: Press Enter to use the default path ~/.ssh/id_ed25519
Passphrase: You can set additional protection (or leave it empty)Passphrase: You can set additional protection (or leave it empty)

3.2 Add the Public Key to GitHub3.2 Add the Public Key to GitHub

bash

# 1. Copy the public key content cat ~/.ssh/id_ed25519.pub | pbcopy # macOS cat ~/.ssh/id_ed25519.pub | xclip # Linux # 2. Open GitHub → Settings → SSH and GPG keys → New SSH key # 3. Paste the public key and save # 4. Test the connection ssh -T git@github.com # Success message: Hi username! You've been authenticated...

3.3 Add the Public Key to a Server3.3 Add the Public Key to a Server

bash

# Method 1: ssh-copy-id (recommended) ssh-copy-id user@your-server # Method 2: Manual copy cat ~/.ssh/id_ed25519.pub | ssh user@server "mkdir -p ~/.ssh && cat >> ~/.ssh/authorized_keys"

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4. SSH Config: Say Goodbye to Long Commands4. SSH Config: Say Goodbye to Long Commands

Configure aliases in ~/.ssh/config — set it up once and benefit forever:Configure aliases in ~/.ssh/config — set it up once and benefit forever:

CODE

Host dev HostName 192.168.1.100 User deploy IdentityFile ~/.ssh/id_ed25519 Host github.com HostName github.com User git IdentityFile ~/.ssh/id_ed25519

After configuration:After configuration:

Before	After
`ssh -i ~/.ssh/id_ed25519 deploy@192.168.1.100`	`ssh dev`
Memorize IP and username every time	Just remember one alias

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5. Common Issue Troubleshooting5. Common Issue Troubleshooting

Problem	Cause	Solution
`Permission denied (publickey)`	Public key not added to server	`ssh-copy-id user@server`
`WARNING: UNPROTECTED PRIVATE KEY FILE`	Private key file permissions too broad	`chmod 600 ~/.ssh/id_ed25519`
`Could not resolve hostname`	SSH Config misconfigured	Check `~/.ssh/config` format
GitHub still asks for password	Using HTTPS instead of SSH	Switch to `git@github.com:user/repo.git`

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6. Summary6. Summary

💡 Tips Praktis💡 Pro Tip

1. Keys > Passwords: Private keys are never transmitted, making them far more secure than passwords 2. Ed25519 recommended: The most modern key algorithm — fast and highly secure 3. Share public keys freely, never leak private keys: Remember this golden rule 4. SSH Config: Set up aliases once, then ssh alias connects instantly 5. GitHub/GitLab: After adding your public key, git push/pull never requires a password again1. Keys > Passwords: Private keys are never transmitted, making them far more secure than passwords 2. Ed25519 recommended: The most modern key algorithm — fast and highly secure 3. Share public keys freely, never leak private keys: Remember this golden rule 4. SSH Config: Set up aliases once, then ssh alias connects instantly 5. GitHub/GitLab: After adding your public key, git push/pull never requires a password again

Next Steps:Next Steps:

[Principles of Ports and Localhost](./ports-localhost) - Understanding the basics of network connections[Principles of Ports and Localhost](./ports-localhost) - Understanding the basics of network connections
[An Introduction to Environment Variables and PATH](./environment-path) - Understanding system configuration[An Introduction to Environment Variables and PATH](./environment-path) - Understanding system configuration

An Introduction to SSH and Key AuthenticationAn Introduction to SSH and Key Authentication

1. Password vs. Key: Why Keys Are Better1. Password vs. Key: Why Keys Are Better

2. Asymmetric Encryption: Public Key and Private Key2. Asymmetric Encryption: Public Key and Private Key

Common Key TypesCommon Key Types

3. Hands-on: Generating and Configuring SSH Keys3. Hands-on: Generating and Configuring SSH Keys

3.1 Generate a Key Pair3.1 Generate a Key Pair

3.2 Add the Public Key to GitHub3.2 Add the Public Key to GitHub

3.3 Add the Public Key to a Server3.3 Add the Public Key to a Server

4. SSH Config: Say Goodbye to Long Commands4. SSH Config: Say Goodbye to Long Commands

5. Common Issue Troubleshooting5. Common Issue Troubleshooting

6. Summary6. Summary