SSH Keys

What it is

An SSH key is a pair of files on your computer — one private (stays on your machine, never shared), one public (you give to services like GitHub). Together they prove your identity without a password.

Why you'd use it

Every time you git push or git pull from GitHub, your machine needs to prove who you are. Without an SSH key, you're typing passwords or dealing with token expiration. With one, it just works. Silently. Every time.

This is the plumbing underneath everything. If the book says "push to GitHub," this is the thing that makes that sentence possible.

How to set it up

Mac

# Generate a new key (Ed25519 is the modern default)
ssh-keygen -t ed25519 -C "your_email@example.com"

# When it asks for a file location, press Enter for the default (~/.ssh/id_ed25519)
# When it asks for a passphrase, either set one or press Enter for none

# Start the SSH agent
eval "$(ssh-agent -s)"

# Add your key to the agent
ssh-add ~/.ssh/id_ed25519

# Copy the public key to your clipboard
pbcopy < ~/.ssh/id_ed25519.pub

Now go to GitHub → Settings → SSH and GPG keys → New SSH Key → paste → save.

Windows

# Open PowerShell (not CMD)
ssh-keygen -t ed25519 -C "your_email@example.com"

# Default location is fine, press Enter
# Passphrase is optional

# Start the SSH agent service
Get-Service ssh-agent | Set-Service -StartupType Automatic
Start-Service ssh-agent

# Add your key
ssh-add $env:USERPROFILE\.ssh\id_ed25519

# Copy the public key to clipboard
Get-Content $env:USERPROFILE\.ssh\id_ed25519.pub | Set-Clipboard

Now go to GitHub → Settings → SSH and GPG keys → New SSH Key → paste → save.

Linux

ssh-keygen -t ed25519 -C "your_email@example.com"

eval "$(ssh-agent -s)"
ssh-add ~/.ssh/id_ed25519

# Copy to clipboard (install xclip if needed: sudo apt install xclip)
xclip -selection clipboard < ~/.ssh/id_ed25519.pub

Now go to GitHub → Settings → SSH and GPG keys → New SSH Key → paste → save.

How to verify it worked

ssh -T git@github.com

You should see:

Hi yourusername! You've successfully authenticated, but GitHub does not provide shell access.

If you see "Permission denied," your key isn't registered with GitHub or the agent isn't running.

How to prompt your AI to do it

Read this page and help me set up SSH keys for GitHub. Check if I already have keys, and if I do, help me figure out if they're already connected to GitHub. If not, walk me through generating new ones and adding them.

Or more directly:

I need to set up SSH keys so I can push to GitHub without passwords. I'm on [Mac/Windows/Linux]. Check if I already have keys at ~/.ssh/ and tell me what to do next.

What can go wrong

Multiple keys: If you have keys for different services (work GitHub, personal GitHub, a server), you may need an ~/.ssh/config file. It tells SSH which key to use where.
Agent not running: If ssh-add says "Could not open connection to authentication agent," the agent isn't started. Run the eval command above.
Wrong permissions: SSH is picky. Your ~/.ssh directory should be 700, your private key 600. Run chmod 700 ~/.ssh && chmod 600 ~/.ssh/id_ed25519 if things look wrong.
Passphrase fatigue: If you set a passphrase and get tired of typing it, you can add it to your Mac keychain with ssh-add --apple-use-keychain ~/.ssh/id_ed25519.

Book connection

This supports every chapter that involves code. The steering file chapter (Chapter 13) assumes you can push to a repo. The pair programming chapter (Chapter 5) assumes you're working in a git-tracked project. SSH keys are the invisible foundation under all of it.