Create and Load an SSH Key

Use a modern ed25519 key for GitHub and GitLab unless a legacy service explicitly requires RSA.

Check for an Existing Key

Before generating a key, inspect the default SSH directory:

ls -al ~/.ssh

Common public-key filenames include id_ed25519.pub, id_ecdsa.pub, and id_rsa.pub. If a matching private key exists without the .pub suffix, it can be loaded into ssh-agent and its public key can be uploaded. Do not overwrite an existing private key unless it is no longer needed.

Generate the Key Pair

Create the SSH directory with restrictive permissions, then generate a key. Replace the comment with an email address or a label that identifies the key owner.

install -d -m 700 ~/.ssh
ssh-keygen -t ed25519 -a 100 -f ~/.ssh/id_ed25519 -C "you@example.com"

Enter a strong passphrase when prompted. The private key remains in ~/.ssh/id_ed25519; the public key to upload is ~/.ssh/id_ed25519.pub.

For a legacy service that does not support Ed25519, generate a 4096-bit RSA key instead:

ssh-keygen -t rsa -b 4096 -f ~/.ssh/id_rsa -C "you@example.com"

Do not generate a DSA (ssh-dss) key. GitHub does not accept new DSA keys, and they are obsolete.

Display the public key:

cat ~/.ssh/id_ed25519.pub

Record its fingerprint so that it can be compared after upload or download:

ssh-keygen -lf ~/.ssh/id_ed25519.pub

caution

Do not copy, upload, email, or commit ~/.ssh/id_ed25519. Anyone with that private key and its passphrase can authenticate as you.

Load the Key into ssh-agent

Start the agent for the current shell, then load the private key:

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

On macOS, save the passphrase in the keychain:

ssh-add --apple-use-keychain ~/.ssh/id_ed25519

To load the key automatically in future macOS sessions, add the following host entry to ~/.ssh/config. Adjust IdentityFile when the key has a different name:

Host github.com gitlab.com
	AddKeysToAgent yes
	UseKeychain yes
	IdentityFile ~/.ssh/id_ed25519

Restrict the configuration file after creating or updating it:

chmod 600 ~/.ssh/config

Confirm that the agent has loaded the expected key:

ssh-add -l

Continue with Add the Key to GitHub and GitLab.