Which SSH key type should I use?

Ed25519 is the modern default. It is faster than RSA, produces smaller keys (68 chars vs 740+ for RSA 4096), and is supported by all modern SSH servers and clients. Use RSA 4096 only when connecting to legacy servers that do not support Ed25519.

What bits size should I choose for RSA?

Use RSA 4096 as a minimum today. RSA 2048 is still technically adequate but 4096 adds marginal cost for significant future-proofing. RSA 1024 is broken and must not be used.

Should I set a passphrase?

Yes, always. A passphrase encrypts the private key file using AES-256. Without it, anyone who gains access to your ~/.ssh/id_ed25519 file can use it immediately. Use ssh-agent or macOS Keychain to avoid retyping the passphrase on every connection.

What is the -C flag (comment)?

The comment is a label appended to the public key. It is visible in authorized_keys files and host logs. Use your email address or a descriptive label (e.g. alice@laptop-2024) to identify which key is which when managing multiple devices.

Where should I store the generated key?

By default, ssh-keygen saves to ~/.ssh/id_ed25519 (private) and ~/.ssh/id_ed25519.pub (public). Use the -f flag to save to a custom path — useful for per-project or per-service keys.

How do I add my public key to a server?

Run: ssh-copy-id user@server — this appends your public key to ~/.ssh/authorized_keys on the server. Or manually append the contents of your .pub file to ~/.ssh/authorized_keys on the target machine.

What is the difference between RSA and ECDSA?

ECDSA (Elliptic Curve DSA) produces small keys similar to Ed25519 but uses the NIST P-256 or P-384 curves, which some security researchers consider potentially backdoored. Ed25519 uses Curve25519 which has no such concerns and is the preferred elliptic-curve option.

How do I convert an existing RSA key to Ed25519?

You cannot convert key types. Generate a new Ed25519 key pair, add the new public key to authorized_keys on all servers, verify you can log in with the new key, then remove the old RSA key from authorized_keys.

SSH Keygen Command Builder

Configure your SSH key options and get the exact ssh-keygen command to run. Nothing is generated on this page — the command runs locally in your terminal.

100% client-side · no keys generated here

tool.ssh-key-gen-info.label-key-type

tool.ssh-key-gen-info.label-key-bits-rsa-only

Comment (optional)

Output file (optional)

No passphrase (not recommended — for automation only)

Command to run in your terminal

Next steps after generating the key

Run the command above in your terminal.
Copy your public key: cat ~/.ssh/id_ed25519.pub
Add it to the target server: ssh-copy-id user@server
Test the connection: ssh -i ~/.ssh/id_ed25519 user@server
Add to ssh-agent for passphrase caching: ssh-add ~/.ssh/id_ed25519

Common use cases

GitHub / GitLab: Generate Ed25519 key, add public key in Settings → SSH Keys.
Server access: Generate RSA 4096 for legacy servers, Ed25519 for modern ones.
CI/CD: Generate a key without passphrase (-N "") for automated deployments, restricted to specific commands via forced commands in authorized_keys.
Multiple identities: Use -f to save to a named path, then configure ~/.ssh/config with Host blocks to select the right key per host.

常見問題

Which SSH key type should I use?: Ed25519 is the modern default. It is faster than RSA, produces smaller keys (68 chars vs 740+ for RSA 4096), and is supported by all modern SSH servers and clients. Use RSA 4096 only when connecting to legacy servers that do not support Ed25519.
What bits size should I choose for RSA?: Use RSA 4096 as a minimum today. RSA 2048 is still technically adequate but 4096 adds marginal cost for significant future-proofing. RSA 1024 is broken and must not be used.
Should I set a passphrase?: Yes, always. A passphrase encrypts the private key file using AES-256. Without it, anyone who gains access to your ~/.ssh/id_ed25519 file can use it immediately. Use ssh-agent or macOS Keychain to avoid retyping the passphrase on every connection.
What is the -C flag (comment)?: The comment is a label appended to the public key. It is visible in authorized_keys files and host logs. Use your email address or a descriptive label (e.g. alice@laptop-2024) to identify which key is which when managing multiple devices.
Where should I store the generated key?: By default, ssh-keygen saves to ~/.ssh/id_ed25519 (private) and ~/.ssh/id_ed25519.pub (public). Use the -f flag to save to a custom path — useful for per-project or per-service keys.
How do I add my public key to a server?: Run: ssh-copy-id user@server — this appends your public key to ~/.ssh/authorized_keys on the server. Or manually append the contents of your .pub file to ~/.ssh/authorized_keys on the target machine.
What is the difference between RSA and ECDSA?: ECDSA (Elliptic Curve DSA) produces small keys similar to Ed25519 but uses the NIST P-256 or P-384 curves, which some security researchers consider potentially backdoored. Ed25519 uses Curve25519 which has no such concerns and is the preferred elliptic-curve option.
How do I convert an existing RSA key to Ed25519?: You cannot convert key types. Generate a new Ed25519 key pair, add the new public key to authorized_keys on all servers, verify you can log in with the new key, then remove the old RSA key from authorized_keys.

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