SSH Key Generator

• Ed25519 — recommended for all new keys since ~2017. Fixed 256-bit keys (no key-size tuning), small public key (~68 bytes), fast verification, immune to timing side channels by construction. Supported by OpenSSH 6.5+ (2014), GitHub since 2014, every major Git host.
• RSA-4096 — still acceptable, slow to generate (a few seconds), large public keys (~750 bytes). Use only if you must support a legacy host with no Ed25519 support.
• RSA-2048 — minimum for new keys, but already considered marginal for 10+ year horizons. NIST recommends migration to ≥3072 bits by 2030.
• ECDSA-P256/P384/P521 — supported but not preferred. Same curves used in TLS; security depends on a good RNG at signing time. Ed25519 is deterministic and avoids that pitfall.
• DSA — deprecated. Removed from OpenSSH 7.0 default builds (2015). Do not generate.

Default to Ed25519 unless you have explicit compatibility requirements. The public key is so small it fits comfortably in a single line in authorized_keys; key generation is fast enough to be interactive; the security margin is generous.

• Without passphrase — convenient. If your laptop is stolen or the file is exfiltrated, the attacker has immediate access to every host that trusts the key. Acceptable only on full-disk-encrypted machines that you trust to sleep-and-wake securely.
• With passphrase — the private key file is encrypted at rest with a key derived from the passphrase. Stolen file is useless without the phrase. Use ssh-agent (or macOS Keychain, gnome-keyring) to cache the decrypted key for the session so you do not retype it every operation.
• Hardware-backed (YubiKey, Touch ID, TPM) — the private key never leaves the secure element. ssh-keygen -t ed25519-sk creates a "security key" type that requires the hardware token to sign. Best for high-trust contexts.

• You are on a machine where you do not want to install openssh-client (Chromebook, locked-down corp laptop) and need a key pair for one-off access.
• You are teaching SSH key concepts and want a visible end-to-end demo where the generated key can be saved and used.
• You are pre-generating keys for a fleet of servers and need OpenSSH and PEM format outputs from the same key material.
• You inherited a system with no working ssh-keygen and need a quick key for emergency access — generate, save, chmod 600.

• It will not enroll your key with GitHub/GitLab/Bitbucket automatically. Copy the public key from the output and paste it into the host's SSH key settings page yourself.
• It will not generate keys backed by hardware tokens. ed25519-sk requires the actual hardware enclave; only ssh-keygen on a machine with the token connected can create it.
• It will not check key strength against compromised-key databases. The keys are freshly generated by Web Crypto getRandomValues — assume they are unique unless your browser's RNG is broken (it is not).