Make an SSH key pair in your browser, ed25519, ECDSA or RSA SSHKEY
The browser's own cryptography makes the key, and the files come out as ssh-keygen writes them, locked with a passphrase the same way. The private key never leaves the tab. Using it on a Chromebook without Linux.
The key
This browser cannot make Ed25519 keys with its own cryptography, so the list starts at ECDSA. Chrome and Edge 137, Firefox 129 and Safari 17 and later can; so can ssh-keygen -t ed25519 on any computer.
Your key pair
Fingerprint
Then, on a Mac or Linux, put both files in ~/.ssh and make the private one readable by you alone:
How it works
SSHKEY makes an SSH key pair with WebCrypto and writes it in OpenSSH's own formats: the private key file, optionally encrypted with your passphrase by bcrypt_pbkdf and AES-256 as ssh-keygen does, and the public key line for authorized_keys or GitHub, with its SHA256 fingerprint. For a machine where ssh-keygen is not to hand. Nothing leaves this tab.
How it is made, and what to know
The key is made by the browser’s own cryptography (WebCrypto’s generateKey, with the operating system’s random number generator) and it never leaves this tab. The files are in the formats ssh-keygen writes: the private key as an OpenSSH private key, and with a passphrase it is locked the way ssh-keygen locks one, with bcrypt_pbkdf and AES-256. Nothing is kept once the tab is closed.
Ed25519 is the one to choose unless a system asks for something else: short, fast and without the pitfalls of the older types. Some older servers and network devices take only RSA; 3072 bits is the least to use now. A key that guards something that matters is better still on a hardware key, which never lets the private part out at all.
Windows’ own OpenSSH reads these files as they are. PuTTY wants its .ppk format: open the private key in PuTTYgen and save it from there. To check a key someone sends you, KEYPRINT reads its fingerprint.
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