Are sufficiently long passwords susceptible to brute force attacks?

Yes. Thought obviously the odds of success go down the longer and more complex that password.

Don't passkeys get that feature by just being longer?

Put simply… no. Passkeys aren’t just ”longer passwords” sent to the same place. Unlike passwords, Passkeys aren’t a “shared secret” that you’re sending to the service you’re authenticating to. Passkeys use asymmetric encryption and are neither sent to nor stored on the server you’re authenticating to. Your passkey is a private key stored on your device and secured by biometrics, the paired public key for which lives on the server you created the passkey to authenticate to.

In a traditional brute force operation, you’re sending guesses to a server that knows your password. If you send the correct guess, you get in. It’s also possible to steal the password from the server and brute force that offline.

With a passkey on the other hand, the server uses your public key to encrypt a string in a challenge message, this string can only be decrypted by your passkey. You then send a response that’s encrypted by your private key, which can then only be decrypted by the public key on the server. So the thing you’re sending to the server to authenticate isn’t your passkey, and it’s unique every time you log in.

So could you perform some kind of operation that would technically still be a kind of brute force? Theoretically yeah. But even so you’d be limited to brute forcing against the server, which isn’t very effective even against passwords. However you would not at all be susceptible to offline brute forcing based on the capture of a passkey either in flight by breaking encryption, or by breaching the server, because your passkey never leaves your device.