How to provide two-factor authentication in Kubuntu without a phone

Is it possible to use two-factor authentication on a Linux laptop running Kubuntu 18.04 without using a phone at all? If so, how?

I'm able to authenticate on my phone without any particular difficulty. But when I log into Coinbase under Kubuntu, I'm asked for a 2-step verification code. Running Authenticator under Kubuntu, I'm asked for a provider (presumably Coinbase), an account name (presumably my account name at Coinbase), and a 2FA token. It seems that both Authenticator and Coinbase are asking for codes, and neither of them are providing codes. It's like trying to connect a male plug to another male plug.

I would guess that I'm making a major conceptual error -- but what?

2 Answers

Looks like Coinbase supports Yubikey. From :

Coinbase supports all WebAuthN / Fido2 standard security keys. An option for a security key is Yubico's yubikey.

See Using security keys

The Yubikey plugs into a USB port, emulates a keyboard, and produces a security code when touched. Should work with Kubuntu 18.04.

Another option is to use a password manager like KeePassXC that has a built-in 2FA function to create TOTP or "Time-Based One-Time Passwords". It allows you to store the secret you get from the service you set it up for, and then create a 2FA code whenever you need it, just like a Yubikey. The only difference is the secrets are stored on your PC (or whereever) in your password manager database, not on a hardware key. It's cheaper, but depending on your threat model may be less secure.

You only need to be sure the 2FA code you need is compatible - some sites use their own proprietary stuff. Anything that works with "Google Authenticator" is usually fine. Coinbase seems to do so.