Along with U2F, see also Authy and Keybase. Each asks for an account ID, then uses a prior authenticated device to assert that the new device is OK to authentic

Along with U2F, see also Authy and Keybase. Each asks for an account ID, then uses a prior authenticated device to assert that the new device is OK to authenticate. Keybase definitely uses a genuine PKI to make this happen. Authy might not -- it's hard to tell from the app's outward behavior. Upside is the convenience of not needing the original account passphrase to add new devices. Depending on your threat model, demanding a passphrase to add a new device might not add anything to the security. Downside (I imagine) is that people can misplace their passphrases for a long time, thereby allowing the account to grow in value, but not realize it until they've lost or destroyed their last authenticated device. Then they're screwed because they no longer have any way to get back in.