Worth noting: The Google Authenticator app lets _anyone_ generate a shared secret seed that can be added to the app (which, afaict, does not communicate with the network at all).
I'm confused as to why you'd need a third-party for this.
The Google Authenticator app is great. I recently got (TOTP) 2-factor auth for an IRC bot going with Google Authenticator; took about 5 minutes to code it up and set it up. It doesn't use any sort of 3rd party service, just the application running locally on my phone.
TOTP/HOTP is dead simple and, with the open source Google Authenticator app, great for the end user.
There is one terrible thing which is that adding an account for a different service but with the same email will overwrite any account with the same email without prompting you.
e.g. setting up 2-factor with MS with the same email you use for Google will overwrite the Google one unless you rename it.
Re-ordering tokens in the Google Authenticator app is janky as hell too, and the only way you'll know if it's going to work is if the regular view switches to one with subtly different font sizes.
Authy's not without its rough edges, but it never looks that bad.
From down-thread, regarding Google Authenticator:
e.g. setting up 2-factor with MS with the same email you use for Google will overwrite the Google one unless you rename it.
I'm confused as to why you'd need a third-party for this.
Example of how to generate a seed and share it with an ssh server: https://scottlinux.com/2013/06/02/use-google-authenticator-f...