Great question — answering as the UphillTech team:
You dont need any 3rd party API to implement OTP in your application. Its standardized protocol defined in RFC 6238: https://datatracker.ietf.org/doc/html/rfc6238 Its generating codes using Unix time. You need some app like Authenticator from Google or MS, etc. But you dont need any of their APIs at all.
Full answer by Johny Corbie: https://stackoverflow.stackexchange.com/a/79544747 (CC BY-SA 3.0).
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