Six-digit verification codes for something like a "forgot password" flow are OTPs -- they're only good for one login -- but they are not HOTP/TOTPs. HOTP/TOTP has a registration step, where you copy a server-generated secret to your phone through a QR-code-encoded otpauth:// URI (https://github.com/google/google-authenticator/wiki/Key-Uri-...). That doesn't happen in a "forgot password" flow.

Incidentally, if you think of TOTP as being HMAC(unix mod 30, secret), one idea would be to do public key crypto instead of symmetric HMAC stuff. That's basically what a security key is.

If you additionally made it so that you couldn't phish the security key -- by having the OS + web browser know which apps can ask for which security keys -- you'd have reinvented WebAuthn.

P.S.: Make you sure you have stuffing protection in place against these kinds of six-digit-code auth schemes. A million possibilities is often acceptable for a secondary factor, but it's useless if attackers can just try all million codes.

Since they're in the thread, nice article 'dogacel! I've never seen an article on this that also took the time to dig into HMAC internals and that gnarly DT function.

It is a bit terse, but there is a 20-line Python implementation which cleared up the ideas for me: https://github.com/susam/mintotp

> Also in some examples like Facebook's password recovery, this secret clock is not shared with the user directly but rather server's generated one-time password is sent via a trusted medium, such as an email to the user.

I’m pretty sure Facebook just makes up a random number and stores it?

Facebook's login/account recovery codes are not TOTP/HOTP, but are random numbers. Also, the author struggled to check their implementation. One can easily compare an implementation of many websites by grabbing the QR codes they use for login and importing into your favorite authenticator app and also decoding the QR code to get the secret. In theory your code should produce the same codes at the same time as the app.

Very helpful article. I recently went down the TOTP rabbit hole and this article would have been great.

I run a suite of servers and setup scripts that go with them. I can create users and secret keys easily enough using our APIs, but I needed a way to generate TOTP codes on the fly. I got it working on my machine, but sharing it with others was a bit difficult because really the only "logic" was generating the secrets while everything else was static data and storing responses from the APIs.

I ended up making my own API to generate TOTP codes from secrets, <https://totpapi.com>. I try to make it clear it should only be used for testing, but it makes this kind of thing much easier for me. Maybe it will help someone else as well. :)