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Behind the 6-digit code: Building HOTP and TOTP from scratch

(blog.dogac.dev)
248 points dogacel | 1 comments | 11 Apr 25 13:06 UTC | HN request time: 0s | source
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ucarion ◴[15 Apr 25 04:57 UTC] No.43689106[source]▶
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.

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1. dogacel ◴[15 Apr 25 05:08 UTC] No.43689155[source]▶
All very valuable comments! Actually I had a small edit on the "forget password" flow.

I agree that an asymmetric key makes much sense. Secret key can be left at the user device while server only contains the public key. That sounds much more secure. I will dig deeper!

True about the stuffing proteciton, I actually want to do further reading on how TOTP is secured from random attacks. Statistically you are expected to crack 1 account in every 1 million attempts in 6 digits codes. Those numbers look pretty huge in the context of security, and a bot-net can potentially brute force couple hundred accounts every day.