My recent journey with 2FA


Of course by 2FA I mean two-factor authentication.

I've been using that for a long time for ssh with my yubikey on OpenBSD but I've never enabled 2FA on the online services I use. The main reason for not doing it before was that I thought that my phone had to play a central role (which in fact is not much the case). While it's the most critical device I have, my phone is the device I trust the least.

However, yesterday I saw a comment on asking about how to use TOTP on OpenBSD. In addition to that, I guess seeing what happened to cloudflare and everything what's happening if you want to cross the US border made me more interested in 2FA than before.

So I began to look into how it works.

How it works

The concept of 2FA is that you may lose your password (or your ssh key) and in that case the person who takes control of it can successfully impersonate you. The goal is that a login system will require something else to verify that it's really you.

One way to achieve this is to use SMS but that sucks: circumventing it is not even restricted to Nation State Actors.

Another can be something biometric but then users need to access to a scanner which is quite impractical in every day life. Even if iPhones have a fingerprint reader, it's not usable by third parties.

It must be something that keeps changing otherwise it's both subject to replay attack and it's just another password.

Here comes the OTP.

One Time Password

One Time Password was defined in RFC2289 (which is quite old: February 1998). Then they made HOTP (H is for HMAC-Based) in RFC4226 and finally the TOTP (T is for Time-Based) in RFC which is an extension of the HOTP to support the time-based moving factor.

To understand in more details you can either read in the RFC4226 5.4. Example of HOTP Computation for Digit = 6 and then the short RFC6238 or you can just read this random blog article on the Internet which explains clearly the same thing.


There's a secret shared and then you compute the HMAC-SHA1 of the shared secret and epoch.

Wait, did you just say sha1?!?1?

Even if there's now a sha1 collision, it's not really a problem. To quote Schneier: "[collision] pretty much puts a bullet into SHA-1 as a hash function for digital signatures (although it doesn't affect applications such as HMAC where collisions aren't important)." (source)

And for a more complete answer, see this answer.

How to use it

Don't be locked out

I wanted to use my phone (something distinct that my computer to compartment things a bit). Obviously the goal is to secure your account without losing it so that means that losing your phone shouldn't prevent you to retrieve access to your accounts. Unusable security is unusable.

If you read about 2FA, you'll see that some services that provide it, give you some backup code to not to be locked out. But I don't want to locked out from services don't provide backup codes either.

So my phone must not be a single point of failure.

We saw earlier that {T,H}OTP are based on a shared secret so let's backup it.

Backuping shared secrets and backup codes

For my regular passwords, I use keepassx which is shared/backuped across my different computers. I created another database to store those. Of course you shouldn't use the same database to keep your passwords and the other secrets in case of you leak one of the two database's password.


Android phone

Now that I'm ready to activate 2FA, let's see how to use it. The plan is to use my android phone. On the Time-based One-time Password Algorithm Wikipedia page there was a list of clients but sadly it was deleted. You can still find it in the history.

I wanted a FOSS application and Google Authenticator is now closed source so I went with FreeOTP which is not completely dead compared to others (but it's not thriving either), so far it works good.


In the case I don't have a phone, I still want to be able to log in my different accounts. In the' link that I gave at the beginning of this article, someone mentioned oath-toolkit which works very easily:

$ oathtool --totp -b deafcafe

(with deafcafe being the shared secret).

Activating it

Now that we're ready to use it, let's do it. So where to activate it? Actually, there's a cool site that lists services that provide or not (and then you can shame them on twitter) 2FA with a link to the service's documentation.

My Feedback

So far I activated 2FA on about half a dozen of website. The first one was the RIPE NCC (if you don't want people to steal your precious IP addresses and/or your atlas credit) and it was actually a good one to try it.

To activate it usually the website gives you a qrcode which is in fact just a URL looking like:


which is fine for my phone but sadly my eyes can't decode qrcode and I need the shared secret to put it my keepassx. Most of the time websites gives you by default the qrcode but also gives you the possibility to access the shared secret.

For now, everything works fine, I use my phone to unlock my different accounts and if anything happens to it, I can just unlock my second keepassx database and use oathtool (or use a backup code) to get my account back.

By Vigdis in
Tags : #opsec, #security,
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