HOTP

Why is U2F better than OTP?

It is not really obvious to many people how U2F is better than OTP for two-factor authentication; in particular I’ve seen it compared with full-blown smartcard-based authentication, and I think that’s a bad comparison to do.

Indeed, since the Security Key is not protected by a PIN, and the NEO-n is designed to be semi-permanently attached to a laptop or desktop. At first this seems pretty insecure, as secure as storing the authorization straight into the computer, but it’s not the case.

But let’s start from the target users: the Security Key is not designed to replace the pure-paranoia security devices such as 16Kibit-per-key smartcards, but rather the on-phone or by-sms OTPs two-factor authenticators, those that use the Google Authenticator or other opensource implementations or that are configured to receive SMS.

Why replacing those? At first sight they all sound like perfectly good idea, what’s to be gained to replace them? Well, there are plenty of things, the first of being the user friendliness of this concept. I know it’s an overuse metaphor, but I do actually consider features on whether my mother would be able to use them or not — she’s not a stupid person and can use a computer mostly just fine, but adding any on more procedures is something that would frustrate her quite a bit.

So either having to open an application and figure out which of many codes to use at one time, or having to receive an SMS and then re-type the code would be not something she’d be happy with. Even more so because she does not have a smartphone, and she does not keep her phone on all the time, as she does not want to be bothered by people. Which makes both the Authenticator and SMS ways not a good choice — and let’s not try to suggests that there are way to not be available on the phone without turning it off, it would be more to learn that she does not care about.

Similar to the “phone-is-not-connected” problem, but for me rather than my mother, is the “wrong-country-for-the-phone” problem: I travel a lot, this year aiming for over a hundred days on the road, and there are very few countries in which I keep my Irish phone number available – namely Italy and the UK, where Three is available and I don’t pay roaming, when the roaming system works… last time I’ve been to London the roaming system was not working – in the others, including the US which is obviously my main destination, I have a local SIM card so I can use data and calls. This means that if my 2FA setup sends an SMS on the Irish number, I won’t receive it easily.

Admittedly, an alternative way to do this would be for me to buy a cheap featurephone, so that instead of losing access to that SIM, I can at least receive calls/SMS.

This is not only a theoretical. I have been at two conferences already (USENIX LISA 13, and Percona MySQL Conference 2014) and realized I cut myself out of my LinkedIn account: the connection comes from a completely different country than usual (US rather than Ireland) and it requires reauthentication… but it was configured to send the SMS to my Irish phone, which I had no access to. Given that at conferences is when you meet people you may want to look up on LinkedIn, it’s quite inconvenient — luckily the authentication on the phone persists.

The authenticator apps are definitely more reliable than that when you travel, but they also come with their set of problems. Beside the not complete coverage of services (LinkedIn noted above for instance does not support authenticator apps), which is going to be a problem for U2F as well, at least at the beginning, neither Google’s or Fedora’s authenticator app allow you to take a backup of the private keys used for OTP authentication, which means that when you change your phone you’ll have to replace, one by one, the OTP generation parameters. For some services such as Gandi, there is also no way to have a backup code, so if you happen to lose, break, or reset your phone without disabling the second factor auth, you’re now in trouble.

Then there are a few more technical problems; HOTP, similarly to other OTP implementations, relies on shared state between the generator and the validator: a counter of how many times the code was generated. The client will increase it with every generation, the server should only increase it after a successful authentication. Even discounting bugs on the server side, a malicious actor whose intent is to lock you out can just make sure to generate enough codes on your device that the server will not look ahead enough to find the valid code.

TOTP instead relies on synchronization of time between server and generator which is a much safer assumption. Unfortunately, this also means you have a limited amount of time to type your code, which is tricky for many people who’re not used to type quickly — Luca, for instance.

There is one more problem with both implementations: they rely on the user to choose the right entry and in the list and copy the right OTP value. This means you can still phish an user to type in an OTP and use it to authenticate against the service: 2FA is a protection against third parties gaining access to your account by having your password posted online rather than a protection against phishing.

U2F helps for this, as it lets the browser to handshake with the service before providing the current token to authenticate the access. Sure there might still be gaps on is implementation and since I have not studied it in depth I’m not going to vouch for it to be untouchable, but I trust the people who worked on it and I feel safer with it than I would be with a simple OTP.