How does Google Authenticator work?

https://prezu.ca/post/2021-07-30-totp-1/

by ecliptik • 5 years ago

252 158 5 years ago

akerl_2• 5 years ago

This article is overall a nice breakdown of how TOTP/HOTP function. My only concern is that it seems to connect the “counter” (time in the case of TOTP) with salting. Salting of passwords is targeted towards a totally different problem. The HMAC functions here don’t take a salt, they take a secret and what the RFC calls a “moving factor”. It exists intentionally to allow the resulting calculation to change over time in unpredictable (to an attacker) ways.

LeoPanthera• 5 years ago

Google Authenticator is an implementation of the catchily-named "Time-based One-Time Password" or TOTP.

https://en.wikipedia.org/wiki/Time-based_One-Time_Password

It is not the only one, any time a site asks you to add a "Google Authenticator" code you can use any TOTP app. 1Password has the functionality built in, for example.

akerl_2• 5 years ago

The author seems to address this directly:

But here we’re focusing on apps like Google Authenticator that use something called TOTP algorithm.
What we want to achieve is understand how apps like Google Authenticator (or any other TOTP app) are solving that?

ocdtrekkie• 5 years ago

It's truly and deeply frustrating to me how many sites and services push "Google Authenticator" branding instead of specifying that it's TOTP or that literally any good 2FA app will work. It's almost to that "Google" = "search" level, but for a more niche aspect.

loloquwowndueo2• 5 years ago

Back in the day it was the best-known and most reliably available cross-platform (iOS, android) code generator - so many sites just assume it as a de facto standard. It also reduces support burden to tell people this is the officially supported code generator, that way you don’t need to struggle understanding why a random app might be having trouble with codes.

These days it’s much less of an issue as there are many more clients - the IdP I manage now mentions authy, tofu and 1password as also supported.

animex• 5 years ago

In Fortune 1000 land, Microsoft Authenticator rules the land. I love it because most have implemented notification push-authentication with biometric verification. Some sites will use that in lieu of any password at all.

The worst is when separate apps & companies roll their own authenticator app.

tialaramex3• 5 years ago

Microsoft's stuff all wants you to install the Microsoft Authenticator which needs a Microsoft account for some reason and that (for me anyway) then immediately wanted me to setup 2FA, for which it required me to install the Microsoft Authenticator, which ... you can see how that goes.

As I began to experience Déjà vu on the first day of a new job I snapped out of it, removed the Microsoft Authenticator and told it nah, I'm OK actually, show me that QR code for "other authenticators" and never looked back.

ezekg• 5 years ago

I'm not sure why you're being down voted, but I agree. I've had several discussions where people think Authy is incompatible with sites that prompt for Google Authenticator, like it's some special more-secure TOTP app.

unclebucknasty2• 5 years ago

It's about the perception of security and reducing friction. Everyone knows Google, so you don't have to convince users of the trustworthiness of a lesser-known brand or open source project within the sensitive context of increasing security.

pilsetnieks• 5 years ago

On the one hand, I agree but on the other, this is the one case where you cannot take any chance of having your user install some random dodgy app that promises the same functionality.

willis9363• 5 years ago

You can even convert TOTP from 2FA into 1.5ish FA by using a password manager that stores the TOTP secret and generates the TOTP key from it.

Godel_unicode2• 5 years ago

An adversary still has to get the manager DB and the key to unlock it (your password manager DB is encrypted with a key, right?).

tshaddox3• 5 years ago

I use 1Password for all of my MFA codes on the web. It's extremely convenient to have them synced across all my devices, but I can't help but think that having my passwords and my MFA private keys stored in the same place almost entirely defeats the purpose of multi-factor authentication.

easton• 5 years ago

There is still a little bit of a point, as it guarantees the password came out of 1Password and wasn't from somewhere else. If someone screencapped your password out of 1Password, it wouldn't of use unless they could get into your vault somehow.

jeromegv• 5 years ago

If your passwords are leaked in a data leak, they still wouldn't get your 2FA. So I wouldn't say it entirely defeats the purpose of 2FA.

temp0826• 5 years ago

Can you use 1password to act as the code generator for logging in to 1password?

lysp• 5 years ago

I actually do the opposite.

Keep passwords on browser, and MFA on phone.

If my browser / desktop got compromised, someone would still need my phone to access my accounts.

agret• 5 years ago

I use LastPass for password sync and Authy for 2FA. Feels a bit safer having them seperate.

tomesco• 5 years ago

1Password had a blog post [1] addressing this concern. Essentially you still have the benefit of one-timeness, but not second factor, and need to compare that to what your security needs are.

[1] https://blog.1password.com/totp-for-1password-users/

kaladin-jasnah• 5 years ago

Have to chime in but my favourite authenticator is andOTP. Seems to be FOSS as well since it's on F-Droid[0] and I hear it has good support from migration from one phone/ROM to another.

[0] https://f-droid.org/en/packages/org.shadowice.flocke.andotp/

teekert2• 5 years ago

AndOTP is really nice. It is the one thing I miss on iOS. On iOS there does not seem to be any totp apps that can just export their db as an encrypted file (or unencrypted if you so choose). So in Apple land I’m stuck with MS Authenticator, coincidentally my only app that needs iCloud (I really want it backed up).

slim• 5 years ago

My favorite is Android Token because it's only 67 kb and it has everything including qrcode scanning :) check it on fdroid

rraghur• 5 years ago

Mine is aegis.. it's on f droid, completely open source, let's you backup your secrets so changing phones isn't a pita

judge2020• 5 years ago

Unfortunately, Stripe does this currently: https://i.judge.sh/youthful/Derpy/chrome_TIDNqlcRvI.png

jvanderbot2• 5 years ago

You can use oathtool if you'd like to "do it yourself". I usually just keep (encrypted with Pass) my secret keys for Authenticators, and execute a one line alias:

oathtool -b --totp `pass show <X>`

... for <X> in github, gmail, blizzard, whatever ...

This way, you can have a backup in case your phone gets lost, and easily port between phones.

stevekemp• 5 years ago

If you're using pass already you could use their OTP plugin:

https://github.com/tadfisher/pass-otp

That gives you "pass otp github.com", etc. You can also export the QRcode, and do similar things.

I put together a simple distribution of pass with a couple of plugins I use, including otp, which is easy to setup - just clone beneath `/opt/pass`:

https://github.com/skx/pass

werid• 5 years ago

at one point i used a python script to provide a totp response to google when using youtube-dl (before logging in became broken and had to switch to cookies)

    import onetimepass as otp
    my_secret = ''  
    my_token = otp.get_totp(my_secret)  
    print my_token

jillesvangurp• 5 years ago

It's really easy to integrate into websites as well. I did so a few years ago. The TOTP algorithm is just a few lines of code. I adapted this implementation https://github.com/j256/two-factor-auth at the time. There are similar libraries available for lots of languages.

You need a library like that and a way to convert an otp:// url into a QR code, for which there are many libaries as well. The rest is just implementing a sane UX around this. Storing the user's TOTP secret server side is a bit tricky. I suspect a plain text field in a database is quite common for this; which of course would be disastrous if that database were ever stolen. Secret stores don't scale for this as they tend to be designed for just a handful of secrets. We ended up encrypting these totp secrets using a key from our secret store.

seanwilson• 5 years ago

You know in Authy (Google Authenticator is similar here?) when there's only 10 seconds or so left to type in the current code so you wait for the next one to appear? Could it not show the next code instead if there wasn't much time left and the app you're authenticating with could accept the current or next code? Feels like an obvious annoyance that's fixable.

pgalvin• 5 years ago

This is mostly a non-issue - services usually accept the current, previous and next TOTP code to account for devices having a slightly incorrect clock.

I’ve never had an Android device that keeps its clock in sync, for example, so they had to address this.

b3morales• 5 years ago

The server sides are typically set up with some flexibility to account for the latency and avoid this UX issue. They will have a window to accept the previous/next code even after the new becomes active.

To avoid replays, though, this should be reset once a code is accepted: only subsequent codes should be accepted afterwards.

So when code N is the "correct" code, any of N-1, N, or N+1 will be accepted. But if N is entered, only N+1 and onwards should be accepted for later attempts.

anchpop• 5 years ago

I’ve thought about that too. It should always show two codes (the current one and the previous one), and websites should accept both. Then when you open the app, you can always start typing the most recent one and have at least the whole interval to type it in

kccqzy• 5 years ago

Servers generally have no problem accepting an expired code, if the code is only expired for a few seconds.

pmlamotte• 5 years ago

Showing two codes would definitely be a nice opt-in feature. Servers generally will compute codes within a sliding window to account for time sync discrepancies so there wouldn't be much harm. I could see how it may be confusing to some end users though.

ecesena2• 5 years ago

Codes are typically valid for 3-5min even if the display only shows 1min. This is to avoid issues with time discrepancy on client/server, and doesn't really make a big difference in terms of security, feel free to try it out.

So, to fix your issue, just use the current code even if it seems expired.

As a side note, it's important to enforce that each code can only be used once. Authy (the server-side service) does that, but many "roll your own" TOTP solutions don't verify this.

JyB2• 5 years ago

Not sure how that's relevant to the parent.

tialaramex3• 5 years ago

A good implementation of this protocol remembers that it saw this code and won't let anybody re-use it. For example suppose I am standing behind Sarah as she logs in to her bank and I happen to already know that her "Favourite carebear" was "Birthday Bear" even though that's not displayed on screen, and I see she types 123 456 as the TOTP code.

If I am very quick and try to also log in to her bank with "Birthday Bear" and 123 456 that shouldn't work even though that's still the "right" code for the next few seconds. The bank login should tell me that I need to wait and enter the next code. I can't see Sarah's next code because she's done and put her phone away.

RFC 6238 says of this: "The verifier MUST NOT accept the second attempt of the OTP after the successful validation has been issued for the first OTP, which ensures one-time only use of an OTP."

JyB2• 5 years ago

Try it. They'll still work if the server side is implemented properly. The server will be lenient on the timestamps.

graftak• 5 years ago

Raivo OTP for iOS does something similar where it displays the previous code (greyed out) too after it has expired. Convenient for when the time runs out while you enter the code, as others mentions there is often some leeway on the token expiry time.

Donckele• 5 years ago

This could be a stupid question, why not just use the first few characters from the hash of the secret and the timestamp?

duskwuff• 5 years ago

That's pretty close to what they're doing, with some small changes (using HMAC, measuring the time in 30-second intervals, and generating a numeric code).

Jtsummers2• 5 years ago

One of the requirements for HOTP (which TOTP is based on) was that the code should be numeric for phone entry or similarly constrained entry mechanisms.

R4 - The value displayed on the token MUST be easily read and entered by the user: This requires the HOTP value to be of reasonable length. The HOTP value must be at least a 6-digit value. It is also desirable that the HOTP value be 'numeric only' so that it can be easily entered on restricted devices such as phones.

See R4 at https://datatracker.ietf.org/doc/html/rfc4226 (split across pages 4 and 5).

cbdumas• 5 years ago

With that scheme an attacker who gets access to a code (no matter how old) can then generate their own going forward. The hash function here means that even if someone obtains a code they can't generate other codes.

TimWolla• 5 years ago

One reason might be that those characters would include letters. A digits-only code is much easier to type, e.g. using a restricted keypad, such as a phone.

tshaddox3• 5 years ago

Don't all hashes start out as numbers anyway, before they get encoded as alphanumeric strings?

jjeaff• 5 years ago

That's basically what they do. Most apps like Google authenticator use totp rather than hotp. The t stands for time, the h is hmac. Except they also use an algorithm to convert those characters to digits only. It's all described under the totp section.

loloquwowndueo2• 5 years ago

There’s even a program called oathtool which you can use to generate TOTP and HOTP (counter-based and much more prone to skew) codes. You just need the random seed that’s generated when you create a device. I’m not sure how easy it is to extract from the qrcode that web services generate, though. Some identity providers will show the raw random seed.

Jtsummers2• 5 years ago

https://github.com/google/google-authenticator/wiki/Key-Uri-...

That's the format that gets encoded into the QR code. If you can decode the QR code you can get the secret key easily.

jmcphers• 5 years ago

It's pretty easy to extract! zbarimg -q -raw <png> will do it.

http://manpages.ubuntu.com/manpages/bionic/man1/zbarimg.1.ht...

hannob• 5 years ago

AFAIR there isn't much to "extract", you simply use any application that can decode the QR code and you get the key which is encoded as a sequence of letters and numbers.

composer• 5 years ago

    oathtool -w1 --base32 --totp $secret

Add a space before the command depending on your shell. Some shells will keep your secret out of history logs.

sleavey• 5 years ago

I found this part interesting:

Notice that when the key is, say, 40 bytes long, it’s padded with zeros to make it 64 bytes long. So the actual entropy of that key is equivalent to the 40-byte long key. But if the key is longer than 64 bytes, it’s effectively being shortened to 20 bytes. So its entropy is lower. If you want to maximize the complexity of the key, and you’re the one choosing the key, go for something around, but never more than, 64 bytes.

Perhaps that's why some sites have a maximum character limit for passwords. (Although I suspect many are just doing it wrong.)

wyager• 5 years ago

There is no sound technical or security justification for having a maximum character limit, up to some obscenely long limit to prevent the user wasting server resources by submitting giant forms.

Also, no one should be using raw HMAC to do password checking. At the very least use something like PBKDF2.

trinovantes• 5 years ago

At this point, I just automatically assume any website that has a maximum character limit and weird character restrictions is storing the password in plaintext

codegeek• 5 years ago

One really good TOTP app for desktops is Authy [0] from Twilio. Some people are not aware that it exists and you don't need an app on your phone for TOTP. Authy works from your desktop as well.

[0] https://authy.com/download/ (scroll down to see Desktop version for download)

robertlagrant• 5 years ago

Interesting! I didn't know that. Pretty handy for corporate stuff - keep the 2FA on the work computer.

banana_giraffe• 5 years ago

On the topic of TOTP apps, I like "OTP Auth" for iOS devices. It's fairly simple, and offers an Apple Watch client that works well.

jumelles• 5 years ago

Seconded, OTP Auth is really nice. It also syncs via iCloud and you can export your info easily.

0x0• 5 years ago

Yep, the Apple Watch app is a killer app!

jcims• 5 years ago

You can also roll your own with https://github.com/pyauth/pyotp

codetrotter• 5 years ago

roll your own

Libraries for a couple of other languages:

https://crates.io/search?q=4226

https://pkg.go.dev/search?q=4226

tyingq2• 5 years ago

It's almost overkill to bother with a library.

  #!/usr/bin/env python3
  import time, hmac, base64, struct
  def totp(seed,curtime,period,len):
      k = base64.b32decode(seed)
      mac = hmac.new(k, struct.pack('>Q', int(curtime/period)), 'sha1').digest()
      offset=mac[-1] & 0x0f
      otp=struct.unpack('>L', mac[offset:offset+4])[0] & 0x7fffffff
      return str(otp)[-len:].zfill(len)

  myseed='KRUGS4ZANFZSAYJAOJQW4ZDPNUQHG5DS' # use gpg or similar to store
  print(totp(myseed,time.time(),60,6))

foxtacles• 5 years ago

I can recommend oathtool (in combination with GPG) on the CLI: https://www.nongnu.org/oath-toolkit/oathtool.1.html

commoner4• 5 years ago

Hard no from me.

I remember when Authy refused to delete my account, prior to their acquisition, despite promising to do so upon request in their terms of service. I wasn't the only one: https://news.ycombinator.com/item?id=9100525

There are plenty of free and open source TOTP authenticators (that don't require you to provide your phone number), and I don't see a good reason to use Authy over them.

ncann• 5 years ago

What about this link? Seems pretty straightforward to delete an account for me:

https://authy.com/account/delete/

thaumasiotes4• 5 years ago

What's the concept behind having an Authy account? It converts a TOTP seed into a time-based TOTP code. No part of that suggests that you'd need an account.

sebazzz• 5 years ago

For those with a Yubikey, those people can use Yubikey Authenticator on both mobile and desktop.

philjohn• 5 years ago

+1 to this. Yes, it's a bit of a pain to setup with multiple keys, but your secret is stored within the Yubikey itself which is far more secure.

tzs2• 5 years ago

Authy also works on Apple Watch. I've found it very difficult to get Siri to understand "open Authy". It usually ends up saying there is no app name "oathy".

Spelling it out usually works: "open A U T H Y".

thaumasiotes4• 5 years ago

Do you pronounce "authy" and "oathy" the same way? I'd expect "authy" to use whatever vowel you use for "caught", not the vowel of "au revoir".

conductor• 5 years ago

Also, KeePassXC[0] (a password manager) has integrated TOTP, which is very handy.

[0] https://keepassxc.org/

sureglymop• 5 years ago

Wow really? I never knew that, I've been using andOTP.

remuskaos• 5 years ago

There is also Aegis[1]. It's FOSS and available in F Droid and Play Store. Authy is pretty good, but doesn't support export of the secrets used for TOTP, whereas Aegis supports export to json and GPG encrypted json.

I've been using Authy for a few years, but switched to Aegis about three years ago and couldn't be happier. Since Authy doesn't support direct export of the secrets, I've had to use a workaround [2].

[1] https://getaegis.app/ [2] https://gist.github.com/JacobJohansen/5f688d45049be440b8ee87...

saganus• 5 years ago

I was interested in using Aegis, but unfortunately I can't import my current keys from Google Authenticator.

It's "supported" but only via accesing the Authenticator's files (or by havitng root access) which in my celphone is not possible (OnePlus Nord).

Weirdly enough, Aegis does not support bulk importing keys from a QR code, which is how you can migrate from one Authenticator instance to another (e.g. to a new phone).

And I am too lazy to go over each key, reset it and load it up again in Aegis.

Hooefully they'll add this feature at some point in the future.

_Anima_• 5 years ago

I deleted my Authy account after the acquisition. The deletion process was a bit of a struggle for reasons I don't recall clearly.

I now use Aegis on phone and the otp plugin of pass on my Linux desktops (+ a ulauncher plugin).

toastal• 5 years ago

There's also an TOTP pass extension.

https://github.com/tadfisher/pass-otp

Like others have mentioned, it unlike Authy this doesn't use your phone number as identity

timwis• 5 years ago

Note that authy’s login (twilio login) uses sms-based 2fa itself, which is very vulnerable to sim-swap attacks. There is no way to disable sms 2fa on it.

To counter this I recommend disabling device sync in Authy, and only enabling it temporarily when you add a new device.

fetzu• 5 years ago

For iOS users, I can really recommend Raivo OTP[0] a FOSS app that offers everything Authy does (backups included) with easy ways to migrate in- and out- of the app through an encrypted ZIP export of all your TOTP keys.

[0] https://github.com/raivo-otp/ios-application

AnonC2• 5 years ago

The mandatory requirement of a phone number to even set this up (with SMS as the verification method) doesn’t suit me. I use other TOTP apps that don’t ask for a phone number or email address or anything else.

jamra• 5 years ago

Authy once deleted local credentials during an app update. I had to restore with my backup keys.. only I lost my backup keys so I had to go the long way to circumvent my 2fa. They didn’t seem very sorry about it either.

jrm43• 5 years ago

Good stuff. One pernicious "lie through technology" that is out there is that TOTP et al are something special that must be done through some sort of secret or proprietary app or worse, hardware key device.

Nothing wrong with having these as an option -- but e.g. recently my workplace mandated using Duo (the 2FA thing, not google) for our 2FA stuff -- which doesn't let you keep your own generating keys. It's literally the only 2FA thing I have that won't let you do this, and its infuriating because I use oathtool for all the others.

And this is at a university -- so effectively this means you must have a modern cell phone with a particular app or buy a dongle or you can't do school.

thaumasiotes4• 5 years ago

One pernicious "lie through technology" that is out there is that TOTP et al are something special that must be done through some sort of secret or proprietary app or worse, hardware key device.

It's a lie, but you only have a second factor if it's true. Your Duo is attempting to deliver what it promises, where Google Authenticator doesn't even make the attempt.

You can extract the secret directly from the app, though, if you have the option to use it as an app.

jrm43• 5 years ago

I'm trying to parse your second sentence, and I think I somewhat agree in a technical sense, but I think it would be fairer to say, Duo forces a second factor on you, whereas Google gives you the option; and in the long run it's better to not babysit people through a "lie."

Last sentence? Serious question. Can you? I was just presuming that you can't; i.e. if I were to build a freedom-reducing proprietary solution like this, I'd keep the the key on my home servers and only deliver the 6 digit codes?

zaphirplane• 5 years ago

I wonder why credit card issuers haven’t gotten on that idea. Whenever I use my credit card I have to enter a onetime /time based amount based pin.

Rather than the system of anyone that knows my card number can charge whatever amount they like for however long they want

karmicthreat• 5 years ago

I like using Yubico Authenticator. I have a primary and backup Yubikey and I copy the TOTP secret to both. Its also nice that the secret is kept on the key rather than my phone.

Unklejoe• 5 years ago

Does Google itself support TOTP with Google Authenticator? When I try to enable 2FA, the only options seem to be a text, call, security key (I’m guessing this is FIDO?), and “get a notification on your device”.

knorker• 5 years ago

How does this article not link to the specs, aka the RFCs?

nick_name• 5 years ago

  # python3 -m pip install pyotp
  SECRET='JBSWY3DPEHPK3PXP' python3 -c "import pyotp; print(pyotp.TOTP('${SECRET}').now())"

dxdm• 5 years ago

I think you forgot to install that 3rd party dependency that does all the heavy lifting here.

muldvarp• 5 years ago

Although "heavy lifting" is a bit of an overstatement here. pyotp has around 200 lines of code.

tedk-42• 5 years ago

I built this to generate OTP codes via CLI

https://github.com/edify42/otp-codegen

bondolo• 5 years ago

Does the password strategy really just truncate passwords over 64 characters? I don't use them often but since text passwords typically have low number of significant bits per character I have always assumed that the input string was hashed to a fixed length password that would increase in quality until the randomness of the text approached random bytes. My current secure passphrase is a paragraph that I have memorized from a book, about 250 characters, but I only type it a couple of times per month. Disappointing if 190 or so characters of this typing is indeed wasted effort and that at least 8 bytes or more (1-3 bits per character) of entropy is wasted per password because text doesn't use all 8 bits.

mullingitover• 5 years ago

How does Google Authenticator work?

Really poorly. Not only my experience, also the prevailing sentiment in app store reviews, where it has a sub-2.5 star rating.

Up until pretty recently you couldn't even back up your passcodes, so it was extremely risky to use it.

Use Authy, there is clearly a lot more effort put into that app.

tim333• 5 years ago

A non techy friend of mine had some bad issues in that she had it on an iPhone, switched to a new iPhone copying everything over via icloud as one usually does and then finding Google Auth didn't copy the auth codes over or warn you that it won't, and hence was locked out of various accounts.

js2• 5 years ago

There’s also a tool that converts Symantec VIP Access tokens to standard TOTP tokens:

https://github.com/dlenski/python-vipaccess

janfromdaito• 5 years ago

Not a big fan of a Google Authentcator myself, and didnt want to stay with Authy as well, thus I started working on my own authenticator, which is web-ased: https://www.daito.io/

The main differentiator is that this a web-based authenticator, not an app-based one. The main goal being a fully separate service (thus full separation of concerns) from a password manager. It's not ready for prime time yet, but if you are interested to test it, please reach out.

lazyant• 5 years ago

https://www.trustedreviews.com/news/google-ignored-a-key-vul...

How does Google Authenticator work?

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