OAuth secrets in mobile apps

When using the OAuth protocol, you need a secret string obtained from the service you want to delegate to. If you are doing this in a web app, you can simply store the secret in your data base or on the file system, but what is the best way to handle it in a mobile app (or a desktop app for that matter)?

Storing the string in the app is obviously not good, as someone could easily find it and abuse it.

Another approach would be to store it on your server, and have the app fetch it on every run, never storing it on the phone. This is almost as bad, because you have to include the URL in the app.

The only workable solution I can come up with is to first obtain the Access Token as normal (preferably using a web view inside the app), and then route all further communication through our server, which would append the secret to the request data and communicate with the provider. Then again, I'm a security noob, so I'd really like to hear some knowledgeable peoples' opinions on this. It doesn't seem to me that most apps are going to these lengths to guarantee security (for example, Facebook Connect seems to assume that you put the secret into a string right in your app).

Another thing: I don't believe the secret is involved in initially requesting the Access Token, so that could be done without involving our own server. Am I correct?

Yes, this is an issue with the OAuth design that we are facing ourselves. We opted to proxy all calls through our own server. OAuth wasn't entirely flushed out in respect of desktop apps. There is no prefect solution to the issue that I've found without changing OAuth.

If you think about it and ask the question why we have secrets, is mostly for provision and disabling apps. If our secret is compromised, then the provider can only really revoke the entire app. Since we have to embed our secret in the desktop app, we are sorta screwed.

The solution is to have a different secret for each desktop app. OAuth doesn't make this concept easy. One way is have the user go and create an secret on their own and enter the key on their own into your desktop app (some facebook apps did something similar for a long time, having the user go and create facebook to setup their custom quizes and crap). It's not a great experience for the user.

I'm working on proposal for a delegation system for OAuth. The concept is that using our own secret key we get from our provider, we could issue our own delegated secret to our own desktop clients (one for each desktop app basically) and then during the auth process we send that key over to the top level provider that calls back to us and re-validates with us. That way we can revoke on own secrets we issue to each desktop client. (Borrowing a lot of how this works from SSL). This entire system would be prefect for value-add webservices as well that pass on calls to a third party webservice.

The process could also be done without delegation verification callbacks if the top level provider provides an API to generate and revoke new delegated secrets. Facebook is doing something similar by allowing facebook apps to allow users to create sub-apps.

There are some talks about the issue online:

http://blog.atebits.com/2009/02/fixing-oauth/ http://groups.google.com/group/twitter-development-talk/browse_thread/thread/629b03475a3d78a1/de1071bf4b820c14#de1071bf4b820c14

Twitter and Yammer's solution is a authentication pin solution: https://dev.twitter.com/oauth/pin-based https://www.yammer.com/api_oauth_security_addendum.html

With OAUth 2.0, you can store the secret on the server. Use the server to acquire an access token that you then move to the app and you can make calls from the app to the resource directly.

With OAuth 1.0 (Twitter), the secret is required to make API calls. Proxying calls through the server is the only way to ensure the secret is not compromised.

Both require some mechanism that your server component knows it is your client calling it. This tends to be done on installation and using a platform specific mechanism to get an app id of some kind in the call to your server.

(I am the editor of the OAuth 2.0 spec)

One solution could be to hard code the OAuth secret into the code, but not as a plain string. Obfuscate it in some way - split it into segments, shift characters by an offset, rotate it - do any or all of these things. A cracker can analyse your byte code and find strings, but the obfuscation code might be hard to figure out.

It's not a foolproof solution, but a cheap one.

Depending on the value of the exploit, some genius crackers can go to greater lengths to find your secret code. You need to weigh the factors - cost of previously mentioned server side solution, incentive for crackers to spend more efforts on finding your secret code, and the complexity of the obfuscation you can implement.

Do not store the secret inside the application.

You need to have a server that can be accessed by the application over https (obviously) and you store the secret on it.

When someone want to login via your mobile/desktop application, your application will simply forward the request to the server that will then append the secret and send it to the service provider. Your server can then tell your application if it was successful or not.

Then if you need to get any sensitive information from the service (facebook, google, twitter, etc), the application ask your server and your server will give it to the application only if it is correctly connected.

There is not really any option except storing it on a server. Nothing on the client side is secure.

Note

That said, this will only protect you against malicious client but not client against malicious you and not client against other malicious clients (phising)...

OAuth is a much better protocol in browser than on desktop/mobile.

There is a new extension to the Authorization Code Grant Type called Proof Key for Code Exchange (PKCE). With it, you don't need a client secret.

> PKCE (RFC 7636) is a technique to secure public clients that don't use > a client secret. > > It is primarily used by native and mobile apps, but the technique can > be applied to any public client as well. It requires additional > support by the authorization server, so it is only supported on > certain providers.

from https://oauth.net/2/pkce/

For more information, you can read the full RFC 7636 or this short introduction.

Here's something to think about. Google offers two methods of OAuth... for web apps, where you register the domain and generate a unique key, and for installed apps where you use the key "anonymous".

Maybe I glossed over something in the reading, but it seems that sharing your webapp's unique key with an installed app is probably more secure than using "anonymous" in the official installed apps method.

With OAuth 2.0 you can simply use the client side flow to obtain an access token and use then this access token to authenticate all further requests. Then you don't need a secret at all.

A nice description of how to implement this can be found here: https://aaronparecki.com/articles/2012/07/29/1/oauth2-simplified#mobile-apps

I don't have a ton of experience with OAuth - but doesn't every request require not only the user's access token, but an application consumer key and secret as well? So, even if somebody steals a mobile device and tries to pull data off of it, they would need an application key and secret as well to be able to actually do anything.

I always thought the intention behind OAuth was so that every Tom, Dick, and Harry that had a mashup didn't have to store your Twitter credentials in the clear. I think it solves that problem pretty well despite it's limitations. Also, it wasn't really designed with the iPhone in mind.

Facebook doesn't implement OAuth strictly speaking (yet), but they have implemented a way for you not to embed your secret in your iPhone app: https://web.archive.org/web/20091223092924/http://wiki.developers.facebook.com/index.php/Session_Proxy

As for OAuth, yeah, the more I think about it, we are a bit stuffed. Maybe this will fix it.

I agree with Felixyz. OAuth whilst better than Basic Auth, still has a long way to go to be a good solution for mobile apps. I've been playing with using OAuth to authenticate a mobile phone app to a Google App Engine app. The fact that you can't reliably manage the consumer secret on the mobile device means that the default is to use the 'anonymous' access.

The Google App Engine OAuth implementation's browser authorization step takes you to a page where it contains text like: "The site <some-site> is requesting access to your Google Account for the product(s) listed below"

YourApp(yourapp.appspot.com) - not affiliated with Google

etc

It takes <some-site> from the domain/host name used in the callback url that you supply which can be anything on the Android if you use a custom scheme to intercept the callback. So if you use 'anonymous' access or your consumer secret is compromised, then anyone could write a consumer that fools the user into giving access to your gae app.

The Google OAuth authorization page also does contain lots of warnings which have 3 levels of severity depending on whether you're using 'anonymous', consumer secret, or public keys.

Pretty scary stuff for the average user who isn't technically savvy. I don't expect to have a high signup completion percentage with that kind of stuff in the way.

This blog post clarifies how consumer secret's don't really work with installed apps. http://hueniverse.com/2009/02/should-twitter-discontinue-their-basic-auth-api/

Here I have answer the secure way to storing your oAuth information in mobile application

https://stackoverflow.com/a/17359809/998483

https://sites.google.com/site/greateindiaclub/mobil-apps/ios/securelystoringoauthkeysiniosapplication

None of these solutions prevent a determined hacker from sniffing packets sent from their mobile device (or emulator) to view the client secret in the http headers.

One solution could be to have a dynamic secret which is made up of a timestamp encrypted with a private 2-way encryption key & algorithm. The service then decrypts the secret and determines if the time stamp is +/- 5 minutes.

In this way, even if the secret is compromised, the hacker will only be able to use it for a maximum of 5 minutes.

I'm also trying to come up with a solution for mobile OAuth authentication, and storing secrets within the application bundle in general.

And a crazy idea just hit me: The simplest idea is to store the secret inside the binary, but obfuscated somehow, or, in other words, you store an encrypted secret. So, that means you've got to store a key to decrypt your secret, which seems to have taken us full circle. However, why not just use a key which is already in the OS, i.e. it's defined by the OS not by your application.

So, to clarify my idea is that you pick a string defined by the OS, it doesn't matter which one. Then encrypt your secret using this string as the key, and store that in your app. Then during runtime, decrypt the variable using the key, which is just an OS constant. Any hacker peeking into your binary will see an encrypted string, but no key.

Will that work?

As others have mentioned, there should be no real issue with storing the secret locally on the device.

On top of that, you can always rely on the UNIX-based security model of Android: only your application can access what you write to the file system. Just write the info to your app's default SharedPreferences object.

In order to obtain the secret, one would have to obtain root access to the Android phone.