Question

I am going to use oAuth to fetch mails and contacts from google. I don’t want to ask the user each time to log in to obtain an access token and secret. From what I understood, I need to store them with my application either in a database or SharedPreferences. But I am a bit worried about security aspects with that. I read that you can encrypt and decrypt the tokens but it is easy for an attacker to just decompile your apk and classes and get the encryption key.
What’s the best method to securely store these tokens in Android?

Answer 1

Store them as shared preferences. Those are by default private, and other apps cannot access them. On a rooted devices, if the user explicitly allows access to some app that is trying to read them, the app might be able to use them, but you cannot protect against that. As for encryption, you have to either require the user to enter the decrypt passphrase every time (thus defeating the purpose of caching credentials), or save the key to a file, and you get the same problem.

There are a few benefits of storing tokens instead of the actual username password:

• Third party apps don’t need to know the password and the user can be sure that they only send it to the original site (Facebook, Twitter, Gmail, etc.)
• Even if someone steals a token, they don’t get to see the password (which the user might be using on other sites too)
• Tokens generally have a lifetime and expire after a certain time
• Tokens can be revoked if you suspect they have been compromised