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The apps we write will soon be enhanced by downloadable “packages” using the in-app purchase API. We would like therefore to start securing our content which we wish to allow the users to download/extract onto their memory card (so as to not use up internal memory for our large applications), however, we need to secure the files somehow so that they can’t simply be taken from the SD.
Can anyone suggest some possible/simple/common techniques used to do so on Android?
I’d think you’d probably want to generate a hash based on some unique device identifier and a strong key of your choosing. A unique identifier for the device can be generated using the technique discussed in this answer. Have the App transmit that hash to your server, and encrypt the package before (or as) it is sent to the user using this hash as a key. Your app will decrypt the data as it is read by generating the key on demand (the same way it was initially generated). Have a look at the MessageDigest class and the javax.crypto package in the API.