Save data on SDCard in a secure way in Android

I want to save some files 开发者_Go百科on SDCard which will be downloaded from net. The user should not be able to have direct access to it. I mean the file should be secure and it should not be transferred to other device and if its transferred then it should not be in readable format. Is there a particular directory on SDCard where these files can be saved and be secure too? Also this files should be automatically deleted on uninstall of the app. I guess there is a direct way of that in Android 2.2 but not below that. So, if someone has any idea of doing that then please let me know.

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Encryption. If you're concerned about the user reading a file that's the closest answer you'll find -- but you need a secure way to store the decryption key also, which is not really attainable.

Realistically, anything that must not be viewed by the device owner must not be on the device.

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If the files are in the physical possession of a dedicated user there is no practical way to make them impossible to copy. You can make them DIFFICULT to copy, but not impossible.

If you use encryption as @mah pointed out, and then don't store the key on the device: fetch the key from a server, retain it in memory and never write out the decrypted file or the key to disk you might be ok in deterring the casual pirate. The dedicated souls will figure it out.