Crypting and Securing Android data

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http://stackoverflow.com – My app picks up the GPS location of the user every 5 minutes, saves it and then sends it to a server. The problem I'm facing is I need the data stored on the android SD card to be Crypted so no one except me on the server can access it. Also I need to think of a way to make sure that no-one has edited the data except me. For the last I've thought writing the MD5 checksum of the file each time I close it so when the app opens the file it checkes the MD5 to see if they match. The problem with this is I think it'll waste resources and battery as I'm already checking for the GPS location every 5 m (HowTos)