In this method malwares uses file read and write permission to select target file and then encrypt them. Doing these task is very easy. But consider that the malware app can just access to limited file including SDCard information if user grant this permission to it.

Sometimes has been saw which this type of ransomwares contains root exploit payloads to first gain root access and then encrypt the files.

This method isn’t efficient most of the time because usually user data are backup in cloud or most of the time the rooting exploit didn’t work.

Code example.