In practical terms, you really cannot, because you don’t know which medium is used for storage and whether its blocks can be reliably overwritten. This is especially true for mobile devices containing flash memory, which has durability to extend its service life and does not guarantee overwriting the same block inside when you overwrite a block in the file system. This would allow a certain adversary to gain physical control over the environment to restore previously written blocks.
You would be better off assuming that your data will be intercepted and encrypted for storage.
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