When you delete a file from your computer, it may seem permanent and irreversible enough, but in fact it is not. It is still able to recover this data through a variety of means, using software designed for this purpose. This may be helpful for accidental deletions, but when private or confidential data is involved, this represents security vulnerability.
File deletion works by just changing the tag on a particular file to the value 'deleted', and making it inaccessible from the default set of menus and directories. However, the data and the file itself actually remains on the hard disk, and only disappear when it is overwritten in the future by new data. But this overwriting may not happen until after the file data has already been recovered!
In fact, some operating systems have a built-in UNDELETE command, which can easily recover recently deleted files. Here, this really is a big vulnerability that can be exploited by malicious parties to gain access to confidential or private information.
An analogous problem arises in disposing of confidential paperwork, and the typical solution is to use so called paper shredders. These shredders tear the paper into fine strips which are then nearly impossible to reconstruct the original from. The solution to the digital counterpart of the problem lies in software, also called shredders.
These utilities are known as file shredders (examples include Super Shredder, among others), and are designed to permanently delete any particular file or group of files. This is done by overwriting the actual data in the file to be deleted with random data. What this means is that no recognizable tracks of the shredded file remains on the hard disk after deletion.
This can be better understood by realizing that the data contained within any type of file can be reduced, fundamentally, to some series of 1's and 0's that then form bits and bytes. Ordinary file deletion leaves the majority of this sequence of bits unchanged, changing only the small portion of it corresponding to the "deleted" or "available" metatag. File shredders, on the other hand, overwrite these bits with random bits, effectively masking the old pattern that was contained in the shredded file.
Because almost all traces of the original data are wiped clean, it becomes next to impossible to retrieve any useful information from files shredded in this manner. Many file shredders perform this random overwriting process several times to ensure that the resultant pattern is truly random, and is no longer correlated with the old data.
These file shredders can sometimes also deal with incoming hard disks at once. This is useful when disposing of or transferring old computers, for instance, to ensure that no recoverable data remains on the hard disk. These hard disk wipes work on the same basic principle as shredding an individual file: all of the data on the hard disk is overwritten with random bits.
Many file shredders (super shredder, for instance, but there are others) are available for download or purchase on the internet. This means that secure deletion is easier than ever to perform.