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Most people have some files that they would rather not show to others - passwords, sensitive information, classified documents from work, the list can be expanded forever. probably you have saved some of this data on your PC where it is easily at your reach, but when the time comes to erase the files from your disk, things get a little bit more difficult and maintaining your privacy is not as easy as it may have seemed at first.
Normal file erasing is insecure
Your first thought may be that when you erase file or folder, the information is gone. Not quite, when you erase file or folder, the Windows does not actually remove the file from the hard driver; it only removes the record about the file from the file system table. The information keeps on the hard driver as long as another file overwrites it, and even after that, it might be possible to recover data by studying the magnetic fields on the disk platter surface. Before the file is overwritten, anyone can without problems view it with a disk maintenance or an undelete utility.
For example, imagine that you have been surfing on the web for a while and afterwards wish to clear all traces revealing what sites you read. You go to your browser's preferences and select to erase the cache and the history file, the data is now gone you think to yourself - well think again. The browser cache files can easily be restored with an undelete utility and your privacy is once again compromised.
To be sure that a file is gone, it must be appropriately overwritten before deleting. As simple as it looks, there are some problems in secure information removal, mostly caused by the construction of a hard driver and the use of files encoding. These problems have been taken into consideration when special eraser tool is designed and because this intuitive design you able to safely and without problems delete sensitive data from the hard disk.
You have most likely already insecurely removed large amount of data from your disk and from time to time applications create (and insecurely remove) temporary files on your disk containing some probably personal data that you would rather not share with other people. This information remains on your hard disk until it gets overwritten and can be retrieved with simple hard disk tool.
This is where the erasing of unused HDD space comes in handy. The erasing of unused disk space means that all free space on the hard disk will be overwritten so that data previously stored on it cannot be retrieved. Good eraser software provides you a convenient way to erase the free hard disk space regularly in order to remove the all temporary files and other private data you possibly have had on the disk.
By now you must be wondering what exactly this software does to my system when erasing files. You have come to the right place, the procedures gone through when erasing data are explained here.
After determining the file type (files encrypted or compressed at the file system level are supported on Windows NT and 2000, but Administrator privileges are required for low-level hard driver access), Eraser needs to determine the file size. When determining the size, the cluster tip area is included so the data stored on it will be erased too.
Once the size is calculated, the file will be overwritten with the special method (see detailed descriptions of the techniques bellow). Eraser software takes care of flushing write buffers to make sure that the data really gets written to the hard disk and is not only saved in a buffer somewhere. If the overwriting was done, the last step is to correctly erase the file.
Before deleting the reference of the file from the file table (standard delete), the file will be truncated to 0 length to remove traces of the allocated clusters, the filename will be overwritten and finally file dates (creation, access, modified) will be scrambled to finish the file removing.
Method of Gutmann
Based on Peter Gutmann's paper "Secure Deletion of Data from Magnetic and Solid-State Memory", the method provides the best security. File will be overwritten 35 times with carefully selected patterns, which makes it unrecoverable.
This method is used as the default for erasing files, but has been proven to be very slow when clearing unused space on a disk (could be many gigabytes).
A Faster Method - US DoD 5220-22.M
Two methods based on United States Department of Defense recommendation 5220-22.M from January 1995. The data will be written 7 times making this method significantly faster than the Gutmann's Method, but also less secure when it comes to hardware recovery.
Pseudorandom Data
All passes will be pseudorandom data, which is very incompressible. Therefore, this is the only method that should be used when deleting unused space or files on a compressed drive. The number of passes is user selectable from one to 65535.
Being the fastest method, this one is used as recommended for removing unused disk space.
Author is founder of Free Security Software site. Visit this page to download open source file deletion solution.
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