Simply deleting a file from your computer via a "trash bin" or "rm" command does not ensure that the file is deleted at all. What most often happens is that the file's name is merely removed from the disk's index, but the data still remains on the disk itself. There are many undelete programs out there which can easily recover this data. By overwriting the data with random junk, the ability to recover it is next to impossible.

More advanced techniques to recover lost data also exist. Overwriting data once is usually not good enough for these solutions. A popular standard states that you should overwrite three times with zeros and ones (zero-one-zero-one-zero-one over every byte), and then with random data. This should even erase the magnetic remains of your data, which still can be detected after it has been overwritten once. Of course, you can overwrite with ones, zeros and random junk many times.

Another thing to consider is the file name, location and date/timestamps. Even if you can erase the data itself, the information about the file may still be availabe in system files somewhere, giving the attacker some information on the deleted files.

Several of the programs below are capable of also erasing the filename and related information from the administration table.

Warning: This approach means that the program will write information directly to the administration table. If something goes wrong, it may corrupt this table. This may result in your entire hard disk becoming corrupt.

Note on logging filesystems However, all of the observations made above about over-writing data in files can be made void by a modern file system, such as a log-structured file-system (LFS). LFS obtains its performance by refraining from seeking for blocks on the disk. Instead, new data is written into new blocks which is placed at the end of the log. This incures a security issue by leaving data on disk that was supposed to be overwritten. Many programs, PGP in particular, assumes that data can be physically overwritten through the file system (by overwriting the data in the file). Although no file system API guarantees that data will actually be overwritten, most systems will do so (assuming the file is not extended). In a LFS, this will almost certainly not be the case.

Further Reading

Why a normal delete is not sufficient. A more detailed analysis of why wiping files is necessary to make sure the data is gone, and what techniques exist to recover erased files.

File Ghosts - Why and how they are left and how they can be retreived.

Tools:

The WIPE utility on PGP is a securely designed utility that works great. Highly reccomended on all platforms. Set your preferences to overwrite files more than 10 times.

DOS/Windows

Real Delete is a DOS TSR, which intercepts all DOS delete calls, and turns them into overwriting deletes. Very useful if you use programs that create temporary files, as these may contain copies of your sensitive data.

Macintosh

Burn 2.5 can overwrite files and free space, while also removing information from the file administration.

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Wiping Your Swap File

Many operating systems use a swapfile to store data from RAM, so you can run more programs than would fit in your memory. This means that anything that is kept in memory can be written to the swapfile. To get rid of the swapfile and the data in it, you can't just erase the swapfile. Wiping your swapfile.

Resources:

Secure Deletion of Data from Magnetic and Solid-State Memory - This whitepaper describes some of the methods available to recover erased data and presents schemes to make this recovery significantly more difficult.

Disk and File Shredders: A Comparison of Windows 95/98/NT utilities.

Directory Snoop - A useful freeware program which lets you view deleted sections of your hard drive, your Windows95 swap file, etc., for residual data.