SysInternals has published the source code of its tools. I took a look at Secure Delete. This is an app that meets C2 compliance for truly deleting file contents on disk. Note that the filename is not fully deleted from the system. The secure delete is accomplished by an overwrite with the secure delete pattern.
All of the security requirements come from 5220.22-M, which is the standard from the Department of Defense. The specifics are that you must overwrite deleted files with (1) a character, (2) the complement of the character, and (3) a random character. This technique is not sufficient for Top Secret material. That would requires a hardware degaussing of the drive.
The source code is provided. However it is copyrighted by Mark Russinovich. The cool thing is that this secure delete works on compressed and encrypted files as well. Good apps like this are probably which Microsoft bought out SysInternals some time ago.
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