PGP (Pretty Good Privacy) is currently the best and most popular data (including texts, e-mails, files, directories, partitions, and whole disk) signing, encrypting, and decrypting program, developed by Phil Zimmermann in 1991. Its source code is open, and it has withstood the challenges from thousands of top hackers around the world. It turns out that PGP is still the most secure encryption software in the world for now (until the popularity of quantum computers, I think). Without a password, no one can decrypt the data encrypted with it!

After all kinds of information leak events, more and more people or companies began to realize the common ways to delete important data is unadvisable. Because that only removes the corresponding data's records from the File Allocation Table, while all data is still intact on the hard drive. Just because of this, many software can restore the data with ease.