Complex sentence: Originally written in 1991, Pretty Good Privacy, or just 'PGP' for short, was one of the first freely and publicly available implementations of public-key cryptography, and was originally used to allow individuals to communicate securely through bulletin board systems.
Split into simpler sentences: Originally written in 1991, Pretty Good Privacy (PGP) was one of the first freely and publicly available implementations of public-key cryptography. It was originally used to allow individuals to communicate securely through bulletin board systems.