Pretty Good Privacy (PGP) was developed during the same period as PEM, in the early 1990’s. PGP was originally designed for securing Internet mail. PGP shares most technical features, such as digital signatures and public-key based encryption, with PEM. Like PEMit uses a proprietary, non-MIME-compatible, message format [3]. However, later MIME-compatible variations have evolved [22]. PGP’s main difference from other proposals is its key management system. It does not use Certificates, but rather a proprietary syntax. Also, it uses a non-hierarchical certification model known as “web of trust”. We will not study PGP further, a good reference is [98], and an account of PGP History can be.
