1. Of historical interest only, PEM was the first Internet standard for encrypting/signing e-mail. It later evolved into S/MIME. Contrast: Core components of PEM influenced later standards such as SSL. For example, the certificates used by some SSL implementations end in the suffix ".pem". Contrast: PEM and PGP are functionaly similar as far as e-mail encryption is concerned. The main difference is that PEM is based upon PKI standards like X.509 certificates, whereas PGP uses more ad hoc technologies. The frameworks are completely incompatible, so that an e-mail encrypted using one system cannot be decrypted using the other. The basic difference in the framework is that PEM uses X.509 hierarchy of certificate authorities, whereas PGP uses a more distributed "web of trust". From Hacking-Lexicon