Pretty Good Privacy (PGP) and Privacy Enhanced Mail (PEM) are used mainly for electronic mail security. [12] They are datagram-like in that they allow users to send single, asynchronous messages to each other.
PEM is more flexible, in that it offers a choice of cryptographic algorithms and certificate authorities, although these can't be negotiated since messages are asynchronous. PEM is also more extensible, since PEM data are passed in named header fields, as specified in [2]. PGP requires less infrastructure and is thus suited for more ad hoc environments.