Secure Multipurpose Internet Mail Extensions
The Secure Multipurpose Internet Mail Extensions (S/MIME) protocol adds security to the existing MIME format which enables any content to be carried in an Simple Mail Transport Protocol (SMTP) message. SMTP is only able to communicate text, so MIME provides an encoding mechanism to carry encoded “binary” files as text characters. S/MIME uses all of public key encryption, secret key encryption, and hash encryption to allow emails, and their attachments, to be encrypted or signed or both. A common alternative to S/MIME is OpenPGP, which enables a similar capability but is based on the pretty good privacy (PGP) set of protocols.