S/MIME

S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public-key encryption and signing of MIME data. S/MIME is on an IETFstandards track and defined in a number of documents, most importantly RFC8551. It was originally developed by RSA Data Security, and the original specification used the IETF MIME specification with the de facto industry standard PKCS #7 secure message format. Change control to S/MIME has since been vested in the IETF, and the specification is now layered on Cryptographic Message Syntax (CMS), an IETF specification that is identical in most respects with PKCS #7. S/MIME functionality is built into the majority of modern email software and interoperates between them. Since it is built on CMS, MIME can also hold an advanced digital signature.