S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key wiki:encryption and signing of MIME data. S/MIME is on an IETF standards track and defined in a number of documents, most importantly RFCs. S/MIME was originally developed by RSA Data Security Inc. The original specification used the recently developed 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, an IETF specification that is identical in most respects with PKCS #7. S/MIME functionality is built into the majority of modern e-mail software and interoperates between them.