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S/MIME (Secure/Multipurpose Internet Mail Extensions) is a standard for public key encryption and signing of MIME data. S/MIME is on an IETF standard track and defined in several documents, most importantly RFC 3369, 3370, 3850, and 3851. RSA Data Security originally developed it, 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). This IETF specification is identical in most respects to PKCS #7. S/MIME functionality is built into most modern email software and interoperates between them. Since it is built on CMS, MIME can also hold an advanced digital signature. S/MIME