RFC 1847 (rfc1847) - Page 2 of 11

Security Multiparts for MIME: Multipart/Signed and Multipart/Encrypted

RFC 1847                  Security Multiparts               October 1995


1.  Introduction

   An Internet electronic mail message consists of two parts: the
   headers and the body.  The headers form a collection of field/value
   pairs structured according to STD 11, RFC 822 [1], whilst the body,
   if structured, is defined according to MIME [2].  The basic MIME
   specification does not provide specific security protection.

   This document defines a framework whereby security protection
   provided by other protocols may be used with MIME in a complementary
   fashion.  By itself, it does not specify security protection.  A MIME
   agent must include support for both the framework defined here and a
   mechanism to interact with a security protocol defined in a separate
   document.  The resulting combined service provides security for
   single-part and multi-part textual and non-textual messages.

   The framework is provided by defining two new security subtypes of
   the MIME multipart content type: signed and encrypted.  In each of
   the security subtypes, there are exactly two related body parts: one
   for the protected data and one for the control information.  The type
   and contents of the control information body parts are determined by
   the value of the protocol parameter of the enclosing multipart/signed
   or multipart/encrypted content type, which is required to be present.
   By registering new values for the required protocol parameter, the
   framework is easily extended to accommodate a variety of protocols.

   A MIME agent that includes support for this framework will be able to
   recognize a security multipart body part and to identify its
   protected data and control information body parts.  If the value of
   the protocol parameter is unrecognized the MIME agent will not be
   able to process the security multipart.  However, a MIME agent may
   continue to process any other body parts that may be present.

2.  Definition of Security Subtypes of Multipart

   The multipart/signed content type specifies how to support
   authentication and integrity services via digital signature.  The
   control information is carried in the second of the two required body
   parts.

   The multipart/encrypted content type specifies how to support
   confidentiality via encryption.  The control information is carried
   in the first of the two required body parts.

   A three-step process is described for the origination and reception
   of the multipart/signed and multipart/encrypted contents.  The
   details of the processing performed during each step is left to be
   specified by the security protocol being used.



Galvin, et al               Standards Track