RFC2480

Network Working Group N. Freed Request for Comments: 2480 Innosoft International, Inc. Category: Standards Track January 1999

             Gateways and MIME Security Multiparts

Status of this Memo

This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1999). All Rights Reserved.

Abstract

This document examines the problems associated with use of MIME security multiparts and gateways to non-MIME environments. A set of requirements for gateway behavior are defined which provide facilities necessary to properly accomodate the transfer of security multiparts through gateways.

Requirements Notation

This document occasionally uses terms that appear in capital letters. When the terms "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" appear capitalized, they are being used to indicate particular requirements of this specification. A discussion of the meanings of the terms "MUST", "SHOULD", and "MAY" appears in RFC 1123 [2]; the terms "MUST NOT" and "SHOULD NOT" are logical extensions of this usage.

The Problem

Security multiparts [RFC-1847] provide an effective way to add integrity and confidentiality services to protocols that employ MIME objects [RFC-2045, RFC-2046]. Difficulties arise, however, in heterogeneous environments involving gateways to environments that don't support MIME. Specifically:

(1)   Security services have to be applied to MIME objects in
      their entirety. Failure to do so can lead to security
      exposures.

      For example, a signature that covers only object data and not
      the object's MIME labels would allow someone to tamper with
      the labels in an undetectable fashion.  Similarly, failure to
      encrypt MIME label information exposes information about the
      content that could facilitate traffic analysis.

      Composite MIME objects (e.g., multipart/mixed, message/rfc822)
      also have to be secured as a unit.  Again, failure to do so
      may facilitate tampering, reveal important information
      unnecessarily, or both.

(2)   Gateways that deal with MIME objects have to be able to
      convert them to non-MIME formats.

      For example, gateways often have to transform MIME labelling
      information into other forms. MIME type information may end up
      being expressed as a file extension or as an OID.

      Gateways also have to take apart composite MIME objects into
      their component parts, converting the resulting set of parts
      into whatever form the non-MIME environments uses for
      composite objects. Failure to do so makes the objects unusable
      in any environment that doesn't support MIME. In many cases
      this also means that multi-level MIME structures have to be
      converted into a sequential list of parts.

(3)   Security services have to be deployed in an end-to-end
      fashion. Failure to do so again can lead to security
      exposures.

      An integrity service deployed at something other than a
      connection end point means a region exists between the point
      where the integrity service is applied and the actual end
      point where object tampering is possible. A confidentiality
      service deployed at something other than a connection end
      point means a region exists where the object is transferred in
      the clear. And worse, distributed private keys are usually
      necessary whenever someone other than the originator applies
      an integrity service or someone other than the recipient
      removes a confidentiality service, which in turn may make
      theft of private key information a possibility.

      All of these issues can be addressed, of course. For example,
      it may be possible to use multiple overlapping security
      services to assure that no exposure exists even though there
      is no end-to-end security per se. And keys can be distributed
      in a secure fashion. However, such designs tend to be quite
      complex, and complexity in a security system is highly

      undesireable.

The preceeding three requirments are fundamentally in conflict: It is possible to satisfy two of them at once, but not all three at once.

In fact the conflict is even worse than it first appears. In most situations of this sort some sort of compromise is possible which, while not satisfying any of the requirements completely, does optimize some sort of average of all the requirements. Such a solution does not exist in this case, however, because many real world situations exist where any one of these requirements absolutely must be satisfied.

Solving the Problem

Since the previously described problem doesn't allow for a single solution the only viable approach is to require that gateways provide multiple solutions. In particular, gateways

(1)   MUST provide the ability to tunnel multipart/signed and
      multipart/encrypted objects as monolithic entities if there is
      any chance whatsoever that MIME capabilities exist on the
      non-MIME side of the gateway. No changes to content of the
      multipart are permitted, even when the content is itself a
      composite MIME object.

      This option must be provided so that entities behind the
      gateway that are capable of processing security multiparts and
      their MIME content will work properly.  As mentioned
      previously, situations exist where application security
      requirements are absolute and must be accomodated, even when
      meeting them causes problems for other agents.

      Exceptions are allowed only when there is no possibility of
      MIME support on one side of the gateway.  For example, a
      gateway to a voice messaging system may have no useful way to
      represent a signed MIME object.

(2)   MUST provide the ability to take apart multipart/signed
      objects, exposing the content (and in the process ruining the
      signature). When this approach is selected, gateways SHOULD
      NOT remove the signature. Instead, gateways SHOULD keep the
      signature intact and add to it a note that it will probably be
      invalid for checking the message contents, but may still be
      contain valuable information about the sender.

      This option must be provided so that entities behind the
      gateway which are incapable of processing MIME will work
      properly.

(3)   SHOULD provide the ability to select between the previous two
      options on per-user basis.

(4)   MAY provide facilities to check signatures and decrypt
      encrypted content. Such facilities MUST NOT be enabled by
      default; the potential security exposure involved has to be
      assessed before such capabilities can be used.

(5)   MAY provide facilities to sign and/or encrypt material passing
      from the non-MIME side to the MIME side of the gateway. Again,
      such facilities MUST NOT be enabled by default; the potential
      security exposure involved in the transfer of unsecured
      content within the application domain behind the gateway has
      to be assessed before such capabilities can be used.

A gateway which complies with the above requirements is considered to be security multiparts compliant.

Security Considerations

This entire document is about security.

References

[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet

          Text Messages", STD 11, RFC 822, August, 1982.

[RFC-1847] Galvin, J., Murphy, S., Crocker, S. and N. Freed,

          "Security Multiparts for MIME: Multipart/Signed and
          Multipart/Encrypted", RFC 1847, October 1995.

[RFC-1123] Braden, R., Ed., "Requirements for Internet Hosts --

          Application and Support", STD 3, RFC 1123, October 1989.

[RFC-2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail

          Extensions (MIME) Part One: Format of Internet Message
          Bodies", RFC 2045, December 1996.

[RFC-2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail

          Extensions (MIME) Part Two: Media Types", RFC 2046,
          December 1996.

[RFC-2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail

          Extensions (MIME) Part Five: Conformance Criteria and
          Examples", RFC 2049, December 1996.

Author's Address

Ned Freed Innosoft International, Inc. 1050 Lakes Drive West Covina, CA 91790 USA

Phone: +1 626 919 3600 Fax: +1 626 919 3614 EMail: [email protected]

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