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.

1. 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.

2. 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 RFC1123 [2]; the
terms "MUST NOT" and "SHOULD NOT" are logical extensions of this
usage.

3. 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.

4. 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.

5. Security Considerations

This entire document is about security.

6. References

[RFC-822] Crocker, D., "Standard for the Format of ARPA Internet
Text Messages", STD 11, RFC822, August, 1982.

[RFC-1847] Galvin, J., Murphy, S., Crocker, S. and N. Freed,
"Security Multiparts for MIME: Multipart/Signed and
Multipart/Encrypted", RFC1847, October 1995.

[RFC-1123] Braden, R., Ed., "Requirements for Internet Hosts --
Application and Support", STD 3, RFC1123, October 1989.

[RFC-2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC2045, December 1996.

[RFC-2046] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", RFC2046,
December 1996.

[RFC-2049] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Five: Conformance Criteria and
Examples", RFC2049, December 1996.

7. 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: ned.freed@innosoft.com

8. Full Copyright Statement

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

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