Internet DRAFT - draft-mahy-dispatch-immi-mls-mime
draft-mahy-dispatch-immi-mls-mime
dispatch R. Mahy
Internet-Draft Wire
Intended status: Informational 7 March 2022
Expires: 8 September 2022
Inside MLS Message Interop (IMMI) MIME type extensions
draft-mahy-dispatch-immi-mls-mime-00
Abstract
This document defines two new extensions to the MLS (Messaging Layer
Security) Protocol to allow for negotiation of MIME types exchanged
among members of an MLS group.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on 8 September 2022.
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Extension Description . . . . . . . . . . . . . . . . . . . . 2
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 3
4.1. accepted_mime_types MLS Extension Type . . . . . . . . . 3
4.2. required_mime_types GroupContext extension . . . . . . . 4
5. Security Considerations . . . . . . . . . . . . . . . . . . . 4
6. Normative References . . . . . . . . . . . . . . . . . . . . 4
7. Informative References . . . . . . . . . . . . . . . . . . . 5
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 5
1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2219].
The terms MLS client, MLS group, and KeyPackage have the same
meanings as in the MLS protocol [I-D.ietf-mls-protocol].
2. Introduction
MLS is a group key establishment protocol motivated by the desire for
group chat with efficient end-to-end encryption. While one of the
motivations of MLS is interoperable standards-based secure messaging,
the MLS protocol does not define or prescribe any format for the
encrypted "application messages" encoded by MLS. This document
describes two extensions to MLS which allow MLS clients to advertise
their supported MIME types, and to specify which MIME types are
required for a particular MLS group. These allow clients to discover
MLS groups with an interoperable and extensible set of content types.
A companion document [I-D.mahy-dispatch-immi-content] describes a
specific profile for interoperable instant messaging body types.
3. Extension Description
This document specifies two MLS extensions of type MimeTypeList:
accepted_mime_types, and required_mime_types.
MimeType is the ASCII string encoded as a TLS vector type containing
a single MIME type and any of its parameters.
MimeTypeList is an ordered list of MimeType objects.
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// Text string representation of a single IANA registered MIME Type.
MimeType mime_type<V>
struct {
MimeType mime_types<V>
} MimeTypeList
Example MIME Types:
image/png
text/plain;charset="UTF-8"
An MLS client which implements this specification SHOULD include the
accepted_mime_types extensions in its KeyPackages, listing all the
MIME types it can receive.
When creating a new MLS group, the group MAY include a
required_mime_type extension in the group Extensions. When used in a
group, the client MUST include the required_mime_types extension in
the list of extensions in RequiredCapabilities.
MLS clients SHOULD NOT add an MLS client to an MLS group with
required_mime_types unless the MLS client advertises it can support
all of the required MIME Types. As an exception, a client could be
preconfigured to know that certain clients support the mandatory
types.
4. IANA Considerations
This document proposes registration of two MLS Extension Types.
4.1. accepted_mime_types MLS Extension Type
The accepted_mime_types MLS Extension Type is used inside KeyPackage
objects. It contains a MimeTypeList representing all the MIME Types
supported by the MLS client publishing the KeyPackage.
Template:
Value: 0x0005
Name: accepted_mime_types
Message(s): This extension may appear in KeyPackage objects
Recommended: Y
Reference: RFC XXXX
Description: list of MIME types supported by the MLS client
advertising the KeyPackage
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4.2. required_mime_types GroupContext extension
The required_mime_types MLS Extension Type is used inside
GroupContext objects. It contains a MimeTypeList representing the
MIME Types which are mandatory for all MLS members of the group to
support.
Template:
Value: 0x0006
Name: required_mime_types
Message(s): This extension may appear in GroupContext objects
Recommended: Y
Reference: RFC XXXX
Description: list of MIME types which every member of the MLS group
is required to support.
5. Security Considerations
The Security Considerations of MLS apply.
Use of the extensions in this document could leak some private
information both in KeyPackages and inside an MLS group. They could
be used to infer a specific implementation, platform, or even
version. Clients should consider carefully the implications in their
environment of making a list of acceptable MIME types available.
A client which can take over group administration could prevent
members from joining or sending messages in an established group, by
requiring a list of required MIME types which the attacker knows is
unsupported. This attack is not especially helpful, as taking over
group administration can have more disruptive effects.
6. Normative References
[I-D.ietf-mls-protocol]
Barnes, R., Beurdouche, B., Robert, R., Millican, J.,
Omara, E., and K. Cohn-Gordon, "The Messaging Layer
Security (MLS) Protocol", Work in Progress, Internet-
Draft, draft-ietf-mls-protocol-12, 11 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-mls-
protocol-12>.
[RFC2219] Hamilton, M. and R. Wright, "Use of DNS Aliases for
Network Services", BCP 17, RFC 2219, DOI 10.17487/RFC2219,
October 1997, <https://www.rfc-editor.org/info/rfc2219>.
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7. Informative References
[I-D.mahy-dispatch-immi-content]
Mahy, R., "Inside MLS Message Interop (IMMI) instant
message content", Work in Progress, Internet-Draft, draft-
mahy-dispatch-immi-content-00, 7 March 2022,
<https://datatracker.ietf.org/doc/html/draft-mahy-
dispatch-immi-content-00>.
Author's Address
Rohan Mahy
Wire
Email: rohan.mahy@wire.com
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