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Network Working Group R. Barnes
Internet-Draft Cisco
Intended status: Informational R. Robert
Expires: 20 May 2021 Wire
16 November 2020
Using Messaging Layer Security (MLS) to Provide Keys for SFrame
draft-barnes-sframe-mls-00
Abstract
Secure Frames (SFrame) defines a compact scheme for encrypting real-
time media. In order for SFrame to address cases where media are
exchanged among many participants (e.g., real-time conferencing), it
needs to be augmented with a group key management protocol. The
Messaging Layer Security (MLS) protocol provides continuous group
authenticated key exchange, allowing a group of participants in a
media session to authenticate each other and agree on a group key.
This document defines how the group keys produced by MLS can be used
with SFrame to secure real-time sessions for groups.
Discussion Venues
This note is to be removed before publishing as an RFC.
Source for this draft and an issue tracker can be found at
https://github.com/bifurcation/sframe-mls.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 20 May 2021.
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Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Simplified BSD License text
as described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. SFrame Key Management . . . . . . . . . . . . . . . . . . . . 3
3. Security Considerations . . . . . . . . . . . . . . . . . . . 5
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
Appendix A. Acknowledgements . . . . . . . . . . . . . . . . . . 5
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5
1. Introduction
Secure Frames (SFrame) defines a compact scheme for encrypting real-
time media. In order for SFrame to address cases where media are
exchanged among many participants (e.g., real-time conferencing), it
needs to be augmented with a group key management protocol. The
Messaging Layer Security (MLS) protocol [!I-D.ietf-mls-protocol]
provides continuous group authenticated key exchange. MLS provides
several important security properties [!I-D.ietf-mls-arch]:
* Group Key Exchange: All members of the group at a given time know
a secret key that is inaccessible to parties outside the group.
* Authentication of group members: Each member of the group can
authenticate the other members of the group.
* Group Agreement: The members of the group all agree on the
identities of the participants in the group.
* Forward Secrecy: There are protocol events such that if a member's
state is compromised after the event, group secrets created before
the event are safe.
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* Post-compromise Security: There are protocol events such that if a
member's state is compromised before the event, the group secrets
created after the event are safe.
When a real-time session uses MLS as the basis for SFrame keys, these
security properties apply to real-time media as well. In the
remainder of this document, we define how to use the secrets produced
by MLS to generate the keys required by SFrame.
[[ OPEN ISSUE: We could define an MLS extension that would provide
negotiation of SFrame parameters, notably the ciphersuite and the
value E defined below. ]]
2. SFrame Key Management
MLS creates a linear sequence of keys, each of which is shared among
the members of a group at a given point in time. When a member joins
or leaves the group, a new key is produced that is known only to the
augmented or reduced group. Each step in the lifetime of the group
is know as an "epoch", and each member of the group is assigned an
"index" that is constant for the time they are in the group.
In SFrame, we derive per-sender "base\_key" values from the group
secret for an epoch, and use the KID field to signal the epoch and
sender index. First, we use the MLS exporter to compute a shared
SFrame secret for the epoch.
sframe_epoch_secret = MLS-Exporter("SFrame 10 MLS", "", AEAD.Nk)
sender_base_key[index] = HKDF-Expand(sframe_epoch_secret,
encode_big_endian(index, 4), AEAD.Nk)
[[ OPEN ISSUE: MLS has its own "secret tree" that provides better
forward secrecy properties within an epoch. (This scheme provides
none.) An alternative approach would be to re-use the MLS secret
tree, either directly or as a data structure. ]]
The Key ID (KID) field in the SFrame header provides the epoch and
index values that are needed to generate the appropriate key from the
MLS key schedule.
KID = (sender_index << E) + (epoch % (1 << E))
For compactness, do not send the whole epoch number. Instead, we
send only its low-order E bits. The participants in the group MUST
agree on the value of E for a given session, through some negotiation
not specified here.
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Note that E effectively defines a re-ordering window, since no more
than 2^E epoch can be active at a given time. The better the
participants are in sync with regard to key roll-over, and the less
reordering of SFrame-protected payloads by the network, the fewer
bits of epoch are necessary.
Receivers MUST be prepared for the epoch counter to roll over,
removing an old epoch when a new epoch with the same E lower bits is
introduced.
[[ OPEN ISSUE: There might be some considerations for new joiners.
Some trial decryption might be necessary to detect whether you're in
epoch N or in epoch N + 1 << E. ]]
Once an SFrame stack has been provisioned with the
"sframe_epoch_secret" for an epoch, it can compute the required KIDs
and "sender_base_key" values on demand, as it needs to encrypt/
decrypt for a given member.
...
|
Epoch 17 +--+-- index=33 -> KID = 0x211
| |
| +-- index=51 -> KID = 0x331
|
|
Epoch 16 +--+-- index=2 --> KID = 0x20
|
|
Epoch 15 +--+-- index=3 --> KID = 0x3f
| |
| +-- index=5 --> KID = 0x5f
|
|
Epoch 14 +--+-- index=3 --> KID = 0x3e
| |
| +-- index=7 --> KID = 0x7e
| |
| +-- index=20 -> KID = 0x14e
|
...
MLS also provides an authenticated signing key pair for each
participant. When SFrame uses signatures, these are the keys used to
generate SFrame signatures.
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3. Security Considerations
The security properties provided by MLS are discussed in detail in
[!I-D.ietf-mls-arch] and [!I-D.ietf-mls-protocol]. This document
extends those guarantees to SFrame.
It should be noted that the per-sender keys derived here do not
provide per-sender authentication, since any member of the group
could derive the same keys (as indeed they must in order to decrypt
the protected payload). Per-sender keys are derived only to avoid
nonce collision among multiple unsynchronized senders. So the
authentication limitations of SFrame remain: There is per-sender
authentication only when signatures are used. Otherwise, SFrame only
authenticates membership in the group, and members are free to
impersonate each other.
4. IANA Considerations
This document makes no request of IANA.
Appendix A. Acknowledgements
TODO
Authors' Addresses
Richard Barnes
Cisco
Email: rlb@ipv.sx
Raphael Robert
Wire
Email: raphael@wire.com
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