Secure Media Frames
charter-ietf-sframe-01

Real-time conferencing sessions increasingly require end-to-end protections
that prevent intermediary servers from decrypting real-time media.  The PERC WG
developed a “double encryption” scheme for end-to-end encryption that was
deeply tied to SRTP as its underlying transport.  This entanglement has
prevented widespread deployment.

This working group will define the SFrame secure encapsulation to provide
authenticated encryption for real-time media content that is independent of the
underlying transport.  The encapsulation will provide the following information
to drive the authenticated encryption for each encryption operation:

* Selection among multiple encryption keys in use during a real-time session

* An algorithm for forming a unique nonce within the scope of the key based on
information in the encapsulation framing

The SFrame specification will detail the specific security properties that the
encapsulation provides, and discuss their implications under common usage
scenarios / threat models.

The transport-independence of this encapsulation means that it can be applied
at a higher level than individual RTP payloads.  For example, it may be
desirable to encrypt whole frames that span multiple packets in order to
amortize the overhead from framing and authentication tags.  It may also be
desirable to encrypt units of intermediate size (e.g., H.264 NALUs or AV1 OBUs)
to allow partial frames to be usable.  The working group will choose what
levels of granularity can be selected in the protocol.

An application using SFrame will need to choose several aspects of its
operation, for example:

* Selecting whether SFrame is to be used for a given media flow

* Specifying which encryption algorithm should be used

* Provisioning keys and key identifiers to endpoints

* Selecting the granularity at which SFrame encryption is applied (if multiple
options are available)

This working group, however, will not specify the signaling required to arrange
SFrame encryption.  In particular, considerations related to SIP or SDP are out
of scope.  This is because SFrame is intended to be applied as an additional
layer on top of the base levels of protection that these protocols provide. 
This working group will, however, define the guidance for how SFrame interacts
with RTP (e.g., with regard to packetization, depacketization, and recovery
algorithms) to ensure that it can be used in environments such as WebRTC. 
Other WebRTC changes such as the payload format and metadata format will be
addressed by the AVTCORE working group.

It is anticipated that several use cases of SFrame will involve its use with
keys derived from the MLS group key exchange protocol.  The working group will
define a mechanism for doing SFrame encryption using keys from MLS, including,
for example, the derivation of SFrame keys per MLS epoch and per sender.  The
availability of this mechanism for using keys from MLS does not preclude the
use of other sources of key material.