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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.