A Solution Framework for Private Media in Privacy-Enhanced RTP Conferencing (PERC)
RFC 8871

Document Type RFC - Proposed Standard (January 2021; No errata)
Authors Paul Jones  , David Benham  , Christian Groves 
Last updated 2021-01-18
Replaces draft-jones-perc-private-media-framework
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Internet Engineering Task Force (IETF)                          P. Jones
Request for Comments: 8871                                         Cisco
Category: Standards Track                                      D. Benham
ISSN: 2070-1721                                                C. Groves
                                                             Independent
                                                            January 2021

     A Solution Framework for Private Media in Privacy-Enhanced RTP
                          Conferencing (PERC)

Abstract

   This document describes a solution framework for ensuring that media
   confidentiality and integrity are maintained end to end within the
   context of a switched conferencing environment where Media
   Distributors are not trusted with the end-to-end media encryption
   keys.  The solution builds upon existing security mechanisms defined
   for the Real-time Transport Protocol (RTP).

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8871.

Copyright Notice

   Copyright (c) 2021 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.  Conventions Used in This Document
   3.  PERC Entities and Trust Model
     3.1.  Untrusted Entities
       3.1.1.  Media Distributor
       3.1.2.  Call Processing
     3.2.  Trusted Entities
       3.2.1.  Endpoint
       3.2.2.  Key Distributor
   4.  Framework for PERC
     4.1.  E2E-Authenticated and HBH-Authenticated Encryption
     4.2.  E2E Key Confidentiality
     4.3.  E2E Keys and Endpoint Operations
     4.4.  HBH Keys and Per-Hop Operations
     4.5.  Key Exchange
       4.5.1.  Initial Key Exchange and Key Distributor
       4.5.2.  Key Exchange during a Conference
   5.  Authentication
     5.1.  Identity Assertions
     5.2.  Certificate Fingerprints in Session Signaling
     5.3.  Conference Identification
   6.  PERC Keys
     6.1.  Key Inventory and Management Considerations
     6.2.  DTLS-SRTP Exchange Yields HBH Keys
     6.3.  The Key Distributor Transmits the KEK (EKT Key)
     6.4.  Endpoints Fabricate an SRTP Master Key
     6.5.  Summary of Key Types and Entity Possession
   7.  Encrypted Media Packet Format
   8.  Security Considerations
     8.1.  Third-Party Attacks
     8.2.  Media Distributor Attacks
       8.2.1.  Denial of Service
       8.2.2.  Replay Attacks
       8.2.3.  Delayed Playout Attacks
       8.2.4.  Splicing Attacks
       8.2.5.  RTCP Attacks
     8.3.  Key Distributor Attacks
     8.4.  Endpoint Attacks
   9.  IANA Considerations
   10. References
     10.1.  Normative References
     10.2.  Informative References
   Acknowledgments
   Authors' Addresses

1.  Introduction

   Switched conferencing is an increasingly popular model for multimedia
   conferences with multiple participants using a combination of audio,
   video, text, and other media types.  With this model, real-time media
   flows from conference participants are not mixed, transcoded,
   translated, recomposed, or otherwise manipulated by a Media
   Distributor, as might be the case with a traditional media server or
   Multipoint Control Unit (MCU).  Instead, media flows transmitted by
   conference participants are simply forwarded by Media Distributors to
   each of the other participants.  Media Distributors often forward
   only a subset of flows based on voice activity detection or other
   criteria.  In some instances, Media Distributors may make limited
   modifications to RTP headers [RFC3550], for example, but the actual
   media content (e.g., voice or video data) is unaltered.

   An advantage of switched conferencing is that Media Distributors can
   be more easily deployed on general-purpose computing hardware,
   including virtualized environments in private and public clouds.
   Virtualized public cloud environments have been viewed as less
   secure, since resources are not always physically controlled by those
   who use them.  This document defines improved security so as to lower
   the barrier to taking advantage of those environments.

   This document defines a solution framework wherein media privacy is
   ensured by making it impossible for a Media Distributor to gain
   access to keys needed to decrypt or authenticate the actual media
   content sent between conference participants.  At the same time, the
   framework allows for the Media Distributors to modify certain RTP
   headers; add, remove, encrypt, or decrypt RTP header extensions; and
   encrypt and decrypt RTP Control Protocol (RTCP) packets [RFC3550].
   The framework also prevents replay attacks by authenticating each
   packet transmitted between a given participant and the Media
   Distributor using a unique key per endpoint that is independent from
   the key for media encryption and authentication.

   This solution framework provides for enhanced privacy in RTP-based
   conferencing environments while utilizing existing security
   procedures defined for RTP with minimal enhancements.

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Additionally, this solution framework uses the following terms and
   abbreviations:

   End-to-End (E2E):  Communications from one endpoint through one or
      more Media Distributors to the endpoint at the other end.

   Hop-by-Hop (HBH):  Communications between an endpoint and a Media
      Distributor or between Media Distributors.

   Trusted Endpoint (or simply endpoint):  An RTP flow-terminating
      entity that has possession of E2E media encryption keys and
      terminates E2E encryption.  This may include embedded user
      conferencing equipment or browsers on computers, media gateways,
      MCUs, media recording devices, and more, that are in the trusted
      domain for a given deployment.  In the context of WebRTC
      [W3C.CR-webrtc], where control of a session is divided between a
      JavaScript application and a browser, the browser acts as the
      Trusted Endpoint for purposes of this framework (just as it acts
      as the endpoint for DTLS-SRTP [RFC5764] in one-to-one calls).

   Media Distributor (MD):  An RTP middlebox that forwards endpoint
      media content (e.g., voice or video data) unaltered -- either a
      subset or all of the flows at any given time -- and is never
      allowed to have access to E2E encryption keys.  It operates
      according to the Selective Forwarding Middlebox RTP topologies
      [RFC7667] per the constraints defined by the Private Media in
      Privacy-Enhanced RTP Conferencing (PERC) system, which includes,
      but is not limited to, having no access to RTP media plaintext and
      having limits on what RTP header fields it can alter.  The header
      fields that may be modified by a Media Distributor are enumerated
      in Section 4 of the double cryptographic transform specification
      [RFC8723] and chosen with respect to utility and the security
      considerations outlined in this document.

   Key Distributor:  An entity that is a logical function that
      distributes keying material and related information to Trusted
      Endpoints and Media Distributor(s) -- only what is appropriate for
      each.  The Key Distributor might be co-resident with another
      entity trusted with E2E keying material.

   Conference:  Two or more participants communicating via Trusted
      Endpoints to exchange RTP flows through one or more Media
      Distributors.

   Call Processing:  All Trusted Endpoints connect to a conference via a
      call processing dialog, e.g., with the "focus" as defined in "A
      Framework for Conferencing with the Session Initiation Protocol
      (SIP)" [RFC4353].

   Third Party:  Any entity that is not an endpoint, Media Distributor,
      Key Distributor, or call processing entity as described in this
      document.

3.  PERC Entities and Trust Model

   Figure 1 depicts the trust relationships, direct or indirect, between
   entities described in the subsequent subsections.  Note that these
   entities may be co-located or further divided into multiple, separate
   physical devices.

   Please note that some entities classified as untrusted in the simple,
   general deployment scenario used most commonly in this document might
   be considered trusted in other deployments.  This document does not
   preclude such scenarios, but it keeps the definitions and examples
   focused by only using the simple, most general deployment scenario.

                                  |
              +----------+        |        +-----------------+
              | Endpoint |        |        | Call Processing |
              +----------+        |        +-----------------+
                                  |
                                  |
           +----------------+     |       +--------------------+
           | Key Distributor|     |       | Media Distributor  |
           +----------------+     |       +--------------------+
                                  |
                Trusted           |             Untrusted
                Entities          |             Entities
                                  |

              Figure 1: Trusted and Untrusted Entities in PERC

3.1.  Untrusted Entities

   The architecture described in this framework document enables
   conferencing infrastructure to be hosted in domains, such as in a
   cloud conferencing provider's facilities, where the trustworthiness
   is below the level needed to assume that the privacy of the
   participant's media is not compromised.  The conferencing
   infrastructure in such a domain is still trusted with reliably
   connecting the participants together in a conference but is not
   trusted with keying material needed to decrypt any of the
   participant's media.  Entities in such less-trustworthy domains are
   referred to as untrusted entities from this point forward.

   It is important to understand that "untrusted" in this document does
   not mean that an entity is not expected to function properly.
   Rather, it means only that the entity does not have access to the E2E
   media encryption keys.

3.1.1.  Media Distributor

   A Media Distributor forwards RTP flows between endpoints in the
   conference while performing per-hop authentication of each RTP
   packet.  The Media Distributor may need access to one or more RTP
   headers or header extensions, potentially adding or modifying a
   certain subset.  The Media Distributor also relays secured messaging
   between the endpoints and the Key Distributor and acquires per-hop
   key information from the Key Distributor.  The actual media content
   must not be decryptable by a Media Distributor, as it is not trusted
   to have access to the E2E media encryption keys.  The key exchange
   mechanisms specified in this framework prevent the Media Distributor
   from gaining access to the E2E media encryption keys.

   An endpoint's ability to connect to a conference serviced by a Media
   Distributor implies that the endpoint is authorized to have access to
   the E2E media encryption keys, although the Media Distributor does
   not have the ability to determine whether an endpoint is authorized.
   Instead, the Key Distributor is responsible for authenticating the
   endpoint (e.g., using WebRTC identity assertions [RFC8827]) and
   determining its authorization to receive E2E and HBH media encryption
   keys.

   A Media Distributor must perform its role in properly forwarding
   media packets while taking measures to mitigate the adverse effects
   of denial-of-service attacks (refer to Section 8) to a level equal to
   or better than traditional conferencing (non-PERC) deployments.

   A Media Distributor or associated conferencing infrastructure may
   also initiate or terminate various messaging techniques related to
   conference control.  This topic is outside the scope of this
   framework document.

3.1.2.  Call Processing

   Call processing is untrusted in the simple, general deployment
   scenario.  When a physical subset of call processing resides in
   facilities outside the trusted domain, it should not be trusted to
   have access to E2E key information.

   Call processing may include the processing of call signaling
   messages, as well as the signing of those messages.  It may also
   authenticate the endpoints for the purpose of call signaling and of
   subsequently joining a conference hosted through one or more Media
   Distributors.  Call processing may optionally ensure the privacy of
   call signaling messages between itself (call processing), the
   endpoint, and other entities.

3.2.  Trusted Entities

   From the PERC model system's perspective, entities considered trusted
   (refer to Figure 1) can be in possession of the E2E media encryption
   keys for one or more conferences.

3.2.1.  Endpoint

   An endpoint is considered trusted and has access to E2E key
   information.  While it is possible for an endpoint to be compromised,
   subsequently performing in undesired ways, defining endpoint
   resistance to compromise is outside the scope of this document.
   Endpoints take measures to mitigate the adverse effects of denial-of-
   service attacks (refer to Section 8) from other entities, including
   from other endpoints, to a level equal to or better than traditional
   conference (non-PERC) deployments.

3.2.2.  Key Distributor

   The Key Distributor, which may be co-located with an endpoint or
   exist standalone, is responsible for providing key information to
   endpoints for both E2E and HBH security and for providing key
   information to Media Distributors for HBH security.

   Interaction between the Key Distributor and call processing is
   necessary for proper conference-to-endpoint mappings.  This is
   described in Section 5.3.

   The Key Distributor needs to be secured and managed in a way that
   prevents exploitation by an adversary, as any kind of compromise of
   the Key Distributor puts the security of the conference at risk.

   The Key Distributor needs to know which endpoints and which Media
   Distributors are authorized to participate in the conference.  How
   the Key Distributor obtains this information is outside the scope of
   this document.  However, Key Distributors MUST reject DTLS
   associations with any unauthorized endpoint or Media Distributor.

4.  Framework for PERC

   The purpose of this framework is to define a means through which
   media privacy is ensured when communicating within a conferencing
   environment consisting of one or more Media Distributors that only
   switch, and hence do not terminate, media.  It does not otherwise
   attempt to hide the fact that a conference between endpoints is
   taking place.

   This framework reuses several specified RTP security technologies,
   including the Secure Real-time Transport Protocol (SRTP) [RFC3711],
   Encrypted Key Transport (EKT) [RFC8870], and DTLS-SRTP.

4.1.  E2E-Authenticated and HBH-Authenticated Encryption

   This solution framework focuses on the E2E privacy and integrity of
   the participant's media by limiting access to only trusted entities
   to the E2E key used for authenticated E2E encryption.  However, this
   framework does give a Media Distributor access to RTP header fields
   and header extensions, as well as the ability to modify a certain
   subset of the header fields and to add or change header extensions.
   Packets received by a Media Distributor or an endpoint are
   authenticated hop by hop.

   To enable all of the above, this framework defines the use of two
   security contexts and two associated encryption keys: an "inner" key
   (a distinct E2E key for each transmitted media flow) for
   authenticated encryption of RTP media between endpoints and an
   "outer" key (a HBH key) known only to a Media Distributor or the
   adjacent endpoint for the hop between an endpoint and a Media
   Distributor or peer endpoint.  An endpoint will receive one or more
   E2E keys from every other endpoint in the conference that correspond
   to the media flows transmitted by those other endpoints, while HBH
   keys are derived from the DTLS-SRTP association with the Key
   Distributor.  Two communicating Media Distributors use DTLS-SRTP
   associations directly with each other to obtain the HBH keys they
   will use.  See Section 4.5 for more details on key exchange.

      +-------------+                                +-------------+
      |             |################################|             |
      |    Media    |------------------------ *----->|    Media    |
      | Distributor |<----------------------*-|------| Distributor |
      |      X      |#####################*#|#|######|      Y      |
      |             |                     | | |      |             |
      +-------------+                     | | |      +-------------+
         #  ^ |  #          HBH Key (XY) -+ | |         #  ^ |  #
         #  | |  #           E2E Key (B) ---+ |         #  | |  #
         #  | |  #           E2E Key (A) -----+         #  | |  #
         #  | |  #                                      #  | |  #
         #  | |  #                                      #  | |  #
         #  | |  *---- HBH Key (AX)    HBH Key (YB) ----*  | |  #
         #  | |  #                                      #  | |  #
         #  *--------- E2E Key (A)      E2E Key (A) ---------*  #
         #  | *------- E2E Key (B)      E2E Key (B) -------* |  #
         #  | |  #                                      #  | |  #
         #  | v  #                                      #  | v  #
      +-------------+                                +-------------+
      | Endpoint A  |                                | Endpoint B  |
      +-------------+                                +-------------+

      Figure 2: E2E and HBH Keys Used for Authenticated Encryption of
                                SRTP Packets

   The double transform [RFC8723] enables endpoints to perform
   encryption using both the E2E and HBH contexts while still preserving
   the same overall interface as other SRTP transforms.  The Media
   Distributor simply uses the corresponding normal (single) AES-GCM
   transform, keyed with the appropriate HBH keys.  See Section 6.1 for
   a description of the keys used in PERC and Section 7 for a diagram of
   how encrypted RTP packets appear on the wire.

   RTCP is only encrypted hop by hop -- not end to end.  This framework
   does not provide an additional step for RTCP-authenticated
   encryption.  Rather, implementations utilize the existing procedures
   specified in [RFC3711]; those procedures use the same outer, HBH
   cryptographic context chosen in the double transform operation
   described above.  For this reason, endpoints MUST NOT send
   confidential information via RTCP.

4.2.  E2E Key Confidentiality

   To ensure the confidentiality of E2E keys shared between endpoints,
   endpoints use a common Key Encryption Key (KEK) that is known only by
   the trusted entities in a conference.  That KEK, defined in the EKT
   specification [RFC8870] as the EKT Key, is used to subsequently
   encrypt the SRTP master key used for E2E-authenticated encryption of
   media sent by a given endpoint.  Each endpoint in the conference
   creates an SRTP master key for E2E-authenticated encryption and keeps
   track of the E2E keys received via the Full EKT Tag for each distinct
   synchronization source (SSRC) in the conference so that it can
   properly decrypt received media.  An endpoint may change its E2E key
   at any time and advertise that new key to the conference as specified
   in [RFC8870].

4.3.  E2E Keys and Endpoint Operations

   Any given RTP media flow is identified by its SSRC, and an endpoint
   might send more than one at a time and change the mix of media flows
   transmitted during the lifetime of a conference.

   Thus, an endpoint MUST maintain a list of SSRCs from received RTP
   flows and each SSRC's associated E2E key information.  An endpoint
   MUST discard old E2E keys no later than when it leaves the
   conference.

   If the packet is to contain RTP header extensions, it should be noted
   that those extensions are only encrypted hop by hop per [RFC8723].
   For this reason, endpoints MUST NOT transmit confidential information
   via RTP header extensions.

4.4.  HBH Keys and Per-Hop Operations

   To ensure the integrity of transmitted media packets, it is REQUIRED
   that every packet be authenticated hop by hop between an endpoint and
   a Media Distributor, as well as between Media Distributors.  The
   authentication key used for HBH authentication is derived from an
   SRTP master key shared only on the respective hop.  Each HBH key is
   distinct per hop, and no two hops ever use the same SRTP master key.

   While endpoints also perform HBH authentication, the ability of the
   endpoints to reconstruct the original RTP header also enables the
   endpoints to authenticate RTP packets end to end.  This design yields
   flexibility to the Media Distributor to change certain RTP header
   values as packets are forwarded.  Values that the Media Distributor
   can change in the RTP header are defined in [RFC8723].  RTCP can only
   be encrypted hop by hop, giving the Media Distributor the flexibility
   to (1) forward RTCP content unchanged, (2) transmit compound RTCP
   packets, (3) initiate RTCP packets for reporting statistics, or
   (4) convey other information.  Performing HBH authentication for all
   RTP and RTCP packets also helps provide replay protection (see
   Section 8).  The use of the replay protection mechanism specified in
   Section 3.3.2 of [RFC3711] is REQUIRED at each hop.

   If there is a need to encrypt one or more RTP header extensions hop
   by hop, the endpoint derives an encryption key from the HBH SRTP
   master key to encrypt header extensions as per [RFC6904].  This still
   gives the Media Distributor visibility into header extensions, such
   as the one used to determine the audio level [RFC6464] of conference
   participants.  Note that when RTP header extensions are encrypted,
   all hops need to decrypt and re-encrypt these encrypted header
   extensions.  Please refer to Sections 5.1, 5.2, and 5.3 of [RFC8723]
   for procedures to perform RTP header extension encryption and
   decryption.

4.5.  Key Exchange

   In brief, the keys used by any given endpoints are determined as
   follows:

   *  The HBH keys that the endpoint uses to send and receive SRTP media
      are derived from a DTLS handshake that the endpoint performs with
      the Key Distributor (following normal DTLS-SRTP procedures).

   *  The E2E key that an endpoint uses to send SRTP media can be either
      set from the DTLS-SRTP association with the Key Distributor or
      chosen by the endpoint.  It is then distributed to other endpoints
      in a Full EKT Tag, encrypted under an EKT Key provided to the
      client by the Key Distributor within the DTLS channel they
      negotiated.  Note that an endpoint MAY create a different E2E key
      per media flow, where a media flow is identified by its SSRC
      value.

   *  Each E2E key that an endpoint uses to receive SRTP media is set by
      receiving a Full EKT Tag from another endpoint.

   *  The HBH keys used between two Media Distributors are derived via
      DTLS-SRTP procedures employed directly between them.

4.5.1.  Initial Key Exchange and Key Distributor

   The Media Distributor maintains a tunnel with the Key Distributor
   (e.g., using the tunnel protocol defined in [PERC-DTLS]), making it
   possible for the Media Distributor to facilitate the establishment of
   a secure DTLS association between each endpoint and the Key
   Distributor as shown in Figure 3.  The DTLS association between
   endpoints and the Key Distributor enables each endpoint to generate
   E2E and HBH keys and receive the KEK.  At the same time, the Key
   Distributor securely provides the HBH key information to the Media
   Distributor.  The key information summarized here may include the
   SRTP master key, the SRTP master salt, and the negotiated
   cryptographic transform.

                             +-----------+
                    KEK info |    Key    | HBH Key info to
                to Endpoints |Distributor| Endpoints & Media Distributor
                             +-----------+
                                # ^ ^ #
                                # | | #--- Tunnel
                                # | | #
   +-----------+             +-----------+             +-----------+
   | Endpoint  |   DTLS      |   Media   |   DTLS      | Endpoint  |
   |    KEK    |<------------|Distributor|------------>|    KEK    |
   |  HBH Key  | to Key Dist | HBH Keys  | to Key Dist |  HBH Key  |
   +-----------+             +-----------+             +-----------+

           Figure 3: Exchanging Key Information between Entities

   In addition to the secure tunnel between the Media Distributor and
   the Key Distributor, there are two additional types of security
   associations utilized as a part of the key exchange, as discussed in
   the following paragraphs.  One is a DTLS-SRTP association between an
   endpoint and the Key Distributor (with packets passing through the
   Media Distributor), and the other is a DTLS-SRTP association between
   peer Media Distributors.

   Endpoints establish a DTLS-SRTP association over the RTP session with
   the Media Distributor and its media ports for the purposes of key
   information exchange with the Key Distributor.  The Media Distributor
   does not terminate the DTLS signaling but instead forwards DTLS
   packets received from an endpoint on to the Key Distributor (and vice
   versa) via a tunnel established between the Media Distributor and the
   Key Distributor.

   When establishing the DTLS association between endpoints and the Key
   Distributor, the endpoint MUST act as the DTLS client, and the Key
   Distributor MUST act as the DTLS server.  The KEK is conveyed by the
   Key Distributor over the DTLS association to endpoints via procedures
   defined in EKT [RFC8870] via the EKTKey message.

   The Key Distributor MUST NOT establish DTLS-SRTP associations with
   endpoints without first authenticating the Media Distributor
   tunneling the DTLS-SRTP packets from the endpoint.

   Note that following DTLS-SRTP procedures for the cipher defined in
   [RFC8723], the endpoint generates both E2E and HBH encryption keys
   and salt values.  Endpoints MUST either use the DTLS-SRTP-generated
   E2E key for transmission or generate a fresh E2E key.  In either
   case, the generated SRTP master salt for E2E encryption MUST be
   replaced with the salt value provided by the Key Distributor via the
   EKTKey message.  That is because every endpoint in the conference
   uses the same SRTP master salt.  The endpoint only transmits the SRTP
   master key (not the salt) used for E2E encryption to other endpoints
   in RTP/RTCP packets per [RFC8870].

   Media Distributors use DTLS-SRTP directly with a peer Media
   Distributor to establish the HBH key for transmitting RTP and RTCP
   packets to that peer Media Distributor.  The Key Distributor does not
   facilitate establishing a HBH key for use between Media Distributors.

4.5.2.  Key Exchange during a Conference

   Following the initial key information exchange with the Key
   Distributor, an endpoint is able to encrypt media end to end with an
   E2E key, sending that E2E key to other endpoints encrypted with the
   KEK, and is able to encrypt and authenticate RTP packets using a HBH
   key.  This framework does not allow the Media Distributor to gain
   access to the KEK information, preventing it from gaining access to
   any endpoint's E2E key and subsequently decrypting media.

   The KEK may need to change from time to time during the lifetime of a
   conference, such as when a new participant joins or leaves a
   conference.  Dictating if, when, or how often a conference is to be
   rekeyed is outside the scope of this document, but this framework
   does accommodate rekeying during the lifetime of a conference.

   When a Key Distributor decides to rekey a conference, it transmits a
   new EKTKey message containing the new EKT Key to each of the
   conference participants.  Upon receipt of the new EKT Key, the
   endpoint MUST create a new SRTP master key and prepare to send that
   key inside a FullEKTField using the new EKT Key as per Section 4.5 of
   [RFC8870].  In order to allow time for all endpoints in the
   conference to receive the new keys, the sender should follow the
   recommendations in Section 4.6 of [RFC8870].  On receiving a new EKT
   Key, endpoints MUST be prepared to decrypt EKT Tags using the new
   key.  The EKT Security Parameter Index (SPI) field is used to
   differentiate between EKT Tags encrypted with the old and new keys.

   After rekeying, an endpoint SHOULD retain prior SRTP master keys and
   EKT Keys for a period of time sufficient for the purpose of ensuring
   that it can decrypt late-arriving or out-of-order packets or packets
   sent by other endpoints that used the prior keys for a period of time
   after rekeying began.  An endpoint MAY retain old keys until the end
   of the conference.

   Endpoints MAY follow the procedures in Section 5.2 of [RFC5764] to
   renegotiate HBH keys as desired.  If new HBH keys are generated, the
   new keys are also delivered to the Media Distributor following the
   procedures defined in [PERC-DTLS] as one possible method.

   At any time, endpoints MAY change the E2E encryption key being used.
   An endpoint MUST generate a new E2E encryption key whenever it
   receives a new EKT Key.  After switching to a new key, the new key is
   conveyed to other endpoints in the conference in RTP/RTCP packets per
   [RFC8870].

5.  Authentication

   It is important that entities can validate the authenticity of other
   entities, especially the Key Distributor and endpoints.  Details on
   this topic are outside the scope of this specification, but a few
   possibilities are discussed in the following sections.  The critical
   requirements are that (1) an endpoint can verify that it is connected
   to the correct Key Distributor for the conference and (2) the Key
   Distributor can verify that the endpoint is the correct endpoint for
   the conference.

   Two possible approaches to resolve this situation are identity
   assertions and certificate fingerprints.

5.1.  Identity Assertions

   A WebRTC identity assertion [RFC8827] is used to bind the identity of
   the user of the endpoint to the fingerprint of the DTLS-SRTP
   certificate used for the call.  This certificate is unique for a
   given call and a conference.  This certificate is unique for a given
   call and a conference, allowing the Key Distributor to ensure that
   only authorized users participate in the conference.  Similarly, the
   Key Distributor can create a WebRTC identity assertion to bind the
   fingerprint of the unique certificate used by the Key Distributor for
   this conference so that the endpoint can verify that it is talking to
   the correct Key Distributor.  Such a setup requires an Identity
   Provider (IdP) trusted by the endpoints and the Key Distributor.

5.2.  Certificate Fingerprints in Session Signaling

   Entities managing session signaling are generally assumed to be
   untrusted in the PERC framework.  However, there are some deployment
   scenarios where parts of the session signaling may be assumed
   trustworthy for the purposes of exchanging, in a manner that can be
   authenticated, the fingerprint of an entity's certificate.

   As a concrete example, SIP [RFC3261] and the Session Description
   Protocol (SDP) [RFC4566] can be used to convey the fingerprint
   information per [RFC5763].  An endpoint's SIP User Agent would send
   an INVITE message containing SDP for the media session along with the
   endpoint's certificate fingerprint, which can be signed using the
   procedures described in [RFC8224] for the benefit of forwarding the
   message to other entities by the focus [RFC4353].  Other entities can
   verify that the fingerprints match the certificates found in the
   DTLS-SRTP connections to find the identity of the far end of the
   DTLS-SRTP connection and verify that it is the authorized entity.

   Ultimately, if using session signaling, an endpoint's certificate
   fingerprint would need to be securely mapped to a user and conveyed
   to the Key Distributor so that it can check that the user in question
   is authorized.  Similarly, the Key Distributor's certificate
   fingerprint can be conveyed to an endpoint in a manner that can be
   authenticated as being an authorized Key Distributor for this
   conference.

5.3.  Conference Identification

   The Key Distributor needs to know what endpoints are being added to a
   given conference.  Thus, the Key Distributor and the Media
   Distributor need to know endpoint-to-conference mappings, which are
   enabled by exchanging a conference-specific unique identifier as
   described in [PERC-DTLS].  How this unique identifier is assigned is
   outside the scope of this document.

6.  PERC Keys

   This section describes the various keys employed by PERC.

6.1.  Key Inventory and Management Considerations

   This section summarizes the several different keys used in the PERC
   framework, how they are generated, and what purpose they serve.

   The keys are described in the order in which they would typically be
   acquired.

   The various keys used in PERC are shown in Table 1 below.

        +===========+=============================================+
        |    Key    |                 Description                 |
        +===========+=============================================+
        | HBH Key   | SRTP master key used to encrypt media hop   |
        |           | by hop.                                     |
        +-----------+---------------------------------------------+
        | KEK       | Key shared by all endpoints and used to     |
        | (EKT Key) | encrypt each endpoint's E2E SRTP master key |
        |           | so receiving endpoints can decrypt media.   |
        +-----------+---------------------------------------------+
        | E2E Key   | SRTP master key used to encrypt media end   |
        |           | to end.                                     |
        +-----------+---------------------------------------------+

                           Table 1: Key Inventory

   While the number of key types is very small, it should be understood
   that the actual number of distinct keys can be large as the
   conference grows in size.

   As an example, with 1,000 participants in a conference, there would
   be at least 1,000 distinct SRTP master keys, all of which share the
   same master salt.  Each of those keys is passed through the Key
   Derivation Function (KDF) as defined in [RFC3711] to produce the
   actual encryption and authentication keys.

   Complicating key management is the fact that the KEK can change and,
   when it does, the endpoints generate new SRTP master keys that are
   associated with a new EKT SPI.  Endpoints might retain old keys for a
   period of time to ensure that they can properly decrypt late-arriving
   or out-of-order packets, which means that the number of keys held
   during that period of time might be substantially higher.

   A more detailed explanation of each of the keys follows.

6.2.  DTLS-SRTP Exchange Yields HBH Keys

   The first set of keys acquired are for HBH encryption and decryption.
   Per the double transform procedures [RFC8723], the endpoint performs
   a DTLS-SRTP exchange with the Key Distributor and receives a key that
   is, in fact, "double" the size that is needed.  The E2E part is the
   first half of the key, so the endpoint discards that information when
   generating its own key.  The second half of the keying material is
   for HBH operations, so that half of the key (corresponding to the
   least significant bits) is assigned internally as the HBH key.

   The Key Distributor informs the Media Distributor of the HBH key.
   Specifically, the Key Distributor sends the least significant bits
   corresponding to the half of the keying material determined through
   DTLS-SRTP with the endpoint to the Media Distributor.  A salt value
   is generated along with the HBH key.  The salt is also longer than
   needed for HBH operations; thus, only the least significant bits of
   the required length (half of the generated salt material) are sent to
   the Media Distributor.  One way to transmit this key and salt
   information is via the tunnel protocol defined in [PERC-DTLS].

   No two endpoints have the same HBH key; thus, the Media Distributor
   MUST keep track of each distinct HBH key (and the corresponding salt)
   and use it only for the specified hop.

   The HBH key is also used for HBH encryption of RTCP.  RTCP is not
   E2E-encrypted in PERC.

6.3.  The Key Distributor Transmits the KEK (EKT Key)

   The Key Distributor sends the KEK (the EKT Key per [RFC8870]) to the
   endpoint via the aforementioned DTLS-SRTP association.  This key is
   known only to the Key Distributor and endpoints; it is the most
   important entity to protect, since having knowledge of this key (and
   the SRTP master salt transmitted as a part of the same message)
   allows an entity to decrypt any media packet in the conference.

   Note that the Key Distributor can send any number of EKT Keys to
   endpoints.  This information is used to rekey the entire conference.
   Each key is identified by an SPI value.  Endpoints MUST expect that a
   conference might be rekeyed when a new participant joins a conference
   or when a participant leaves a conference, in order to protect the
   confidentiality of the conversation before and after such events.

   The SRTP master salt to be used by the endpoint is transmitted along
   with the EKT Key.  All endpoints in the conference utilize the same
   SRTP master salt that corresponds with a given EKT Key.

   The Full EKT Tag in media packets is encrypted using a cipher
   specified via the EKTKey message (e.g., AES Key Wrap with a 128-bit
   key).  This cipher is different than the cipher used to protect media
   and is only used to encrypt the endpoint's SRTP master key (and other
   EKT Tag data as per [RFC8870]).

   The KEK is not given to the Media Distributor.

6.4.  Endpoints Fabricate an SRTP Master Key

   As stated earlier, the E2E key determined via DTLS-SRTP MAY be
   discarded in favor of a locally generated E2E SRTP master key.  While
   the DTLS-SRTP-derived SRTP master key can be used initially, the
   endpoint might choose to change the SRTP master key periodically and
   MUST change the SRTP master key as a result of the EKT Key changing.

   A locally generated SRTP master key is used along with the master
   salt transmitted to the endpoint from the Key Distributor via the
   EKTKey message to encrypt media end to end.

   Since the Media Distributor is not involved in E2E functions, it does
   not create this key, nor does it have access to any endpoint's E2E
   key.  Note, too, that even the Key Distributor is unaware of the
   locally generated E2E keys used by each endpoint.

   The endpoint transmits its E2E key to other endpoints in the
   conference by periodically including it in SRTP packets in a Full EKT
   Tag.  When placed in the Full EKT Tag, it is encrypted using the EKT
   Key provided by the Key Distributor.  The master salt is not
   transmitted, though, since all endpoints receive the same master salt
   via the EKTKey message from the Key Distributor.  The recommended
   frequency with which an endpoint transmits its SRTP master key is
   specified in [RFC8870].

6.5.  Summary of Key Types and Entity Possession

   All endpoints have knowledge of the KEK.

   Every HBH key is distinct for a given endpoint; thus, Endpoint A and
   Endpoint B do not have knowledge of the other's HBH key.  Since HBH
   keys are derived from a DTLS-SRTP association, there is at most one
   HBH key per endpoint.  (The only exception is where the DTLS-SRTP
   association might be rekeyed per Section 5.2 of [RFC5764] and a new
   key is created to replace the former key.)

   Each endpoint generates its own E2E key (SRTP master key); thus,
   there is a distinct E2E key per endpoint.  This key is transmitted
   (encrypted) via the Full EKT Tag to other endpoints.  Endpoints that
   receive media from a given transmitting endpoint gain knowledge of
   the transmitter's E2E key via the Full EKT Tag.

   Table 2 summarizes the various keys and which entity is in possession
   of a given key.

     +=======================+============+======+======+============+
     |       Key/Entity      | Endpoint A | MD X | MD Y | Endpoint B |
     +=======================+============+======+======+============+
     | KEK (EKT Key)         |    Yes     |  No  |  No  |    Yes     |
     +-----------------------+------------+------+------+------------+
     | E2E Key (A and B)     |    Yes     |  No  |  No  |    Yes     |
     +-----------------------+------------+------+------+------------+
     | HBH Key (A<=>MD X)    |    Yes     | Yes  |  No  |     No     |
     +-----------------------+------------+------+------+------------+
     | HBH Key (B<=>MD Y)    |     No     |  No  | Yes  |    Yes     |
     +-----------------------+------------+------+------+------------+
     | HBH Key (MD X<=>MD Y) |     No     | Yes  | Yes  |     No     |
     +-----------------------+------------+------+------+------------+

                  Table 2: Key Types and Entity Possession

7.  Encrypted Media Packet Format

   Figure 4 presents a complete picture of what an encrypted media
   packet per this framework looks like when transmitted over the wire.
   The packet format shown in the figure is encrypted using the double
   cryptographic transform with an EKT Tag appended to the end.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<++
    |V=2|P|X|  CC   |M|     PT      |       sequence number         | IO
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
    |                           timestamp                           | IO
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
    |           synchronization source (SSRC) identifier            | IO
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ IO
    |            contributing source (CSRC) identifiers             | IO
    |                               ....                            | IO
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
    |                    RTP extension (OPTIONAL) ...               | |O
+>+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O I |                          payload  ...                         | IO
O I |                               +-------------------------------+ IO
O I |                               | RTP padding   | RTP pad count | IO
O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O | |                    E2E authentication tag                     | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | |                            OHB ...                            | |O
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | |                    HBH authentication tag                     | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| | |   EKT Tag ...   | R                                             ||
| | +-+-+-+-+-+-+-+-+-+ |                                             ||
| |                     +- Neither encrypted nor authenticated;       ||
| |                        appended after the double transform        ||
| |                        is performed                               ||
| |                                                                   ||
| +- E2E-Encrypted Portion               E2E-Authenticated Portion ---+|
|                                                                      |
+--- HBH-Encrypted Portion               HBH-Authenticated Portion ----+

    I = Inner (E2E) encryption/authentication
    O = Outer (HBH) encryption/authentication

               Figure 4: Encrypted Media Packet Format

8.  Security Considerations

8.1.  Third-Party Attacks

   Third-party attacks are attacks attempted by an adversary that is not
   supposed to have access to keying material or is otherwise not an
   authorized participant in the conference.

   On-path attacks are mitigated by HBH integrity protection and
   encryption.  The integrity protection mitigates packet modification.
   Encryption makes selective blocking of packets harder, but not
   impossible.

   Off-path attackers could try connecting to different PERC entities to
   send specifically crafted packets with an aim of forcing the receiver
   to forward or render bogus media packets.  Endpoints and Media
   Distributors mitigate such an attack by performing HBH authentication
   and discarding packets that fail authentication.

   Another attack vector is a third party claiming to be a Media
   Distributor, fooling endpoints into sending packets to the false
   Media Distributor instead of the correct one.  The deceived sending
   endpoints could incorrectly assume that their packets have been
   delivered to endpoints when they in fact have not.  While this attack
   is possible, the result is a simple denial of service with no leakage
   of confidential information, since the false Media Distributor would
   not have access to either HBH or E2E encryption keys.

   A third party could cause a denial of service by transmitting many
   bogus or replayed packets toward receiving devices and ultimately
   degrading conference or device performance.  Therefore,
   implementations might wish to devise mechanisms to safeguard against
   such illegitimate packets, such as utilizing rate-limiting or
   performing basic sanity checks on packets (e.g., looking at packet
   length or expected sequence number ranges), before performing
   decryption operations that are more expensive.

   The use of mutual DTLS authentication (as required by DTLS-SRTP) also
   helps to prevent a denial-of-service attack by preventing a false
   endpoint or false Media Distributor from successfully participating
   as a perceived valid media sender that could otherwise carry out an
   on-path attack.  When mutual authentication fails, a receiving
   endpoint would know that it could safely discard media packets
   received from the endpoint without inspection.

8.2.  Media Distributor Attacks

   A malicious or compromised Media Distributor can attack the session
   in a number of possible ways, as discussed below.

8.2.1.  Denial of Service

   A simple form of attack is discarding received packets that should be
   forwarded.  This solution framework does not provide any mitigation
   mechanisms for Media Distributors that fail to forward media packets.

   Another form of attack is modifying received packets before
   forwarding.  With this solution framework, any modification of the
   E2E-authenticated data results in the receiving endpoint getting an
   integrity failure when performing authentication on the received
   packet.

   The Media Distributor can also attempt to perform resource
   consumption attacks on the receiving endpoint.  One such attack would
   be to insert random SSRC/CSRC values in any RTP packet along with a
   Full EKT Tag.  Since such a message would trigger the receiver to
   form a new cryptographic context, the Media Distributor can attempt
   to consume the receiving endpoint's resources.  While E2E
   authentication would fail and the cryptographic context would be
   destroyed, the key derivation operation would nonetheless consume
   some computational resources.  While resource consumption attacks
   cannot be mitigated entirely, rate-limiting packets might help reduce
   the impact of such attacks.

8.2.2.  Replay Attacks

   A replay attack is an attack where an already-received packet from a
   previous point in the RTP stream is replayed as a new packet.  This
   could, for example, allow a Media Distributor to transmit a sequence
   of packets identified as a user saying "yes", instead of the "no" the
   user actually said.

   A replay attack is mitigated by the requirement to implement replay
   protection as described in Section 3.3.2 of [RFC3711].  E2E replay
   protection MUST be provided for the duration of the conference.

8.2.3.  Delayed Playout Attacks

   A delayed playout attack is an attack where media is received and
   held by a Media Distributor and then forwarded to endpoints at a
   later point in time.

   This attack is possible even if E2E replay protection is in place.
   Because the Media Distributor is allowed to select a subset of
   streams and not forward the rest to a receiver, such as in forwarding
   only the most active speakers, the receiver has to accept gaps in the
   E2E packet sequence.  The problem here is that a Media Distributor
   can choose to not deliver a particular stream for a while.

   While the Media Distributor can purposely stop forwarding media
   flows, it can also select an arbitrary starting point to resume
   forwarding those media flows, perhaps forwarding old packets rather
   than current packets.  As a consequence, what the media source sent
   can be substantially delayed at the receiver with the receiver
   believing that newly arriving packets are delayed only by transport
   delay when the packets may actually be minutes or hours old.

   While this attack cannot be eliminated entirely, its effectiveness
   can be reduced by rekeying the conference periodically, since
   significantly delayed media encrypted with expired keys would not be
   decrypted by endpoints.

8.2.4.  Splicing Attacks

   A splicing attack is an attack where a Media Distributor receiving
   multiple media sources splices one media stream into the other.  If
   the Media Distributor were able to change the SSRC without the
   receiver having any method for verifying the original source ID, then
   the Media Distributor could first deliver stream A and then later
   forward stream B under the same SSRC that stream A was previously
   using.  By including the SSRC in the integrity check for each packet
   -- both HBH and E2E -- PERC prevents splicing attacks.

8.2.5.  RTCP Attacks

   PERC does not provide E2E protection of RTCP messages.  This allows a
   compromised Media Distributor to impact any message that might be
   transmitted via RTCP, including media statistics, picture requests,
   or loss indication.  It is also possible for a compromised Media
   Distributor to forge requests, such as requests to the endpoint to
   send a new picture.  Such requests can consume significant bandwidth
   and impair conference performance.

8.3.  Key Distributor Attacks

   As stated in Section 3.2.2, the Key Distributor needs to be secured,
   since exploiting the Key Server can allow an adversary to gain access
   to the keying material for one or more conferences.  Having access to
   that keying material would then allow the adversary to decrypt media
   sent from any endpoint in the conference.

   As a first line of defense, the Key Distributor authenticates every
   security association -- associations with both endpoints and Media
   Distributors.  The Key Distributor knows which entities are
   authorized to have access to which keys, and inspection of
   certificates will substantially reduce the risk of providing keys to
   an adversary.

   Both physical and network access to the Key Distributor should be
   severely restricted.  This may be more difficult to achieve when the
   Key Distributor is embedded within, for example, an endpoint.
   Nonetheless, consideration should be given to shielding the Key
   Distributor from unauthorized access or any access that is not
   strictly necessary for the support of an ongoing conference.

   Consideration should be given to whether access to the keying
   material will be needed beyond the conclusion of a conference.  If
   not needed, the Key Distributor's policy should be to destroy the
   keying material once the conference concludes or when keying material
   changes during the course of the conference.  If keying material is
   needed beyond the lifetime of the conference, further consideration
   should be given to protecting keying material from future exposure.
   While it might seem obvious, it is worth making this point, to avoid
   any doubt that if an adversary were to record the media packets
   transmitted during a conference and then gain unauthorized access to
   the keying material left unsecured on the Key Distributor even years
   later, the adversary could decrypt the content of every packet
   transmitted during the conference.

8.4.  Endpoint Attacks

   A Trusted Endpoint is so named because conference confidentiality
   relies heavily on the security and integrity of the endpoint.  If an
   adversary successfully exploits a vulnerability in an endpoint, it
   might be possible for the adversary to obtain all of the keying
   material used in the conference.  With that keying material, an
   adversary could decrypt any of the media flows received from any
   other endpoint, either in real time or at a later point in time
   (assuming that the adversary makes a copy of the media packets).

   Additionally, if an adversary successfully exploits an endpoint, the
   adversary could inject media into the conference.  For example, an
   adversary could manipulate the RTP or SRTP software to transmit
   whatever media the adversary wishes to send.  This could involve the
   reuse of the compromised endpoint's SSRC or, since all conference
   participants share the same KEK, the use of a new SSRC or the SSRC
   value of another endpoint.  Only a single SRTP cipher suite defined
   provides source authentication properties that allow an endpoint to
   cryptographically assert that it sent a particular E2E-protected
   packet (namely, Timed Efficient Stream Loss-Tolerant Authentication
   (TESLA) [RFC4383]), and its usage is presently not defined for PERC.
   The suite defined in PERC only allows an endpoint to determine that
   whoever sent a packet had received the KEK.

   However, attacks on the endpoint are not limited to the PERC-specific
   software within the endpoint.  An attacker could inject media or
   record media by manipulating the software that sits between the PERC-
   enabled application and the hardware microphone of a video camera,
   for example.  Likewise, an attacker could potentially access
   confidential media by accessing memory, cache, disk storage, etc. if
   the endpoint is not secured.

9.  IANA Considerations

   This document has no IANA actions.

10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, DOI 10.17487/RFC3550,
              July 2003, <https://www.rfc-editor.org/info/rfc3550>.

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,
              <https://www.rfc-editor.org/info/rfc3711>.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904,
              DOI 10.17487/RFC6904, April 2013,
              <https://www.rfc-editor.org/info/rfc6904>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8723]  Jennings, C., Jones, P., Barnes, R., and A.B. Roach,
              "Double Encryption Procedures for the Secure Real-Time
              Transport Protocol (SRTP)", RFC 8723,
              DOI 10.17487/RFC8723, April 2020,
              <https://www.rfc-editor.org/info/rfc8723>.

   [RFC8870]  Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", RFC 8870, DOI 10.17487/RFC8870, January 2021,
              <https://www.rfc-editor.org/info/rfc8870>.

10.2.  Informative References

   [PERC-DTLS]
              Jones, P. E., Ellenbogen, P. M., and N. H. Ohlmeier, "DTLS
              Tunnel between a Media Distributor and Key Distributor to
              Facilitate Key Exchange", Work in Progress, Internet-
              Draft, draft-ietf-perc-dtls-tunnel-06, 16 October 2019,
              <https://tools.ietf.org/html/draft-ietf-perc-dtls-tunnel-
              06>.

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,
              <https://www.rfc-editor.org/info/rfc3261>.

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              DOI 10.17487/RFC4353, February 2006,
              <https://www.rfc-editor.org/info/rfc4353>.

   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient
              Stream Loss-Tolerant Authentication (TESLA) in the Secure
              Real-time Transport Protocol (SRTP)", RFC 4383,
              DOI 10.17487/RFC4383, February 2006,
              <https://www.rfc-editor.org/info/rfc4383>.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <https://www.rfc-editor.org/info/rfc4566>.

   [RFC5763]  Fischl, J., Tschofenig, H., and E. Rescorla, "Framework
              for Establishing a Secure Real-time Transport Protocol
              (SRTP) Security Context Using Datagram Transport Layer
              Security (DTLS)", RFC 5763, DOI 10.17487/RFC5763, May
              2010, <https://www.rfc-editor.org/info/rfc5763>.

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,
              <https://www.rfc-editor.org/info/rfc5764>.

   [RFC6464]  Lennox, J., Ed., Ivov, E., and E. Marocco, "A Real-time
              Transport Protocol (RTP) Header Extension for Client-to-
              Mixer Audio Level Indication", RFC 6464,
              DOI 10.17487/RFC6464, December 2011,
              <https://www.rfc-editor.org/info/rfc6464>.

   [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
              DOI 10.17487/RFC7667, November 2015,
              <https://www.rfc-editor.org/info/rfc7667>.

   [RFC8224]  Peterson, J., Jennings, C., Rescorla, E., and C. Wendt,
              "Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 8224,
              DOI 10.17487/RFC8224, February 2018,
              <https://www.rfc-editor.org/info/rfc8224>.

   [RFC8827]  Rescorla, E., "WebRTC Security Architecture", RFC 8827,
              DOI 10.17487/RFC8827, January 2021,
              <https://www.rfc-editor.org/info/rfc8827>.

   [W3C.CR-webrtc]
              Jennings, C., Boström, H., and J-I. Bruaroey, "WebRTC 1.0:
              Real-time Communication Between Browsers", W3C Proposed
              Recommendation, <https://www.w3.org/TR/webrtc/>.

Acknowledgments

   The authors would like to thank Mo Zanaty, Christian Oien, and
   Richard Barnes for invaluable input on this document.  Also, we would
   like to acknowledge Nermeen Ismail for serving on the initial draft
   versions of this document as a coauthor.  We would also like to
   acknowledge John Mattsson, Mats Naslund, and Magnus Westerlund for
   providing some of the text in the document, including much of the
   original text in the Security Considerations section (Section 8).

Authors' Addresses

   Paul E. Jones
   Cisco
   7025 Kit Creek Rd.
   Research Triangle Park, North Carolina 27709
   United States of America

   Phone: +1 919 476 2048
   Email: paulej@packetizer.com

   David Benham
   Independent
   California
   United States of America

   Email: dabenham@gmail.com

   Christian Groves
   Independent
   Melbourne
   Australia

   Email: cngroves.std@gmail.com