Network Working Group                                           P. Jones
Internet-Draft                                                     Cisco
Intended status: Standards Track                               D. Benham
Expires: March 8, 2019                                         C. Groves
                                                       September 4, 2018

     A Solution Framework for Private Media in Privacy Enhanced RTP


   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 aims to build upon existing security mechanisms
   defined for the real-time transport protocol (RTP).

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on March 8, 2019.

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   Copyright (c) 2018 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
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   carefully, as they describe your rights and restrictions with respect

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions Used in This Document . . . . . . . . . . . . . .   4
   3.  PERC Entities and Trust Model . . . . . . . . . . . . . . . .   5
     3.1.  Untrusted Entities  . . . . . . . . . . . . . . . . . . .   5
       3.1.1.  Media Distributor . . . . . . . . . . . . . . . . . .   6
       3.1.2.  Call Processing . . . . . . . . . . . . . . . . . . .   6
     3.2.  Trusted Entities  . . . . . . . . . . . . . . . . . . . .   7
       3.2.1.  Endpoint  . . . . . . . . . . . . . . . . . . . . . .   7
       3.2.2.  Key Distributor . . . . . . . . . . . . . . . . . . .   7
   4.  Framework for PERC  . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  End-to-End and Hop-by-Hop Authenticated Encryption  . . .   8
     4.2.  E2E Key Confidentiality . . . . . . . . . . . . . . . . .   9
     4.3.  E2E Keys and Endpoint Operations  . . . . . . . . . . . .   9
     4.4.  HBH Keys and Hop Operations . . . . . . . . . . . . . . .  10
     4.5.  Key Exchange  . . . . . . . . . . . . . . . . . . . . . .  10
       4.5.1.  Initial Key Exchange and Key Distributor  . . . . . .  11
       4.5.2.  Key Exchange during a Conference  . . . . . . . . . .  12
   5.  Authentication  . . . . . . . . . . . . . . . . . . . . . . .  13
     5.1.  Identity Assertions . . . . . . . . . . . . . . . . . . .  13
     5.2.  Certificate Fingerprints in Session Signaling . . . . . .  13
     5.3.  Conferences Identification  . . . . . . . . . . . . . . .  14
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
     6.1.  Third Party Attacks . . . . . . . . . . . . . . . . . . .  14
     6.2.  Media Distributor Attacks . . . . . . . . . . . . . . . .  15
       6.2.1.  Denial of service . . . . . . . . . . . . . . . . . .  15
       6.2.2.  Replay Attack . . . . . . . . . . . . . . . . . . . .  16
       6.2.3.  Delayed Playout Attack  . . . . . . . . . . . . . . .  16
       6.2.4.  Splicing Attack . . . . . . . . . . . . . . . . . . .  16
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  17
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Appendix A.  PERC Key Inventory . . . . . . . . . . . . . . . . .  19
     A.1.  DTLS-SRTP Exchange Yields HBH Keys  . . . . . . . . . . .  20
     A.2.  The Key Distributor Transmits the KEK (EKT Key) . . . . .  20
     A.3.  Endpoints fabricate an SRTP Master Key  . . . . . . . . .  21
     A.4.  Who has What Key  . . . . . . . . . . . . . . . . . . . .  21
   Appendix B.  PERC Packet Format . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

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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,
   transrated, 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 the Media Distributor
   to each of the other participants, often forwarding only a subset of
   flows based on voice activity detection or other criteria.  In some
   instances, the Media Distributors may make limited modifications to
   RTP [RFC3550] headers, 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.
   Deploying conference resources in a public cloud environment might
   introduce a higher security risk.  Whereas traditional conference
   resources were usually deployed in private networks that were
   protected, cloud-based conference resources might be viewed as less
   secure since they are not always physically controlled by those who
   use them.  Additionally, there are usually several ports open to the
   public in cloud deployments, such as for remote administration, and
   so on.

   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 RTCP packets.  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

   A goal of this document is to define a framework for enhanced privacy
   in RTP-based conferencing environments while utilizing existing
   security procedures defined for RTP with minimal enhancements.

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2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119] when they
   appear in ALL CAPS.  These words may also appear in this document in
   lower case as plain English words, absent their normative meanings.

   Additionally, this solution framework uses the following terms and

   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: 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 device and more that
   are in the trusted domain for a given deployment.

   Media Distributor (MD): An RTP middlebox that is not allowed to to
   have access to E2E encryption keys.  It operates according to the
   Selective Forwarding Middlebox RTP topologies [RFC7667] per the
   constraints defined by the PERC system, which includes, but not
   limited to, having no access to RTP media unencrypted and having
   limits on what RTP header field it can alter.

   Key Distributor: An entity that is a logical function which
   distributes keying material and related information to trusted
   endpoints and Media Distributor(s), only that which 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

   Call Processing: All trusted endpoints in the conference connect to
   it by a call processing dialog, such as with the Focus defined in the
   Framework for Conferencing with SIP [RFC4353].

   Third Party: Any entity that is not an Endpoint, Media Distributor,
   Key Distributor or Call Processing entity as described in this

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3.  PERC Entities and Trust Model

   The following figure depicts the trust relationships, direct or
   indirect, between entities described in the subsequent sub-sections.
   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 will keep the definitions and examples
   focused by only using the the simple, most general deployment

              +----------+        |        +-----------------+
              | 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 the privacy of participant's
   media will not be compromised.  The conferencing infrastructure in
   such a domain is still trusted with reliably connecting the
   participants together in a conference, but not trusted with keying
   material needed to decrypt any of the participant's media.  Entities
   in such lower trustworthiness domains will simply be referred to as
   untrusted entities from this point forward.

   It is important to understand that untrusted in this document does
   not mean 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.

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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 will also relay secured
   messaging between the endpoints and the Key Distributor and will
   acquire per-hop key information from the Key Distributor.  The actual
   media content MUST NOT not be decryptable by a Media Distributor, so
   it is untrusted to have access to the E2E media encryption keys.  The
   key exchange mechanisms specified in this framework will prevent the
   Media Distributor from gaining access to the E2E media encryption

   An endpoint's ability to join a conference hosted by a Media
   Distributor MUST NOT alone be interpreted as being authorized to have
   access to the E2E media encryption keys, as the Media Distributor
   does not have the ability to determine whether an endpoint is
   authorized.  Instead, the Key Distributor is responsible for
   authenticating endpoints (e.g., using WebRTC Identity
   [I-D.ietf-rtcweb-security-arch]) and determining their authorization
   to receive E2E 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 6), etc, to a level
   equal to or better than traditional conferencing (i.e. non-PERC)

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

3.1.2.  Call Processing

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

   The call processing function 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
   subsequently joining of a conference hosted through one or more Media
   Distributors.  Call processing may optionally ensure the privacy of
   call signaling messages between itself, the endpoint, and other

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   In any deployment scenario where the call processing function is
   considered trusted, the call processing function MUST ensure the
   integrity of received messages before forwarding to other entities.

3.2.  Trusted Entities

   From the PERC model system 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 will have 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 will take measures to mitigate the adverse effects of
   denial of service attacks (refer to Section 6) from other entities,
   including from other endpoints, to a level equal to or better than
   traditional conference (i.e., non-PERC) deployments.

3.2.2.  Key Distributor

   The Key Distributor, which may be colocated with an endpoint or exist
   standalone, is responsible for providing key information to endpoints
   for both end-to-end and hop-by-hop security and for providing key
   information to Media Distributors for the hop-by-hop security.

   Interaction between the Key Distributor and the call processing
   function is necessary to 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 to
   prevent exploitation by an adversary, as any kind of compromise of
   the Key Distributor puts the security of the conference at risk.

4.  Framework for PERC

   The purpose for this framework is to define a means through which
   media privacy can be ensured when communicating within a conferencing
   environment consisting of one or more Media Distributors that only
   switch, hence 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 SRTP [RFC3711], PERC EKT [I-D.ietf-perc-srtp-ekt-diet], and
   DTLS-SRTP [RFC5764].

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4.1.  End-to-End and Hop-by-Hop Authenticated Encryption

   This solution framework focuses on the end-to-end privacy and
   integrity of the participant's media by limiting access of the end-
   to-end key information to trusted entities.  However, this framework
   does give a Media Distributor access to RTP headers and all or most
   header extensions, as well as the ability to modify a certain subset
   of those headers and to add 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
   (an E2E key distinct for each transmitted media flow) for
   authenticated encryption of RTP media between endpoints and an
   "outer" key (HBH key) known only to media distributor and the
   adjacent endpoint) for the hop between an endpoint and a Media
   Distributor or between Media Distributor.

      +-------------+                                +-------------+
      |             |################################|             |
      |    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

   The PERC Double transform [I-D.ietf-perc-double] 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 Appendix A

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   for a description of the keys used in PERC and Appendix B for an
   overview of how the packet appears on the wire.

   RTCP can only be encrypted hop-by-hop, not end-to-end.  This
   framework introduces no additional step for RTCP authenticated
   encryption, so the procedures needed are specified in [RFC3711] and
   use the same outer, hop-by-hop cryptographic context chosen in the
   Double operation described above.

4.2.  E2E Key Confidentiality

   To ensure the confidentiality of E2E keys shared between endpoints,
   endpoints will make use of a common Key Encryption Key (KEK) that is
   known only by the trusted entities in a conference.  That KEK,
   defined in the PERC EKT [I-D.ietf-perc-srtp-ekt-diet] as the EKT Key,
   will be 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 will create a random SRTP master key for
   E2E authenticated encryption, thus participants in the conference
   MUST keep track of the E2E keys received via the Full EKT Field for
   each distinct SSRC in the conference so that it can properly decrypt
   received media.  Note, too, that an endpoint may change its E2E key
   at any time and advertise that new key to the conference as specified
   in [I-D.ietf-perc-srtp-ekt-diet].

4.3.  E2E Keys and Endpoint Operations

   Any given RTP media flow can be identified by its SSRC, and endpoints
   might send more than one at a time and change the mix of media flows
   transmitted during the life of a conference.

   Thus, endpoints MUST maintain a list of SSRCs from received RTP flows
   and each SSRC's associated E2E key information.  Following a change
   in an E2E key, prior E2E keys SHOULD be retained by receivers for a
   period long enough to ensure that late-arriving or out-of-order
   packets from the endpoint can be successfully decrypted.  Receiving
   endpoints MUST discard old E2E keys no later than when it leaves the

   If there is a need to encrypt one or more RTP header extensions end-
   to-end, an encryption key is derived from the end-to-end SRTP master
   key to encrypt header extensions as per [RFC6904].  The Media
   Distributor will not be able use the information contained in those
   header extensions encrypted with an E2E key.

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4.4.  HBH Keys and Hop Operations

   To ensure the integrity of transmitted media packets, this framework
   requires that every packet be authenticated hop-by-hop (HBH) between
   an endpoint and a Media Distributor, as well between Media
   Distributors.  The authentication key used for hop-by-hop
   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.

   Using hop-by-hop authentication gives the Media Distributor the
   ability to change certain RTP header values.  Which values the Media
   Distributor can change in the RTP header are defined in
   [I-D.ietf-perc-double].  RTCP can only be encrypted HBH, giving the
   Media Distributor the flexibility to forward RTCP content unchanged,
   transmit compound RTCP packets or to initiate RTCP packets for
   reporting statistics or conveying other information.  Performing hop-
   by-hop authentication for all RTP and RTCP packets also helps provide
   replay protection (see Section 6).

   If there is a need to encrypt one or more RTP header extensions hop-
   by-hop, an encryption key is derived from the hop-by-hop SRTP master
   key to encrypt header extensions as per [RFC6904].  This will still
   give the Media Distributor visibility into header extensions, such as
   the one used to determine audio level [RFC6464] of conference
   participants.  Note that when RTP header extensions are encrypted,
   all hops - in the untrusted domain at least - will need to decrypt
   and re-encrypt these encrypted header extensions.

4.5.  Key Exchange

   In brief, the keys used by any given endpoints are determined in the
   following way:

   o  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).

   o  The E2E key that an endpoint uses to send SRTP media can either be
      set from DTLS or chosen by the endpoint.  It is then distributed
      to other endpoints in a Full EKT Field, encrypted under an EKTKey
      provided to the client by the Key Distributor within the DTLS
      channel they negotiated.

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

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4.5.1.  Initial Key Exchange and Key Distributor

   The Media Distributor maintains a tunnel with the Key Distrubutor
   (e.g., using [I-D.ietf-perc-dtls-tunnel]), 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
   the following figure.  The DTLS association between endpoints and the
   Key Distributor will enable each endpoint to generate E2E and HBH
   keys and receive the Key Encryption Key (KEK) (i.e., EKT Key).  At
   the same time, the Key Distributor can securely provide the HBH key
   information to the Media Distributor.  The key information summarized
   here may include the SRTP master key, 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

   Endpoints will establish a DTLS-SRTP [RFC5764] association over the
   RTP session's media ports for the purposes of key information
   exchange with the Key Distributor.  The Media Distributor will not
   terminate the DTLS signaling, but will instead forward DTLS packets
   received from an endpoint on to the Key Distributor (and vice versa)
   via a tunnel established between Media Distributor and the Key
   Distributor.  This tunnel is used to encapsulate the DTLS-SRTP
   signaling between the Key Distributor and endpoints will also be used
   to convey HBH key information from the Key Distributor to the Media
   Distributor, so no additional protocol or interface is required.

   In 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 Key Encryption Key
   (KEK) (i.e., EKT Key) is conveyed by the Key Distributor over the
   DTLS association to endpoints via procedures defined in PERC EKT
   [I-D.ietf-perc-srtp-ekt-diet] via the EKTKey message.

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   Note that following DTLS-SRTP procedures for the
   [I-D.ietf-perc-double] cipher, the endpoint will generate both E2E
   and HBH encryption keys and salt values.  Endpoints MAY use the DTLS-
   SRTP generated E2E key for transmission or MAY 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

   Media Distributors use DTLS-SRTP [RFC5764] 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 will be able to encrypt media end-to-end
   with an E2E key, sending that E2E key to other endpoints encrypted
   with the KEK, and will be able to encrypt and authenticate RTP
   packets using a HBH key.  The procedures defined do 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 (i.e., EKT Key) may need to change from time-to-time during
   the life 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 re-keyed is outside the scope of this document, but this
   framework does accommodate re-keying during the life of a conference.

   When a Key Distributor decides to re-key a conference, it transmits a
   specific message defined in PERC EKT [I-D.ietf-perc-srtp-ekt-diet] to
   each of the conference participants.  The endpoint MUST create a new
   SRTP master key and prepare to send that key inside a Full EKT Field
   using the new EKTKey.  Since it may take some time for all of the
   endpoints in conference to finish re-keying, senders MUST delay a
   short period of time before sending media encrypted with the new
   master key, but it MUST be prepared to make use of the information
   from a new inbound EKT Key immediately.  See Section 2.2.2 of

   Endpoints MAY follow the procedures in section 5.2 of [RFC5764] to
   re-negotiate HBH keys as desired.  If new HBH keys are generated, the

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   new keys are also delivered to the Media Distributor following the
   procedures defined in [I-D.ietf-perc-dtls-tunnel].

   Endpoints are at liberty to change the E2E encryption key used at any
   time.  Endpoints MUST generate a new E2E encryption key whenever it
   receives a new EKT Key.  After switching to a new key, the new key
   will be conveyed to other endpoints in the conference in RTP/RTCP
   packets per [I-D.ietf-perc-srtp-ekt-diet].

5.  Authentication

   It is important to this solution framework that the entities can
   validate the authenticity of other entities, especially the Key
   Distributor and endpoints.  The details of this are outside the scope
   of specification but a few possibilities are discussed in the
   following sections.  The key requirements is that endpoints can
   verify they are connected to the correct Key Distributor for the
   conference and the Key Distributor can verify the endpoints are the
   correct endpoints for the conference.

   Two possible approaches to solve this are Identity Assertions and
   Certificate Fingerprints.

5.1.  Identity Assertions

   WebRTC Identity assertion [I-D.ietf-rtcweb-security-arch] can be 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 allows 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 validate 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 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

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   session along with the endpoint's certificate fingerprint, which can
   be signed using the procedures described in [RFC4474] for the benefit
   of forwarding the message to other entities by the Focus [RFC4353].
   Other entities can now verify 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 check that is the authorized

   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 that user is
   authorized.  Similarly, the Key Distributor's certificate fingerprint
   can be conveyed to endpoint in a manner that can be authenticated as
   being an authorized Key Distributor for this conference.

5.3.  Conferences Identification

   The Key Distributor needs to know what endpoints are being added to a
   given conference.  Thus, the Key Distributor and the Media
   Distributor will need to know endpoint-to-conference mappings, which
   is enabled by exchanging a conference-specific unique identifier as
   defined in [I-D.ietf-perc-dtls-tunnel].  How this unique identifier
   is assigned is outside the scope of this document.

6.  Security Considerations

   This framework, and the individual protocols defined to support it,
   must take care to not increase the exposure to Denial of Service
   (DoS) attacks by untrusted or third-party entities and should take
   measures to mitigate, where possible, more serious DoS attacks from
   on-path and off-path attackers.

   The following section enumerates the kind of attacks that will be
   considered in the development of this framework's solution.

6.1.  Third Party Attacks

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

   Off-path attackers may try connecting to different PERC entities and
   send specifically crafted packets.  A successful attacker might be
   able to get the Media Distributor to forward such packets.  If not
   making use of HBH authentication on the Media Distributor, such an
   attack could only be detected in the receiving endpoints where the
   forged packets would finally be dropped.

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   Another potential attack is a third party claiming to be a Media
   Distributor, fooling endpoints in to sending packets to the false
   Media Distributor instead of the correct one.  The deceived sending
   endpoints could incorrectly assuming their packets have been
   delivered to endpoints when they in fact have not.  Further, the
   false Media Distributor may cascade to another legitimate Media
   Distributor creating a false version of the real conference.

   This attack can be mitigated by the false Media Distributor not being
   authenticated by the Key Distributor during PERC Tunnel
   establishment.  Without the tunnel in place, endpoints will not
   establish secure associations with the Key Distributor and receive
   the KEK, causing the conference to not proceed.

6.2.  Media Distributor Attacks

   The Media Distributor can attack the session in a number of possible

6.2.1.  Denial of service

   Any modification of the end-to-end authenticated data will result 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 with an inband
   key-distribution message attached (i.e., Full EKT Field).  Since such
   a message would trigger the receiver to form a new cryptographic
   context, the Media Distributor can attempt to consume the receiving
   endpoints resources.

   Another denial of service attack is where the Media Distributor
   rewrites the PT field to indicate a different codec.  The effect of
   this attack is that any payload packetized and encoded according to
   one RTP payload format is then processed using another payload format
   and codec.  Assuming that the implementation is robust to random
   input, it is unlikely to cause crashes in the receiving software/
   hardware.  However, it is not unlikely that such rewriting will cause
   severe media degradation.

   For audio formats, this attack is likely to cause highly disturbing
   audio and/or can be damaging to hearing and playout equipment.

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6.2.2.  Replay Attack

   Replay attack is when an already received packets from a previous
   point in the RTP stream is replayed as 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.

   The mitigation for a replay attack is to prevent old packets beyond a
   small-to-modest jitter and network re-ordering sized window to be
   rejected.  End-to-end replay protection MUST be provided for the
   whole duration of the conference.

6.2.3.  Delayed Playout Attack

   The delayed playout attack is a variant of the replay attack.  This
   attack is possible even if E2E replay protection is in place.
   However, due to fact that the Media Distributor is allowed to select
   a sub-set 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 issue with this is that
   a Media Distributor can select to not deliver a particular stream for
   a while.

   Within the window from last packet forwarded to the receiver and the
   latest received by the Media Distributor, the Media Distributor can
   select an arbitrary starting point when resuming forwarding packets.
   Thus what the media source said can be substantially delayed at the
   receiver with the receiver believing that it is what was said just
   now, and only delayed due to transport delay.

6.2.4.  Splicing Attack

   The splicing attack is an attack where a Media Distributor receiving
   multiple media sources splices one media stream into the other.  If
   the Media Distributor is 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 as stream A was previously using.  Not
   allowing the Media Distributor to change the SSRC mitigates this

7.  IANA Considerations

   There are no IANA considerations for this document.

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8.  Acknowledgments

   The authors would like to thank Mo Zanaty and Christian Oien for
   invaluable input on this document.  Also, we would like to
   acknowledge Nermeen Ismail for serving on the initial versions of
   this document as a co-author.

9.  References

9.1.  Normative References

              Jennings, C., Jones, P., Barnes, R., and A. Roach, "SRTP
              Double Encryption Procedures", draft-ietf-perc-double-09
              (work in progress), May 2018.

              Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel
              between a Media Distributor and Key Distributor to
              Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-03
              (work in progress), April 2018.

              Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", draft-ietf-perc-srtp-ekt-diet-08 (work in progress),
              July 2018.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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, <>.

   [RFC6904]  Lennox, J., "Encryption of Header Extensions in the Secure
              Real-time Transport Protocol (SRTP)", RFC 6904,
              DOI 10.17487/RFC6904, April 2013,

9.2.  Informative References

              Rescorla, E., "WebRTC Security Architecture", draft-ietf-
              rtcweb-security-arch-15 (work in progress), July 2018.

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   [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,

   [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,

   [RFC4353]  Rosenberg, J., "A Framework for Conferencing with the
              Session Initiation Protocol (SIP)", RFC 4353,
              DOI 10.17487/RFC4353, February 2006,

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474,
              DOI 10.17487/RFC4474, August 2006,

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, DOI 10.17487/RFC4566,
              July 2006, <>.

   [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, <>.

   [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,

   [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,

   [RFC7667]  Westerlund, M. and S. Wenger, "RTP Topologies", RFC 7667,
              DOI 10.17487/RFC7667, November 2015,

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Appendix A.  PERC Key Inventory

   PERC specifies the use of a number of different keys and,
   understandably, it looks complicated or confusing on the surface.
   This section summarizes the various keys used in the system, how they
   are generated, and what purpose they serve.

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

   The various keys used in PERC are shown in Figure 4 below.

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

                          Figure 4: Key Inventory

   As you can see, the number key types is very small.  However, what
   can be challenging is keeping track of all of the distinct E2E keys
   as the conference grows in size.  With 1,000 participants in a
   conference, there will be 1,000 distinct SRTP master keys, all of
   which share the same master salt.  Each of those keys are passed
   through the KDF 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.  And, of course, there is a new SRTP master salt to
   go with those keys.  Endpoints have to retain old keys for a period
   of time to ensure they can properly decrypt late-arriving or out-of-
   order packets.

   The time required to retain old keys (either EKT Keys or SRTP master
   keys) is not specified, but they should be retained at least for the
   period of time required to re-key the conference or handle late-
   arriving or out-of-order packets.  A period of 60s should be
   considered a generous retention period, but endpoints may keep old
   keys on hand until the end of the conference.

   Or more detailed explanation of each of the keys follows.

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A.1.  DTLS-SRTP Exchange Yields HBH Keys

   The first set of keys acquired are for hop-by-hop encryption and
   decryption.  Assuming the use of Double [I-D.ietf-perc-double], the
   endpoint would perform DTLS-SRTP exchange with the key distributor
   and receive a key that is, in fact, "double" the size that is needed.
   Per the Double specification, the E2E part is the first half of the
   key, so the endpoint will just discard that information in PERC.  It
   is not used.  The second half of the key 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 media distributor doesn't perform DTLS-SRTP, but it is at this
   point that the key distributor will inform the media distributor of
   the HBH key value via the tunnel protocol
   ([I-D.ietf-perc-dtls-tunnel]).  The key distributor will send the
   least significant bits corresponding to the half of the keying
   material determined through DTLS-SRTP with the endpoint to the media
   distributor via the tunnel protocol.  There is a salt 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 (i.e., half of the generated salt material) are sent to the
   media distributor via the tunnel protocol.

   No two endpoints will have the same HBH key, thus the media
   distributor must keep track each distinct HBH key (and the
   corresponding salt) and use it only for the specified hop.

   This key is also used for HBH encryption of RTCP.  RTCP is not end-
   to-end encrypted in PERC.

A.2.  The Key Distributor Transmits the KEK (EKT Key)

   Via the aforementioned DTLS-SRTP association, the key distributor
   will send the endpoint the KEK (i.e., EKT Key per
   [I-D.ietf-perc-srtp-ekt-diet]).  This key is known only to the key
   distributor and endpoints.  This key is the most important to protect
   since having knowledge of this key (and the SRTP master salt
   transmitted as a part of the same message) will allow 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 can be used to re-key the entire conference.  Each
   key is identified by a "Security Parameter Index" (SPI) value.
   Endpoints should expect that a conference might be re-keyed 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.

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   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 EKT Field 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 Field
   data as per [I-D.ietf-perc-srtp-ekt-diet]).

   The media distributor is not given the KEK (i.e., EKT Key).

A.3.  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 SRTP master key.  While the
   DTLS-SRTP-derived key could be used, the fact that an endpoint might
   need to change the SRTP master key periodically or is forced to
   change the SRTP master key as a result of the EKT key changing means
   using it has only limited utility.  To reduce complexity, PERC
   *RECOMMENDS* that endpoints create random SRTP master keys locally to
   be used for E2E encryption.

   This 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 will
   not create this key nor 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 will transmit its E2E key to other endpoints in the
   conference by periodically including it in SRTP packets in a Full EKT
   Field.  When placed in the Full EKT Field, it is encrypted using the
   EKT Key provided by the key distributor.  The master salt is not
   transmitted, though, since all endpoints will have received the same
   master salt via the EKTKey message.  The recommended frequency with
   which an endpoint transmits its SRTP master key is specified in

A.4.  Who has What Key

   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.

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   Each endpoint generates its own E2E Key (SRTP master key), thus the
   key distinct per endpoint.  This key is transmitted (encrypted) via
   the EKT Field to other endpoints.  Endpoints that receive media from
   a given transmitting endpoint will therefore have knowledge of the
   transmitter's E2E key.

   To summarize the various keys and which entity is in possession of a
   given key, refer to Figure 5.

    | Key     /    Entity  | Endpoint A |  MD X |  MD Y | Endpoint B |
    | KEK                  |    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     |

                         Figure 5: Keys per Entity

Appendix B.  PERC Packet Format

   Figure 6 presents a complete picture of what a PERC packet looks like
   when transmitted over the wire.

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       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
    A |V=2|P|X|  CC   |M|     PT      |       sequence number         |
    A |                           timestamp                           |
    A |           synchronization source (SSRC) identifier            |
    A |            contributing source (CSRC) identifiers             |
    A |                               ....                            |
    A |                    RTP extension (OPTIONAL)                   |
    A |                      (including the OHB)                      |
    C :                                                               :
    C :                       Ciphertext Payload                      :
    C :                                                               :
    R :                                                               :
    R :                        EKT Field                              :
    R :                                                               :

                 C = Ciphertext (encrypted and authenticated)
                 A = Associated Data (authenticated only)
                 R = neither encrypted nor authenticated, added
                     after Authenticated Encryption completed

                       Figure 6: PERC Packet Format

Authors' Addresses

   Paul E. Jones
   7025 Kit Creek Rd.
   Research Triangle Park, North Carolina  27709

   Phone: +1 919 476 2048

   David Benham


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   Christian Groves


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