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  <front>
    <title abbrev="Private Media Framework">A Solution Framework for Private Media in Privacy Enhanced Privacy-Enhanced RTP Conferencing (PERC)</title><author (PERC)</title>

   <seriesInfo name="RFC" value="8871"/>
    <author initials="P." surname="Jones" fullname="Paul E. Jones"><organization>Cisco</organization><address><postal><street>7025 Jones">
      <organization>Cisco</organization>
      <address>
        <postal>
          <street>7025 Kit Creek Rd.</street>
          <city>Research Triangle Park</city>
<code>27709</code>
<country>USA</country>
          <region>North Carolina</region>
</postal><phone>+1
          <code>27709</code>
          <country>United States of America</country>
        </postal>
        <phone>+1 919 476 2048</phone>
        <email>paulej@packetizer.com</email>
</address></author>
      </address>
    </author>
    <author initials="D." surname="Benham" fullname="David Benham"><organization>Independent</organization><address><postal><street></street>
</postal><email>dabenham@gmail.com</email>
</address></author> Benham">
      <organization>Independent</organization>
      <address>
        <postal>
          <region>California</region>
          <country>United States of America</country>
	</postal>
        <email>dabenham@gmail.com</email>
      </address>
    </author>
    <author initials="C." surname="Groves" fullname="Christian Groves"><organization>Independent</organization><address><postal><street></street> Groves">
      <organization>Independent</organization>
      <address>
        <postal>
          <street/>
          <city>Melbourne</city>
          <country>Australia</country>
</postal><email>cngroves.std@gmail.com</email>
</address></author>
<date/>
<area>Internet</area><workgroup></workgroup><keyword>PERC</keyword>
        </postal>
        <email>cngroves.std@gmail.com</email>
      </address>
    </author>
    <date month="January" year="2021"/>
    <keyword>PERC</keyword>
    <keyword>Private Media Framework</keyword>
    <keyword>conferencing</keyword>

<abstract><t>This
    <abstract>
      <t>This document describes a solution framework for ensuring that media
confidentiality and integrity are maintained end-to-end end to end within the
context of a switched conferencing environment where media
distributors 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 Real-time Transport Protocol (RTP).</t>
    </abstract>
  </front>
  <middle>
    <section anchor="introduction" title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>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,
translated, recomposed, or otherwise manipulated by a Media
Distributor, as might be the case with a traditional media server or
multipoint control unit
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 <xref target="RFC3550"></xref> headers, target="RFC3550" format="default"/>, for example, but the actual media content
(e.g., voice or video data) is unaltered.</t>
      <t>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
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.</t>
      <t>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 <xref target="RFC3550"></xref> packets. target="RFC3550" format="default"/>.
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
endpoint that is independent from the key for media encryption and
authentication.</t>
      <t>This solution framework provides for enhanced privacy
in RTP-based conferencing environments while utilizing existing
security procedures defined for RTP with minimal enhancements.</t>
    </section>
    <section anchor="conventions-used-in-this-document" title="Conventions numbered="true" toc="default">
      <name>Conventions Used in This Document"> Document</name>
       <t>The key words &quot;MUST&quot;, &quot;MUST NOT&quot;, &quot;REQUIRED&quot;, &quot;SHALL&quot;,
&quot;SHALL NOT&quot;, &quot;SHOULD&quot;, &quot;SHOULD NOT&quot;, &quot;RECOMMENDED&quot;,
&quot;NOT RECOMMENDED&quot;, &quot;MAY&quot;, "<bcp14>MUST</bcp14>", "<bcp14>MUST NOT</bcp14>",
       "<bcp14>REQUIRED</bcp14>", "<bcp14>SHALL</bcp14>",
       "<bcp14>SHALL NOT</bcp14>", "<bcp14>SHOULD</bcp14>",
       "<bcp14>SHOULD NOT</bcp14>",
       "<bcp14>RECOMMENDED</bcp14>", "<bcp14>NOT RECOMMENDED</bcp14>",
       "<bcp14>MAY</bcp14>", and &quot;OPTIONAL&quot; "<bcp14>OPTIONAL</bcp14>" in this document
       are to be interpreted as described in BCP 14 BCP&nbsp;14
       <xref target="RFC2119"></xref> target="RFC2119"/> <xref target="RFC8174"></xref> target="RFC8174"/> when, and only
       when, they appear in all capitals, as shown here.</t>
      <t>Additionally, this solution framework uses the following
terms and acronyms:</t>
<t>End-to-End (E2E): Communications abbreviations:</t>
      <dl newline="false" spacing="normal">
      <dt>End-to-End (E2E):</dt><dd>Communications from one Endpoint endpoint through one or more
Media Distributors to the Endpoint endpoint at the other end.</t>
<t>Hop-by-Hop (HBH): Communications end.</dd>
      <dt>Hop-by-Hop (HBH):</dt><dd>Communications between an Endpoint endpoint and a Media
Distributor or between Media Distributors.</t>
<t>Trusted Distributors.</dd>
      <dt>Trusted Endpoint (or simply Endpoint): An endpoint):</dt><dd>An RTP flow terminating 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 devices, and more more, that are in the
trusted domain for a given deployment. In the context of WebRTC
<xref target="W3C.CR-webrtc-20180927"></xref>, target="W3C.CR-webrtc" format="default"/>, 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 <xref target="RFC5764"></xref> target="RFC5764" format="default"/> in
one-to-one calls).</t>
<t>Media calls).</dd>
      <dt>Media Distributor (MD): An (MD):</dt><dd>An RTP middlebox that forwards Endpoint endpoint media
content (e.g., voice or video data) unaltered, unaltered -- either a subset or all
of the flows at any given time, time -- and is never allowed to have access
to E2E encryption keys.  It operates according to the
Selective Forwarding Middlebox RTP topologies <xref target="RFC7667"></xref> target="RFC7667" format="default"/> per the
constraints defined by the PERC 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 field fields it can alter.  The header fields that may be
modified by a Media Distributor are enumerated in Section 4 of the Double <xref target="RFC8723"
sectionFormat="of" section="4">the double cryptographic transform specification <xref target="I-D.ietf-perc-double"></xref>
specification</xref> and chosen
with respect to utility and the security considerations outlined in this
document.</t>
<t>Key Distributor: An
document.</dd>
      <dt>Key Distributor:</dt><dd>An entity that is a logical function which that
distributes keying material and related information to Trusted
Endpoints and Media Distributor(s), Distributor(s) -- only that which what is appropriate for
each.  The Key Distributor might be co-resident with another entity
trusted with E2E keying material.</t>
<t>Conference: Two material.</dd>
      <dt>Conference:</dt><dd>Two or more participants communicating via Trusted
Endpoints to exchange RTP flows through one or more Media Distributor.</t>
<t>Call Processing: All Distributors.</dd>
      <dt>Call Processing:</dt><dd>All Trusted Endpoints in the conference connect to it
by a
conference via a call processing dialog, such as e.g., with the Focus "focus" as defined in the
<xref target="RFC4353" format="default">"A Framework for Conferencing with SIP <xref target="RFC4353"></xref>.</t>
<t>Third Party: Any the
Session Initiation Protocol (SIP)"</xref>.</dd>
      <dt>Third Party:</dt><dd>Any entity that is not an Endpoint, endpoint, Media Distributor,
Key Distributor Distributor, or Call Processing call processing entity as described in this
document.</t>
document.</dd>
      </dl>
    </section>
    <section anchor="perc-entities-and-trust-model" title="PERC numbered="true" toc="default">
      <name>PERC Entities and Trust Model">
<t>The following figure Model</name>
      <t><xref target="fig-trust-model"/> depicts the trust relationships, direct or
indirect, between entities described in the subsequent sub-sections. subsections.
Note that these entities may be co-located or further divided into
multiple, separate physical devices.</t>
      <t>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.</t>
      <figure anchor="fig-trust-model" align="center" title="Trusted anchor="fig-trust-model">
        <name>Trusted and Untrusted Entities in PERC
"> PERC</name>
        <artwork align="center"> align="center" name="" type="" alt=""><![CDATA[
                       |
   +----------+        |        +-----------------+
   | Endpoint |        |        | Call Processing |
   +----------+        |        +-----------------+
                       |
                       |
+----------------+     |       +--------------------+
| Key Distributor|     |       | Media Distributor  |
+----------------+     |       +--------------------+
                       |
     Trusted           |             Untrusted
     Entities          |             Entities
                       |

</artwork>
                       |]]></artwork>
      </figure>
      <section anchor="untrusted-entities" title="Untrusted Entities"> numbered="true" toc="default">
        <name>Untrusted Entities</name>
        <t>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, conference but is not trusted with keying material needed
to decrypt any of the participant's media.  Entities in such lower
trustworthiness
less-trustworthy domains are referred to as untrusted
entities from this point forward.</t>
        <t>It is important to understand that untrusted "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.</t>
        <section anchor="media-distributor" title="Media Distributor"> numbered="true" toc="default">
          <name>Media Distributor</name>
          <t>A Media Distributor forwards RTP flows between Endpoints 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
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 untrusted 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.</t>
          <t>An Endpoint's endpoint's ability to connect to a conference serviced by a Media
Distributor does imply implies that the Endpoint endpoint is authorized to
have access to the E2E media encryption keys, as although the Media Distributor
does not have the ability to determine whether an Endpoint endpoint is
authorized.  Instead, the Key Distributor is responsible for
authenticating the Endpoint endpoint (e.g., using WebRTC Identity identity assertions
<xref target="I-D.ietf-rtcweb-security-arch"></xref>) target="RFC8827" format="default"/>) and determining its
authorization to receive E2E and HBH media encryption keys.</t> keys.
</t>
          <t>A Media Distributor must perform its role in properly forwarding
media packets while taking measures to mitigate the adverse effects of
denial of service
denial-of-service attacks (refer to <xref target="attacks"></xref>) target="attacks" format="default"/>) to a level equal
to or better than traditional conferencing (non-PERC)
deployments.</t>
          <t>A Media Distributor or associated conferencing infrastructure may also
initiate or terminate various conference control messaging techniques related messaging,
which to conference
control. This topic is outside the scope of this framework document.</t>
        </section>
        <section anchor="call-processing" title="Call Processing">
<t>The call numbered="true" toc="default">
          <name>Call Processing</name>
          <t>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.</t>
<t>The call
          <t>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 endpoints for the purpose of call signaling and of
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, itself (call processing), the Endpoint, endpoint, and other
entities.</t>
        </section>
      </section>
      <section anchor="trusted-entities" title="Trusted Entities"> numbered="true" toc="default">
        <name>Trusted Entities</name>
        <t>From the PERC model system system's perspective, entities considered trusted
(refer to <xref target="fig-trust-model"></xref>) target="fig-trust-model" format="default"/>) can be in possession of the E2E media
encryption keys for one or more conferences.</t>
        <section anchor="endpoint" title="Endpoint"> numbered="true" toc="default">
          <name>Endpoint</name>
          <t>An Endpoint endpoint is considered trusted and has access to E2E key
information.  While it is possible for an Endpoint endpoint to be compromised,
subsequently performing in undesired ways, defining Endpoint endpoint
resistance to compromise is outside the scope of this document.
Endpoints take measures to mitigate the adverse effects of denial
of service denial-of-service attacks (refer to <xref target="attacks"></xref>) target="attacks" format="default"/>) from other entities,
including from other Endpoints, endpoints, to a level equal to or better than
traditional conference (non-PERC) deployments.</t>
        </section>
        <section anchor="key_distributor" title="Key Distributor"> numbered="true" toc="default">
          <name>Key Distributor</name>
          <t>The Key Distributor, which may be colocated co-located with an Endpoint endpoint or exist
standalone, is responsible for providing key information to Endpoints endpoints
for both end-to-end (E2E) E2E and hop-by-hop (HBH) HBH security and for providing key
information to Media Distributors for the hop-by-hop HBH security.</t>
          <t>Interaction between the Key Distributor and the call processing
function
is necessary for proper conference-to-Endpoint conference-to-endpoint
mappings. This is described in <xref target="conf-id"></xref>.</t> target="conf-id" format="default"/>.</t>
          <t>The Key Distributor needs to be secured and managed in a way to
prevent that
prevents exploitation by an adversary, as any kind of compromise of the
Key Distributor puts the security of the conference at risk.</t>
<t>They
          <t>The Key Distributor needs to know which Endpoints 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 <bcp14>MUST</bcp14> reject DTLS associations
with any unauthorized Endpoint endpoint or Media Distributor.</t>
        </section>
      </section>
    </section>
    <section anchor="framework-for-perc" title="Framework numbered="true" toc="default">
      <name>Framework for PERC"> PERC</name>
      <t>The purpose for 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 endpoints is taking place.</t>
      <t>This framework reuses several specified RTP security technologies,
including the Secure Real-time Transport Protocol (SRTP) <xref target="RFC3711"></xref>, target="RFC3711" format="default"/>,
Encrypted Key Transport (EKT) <xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>, target="RFC8870" format="default"/>,
and DTLS-SRTP.</t>
      <section anchor="end-to-end-and-hop-by-hop-authenticated-encryption" title="End-to-End numbered="true" toc="default">
        <name>E2E-Authenticated and Hop-by-Hop Authenticated Encryption"> HBH-Authenticated Encryption</name>
        <t>This solution framework focuses on the end-to-end E2E privacy and
integrity of the participant's media by limiting access to only trusted
entities to the E2E key used for authenticated end-to-end E2E encryption.
However, this framework does give a Media Distributor access to RTP headers 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 endpoint are authenticated
hop-by-hop.</t>
hop by hop.</t>
        <t>To enable all of the above, this framework defines the use of two
security contexts and two associated encryption keys: an &quot;inner&quot; "inner" key
(an
(a distinct E2E key distinct for each transmitted media flow) for authenticated
encryption of RTP media between Endpoints endpoints and an &quot;outer&quot; "outer" key (HBH (a HBH key)
known only to a Media Distributor or the adjacent Endpoint endpoint
for the hop between an Endpoint endpoint and a Media Distributor or peer Endpoint. endpoint.
An Endpoint endpoint will receive one or more E2E keys from
every other Endpoint endpoint in the conference that correspond to the media flows
transmitted by those other Endpoints, 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 <xref target="key_exchange"></xref> target="key_exchange" format="default"/> for more details
on key exchange.</t>
        <figure anchor="fig-e2e-and-hbh-keys-used" align="center" title="E2E anchor="fig-e2e-and-hbh-keys-used">
          <name>E2E and HBH Keys Used for Authenticated Encryption of SRTP
Packets
"> Packets</name>
          <artwork align="center">+-------------+ align="center" name="" type="" alt=""><![CDATA[+-------------+                                +-------------+
|             |################################|             |
|    Media    |------------------------ *-----&gt;| *----->|    Media    |
| Distributor |&lt;----------------------*-|------| |<----------------------*-|------| 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  |
+-------------+                                +-------------+
</artwork>                                +-------------+]]></artwork>
        </figure>
        <t>The Double double transform <xref target="I-D.ietf-perc-double"></xref> target="RFC8723" format="default"/> enables Endpoints endpoints
to perform encryption using both the end-to-end E2E and hop-by-hop 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 <xref target="keyinventory"></xref> target="keyinventory" format="default"/> for a description of the keys used in PERC
and <xref target="packetformat"></xref> target="packetformat" format="default"/> for a diagram of how encrypted RTP packets appear on the
wire.</t>
        <t>RTCP is only encrypted hop-by-hop, hop by hop -- not end-to-end. end to end. This framework
introduces no
does not provide an additional step for RTCP authenticated encryption, so RTCP-authenticated
encryption.  Rather, implementations utilize the existing procedures needed are
specified in <xref target="RFC3711"></xref> and target="RFC3711" format="default"/>; those procedures use
the same outer, hop-by-hop HBH cryptographic context chosen in the Double double transform operation
described above. For this reason, Endpoints MUST NOT endpoints <bcp14>MUST NOT</bcp14> send
confidential information via RTCP.</t>
      </section>
      <section anchor="e2e-key-confidentiality" title="E2E numbered="true" toc="default">
        <name>E2E Key Confidentiality"> Confidentiality</name>
        <t>To ensure the confidentiality of E2E keys shared between Endpoints,
Endpoints 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 <xref target="I-D.ietf-perc-srtp-ekt-diet"></xref> target="RFC8870" format="default"/> as the EKT Key, is
used to subsequently encrypt the SRTP master key used for E2E
authenticated E2E-authenticated encryption of media sent by a given Endpoint. endpoint.
Each Endpoint endpoint in the conference creates an SRTP master
key for E2E authenticated E2E-authenticated encryption and
keep
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 endpoint may change its E2E key at any
time and advertise that new key to the conference as specified in
<xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>.</t> target="RFC8870" format="default"/>.</t>
      </section>
      <section anchor="e2e_keys_ops" title="E2E numbered="true" toc="default">
        <name>E2E Keys and Endpoint Operations"> Operations</name>
        <t>Any given RTP media flow is identified by its SSRC, and an Endpoint endpoint
might send more than one at a time and change the mix of media flows
transmitted during the life lifetime of a conference.</t>
        <t>Thus, an Endpoint MUST endpoint <bcp14>MUST</bcp14> maintain a list of SSRCs from received RTP
flows and each SSRC's associated E2E key information.  An Endpoint MUST endpoint <bcp14>MUST</bcp14>
discard old E2E keys no later than when it leaves the conference
(see <xref target="keyexchange"></xref>).</t> conference.</t>
        <t>If the packet is to contain RTP header extensions, it should be noted
that those extensions are only encrypted HBH hop by hop per <xref target="I-D.ietf-perc-double"></xref>. target="RFC8723" format="default"/>.  For
this reason, Endpoints MUST NOT endpoints <bcp14>MUST NOT</bcp14> transmit confidential information
via RTP header extensions.</t>
      </section>
      <section anchor="hbh-keys-and-per-hop-operations" title="HBH numbered="true" toc="default">
        <name>HBH Keys and Per-hop Operations"> Per-Hop Operations</name>
        <t>To ensure the integrity of transmitted media packets, it is
REQUIRED
<bcp14>REQUIRED</bcp14> that every packet be authenticated hop-by-hop hop by hop between
an Endpoint endpoint and a Media Distributor, as well as between Media
Distributors.  The authentication key used for hop-by-hop HBH
authentication is derived from an SRTP master key shared only on the
respective hop.  Each HBH key is distinct per hop hop, and no two hops ever
use the same SRTP master key.</t>
        <t>While Endpoints endpoints also perform HBH authentication, the ability of the Endpoints endpoints
to reconstruct the original RTP header also enables the Endpoints endpoints to
authenticate RTP packets E2E. end to end.  This design yields flexibility to the Media
Distributor to change certain RTP header values as packets are
forwarded.  Which values  Values that the Media Distributor can change in the RTP header
are defined in
<xref target="I-D.ietf-perc-double"></xref>. target="RFC8723" format="default"/>.  RTCP can only be encrypted hop-by-hop, hop by
hop, giving the Media Distributor the flexibility to forward (1)&nbsp;forward RTCP
content unchanged,
transmit (2)&nbsp;transmit compound RTCP packets or to initiate packets, (3)&nbsp;initiate
RTCP packets for reporting statistics statistics, or conveying (4)&nbsp;convey other information.
Performing
hop-by-hop HBH authentication for all RTP and RTCP packets also helps
provide replay protection (see <xref target="attacks"></xref>). target="attacks" format="default"/>).  The use of the replay
protection mechanism specified in Section 3.3.2 of <xref target="RFC3711"></xref> target="RFC3711" sectionFormat="of"
section="3.3.2"/> is
REQUIRED <bcp14>REQUIRED</bcp14> at each hop.</t>
        <t>If there is a need to encrypt one or more RTP header extensions
hop-by-hop,
hop by hop, the Endpoint endpoint derives an encryption key from the HBH SRTP
master key to encrypt header extensions as per <xref target="RFC6904"></xref>. target="RFC6904" format="default"/>.  This
still gives the Media Distributor visibility into header extensions,
such as the one used to determine the audio level <xref target="RFC6464"></xref> target="RFC6464" format="default"/> 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 through 5.3
Sections&nbsp;<xref target="RFC8723" section="5.1"
 sectionFormat="bare"/>, <xref target="RFC8723" section="5.2"
 sectionFormat="bare"/>, and <xref target="RFC8723" section="5.3" sectionFormat="bare"/> of <xref target="I-D.ietf-perc-double"></xref> target="RFC8723"/> for procedures
to perform RTP header extension encryption and decryption.</t>
      </section>
      <section anchor="key_exchange" title="Key Exchange"> numbered="true" toc="default">
        <name>Key Exchange</name>
        <t>In brief, the keys used by any given Endpoints endpoints are determined in the
following way:</t>
<t>
<list style="symbols">
<t>The as
follows:</t>
        <ul spacing="normal">
          <li>The HBH keys that the Endpoint endpoint uses to send and receive SRTP media
are derived from a DTLS handshake that the Endpoint endpoint performs with
the Key Distributor (following normal DTLS-SRTP procedures).</t>
<t>The procedures).</li>
          <li>The E2E key that an Endpoint endpoint uses to send SRTP media can either be
either set from the DTLS-SRTP association with the Key Distributor or chosen
by the Endpoint. endpoint.  It is then distributed to other Endpoints 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 endpoint <bcp14>MAY</bcp14> create a different E2E key per media flow, where a
media flow is identified by its SSRC value.</t>
<t>Each value.</li>
          <li>Each E2E key that an Endpoint endpoint uses to receive SRTP media is set
by receiving a Full EKT Tag from another Endpoint.</t>
<t>The endpoint.</li>
          <li>The HBH keys used between two Media Distributors is are derived from via
DTLS-SRTP procedures employed directly between them.</t>
</list>
</t> them.</li>
        </ul>
        <section anchor="initial-key-exchange-and-key-distributor" title="Initial numbered="true" toc="default">
          <name>Initial Key Exchange and Key Distributor"> Distributor</name>
          <t>The Media Distributor maintains a tunnel with the Key Distributor
(e.g., using the tunnel protocol defined in <xref target="I-D.ietf-perc-dtls-tunnel"></xref>), target="I-D.ietf-perc-dtls-tunnel" format="default"/>), making it
possible for the Media Distributor to facilitate the establishment of
a secure DTLS association between each Endpoint endpoint and the Key
Distributor as shown the following figure. in <xref target="fig-initial-key-exchange"/>.  The DTLS association
between Endpoints endpoints and the Key Distributor enables each Endpoint 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.</t>
          <figure anchor="fig-initial-key-exchange" align="center" title="Exchanging anchor="fig-initial-key-exchange">
            <name>Exchanging Key Information Between Entities
"> between Entities</name>
            <artwork align="center"> align="center" name="" type="" alt=""><![CDATA[
                          +-----------+
                 KEK info |    Key    | HBH Key info to
             to Endpoints |Distributor| Endpoints &amp; & Media Distributor
                          +-----------+
                             # ^ ^ #
                             # | | #--- Tunnel
                             # | | #
+-----------+             +-----------+             +-----------+
| Endpoint  |   DTLS      |   Media   |   DTLS      | Endpoint  |
|    KEK    |&lt;------------|Distributor|------------&gt;|    |<------------|Distributor|------------>|    KEK    |
|  HBH Key  | to Key Dist | HBH Keys  | to Key Dist |  HBH Key  |
+-----------+             +-----------+             +-----------+

</artwork>             +-----------+]]></artwork>
          </figure>
          <t>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 exchange, as discussed in the following
paragraphs.  One is a DTLS-SRTP association between an Endpoint endpoint and the Key
Distributor (with packets passing through the Media Distributor) Distributor), and the
other is a DTLS-SRTP association between peer Media Distributors.</t>
          <t>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, signaling but instead forwards DTLS packets received
from an Endpoint endpoint on to the Key Distributor (and vice versa) via a
tunnel established between the Media Distributor and the Key Distributor.</t>
<t>In
          <t>When establishing the DTLS association between Endpoints endpoints and the
Key Distributor, the Endpoint MUST endpoint <bcp14>MUST</bcp14> act as the DTLS client client, and the
Key Distributor MUST <bcp14>MUST</bcp14> act as the DTLS server.  The KEK
is conveyed by the Key Distributor over the DTLS
association to Endpoints endpoints via procedures defined in EKT
<xref target="I-D.ietf-perc-srtp-ekt-diet"></xref> target="RFC8870" format="default"/> via the EKTKey message.</t>
          <t>The Key Distributor MUST NOT <bcp14>MUST NOT</bcp14> establish DTLS-SRTP associations with
Endpoints
endpoints without first authenticating the Media Distributor tunneling the
DTLS-SRTP packets from the Endpoint.</t> endpoint.</t>
          <t>Note that following DTLS-SRTP procedures for the cipher defined
in <xref target="I-D.ietf-perc-double"></xref>
cipher, target="RFC8723" format="default"/>, the Endpoint endpoint generates both E2E and HBH encryption keys
and salt values.  Endpoints MUST <bcp14>MUST</bcp14> either use the DTLS-SRTP generated 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 <bcp14>MUST</bcp14> be replaced with the salt value
provided by the Key Distributor via the EKTKey message.  That is because
every Endpoint endpoint in the conference uses the same SRTP master salt.  The
Endpoint
endpoint only transmits the SRTP master key (not the salt) used for E2E
encryption to other Endpoints endpoints in RTP/RTCP packets per
<xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>.</t> target="RFC8870" format="default"/>.</t>
          <t>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.</t>
        </section>
        <section anchor="keyexchange" title="Key numbered="true" toc="default">
          <name>Key Exchange during a Conference"> Conference</name>
          <t>Following the initial key information exchange with the Key
Distributor, an Endpoint endpoint is able to encrypt media end-to-end end to end with
an E2E key, sending that E2E key to other Endpoints endpoints encrypted with the
KEK, and is able to encrypt and authenticate RTP packets
using a HBH key.  The procedures defined do  This framework does not allow the Media Distributor
to gain access to the KEK information, preventing it from
gaining access to any Endpoint's endpoint's E2E key and subsequently decrypting
media.</t>
          <t>The KEK may need to change from time-to-time time to time during the
life
lifetime of a conference, such as when a new participant joins or leaves a
conference.  Dictating if, when when, or how often a conference is to be
re-keyed
rekeyed is outside the scope of this document, but this framework
does accommodate re-keying rekeying during the life lifetime of a conference.</t>
          <t>When a Key Distributor decides to re-key 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 endpoint <bcp14>MUST</bcp14> create a
new SRTP master key and prepare to send that key inside a Full EKT
Field FullEKTField using
the new EKT Key as per Section 4.5 of <xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>. target="RFC8870" sectionFormat="of"
section="4.5"/>. In order to allow time for all Endpoints endpoints in the conference to receive the new
keys, the sender should follow the recommendations in Section 4.7 of
[I-D.ietf-perc-srtp-ekt-diet]. <xref target="RFC8870"
sectionFormat="of" section="4.6"/>. On receiving a new EKT Key, Endpoints MUST endpoints <bcp14>MUST</bcp14>
be prepared to decrypt EKT tags Tags using the new key.  The EKT SPI Security Parameter
Index (SPI) field is
used to differentiate between tags EKT Tags encrypted with the old and new keys.</t>
          <t>After re-keying, rekeying, an Endpoint SHOULD endpoint <bcp14>SHOULD</bcp14> retain prior SRTP master keys and
EKT Key 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
endpoints that used the prior keys for a period of time after re-keying rekeying began.
An Endpoint MAY endpoint <bcp14>MAY</bcp14> retain old keys until the end of the conference.</t>
          <t>Endpoints MAY <bcp14>MAY</bcp14> follow the procedures in section 5.2 of <xref target="RFC5764"></xref> target="RFC5764" sectionFormat="of" section="5.2"/>
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 <xref target="I-D.ietf-perc-dtls-tunnel"></xref> target="I-D.ietf-perc-dtls-tunnel" format="default"/> as one possible method.</t>
<t>Endpoints MAY
          <t>At any time, endpoints <bcp14>MAY</bcp14> change the E2E
encryption key used at
any time. being used.  An Endpoint MUST endpoint <bcp14>MUST</bcp14> 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 endpoints in the conference
in RTP/RTCP packets per <xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>.</t> target="RFC8870" format="default"/>.</t>
        </section>
      </section>
    </section>
    <section anchor="authentication" title="Authentication"> numbered="true" toc="default">
      <name>Authentication</name>
      <t>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 endpoints. Details on this topic are outside the scope
of specification this specification, but a few possibilities are discussed in the
following sections.  The critical requirements are that an Endpoint (1)&nbsp;an endpoint
can verify that it is connected to the correct Key Distributor for the
conference and the (2)&nbsp;the Key Distributor can verify that the Endpoint endpoint is
the correct
Endpoint endpoint for the conference.</t>
      <t>Two possible approaches to solve resolve this situation are Identity Assertions identity assertions and
Certificate Fingerprints.</t>
certificate fingerprints.</t>
      <section anchor="identity-assertions" title="Identity Assertions">
<t>WebRTC Identity numbered="true" toc="default">
        <name>Identity Assertions</name>
        <t>A WebRTC identity assertion <xref target="I-D.ietf-rtcweb-security-arch"></xref> target="RFC8827" format="default"/> is used
to bind the identity of the user of the Endpoint 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 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 Similarly, the Key Distributor can create a WebRTC Identity identity
assertion to bind the fingerprint of the unique certificate used by
the Key Distributor for this conference so that the Endpoint endpoint can
validate
verify that it is talking to the correct Key Distributor. Such a setup
requires an Identity Provider (Idp) (IdP) trusted by the Endpoints endpoints and the
Key Distributor.</t>
      </section>
      <section anchor="certificate-fingerprints-in-session-signaling" title="Certificate numbered="true" toc="default">
        <name>Certificate Fingerprints in Session Signaling"> Signaling</name>
        <t>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.</t>
        <t>As a concrete example, SIP <xref target="RFC3261"></xref> target="RFC3261" format="default"/> and
the Session Description Protocol (SDP) <xref target="RFC4566"></xref> target="RFC4566" format="default"/> can be used
to convey the fingerprint information per <xref target="RFC5763"></xref>. target="RFC5763" format="default"/>.  An Endpoint's endpoint's
SIP User Agent would send an INVITE message containing SDP for the
media session along with the Endpoint's endpoint's certificate fingerprint, which
can be signed using the procedures described in <xref target="RFC8224"></xref> target="RFC8224" format="default"/> for the
benefit of forwarding the message to other entities by the Focus focus
<xref target="RFC4353"></xref>. target="RFC4353" format="default"/>.  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.</t>
        <t>Ultimately, if using session signaling, an Endpoint's 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 the user in question is authorized.
Similarly, the Key Distributor's certificate fingerprint can be
conveyed to an Endpoint endpoint in a manner that can be authenticated as being an
authorized Key Distributor for this conference.</t>
      </section>
      <section anchor="conf-id" title="Conference Identification"> numbered="true" toc="default">
        <name>Conference Identification</name>
        <t>The Key Distributor needs to know what Endpoints endpoints are being added to a
given conference. Thus, the Key Distributor and the Media Distributor
need to know Endpoint-to-conference endpoint-to-conference mappings, which is are enabled by
exchanging a conference-specific unique identifier defined as described in
<xref target="I-D.ietf-perc-dtls-tunnel"></xref>. target="I-D.ietf-perc-dtls-tunnel" format="default"/>.  How this unique
identifier is assigned is outside the scope of this document.</t>
      </section>
    </section>
    <section anchor="perc-keys" title="PERC Keys"> numbered="true" toc="default">
      <name>PERC Keys</name>
      <t>This section describes the various keys employed by PERC.</t>
      <section anchor="keyinventory" title="Key numbered="true" toc="default">
        <name>Key Inventory and Management Considerations"> Considerations</name>
        <t>This section summarizes the several different keys used in the PERC framework,
how they are generated, and what purpose they serve.</t>
        <t>The keys are described in the order in which they would typically be
acquired.</t>
        <t>The various keys used in PERC are shown in
<xref target="key-inventory-table"></xref> target="key-inventory-table" format="default"/> below.</t>
<figure anchor="key-inventory-table" align="center" title="Key Inventory
">
<artwork align="center">+-----------+----------------------------------------------------+
| Key       | Description                                        |
+-----------+----------------------------------------------------+
| HBH Key   | SRTP

<table anchor="key-inventory-table">
  <name>Key Inventory</name>
  <thead>
    <tr>
      <th align="center">Key</th>
      <th align="center">Description</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>HBH Key</td>
      <td>SRTP master key used to encrypt media hop-by-hop.  |
+-----------+----------------------------------------------------+
| KEK       | Key hop by hop.</td>
    </tr>
    <tr>
      <td>KEK (EKT&nbsp;Key)</td>
      <td>Key shared by all Endpoints endpoints and used to encrypt    |
| (EKT Key) | each Endpoint's endpoint's E2E
      SRTP master key so receiving   |
|           | Endpoints endpoints can decrypt media.                       |
+-----------+----------------------------------------------------+
| E2E Key   | SRTP media.</td>
    </tr>
    <tr>
      <td>E2E Key</td>
      <td>SRTP master key used to encrypt media end-to-end.  |
+-----------+----------------------------------------------------+
</artwork>
</figure> end to end.</td>
    </tr>
  </tbody>
</table>

        <t>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.</t>
        <t>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 are is passed through the KDF Key Derivation Function (KDF) as defined in <xref target="RFC3711"></xref> target="RFC3711" format="default"/> to produce
the actual encryption and authentication keys.</t>
        <t>Complicating key management is the fact that the KEK can change and, when
it does, the Endpoints 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
more.</t> higher.</t>
        <t>A more detailed explanation of each of the keys follows.</t>
      </section>
      <section anchor="dtls-srtp-exchange-yields-hbh-keys" title="DTLS-SRTP numbered="true" toc="default">
        <name>DTLS-SRTP Exchange Yields HBH Keys"> Keys</name>
        <t>The first set of keys acquired are for hop-by-hop HBH encryption and
decryption.  Per the Double double transform procedures <xref target="I-D.ietf-perc-double"></xref> procedures, target="RFC8723" format="default"/>, the
Endpoint
endpoint performs a DTLS-SRTP exchange with the Key Distributor
and receives a key that is, in fact, &quot;double&quot; "double" the size that is needed.
The end-to-end E2E part is the first half of the key, so the Endpoint endpoint discards
that information when generating its own key.  The second half of the key keying
material is for hop-by-hop HBH operations, so that half of the key
(corresponding to the least significant bits) is assigned internally as
the HBH key.</t>
        <t>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 endpoint
to the Media Distributor.  A salt value is
generated along with the HBH key.  The salt is also longer than needed
for hop-by-hop operations, thus 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 <xref target="I-D.ietf-perc-dtls-tunnel"></xref>.</t> target="I-D.ietf-perc-dtls-tunnel" format="default"/>.</t>
        <t>No two Endpoints endpoints have the same HBH key, thus key; thus, the Media Distributor
MUST
<bcp14>MUST</bcp14> keep track of each distinct HBH key (and the corresponding salt) and
use it only for the specified hop.</t>
        <t>The HBH key is also used for hop-by-hop HBH encryption of RTCP.  RTCP is not
end-to-end encrypted
E2E-encrypted in PERC.</t>
      </section>
      <section anchor="the-key-distributor-transmits-the-kek-ekt-key" title="The numbered="true" toc="default">
        <name>The Key Distributor Transmits the KEK (EKT Key)">
<t>Via the aforementioned DTLS-SRTP association, the Key)</name>
        <t>The Key Distributor sends the Endpoint the KEK (EKT (the EKT Key per
<xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>). target="RFC8870" format="default"/>) to the endpoint via the aforementioned DTLS-SRTP association.  This key is known only to
the Key Distributor and Endpoints.  This key endpoints; it is the most important entity to
protect
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.</t>
        <t>Note that the Key Distributor can send any number of EKT Keys to
Endpoints.
endpoints.  This information is used to re-key rekey the entire conference.  Each
key is identified by a &quot;Security Parameter Index&quot; (SPI) an SPI value.
Endpoints MUST <bcp14>MUST</bcp14> expect that a conference might be re-keyed rekeyed
when a new participant joins a conference or when a participant
leaves a conference conference, in order to protect the confidentiality of
the conversation before and after such events.</t>
        <t>The SRTP master salt to be used by the Endpoint endpoint is transmitted along
with the EKT Key.  All Endpoints endpoints in the conference utilize
the same SRTP master salt that corresponds with a given EKT Key.</t>
        <t>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 endpoint's SRTP master key (and other EKT Tag data
as per <xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>).</t> target="RFC8870" format="default"/>).</t>
        <t>The Media Distributor KEK is not given to the KEK.</t> Media Distributor.</t>
      </section>
      <section anchor="endpoints-fabricate-an-srtp-master-key" title="Endpoints fabricate numbered="true" toc="default">
        <name>Endpoints Fabricate an SRTP Master Key"> Key</name>
        <t>As stated earlier, the E2E key determined via DTLS-SRTP MAY <bcp14>MAY</bcp14> be
discarded in favor of a locally-generated locally generated E2E SRTP master key.  While the
DTLS-SRTP-derived SRTP master key can be used initially, the Endpoint endpoint might
choose to change the SRTP master key periodically and MUST <bcp14>MUST</bcp14> change the
SRTP master key as a result of the EKT key Key changing.</t>
        <t>A locally-generated locally generated SRTP master key is used along with the master salt
transmitted to the Endpoint endpoint from the Key Distributor via the EKTKey
message to encrypt media end-to-end.</t> end to end.</t>
        <t>Since the Media Distributor is not involved in E2E functions, it does not
create this key key, nor does it have access to any Endpoint's endpoint's E2E key.  Note, too,
that even the Key Distributor is unaware of the locally-generated locally generated E2E keys
used by each Endpoint.</t> endpoint.</t>
        <t>The Endpoint endpoint transmits its E2E key to other Endpoints 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 endpoints receive the same master salt via the EKTKey
message from the Key Distributor.  The recommended frequency with which an
Endpoint
endpoint transmits its SRTP master key is specified in
<xref target="I-D.ietf-perc-srtp-ekt-diet"></xref>.</t> target="RFC8870" format="default"/>.</t>
      </section>
      <section anchor="summary-of-key-types-and-entity-possession" title="Summary numbered="true" toc="default">
        <name>Summary of Key Types and Entity Possession"> Possession</name>
        <t>All Endpoints endpoints have knowledge of the KEK.</t>
        <t>Every HBH key is distinct for a given Endpoint, thus 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, endpoint.  (The only exception is where the DTLS-SRTP association might
be rekeyed per Section 5.2 of <xref target="RFC5764"></xref> target="RFC5764" sectionFormat="of" section="5.2"/> and a new key is created to
replace the former key.)</t>
        <t>Each Endpoint endpoint generates its own E2E key (SRTP master key), thus key); thus,
there is a distinct E2E key per endpoint.  This key is transmitted (encrypted) via
the Full EKT Tag to other Endpoints. endpoints.  Endpoints that receive media from
a given transmitting Endpoint endpoint gain knowledge of the
transmitter's E2E key via the Full EKT Tag.</t>
<t>To summarize
        <t><xref target="who-has-what-key-table" format="default"/> summarizes
        the various keys and which entity is in possession of a given key, refer to <xref target="fig-who-has-what-key"></xref>.</t>
<figure anchor="fig-who-has-what-key" align="center" title="Keys key.</t>

<table anchor="who-has-what-key-table">
  <name>Key Types and Entity Possession
">
<artwork align="center">+----------------------+------------+-------+-------+------------+
| Key     /    Entity  | Endpoint A |  MD X |  MD Y | Endpoint B |
+----------------------+------------+-------+-------+------------+
| KEK Possession</name>
  <thead>
    <tr>
      <th align="center">Key/Entity</th>
      <th align="center">Endpoint A</th>
      <th align="center">MD X</th>
      <th align="center">MD Y</th>
      <th align="center">Endpoint B</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>KEK (EKT Key)        |    Yes     |  No   |  No   |     Yes    |
+----------------------+------------+-------+-------+------------+
| E2E Key)</td>
      <td align="center">Yes</td>
      <td align="center">No</td>
      <td align="center">No</td>
      <td align="center">Yes</td>
    </tr>
    <tr>
      <td>E2E Key (A and B)    |    Yes     |  No   |  No   |     Yes    |
+----------------------+------------+-------+-------+------------+
| HBH B)</td>
      <td align="center">Yes</td>
      <td align="center">No</td>
      <td align="center">No</td>
      <td align="center">Yes</td>
    </tr>
    <tr>
      <td>HBH Key (A&lt;=&gt;MD X)   |    Yes     |  Yes  |  No   |     No     |
+----------------------+------------+-------+-------+------------+
| HBH X)</td>
      <td align="center">Yes</td>
      <td align="center">Yes</td>
      <td align="center">No</td>
      <td align="center">No</td>
    </tr>
    <tr>
      <td>HBH Key (B&lt;=&gt;MD Y)   |    No      |  No   |  Yes  |     Yes    |
+----------------------+------------+---------------+------------+
| HBH Y)</td>
      <td align="center">No</td>
      <td align="center">No</td>
      <td align="center">Yes</td>
      <td align="center">Yes</td>
    </tr>
    <tr>
      <td>HBH Key (MD X&lt;=&gt;MD Y)|    No      |  Yes  |  Yes  |     No     |
+----------------------+------------+---------------+------------+
</artwork>
</figure> Y)</td>
      <td align="center">No</td>
      <td align="center">Yes</td>
      <td align="center">Yes</td>
      <td align="center">No</td>
    </tr>
  </tbody>
</table>
      </section>
    </section>
    <section anchor="packetformat" title="Encrypted numbered="true" toc="default">
      <name>Encrypted Media Packet Format"> Format</name>
      <t><xref target="fig-perc-packet-format"></xref> target="fig-perc-packet-format" format="default"/> 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 double cryptographic transform
with an EKT Tag appended to the end.</t>
      <figure anchor="fig-perc-packet-format" align="center" title="Encrypted anchor="fig-perc-packet-format">
        <name>Encrypted Media Packet Format
"> Format</name>
        <artwork align="center"> align="center" name="" type="" alt=""><![CDATA[
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+&lt;++
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<++
    |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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+&lt;+O
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
    |                    RTP extension (OPTIONAL) ...               | |O
+&gt;+&gt;+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+&lt;+O
+>+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O I |                          payload  ...                         | IO
O I |                               +-------------------------------+ IO
O I |                               | RTP padding   | RTP pad count | IO
O +&gt;+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+&lt;+O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O | |                    E2E authentication tag                     | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | |                            OHB ...                            | |O
+&gt;|
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | |                    HBH authentication tag                     | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| | |   EKT Tag ...   | R                                             ||
| | +-+-+-+-+-+-+-+-+-+ |                                             ||
| |                     +- Neither encrypted nor authenticated;       ||
| |                        appended after Double is performed the double transform        ||
| |                        is performed                               ||
| |                                                                   ||
| +- E2E Encrypted E2E-Encrypted Portion               E2E Authenticated               E2E-Authenticated Portion ---+|
|                                                                      |
+--- HBH Encrypted HBH-Encrypted Portion               HBH Authenticated               HBH-Authenticated Portion ----+

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

</artwork> encryption/authentication]]></artwork>
      </figure>
    </section>
    <section anchor="attacks" title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <section anchor="third-party-attacks" title="Third Party Attacks">
<t>Third party numbered="true" toc="default">
        <name>Third-Party Attacks</name>
        <t>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.</t>
        <t>On-path attacks are mitigated by hop-by-hop HBH integrity protection and
encryption.  The integrity protection mitigates packet modification
and encryption modification.
Encryption makes selective blocking of packets harder, but not
impossible.</t>
        <t>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 hop-by-hop HBH authentication and discarding packets
that fail authentication.</t>
        <t>Another attack vector is a third party claiming to be a Media
Distributor, fooling Endpoints endpoints into sending packets to the false
Media Distributor instead of the correct one.  The deceived sending
Endpoints
endpoints could incorrectly assume that their packets have been delivered
to Endpoints 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.</t>
        <t>A third party could cause a denial-of-service denial of service by transmitting many bogus
or replayed packets toward receiving devices that and ultimately degrade 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 sanity checks on packets
(e.g., looking at packet length or expected sequence number ranges) ranges), before
performing more expensive decryption operations.</t>
<t>Use operations that are more expensive.</t>
        <t>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 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 endpoint would know that it could safely
discard media packets received from the Endpoint endpoint without inspection.</t>
      </section>
      <section anchor="media-distributor-attacks" title="Media numbered="true" toc="default">
        <name>Media Distributor Attacks"> Attacks</name>
        <t>A malicious or compromised Media Distributor can attack the session in a
number of possible ways.</t> ways, as discussed below.</t>
        <section anchor="denial-of-service" title="Denial numbered="true" toc="default">
          <name>Denial of service"> Service</name>
          <t>A simple form of attack is discarding received packets that should be
forwarded.  This solution framework does not introduce provide any mitigation
mechanisms for Media Distributors that fail to forward media packets.</t>
          <t>Another form of attack is modifying received packets before forwarding.
With this solution framework, any modification of the end-to-end
authenticated E2E-authenticated data
results in the receiving Endpoint endpoint getting an integrity failure when performing authentication on the received packet.</t>
          <t>The Media Distributor can also attempt to perform resource consumption
attacks on the receiving Endpoint. endpoint.  One such attack would be to insert
random SSRC/CSRC values in any RTP packet along with a Full EKT Tag.
Since &nbsp;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
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.</t>
        </section>
        <section anchor="replay-attack" title="Replay Attack"> numbered="true" toc="default">
          <name>Replay Attacks</name>
          <t>A replay attack is when an already received 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 &quot;yes&quot;, "yes", instead of the &quot;no&quot; "no" the user
actually said.</t>
          <t>A replay attack is mitigated by the requirement to implement
replay protection as
described in Section 3.3.2 of <xref target="RFC3711"></xref>.
End-to-end target="RFC3711" sectionFormat="of" section="3.3.2"/>.
E2E replay protection MUST <bcp14>MUST</bcp14> be provided for the
duration of the conference.</t>
        </section>
        <section anchor="delayed-playout-attack" title="Delayed numbered="true" toc="default">
          <name>Delayed Playout Attack"> Attacks</name>
          <t>A delayed playout attack is one an attack where media is received and held by
a Media Distributor and then forwarded to Endpoints endpoints at a later point
in time.</t>
          <t>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 issue with this problem here is that a Media
Distributor can select choose to not deliver a particular stream for a while.</t>
          <t>While the Media Distributor can purposely stop forwarding media flows, it
can also select an arbitrary starting point to resume forwarding those
media flow, 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.</t>
          <t>While this attack cannot be eliminated entirely, its effectiveness
can be reduced by re-keying rekeying the conference periodically periodically, since
significantly-delayed
significantly delayed media encrypted with expired keys would not be
decrypted by Endpoints.</t> endpoints.</t>
        </section>
        <section anchor="splicing-attack" title="Splicing Attack"> numbered="true" toc="default">
          <name>Splicing Attacks</name>
          <t>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 as that stream A was previously using.  By including
the SSRC in the integrity check for each packet, packet -- both HBH and E2E, E2E -- PERC
prevents splicing attacks.</t>
        </section>
        <section anchor="rtcp-attacks" title="RTCP Attacks"> numbered="true" toc="default">
          <name>RTCP Attacks</name>
          <t>PERC does not provide end-to-end 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 endpoint to send a new picture.  Such
requests can consume significant bandwidth and impair conference performance.</t>
        </section>
      </section>
      <section anchor="key-distributor-attacks" title="Key numbered="true" toc="default">
        <name>Key Distributor Attacks"> Attacks</name>
        <t>As stated in <xref target="key_distributor"></xref>, target="key_distributor" format="default"/>, the Key Distributor needs to be secured 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 endpoint in the conference.</t>
        <t>As a first line of defense, the Key Distributor authenticates every
security association, both association -- associations with Endpoints both endpoints and Media
Distributors.  The Key Distributor knows which entities are authorized to
have access to which keys keys, and inspection of certificates will substantially
reduce the risk of providing keys to an adversary.</t>
        <t>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 and Endpoint, within, for example. 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.</t>
        <t>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 be seem
obvious, it is worth stating 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.</t>
      </section>
      <section anchor="endpoint-attacks" title="Endpoint Attacks"> numbered="true" toc="default">
        <name>Endpoint Attacks</name>
        <t>A Trusted Endpoint is so named because conference confidentiality relies
heavily on the security and integrity of the Endpoint. endpoint.  If an adversary
successfully exploits a vulnerability in an Endpoint, 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, endpoint, either in real-time real time
or at a later point in time (assuming that the adversary makes a copy of the
media packets).</t>
        <t>Additionally, if an adversary successfully exploits an Endpoint, endpoint, the
adversary could inject media into the conference. One way For example, an adversary
could do that is by manipulating manipulate the RTP or SRTP software to transmit
whatever media the adversary wishes to send.
This could involve
re-use of the Endpoint's SSRC, a new SSRC, or reuse of the compromised endpoint's SSRC value of an existing
endpoint.  This is made possible or,
since all conference participants share the same KEK. 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 E2E-protected packet (namely, TESLA Timed Efficient
Stream Loss-Tolerant Authentication (TESLA)
<xref target="RFC4383"></xref>), target="RFC4383" format="default"/>), and its usage is presently not
defined for PERC.  The suite
defined in PERC only allows an Endpoint endpoint to determine that whoever sent a
packet had received the KEK.</t>
        <t>However, attacks on the endpoint are not limited to the PERC-specific
software within the Endpoint. 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 no not secured.</t>
      </section>
    </section>
    <section anchor="iana-considerations" title="IANA Considerations">
<t>There are numbered="true" toc="default">
      <name>IANA Considerations</name>
      <t>This document has no IANA considerations for this document.</t> actions.</t>
    </section>
  </middle>
  <back>

<displayreference target="I-D.ietf-perc-dtls-tunnel" to="PERC-DTLS"/>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
<!-- draft-ietf-perc-double (RFC 8723; Published) -->
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8723.xml"/>

<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3711.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3550.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>

<!-- draft-ietf-perc-srtp-ekt-diet (RFC 8870) -->
<reference anchor="RFC8870" target="https://www.rfc-editor.org/info/rfc8870">
 <front>
  <title>Encrypted Key Transport for DTLS and Secure RTP</title>
    <author initials="C" surname="Jennings" fullname="Cullen Jennings">
    <organization>company</organization>
    </author>
    <author initials="J" surname="Mattsson" fullname="John Mattsson">
    <organization>company</organization>
    </author>
    <author initials="D" surname="McGrew" fullname="David A. McGrew">
    <organization>company</organization>
    </author>
    <author initials="D" surname="Wing" fullname="Dan Wing">
    <organization>company</organization>
    </author>
    <author initials="F" surname="Andreasen" fullname="Flemming Andreasen">
    <organization>company</organization>
    </author>
   <date month="January" year="2021"/>
  </front>
  <seriesInfo name="RFC" value="8870"/>
  <seriesInfo name="DOI" value="10.17487/RFC8870"/>
</reference>

<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6904.xml"/>
      </references>
      <references>
        <name>Informative References</name>

   <reference anchor="W3C.CR-webrtc" target="https://www.w3.org/TR/webrtc/">
     <front>
       <title>WebRTC 1.0: Real-time Communication Between Browsers</title>
       <author initials="C." surname="Jennings" fullname="Cullen Jennings">
         <organization/>
       </author>
       <author initials="H." surname="Boström" fullname="Henrik Boström">
         <organization/>
       </author>
       <author initials="J-I." surname="Bruaroey" fullname="Jan-Ivar Bruaroey">
         <organization/>
       </author>
       <date/>
     </front>
       <refcontent>W3C Proposed Recommendation</refcontent>
   </reference>

<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5764.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7667.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4353.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3261.xml"/>

<!-- draft-ietf-perc-dtls-tunnel (Expired) -->
<xi:include href="https://datatracker.ietf.org/doc/bibxml3/reference.I-D.ietf-perc-dtls-tunnel.xml"/>

<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4566.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4383.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6464.xml"/>
<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5763.xml"/>

<!-- draft-ietf-rtcweb-security-arch (RFC 8827) -->
 <reference anchor="RFC8827" target="https://www.rfc-editor.org/info/rfc8827">
 <front>
 <title>WebRTC Security Architecture</title>
 <author initials='E.' surname='Rescorla' fullname='Eric Rescorla'>
   <organization/>
 </author>
 <date month='January' year='2021'/>
 </front>
 <seriesInfo name="RFC" value="8827"/>
 <seriesInfo name="DOI" value="10.17487/RFC8827"/>
 </reference>

<xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8224.xml"/>
      </references>
    </references>

    <section anchor="acknowledgments" title="Acknowledgments"> numbered="false" toc="default">
      <name>Acknowledgments</name>

      <t>The authors would like to thank Mo Zanaty, Christian Oien, <contact fullname="Mo Zanaty"/>,
      <contact fullname="Christian Oien"/>, and Richard Barnes <contact fullname="Richard
      Barnes"/> for invaluable input on this document.  Also, we would like to
      acknowledge
Nermeen Ismail <contact fullname="Nermeen Ismail"/> for serving on the
      initial draft versions of this document as a co-author. coauthor.  We would also
      like to acknowledge John Mattsson, Mats Naslund,
and Magnus Westerlund <contact fullname="John Mattsson"/>, <contact
      fullname="Mats Naslund"/>, and <contact fullname="Magnus Westerlund"/>
      for providing some of the text in the document, including much of the
      original text in the security considerations section.</t> Security Considerations section (<xref
      target="attacks"/>).</t>
    </section>

</middle>

<back>
<references title="Normative References">
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml-ids/reference.I-D.ietf-perc-double.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3711.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3550.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml-ids/reference.I-D.ietf-perc-srtp-ekt-diet.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6904.xml"?>
</references>
<references title="Informative References">

<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml-w3c/reference.W3C.CR-webrtc-20180927.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5764.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7667.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4353.xml"?>

<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.3261.xml"?>

<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml-ids/reference.I-D.ietf-perc-dtls-tunnel.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4566.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.4383.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6464.xml"?>
<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.5763.xml"?>

<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml-ids/reference.I-D.ietf-rtcweb-security-arch.xml"?>

<?rfc include="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8224.xml"?>
</references>
  </back>
</rfc>