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<!-- updated by Jean Mahoney /12/12/19 -->
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<rfc ipr="trust200902"
xmlns:xi="http://www.w3.org/2001/XInclude"
number="8723"
updates=""
obsoletes=""
category="std" docName="draft-ietf-perc-double-12">
<?rfc toc="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<?rfc private=""?>
<?rfc topblock="yes"?>
<?rfc comments="no"?>
consensus="true"
submissionType="IETF"
ipr="trust200902"
sortRefs="true"
symRefs="true"
xml:lang="en"
docName="draft-ietf-perc-double-12"
tocInclude="true"
version="3">
  <!-- xml2rfc v2v3 conversion 2.37.1 -->
  <front>
    <title abbrev="Double SRTP">SRTP Double SRTP">Double Encryption Procedures</title> Procedures for Secure Real-Time Transport Protocol (SRTP)</title>
    <seriesInfo name="RFC" value="8723"/>
    <author initials="C." surname="Jennings" fullname="Cullen Jennings">
      <organization>Cisco Systems</organization>
      <address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
        <email>fluffy@iii.ca</email>
<uri></uri>
      </address>
    </author>
    <author initials="P." surname="Jones" fullname="Paul E. Jones">
      <organization>Cisco Systems</organization>
      <address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
        <email>paulej@packetizer.com</email>
<uri></uri>
      </address>
    </author>
    <author initials="R." surname="Barnes" fullname="Richard Barnes">
      <organization>Cisco Systems</organization>
      <address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
        <email>rlb@ipv.sx</email>
<uri></uri>
      </address>
    </author>
    <author initials="A.B." surname="Roach" fullname="Adam Roach">
      <organization>Mozilla</organization>
      <address>
<postal>
<street></street>
<city></city>
<code></code>
<country></country>
<region></region>
</postal>
<phone></phone>
        <email>adam@nostrum.com</email>
<uri></uri>
      </address>
    </author>
    <date year="2019" month="August" day="29"/> year="2020" month="January"/>
    <area>Internet</area>
<workgroup></workgroup>
    <workgroup/>
    <keyword>PERC</keyword>
    <keyword>SRTP</keyword>
    <keyword>RTP</keyword>
    <keyword>conferencing</keyword>
    <keyword>encryption</keyword>
    <abstract>
      <t>In some conferencing scenarios, it is desirable for an intermediary
to be able to manipulate some parameters in Real Time Real-time Transport Protocol (RTP)
packets, while still providing strong end-to-end security
guarantees. This document defines a cryptographic transform for the
Secure Real Time Real-time Transport Protocol (SRTP) that uses two separate but related
cryptographic operations to provide hop-by-hop and end-to-end
security guarantees.  Both the end-to-end and hop-by-hop
cryptographic algorithms can utilize an authenticated encryption
with associated data (AEAD) algorithm or take advantage of future SRTP
transforms with different properties.
</t>
    </abstract>
  </front>
  <middle>
    <section anchor="introduction" title="Introduction"> numbered="true" toc="default">
      <name>Introduction</name>
      <t>Cloud conferencing systems that are based on switched conferencing
have a central Media Distributor (MD) device that receives media from
endpoints and distributes it to other endpoints, but does not need
to interpret or change the media content.  For these systems, it is
desirable to have one cryptographic key that enables encryption and
authentication of the media
end-to-end while still allowing certain information in the header of
a Real Time Real-time Transport Protocol (RTP) packet to be changed by the Media
Distributor.  At the same time, a separate cryptographic key
provides integrity and optional confidentiality for the media
flowing between the Media Distributor and the endpoints.  The
framework document <xref target="I-D.ietf-perc-private-media-framework"/> target="I-D.ietf-perc-private-media-framework" format="default"/>
describes this concept in more detail.
</t>
      <t>This specification defines a transform for the Secure Real Time Real-time Transport
Protocol (SRTP) that uses 1) the AES-GCM AES Galois/Counter Mode (AES-GCM) algorithm <xref target="RFC7714"/> target="RFC7714" format="default"/> to
provide encryption and integrity for an RTP packet for the
end-to-end cryptographic key as well as and 2) a hop-by-hop cryptographic
encryption and integrity between the endpoint and the Media
Distributor. The Media Distributor decrypts and checks integrity of
the hop-by-hop security. The Media Distributor MAY <bcp14>MAY</bcp14> change some of
the RTP header information that would impact the end-to-end
integrity. In that case, the original value of any RTP header field
that is changed is included in an &quot;Original "Original Header Block&quot; Block" that is
added to the packet. The new RTP packet is encrypted with the
hop-by-hop cryptographic algorithm before it is sent. The receiving
endpoint decrypts and checks integrity using the hop-by-hop
cryptographic algorithm and then replaces any parameters the Media
Distributor changed using the information in the Original Header
Block before decrypting and checking the end-to-end integrity.
</t>
      <t>One can think of the double transform as a normal SRTP transform for encrypting
the RTP in a way where such that things that only know half of the key, can
decrypt and modify part of the RTP packet but not other parts,
including the media payload.
</t>
    </section>
    <section anchor="terminology" title="Terminology"> numbered="true" toc="default">
      <name>Terminology</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"/> target="RFC2119" format="default"/> <xref target="RFC8174"/> target="RFC8174" format="default"/>
    when, and only when, they appear in all capitals, as shown here.
</t>
      <t>Terms used throughout this document include:
</t>
<t>
<list style="symbols">
<t>Media Distributor: A
      <dl spacing="normal">
        <dt>Media Distributor:</dt>
        <dd>A device that receives media from endpoints and
distributes it to other endpoints, but does not need to interpret
or change the media content (see also
<xref target="I-D.ietf-perc-private-media-framework"/>)</t>
<t>end-to-end: The target="I-D.ietf-perc-private-media-framework" format="default"/>).</dd>
        <dt>end-to-end:</dt>
        <dd>The path from one endpoint through one or
more Media Distributors to the endpoint at the other end.</t>
<t>hop-by-hop: The end.</dd>
        <dt>hop-by-hop:</dt>
        <dd>The path from the endpoint to or from the
Media Distributor.</t>
<t>Original Distributor.</dd>
        <dt>Original Header Block (OHB): An (OHB):</dt>
        <dd>An octet string that contains the
original values from the RTP header that might have been changed
by a Media Distributor.</t>
</list>
</t> Distributor.</dd>
      </dl>
    </section>
    <section anchor="cryptographic-context" title="Cryptographic Context"> numbered="true" toc="default">
      <name>Cryptographic Context</name>

      <t>This specification uses a cryptographic context with two parts:
</t>
<t>
<list style="symbols">
<t>An
      <ul spacing="normal">
        <li>An inner (end-to-end) part that is used by endpoints that originate and
consume media to ensure the integrity of media end-to-end, and</t>
<t>An and</li>
        <li>An outer (hop-by-hop) part that is used between endpoints and Media
Distributors to ensure the integrity of media over a single hop and to
enable a Media Distributor to modify certain RTP header fields. RTCP
is also handled using the hop-by-hop cryptographic part.</t>
</list>
</t> part.</li>
      </ul>
      <t>The RECOMMENDED <bcp14>RECOMMENDED</bcp14> cipher for the hop-by-hop and end-to-end algorithm algorithms is
AES-GCM.  Other combinations of SRTP ciphers that support the
procedures in this document can be added to the IANA registry.
</t>
      <t>The keys and salt for these algorithms are generated with the following
steps:
</t>
<t>
<list style="symbols">
<t>Generate
      <ul spacing="normal">
        <li>Generate key and salt values of the length required for the combined
inner (end-to-end) and outer (hop-by-hop) algorithms.</t>
<t>Assign algorithms.</li>
        <li>Assign the key and salt values generated for the inner (end-to-end)
algorithm to the first half of the key and the first half of the
salt for the double algorithm.</t>
<t>Assign algorithm.</li>
        <li>Assign the key and salt values for the outer (hop-by-hop) algorithm
to the second half of the key and second half of the salt for the
double algorithm. The first half of the key is referred to as the
inner key while the second half is referred to as the outer key.
When a key is used by a cryptographic algorithm, the salt that is used is
the part of the salt generated with that key.</t>
<t>the SSRC key.</li>
        <li>the synchronization source (SSRC) is the same for both the inner and out outer algorithms as
it can not cannot be changed.</t>
<t>The SEQ changed.</li>
        <li>The sequence number (SEQ) and ROC rollover counter (ROC) are tracked independently for the inner and outer
algorithms.</t>
</list>
</t>
algorithms.</li>
      </ul>
      <t>If the Media Distributor is to be able to modify header fields but
not decrypt the payload, then it must have a cryptographic key for the
outer algorithm, algorithm but not the inner (end-to-end) algorithm.  This
document does not define how the Media Distributor should be
provisioned with this information.  One possible way to provide
keying material for the outer (hop-by-hop) algorithm is to use
<xref target="I-D.ietf-perc-dtls-tunnel"/>. target="I-D.ietf-perc-dtls-tunnel" format="default"/>.
</t>
      <section anchor="key-derivation" title="Key Derivation"> numbered="true" toc="default">
        <name>Key Derivation</name>
        <t>Although SRTP uses a single master key to derive keys for an SRTP
session, this transform requires separate inner and outer keys.
In order to allow the inner and outer keys to be managed
independently via the master key, the transforms defined in this
document MUST <bcp14>MUST</bcp14> be used with the following pseudo-random pseudorandom function
(PRF), which preserves the separation between the two halves of the
key.  Given a positive integer <spanx style="verb">n</spanx> <tt>n</tt> representing the desired output
length, a master key <spanx style="verb">k_master</spanx>, <tt>k_master</tt>, and an input <spanx style="verb">x</spanx>: <tt>x</tt>:
</t>

<figure align="center"><artwork align="center">
PRF\_double\_n(k\_master,x)
        <artwork align="center" name="" type="" alt=""><![CDATA[
PRF_double_n(k_master,x) = PRF\_(n/2)(inner(k\_master),x) PRF_(n/2)(inner(k_master),x) ||
                              PRF\_(n/2)(outer(k\_master),x)
</artwork></figure>
                           PRF_(n/2)(outer(k_master),x)
]]></artwork>

        <t>Here <spanx style="verb">PRF_n(k, x)</spanx> <tt>PRF_double_n(k_master, x)</tt> represents the AES_CM PRF KDF Key Derivation Function (KDF) (see Section 4.3.3
of
<xref target="RFC3711"/>) target="RFC3711" section="4.3.3" sectionFormat="of" format="default"/>) for
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM algorithm and
AES_256_CM_PRF KDF <xref target="RFC6188"/> target="RFC6188" format="default"/> for DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM
algorithm. <spanx style="verb">inner(key)</spanx> The term <tt>inner(k_master)</tt> represents the first half of
the key, and <spanx style="verb">outer(key)</spanx> key; <tt>outer(k_master)</tt>
represents the second half of the key.
</t>
      </section>
    </section>
    <section anchor="ohb" title="Original numbered="true" toc="default">
      <name>Original Header Block"> Block</name>
      <t>The Original Header Block (OHB) contains the original values of any
modified RTP header fields. In the encryption process, the OHB is
included in an SRTP packet as described in <xref target="rtp-operations"/>. target="rtp-operations" format="default"/>.  In the
decryption process, the receiving endpoint uses it to reconstruct
the original RTP header, header so that it can pass the proper AAD additional authenticated data (AAD) value to
the inner transform.
</t>
      <t>The OHB can reflect modifications to the following fields in an RTP header: the
payload type, type (PT), the sequence number, number (SEQ), and the marker bit.  All other fields in the
RTP header MUST <bcp14>MUST</bcp14> remain unmodified; since the OHB cannot reflect their original
values, the receiver will be unable to verify the E2E end-to-end (E2E) integrity of the packet.
</t>
      <t>The OHB has the following syntax (in ABNF <xref target="RFC5234"/>): target="RFC5234" format="default"/>):
</t>

<figure align="left"><artwork align="left">
      <sourcecode name="" src="" type="abnf" markers="false"><![CDATA[
OCTET = %x00-FF

PT = OCTET
SEQ = 2OCTET
Config = OCTET
OHB = [ PT ] [ SEQ ] Config
</artwork></figure>
]]></sourcecode>
      <t>If present, the PT and SEQ parts of the OHB contain the original payload type
and sequence number fields, respectively. The final &quot;config&quot; "Config" octet of the OHB
specifies whether these fields are present, and the original value of the
marker bit (if necessary):
</t>

<figure align="left"><artwork align="left">
      <artwork align="left" name="" type="" alt=""><![CDATA[
+-+-+-+-+-+-+-+-+
|R R R R B M P Q|
+-+-+-+-+-+-+-+-+
</artwork></figure>
<t>
<list style="symbols">
<t>P:
]]></artwork>
      <ul spacing="normal">
        <li>P: PT is present</t>
<t>Q: present</li>
        <li>Q: SEQ is present</t>
<t>M: present</li>
        <li>M: Marker bit is present</t>
<t>B: present</li>
        <li>B: Value of marker bit</t>
<t>R: bit</li>
        <li>R: Reserved, MUST <bcp14>MUST</bcp14> be set to 0</t>
</list>
</t> 0</li>
      </ul>

      <t>In particular, an all-zero OHB config Config octet (0x00) (<tt>0x00</tt>) indicates that
there have been no modifications from the original header.
</t>
      <t>If the marker bit is not present (M=0), then B MUST <tt>B</tt> <bcp14>MUST</bcp14> be set to zero.
That is, if <spanx style="verb">C</spanx> <tt>C</tt> represents the value of the config Config octet, then the
masked value <spanx style="verb">C <tt>C &amp; 0x0C</spanx> MUST NOT 0x0C</tt> <bcp14>MUST NOT</bcp14> have the value <spanx style="verb">0x80</spanx>. <tt>0x80</tt>.
</t>
    </section>
    <section anchor="rtp-operations" title="RTP Operations"> numbered="true" toc="default">
      <name>RTP Operations</name>
      <t>As implied by the use of the word &quot;double&quot; "double" above, this transform
applies AES-GCM to the SRTP packet twice.  This allows media
distributors to be able to modify some header fields while allowing
endpoints to verify the end-to-end integrity of a packet.
</t>
      <t>The first, &quot;inner&quot; "inner" application of AES-GCM encrypts the SRTP payload
and integrity-protects protects the integrity of a version of the SRTP header with extensions
truncated.  Omitting extensions from the inner integrity check means
that they can be modified by a media distributor Media Distributor holding only the
&quot;outer&quot;
outer key.
</t>
      <t>The second, &quot;outer&quot; "outer" application of AES-GCM encrypts the ciphertext
produced by the inner encryption (i.e., the encrypted payload and
authentication tag), plus an OHB that expresses any changes made
between the inner and outer transforms.
</t>
      <t>A media distributor Media Distributor that has the outer key but not the inner key may
modify the header fields that can be included in the OHB by
decrypting, modifying, and re-encrypting the packet.
</t>
      <section anchor="encrypt" title="Encrypting a Packet">
<t>To encrypt numbered="true" toc="default">
        <name>Encrypting a packet, the Packet</name>
        <t>An endpoint encrypts the a packet by using the inner (end-to-end)
cryptographic key and then encrypts using the outer (hop-by-hop) cryptographic key.
The encryption also supports a mode
for repair packets that only does the outer (hop-by-hop) encryption.
The processes is as follows:
</t>
<t>
<list style="numbers">
<t>Form
        <ol spacing="normal" type="1">
          <li>Form an RTP packet.  If there are any header extensions,
they MUST <bcp14>MUST</bcp14> use <xref target="RFC8285"/>.</t>
<t>If target="RFC8285" format="default"/>.</li>
          <li>If the packet is for repair mode data, skip to step 6.</t> <xref
	  target="step6" format="none">step 6</xref>.</li>
          <li>
            <t>Form a synthetic RTP packet with the following contents:
<list style="symbols"> contents:</t>
            <ul spacing="normal">
              <li>
                  <t>Header: The RTP header of the original packet with
the following modifications:</t>
<t>The
                  <ul spacing="normal">
                     <li>The X bit is set to zero</t>
<t>The zero.</li>
                     <li>The header is truncated to remove any extensions
(i.e., keep only the first 12 + 4 * CC CSRC count (CC) bytes of the header)</t>
<t>Payload: header).</li>
                  </ul>
              </li>
              <li>Payload: The RTP payload of the original packet (including
padding when present) </t>
</list></t>
<t>Apply present).</li>
            </ul>
          </li>
          <li anchor="step4a">Apply the inner cryptographic algorithm to the synthetic RTP packet from the previous step.</t>
<t>Replace step.</li>
          <li>Replace the header of the protected RTP packet with the header of
the original packet (to restore any header extensions and reset
the X bit), and append an empty OHB (0x00) (<tt>0x00</tt>) to the encrypted
payload (with the authentication tag) obtained from the step 4.</t>
<t>Apply <xref
target="step4a" format="none">step 4</xref>.</li>
          <li anchor="step6">Apply the outer cryptographic algorithm to the RTP packet.  If
encrypting RTP header extensions hop-by-hop, then <xref target="RFC6904"/> MUST target="RFC6904" format="default"/> <bcp14>MUST</bcp14>
be used when encrypting the RTP packet using the outer
cryptographic key.</t>
</list>
</t> key.</li>
        </ol>
        <t>When using EKT Encrypted Key Transport (EKT) <xref target="I-D.ietf-perc-srtp-ekt-diet"/>, target="I-D.ietf-perc-srtp-ekt-diet" format="default"/>, the EKT Field EKTField comes
after the SRTP packet packet, exactly like using EKT with any other SRTP
transform.
</t>
      </section>
      <section anchor="relay" title="Relaying numbered="true" toc="default">
        <name>Relaying a Packet"> Packet</name>
        <t>The Media Distributor has the part of the key for the outer
(hop-by-hop) cryptographic algorithm, but it does not have the part
of the key for the inner (end-to-end) cryptographic algorithm.  The
cryptographic algorithm and key used to decrypt a packet and
any encrypted RTP header extensions would be the same as those
used in the endpoint's outer algorithm and key.
</t>
        <t>In order to modify a packet, the Media Distributor decrypts the
received packet, modifies the packet, updates the OHB with
any modifications not already present in the OHB, and re-encrypts
the packet using the the outer (hop-by-hop) cryptographic key
before transmitting. transmitting using the following steps:
	</t>
<t>
<list style="numbers">
<t>Apply
        <ol spacing="normal" type="1">
          <li anchor="step1">Apply the outer (hop-by-hop) cryptographic algorithm to decrypt the
packet.  If decrypting RTP header extensions hop-by-hop, then
<xref target="RFC6904"/> MUST target="RFC6904" format="default"/> <bcp14>MUST</bcp14> be used.  Note that the RTP payload produced by
this decryption operation contains the original encrypted payload
with the tag from the inner transform and the OHB appended.</t>
<t>Make appended.</li>
          <li>Make any desired changes to the fields that are allowed to be changed,
i.e., PT, SEQ, and M.  The Media Distributor MAY <bcp14>MAY</bcp14> also make
modifications to header extensions, without the need to reflect
these changes in the OHB.</t> OHB.</li>
          <li>
            <t>Reflect any changes to header fields in the OHB:
<list style="symbols">
<t>If
</t>
            <ul spacing="normal">
              <li>If the Media Distributor changed a field that is not already in the
OHB, then it MUST <bcp14>MUST</bcp14> add the original value of the field to the
OHB.  Note that this might result in an increase in the size of
the OHB.</t>
<t>If OHB.</li>
              <li>If the Media Distributor took a field that had previously been
modified and reset to its original value, then it SHOULD <bcp14>SHOULD</bcp14> drop
the corresponding information from the OHB.  Note that this
might result in a decrease in the size of the OHB.</t>
<t>Otherwise, OHB.</li>
              <li>Otherwise, the Media Distributor MUST NOT <bcp14>MUST NOT</bcp14> modify the OHB.</t>
</list></t>
<t>Apply OHB.</li>
            </ul>
          </li>
          <li anchor="step4">Apply the outer (hop-by-hop) cryptographic algorithm to the
packet. If the RTP Sequence Number sequence number has been modified, SRTP
processing happens as defined in SRTP and will end up using the new
Sequence Number.
sequence number. If encrypting RTP header extensions hop-by-hop,
then <xref target="RFC6904"/> MUST target="RFC6904" format="default"/> <bcp14>MUST</bcp14> be used.</t>
</list>
</t> used.</li>
        </ol>
        <t>In order to avoid nonce reuse, the cryptographic contexts
	used in
step 1 steps
<xref target="step1" format="counter">1</xref> and step 5 MUST <xref
target="step4" format="counter">4</xref> <bcp14>MUST</bcp14> use different, independent master keys.  Note
that this means that the key used for decryption by the MD MUST <bcp14>MUST</bcp14> be
different from the key used for re-encryption to the end recipient.
</t>
        <t>Note that if multiple MDs modify the same packet, then the first MD
to alter a given header field is the one that adds it to the OHB.
If a subsequent MD changes the value of a header field that has
already been changed, then the original value will already be in the
OHB, so no update to the OHB is required.
</t>
        <t>A Media Distributor that decrypts, modifies, and re-encrypts
packets in this way MUST <bcp14>MUST</bcp14> use an independent key for each recipient,
and MUST NOT <bcp14>MUST NOT</bcp14> re-encrypt the packet using the sender's keys.  If the
Media Distributor decrypts and re-encrypts with the same key and
salt, it will result in the reuse of a (key, nonce) pair,
undermining the security of AES-GCM.
</t>
      </section>
      <section anchor="decrypt" title="Decrypting numbered="true" toc="default">
        <name>Decrypting a Packet"> Packet</name>
        <t>To decrypt a packet, the endpoint first decrypts and verifies using
the outer (hop-by-hop) cryptographic key, then uses the OHB to
reconstruct the original packet, which it decrypts and verifies with
the inner (end-to-end) cryptographic key. key using the following steps:
</t>
<t>
<list style="numbers">
<t>Apply
        <ol spacing="normal" type="1">
          <li>Apply the outer cryptographic algorithm to the packet.  If the
integrity check does not pass, discard the packet.  The result of
this is referred to as the outer SRTP packet.  If decrypting RTP
header extensions hop-by-hop, then <xref target="RFC6904"/> MUST target="RFC6904" format="default"/> <bcp14>MUST</bcp14> be used when
decrypting the RTP packet using the outer cryptographic key.</t>
<t>If key.</li>
          <li>If the packet is for repair mode data, skip the rest of the
steps. Note that the packet that results from the repair algorithm
will still have encrypted data that needs to be decrypted as
specified by the repair algorithm sections.</t>
<t>Remove sections.</li>
          <li>Remove the inner authentication tag and the OHB from the end of the
payload of the outer SRTP packet.</t> packet.</li>
          <li>
            <t>Form a new synthetic SRTP packet with:
<list style="symbols"> </t>
            <ul spacing="normal">
              <li>
                 <t>Header = Received header, with the following modifications:</t>
<t>Header
                 <ul spacing="normal">
                    <li>Header fields replaced with values from OHB (if any)</t>
<t>The any).</li>
                    <li>The X bit is set to zero</t>
<t>The zero.</li>
                    <li>The header is truncated to remove any extensions (i.e., keep
only the first 12 + 4 * CC bytes of the header)</t>
<t>Payload header).</li>
                 </ul>
              </li>
              <li>Payload is the encrypted payload from the outer SRTP packet (after
the inner tag and OHB have been stripped).</t>
<t>Authentication stripped).</li>
              <li>Authentication tag is the inner authentication tag from the outer
SRTP packet.</t>
</list></t>
<t>Apply packet.</li>
            </ul>
          </li>
          <li>Apply the inner cryptographic algorithm to this synthetic SRTP
packet.  Note if the RTP Sequence Number sequence number was changed by the Media
Distributor, the synthetic packet has the original Sequence
Number. sequence
number. If the integrity check does not pass, discard the packet.</t>
</list>
</t> packet.</li>
        </ol>
        <t>Once the packet has been successfully decrypted, the application needs
to be careful about which information it uses to get the correct
behavior.  The application MUST <bcp14>MUST</bcp14> use only the information found in the
synthetic SRTP packet and MUST NOT <bcp14>MUST NOT</bcp14> use the other data that was in the
outer SRTP packet with the following exceptions:
</t>
<t>
<list style="symbols">
<t>The
        <ul spacing="normal">
          <li>The PT from the outer SRTP packet is used for normal matching to SDP
Session Description Protocol (SDP) and codec selection.</t>
<t>The selection.</li>
          <li>The sequence number from the outer SRTP packet is used for normal
RTP ordering.</t>
</list>
</t> ordering.</li>
        </ul>
        <t>The PT and sequence number from the inner SRTP packet can be used for
collection of various statistics.
</t>
        <t>If the RTP header of the outer packet contains extensions, they MAY <bcp14>MAY</bcp14>
be used.  However, because extensions are not protected end-to-end,
implementations SHOULD <bcp14>SHOULD</bcp14> reject an RTP packet containing headers that
would require end-to-end protection.
</t>
      </section>
    </section>
    <section anchor="rtcp-operations" title="RTCP Operations"> numbered="true" toc="default">
      <name>RTCP Operations</name>
      <t>Unlike RTP, which is encrypted both hop-by-hop and end-to-end using
two separate cryptographic keys, RTCP is encrypted using only the outer
(hop-by-hop) cryptographic key. The procedures for RTCP encryption
are specified in <xref target="RFC3711"/> target="RFC3711" format="default"/>, and this document introduces no
additional steps.
</t>
    </section>
    <section anchor="use-with-other-rtp-mechanisms" title="Use numbered="true" toc="default">
      <name>Use with Other RTP Mechanisms"> Mechanisms</name>
      <t>Media Distributors sometimes interact with RTP media packets sent
by endpoints, e.g., to provide recovery or receive commands via
DTMF.
dual-tone multi-frequency (DTMF) signaling.  When media packets are encrypted end-to-end, these procedures
require modification.  (End-to-end interactions, including
end-to-end recovery, are not affected by end-to-end encryption.)
</t>
      <t>Repair mechanisms, in general, will need to perform recovery on
encrypted packets (double-encrypted when using this transform),
since the Media Distributor does not have access to the plaintext of
the packet, only an intermediate, E2E-encrypted form.
</t>
      <t>When the recovery mechanism calls for the recovery packet itself to
be encrypted, it is encrypted with only the outer, hop-by-hop key.  This
allows a media distributor Media Distributor to generate recovery packets without
having access to the inner, end-to-end keys.  However, it also results in
recovery packets being triple-encrypted, twice for the base
transform, and once for the recovery protection.
</t>
      <section anchor="rtx" title="RTP numbered="true" toc="default">
        <name>RTP Retransmission (RTX)"> (RTX)</name>
        <t>When using RTX <xref target="RFC4588"/> target="RFC4588" format="default"/> with double, the double transform, the cached payloads MUST <bcp14>MUST</bcp14> be the
double-encrypted packets, i.e., the bits that are sent over the wire to the
other side. When encrypting a retransmission packet, it MUST <bcp14>MUST</bcp14> be
encrypted the like a packet in repair mode (i.e., with only the hop-by-hop key).
</t>
        <t>If the Media Distributor were to cache the inner, E2E-encrypted
payload and retransmit that it with an RTX OSN original sequence number field prepended, then
the modifications to the payload would cause the inner integrity
check to fail at the receiver.
</t>
        <t>A typical RTX receiver would decrypt the packet, undo the RTX
transformation, then process the resulting packet normally by
using the steps in <xref target="decrypt"/>. target="decrypt" format="default"/>.
</t>
      </section>
      <section anchor="red" title="Redundant numbered="true" toc="default">

        <name>Redundant Audio Data (RED)"> (RED)</name>
        <t>When using RED <xref target="RFC2198"/> target="RFC2198" format="default"/> with double, the double transform, the processing at the sender
and receiver is the same as when using RED with any other SRTP
transform.
</t>
        <t>The main difference between the double transform and any other transform is that
in an intermediated environment, usage of RED must be end-to-end.  A
Media Distributor cannot synthesize RED packets, because it lacks
access to the plaintext media payloads that are combined to form a
RED payload.
</t>
        <t>Note that FlexFEC Flexible Forward Error Correction (Flex FEC) may often provide similar or better repair
capabilities compared to RED.  For most applications, FlexFEC Flex FEC is a
better choice than RED; in particular, FlexFEC Flex FEC has modes in which
the Media Distributor can synthesize recovery packets.
</t>
      </section>
      <section anchor="fec" title="Forward numbered="true" toc="default">
        <name>Forward Error Correction (FEC)"> (FEC)</name>
        <t>When using Flex FEC <xref target="I-D.ietf-payload-flexible-fec-scheme"/> target="RFC8627" format="default"/> with
double,
the double transform, repair packets MUST <bcp14>MUST</bcp14> be constructed by first
double-encrypting the packet, then performing FEC.  Processing of
repair packets proceeds in the opposite order, performing FEC
recovery and then decrypting.  This ensures that the original media
is not revealed to the Media Distributor but but, at the same time time, allows
the Media Distributor to repair media.  When encrypting a packet
that contains the Flex FEC data, which is already encrypted, it MUST <bcp14>MUST</bcp14>
be encrypted with only the outer, hop-by-hop transform.
</t>
        <t>The algorithm recommended in <xref target="I-D.ietf-rtcweb-fec"/> target="I-D.ietf-rtcweb-fec" format="default"/> for repair of video
is Flex FEC <xref target="I-D.ietf-payload-flexible-fec-scheme"/>. target="RFC8627" format="default"/>.  Note that for
interoperability with WebRTC, <xref target="I-D.ietf-rtcweb-fec"/> target="I-D.ietf-rtcweb-fec" format="default"/> recommends not
using additional FEC only m-line FEC-only "m=" lines in SDP for the repair packets.
</t>
      </section>
      <section anchor="dtmf" title="DTMF"> numbered="true" toc="default">
        <name>DTMF</name>
        <t>When DTMF is sent using the mechanism in <xref target="RFC4733"/>, target="RFC4733" format="default"/>, it is
end-to-end encrypted and encrypted; the relay can not cannot read it, so it cannot be
used to control the relay. Other out of band out-of-band methods to control the
relay need to be used instead.
</t>
      </section>
    </section>
    <section anchor="recommended-inner-and-outer-cryptographic-algorithms" title="Recommended numbered="true" toc="default">
      <name>Recommended Inner and Outer Cryptographic Algorithms"> Algorithms</name>

      <t>This specification recommends and defines AES-GCM as both the inner
and outer cryptographic algorithms, identified as
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM and
DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM.  These algorithm algorithms provide for
authenticated encryption and will consume additional processing time
double-encrypting for hop-by-hop and end-to-end.  However, the
approach is secure and simple, and simple; thus, it is thus  viewed as an acceptable
trade-off in processing efficiency.
</t>
      <t>Note that names for the cryptographic transforms are of the form
DOUBLE_(inner algorithm)_(outer algorithm).
</t>
      <t>While this document only defines a profile based on AES-GCM, it is
possible for future documents to define further profiles with
different inner and outer algorithms in this same framework.  For example,
if a new SRTP transform was were defined that encrypts some or all of the
RTP header, it would be reasonable for systems to have the option of
using that for the outer algorithm.  Similarly, if a new transform was were
defined that provided only integrity, that would also be reasonable to
use for the outer transform as the payload data is already encrypted by the
inner transform.
</t>
      <t>The AES-GCM cryptographic algorithm introduces an additional 16
octets to the length of the packet.  When using AES-GCM for both the
inner and outer cryptographic algorithms, the total additional
length is 32 octets.  The OHB will consume an additional 1-4 octets.
Packets in repair mode will carry additional repair data, further
increasing their size.
</t>
    </section>
    <section anchor="sec" title="Security Considerations"> numbered="true" toc="default">
      <name>Security Considerations</name>
      <t>This SRTP transform provides protection against two classes of
attacker: An  a network attacker that knows neither the inner nor outer
keys,
keys and a malicious MD that knows the outer key.  Obviously, it
provides no protections against an attacker that holds both the
inner and outer keys.
</t>
      <t>The protections with regard to the network are the same as with the
normal SRTP AES-GCM transforms.  The major difference is that the
double transforms are designed to work better in a group context.
In such contexts, it is important to note that because these
transforms are symmetric, they do not protect against attacks within
the group.  Any member of the group can generate valid SRTP packets
for any SSRC in use by the group.
</t>
      <t>With regard to a malicious MD, the recipient can verify the
integrity of the base header fields and confidentiality and
integrity of the payload.  The recipient has no assurance, however,
of the integrity of the header extensions in the packet.
</t>
      <t>The main innovation of this transform relative to other SRTP
transforms is that it allows a partly-trusted partly trusted MD to decrypt, modify,
and re-encrypt a packet.  When this is done, the cryptographic
contexts used for decryption and re-encryption MUST <bcp14>MUST</bcp14> use different,
independent master keys.  If the same context is
used, the nonce formation rules for SRTP will cause the same key and
nonce to be used with two different plaintexts, which substantially
degrades the security of AES-GCM.
</t>
      <t>In other words, from the perspective of the MD, re-encrypting
packets using this protocol will involve the same cryptographic
operations as if it had established independent AES-GCM crypto
contexts with the sender and the receiver.  If This property allows
the MD doesn't modify
any header fields, then use of an MD that supports AES-GCM could be unused
unmodified. but does not modify any
header fields, without requiring any modification to the MD.
</t>
    </section>
    <section anchor="iana" title="IANA Considerations"> numbered="true" toc="default">
      <name>IANA Considerations</name>
      <section anchor="dtlssrtp" title="DTLS-SRTP">
<t>We request IANA to add numbered="true" toc="default">

        <name>DTLS-SRTP</name>

        <t>IANA has added the following values protection profiles to defines a DTLS-SRTP
&quot;SRTP the
"DTLS-SRTP Protection Profile&quot; Profiles" registry defined in <xref target="RFC5764"/>. target="RFC5764" format="default"/>.
</t>
<texttable>
<ttcol align="left">Value</ttcol>
<ttcol align="left">Profile</ttcol>
<ttcol align="left">Reference</ttcol>

<c>{0x00, 0x09}</c><c>DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM</c><c>RFCXXXX</c>
<c>{0x00, 0x0A}</c><c>DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM</c><c>RFCXXXX</c>
</texttable>
<t>Note to IANA: Please assign value RFCXXXX and update table
<table align="center">
  <name>Updates to point at
this RFC for these values.
</t> the DTLS-SRTP Protection Profiles Registry</name>
          <thead>
            <tr>
              <th align="left">Value</th>
              <th align="left">Profile</th>
              <th align="left">Reference</th>
            </tr>
          </thead>
          <tbody>
            <tr>
              <td align="left">{0x00, 0x09}</td>
              <td align="left">DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM</td>
              <td align="left">RFC 8723</td>
            </tr>
            <tr>
              <td align="left">{0x00, 0x0A}</td>
              <td align="left">DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM</td>
              <td align="left">RFC 8723</td>
            </tr>
          </tbody>
        </table>

        <t>The SRTP transform parameters for each of these protection profiles are:
</t>

<figure align="left"><artwork align="left">
DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM
    cipher:                 AES_128_GCM
<table align="center">
  <name>SRTP Transform Parameters</name>
   <tbody>
      <tr>
         <th colspan="2">DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM</th>
      </tr>
      <tr>
         <td>cipher:</td>
         <td>AES_128_GCM then AES_128_GCM
    cipher_key_length:      256 bits
    cipher_salt_length:     192 bits
    aead_auth_tag_length:   256 bits
    auth_function:          NULL
    auth_key_length:        N/A
    auth_tag_length:        N/A
    maximum lifetime:       at AES_128_GCM</td>
      </tr>
      <tr>
         <td>cipher_key_length:</td><td>256 bits</td>
      </tr>
      <tr>
         <td>cipher_salt_length:</td><td>192 bits</td>
      </tr>
      <tr>
         <td>aead_auth_tag_length:</td><td>256 bits</td>
      </tr>
      <tr>
         <td>auth_function:</td><td>NULL</td>
      </tr>
      <tr>
         <td>auth_key_length:</td><td>N/A</td>
      </tr>
      <tr>
         <td>auth_tag_length:</td><td>N/A</td>
      </tr>
      <tr>
         <td>maximum lifetime:</td><td>at most 2^31 2<sup>31</sup> SRTCP packets and at most 2^48 2<sup>48</sup> SRTP packets

DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM
    cipher:                 AES_256_GCM packets</td>
     </tr>
      <tr>
         <th colspan="2">DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM</th>
      </tr>
      <tr>
         <td>cipher:</td><td>AES_256_GCM then AES_256_GCM
    cipher_key_length:      512 bits
    cipher_salt_length:     192 bits
    aead_auth_tag_length:   256 bits
    auth_function:          NULL
    auth_key_length:        N/A
    auth_tag_length:        N/A
    maximum lifetime:       at AES_256_GCM</td>
      </tr>
      <tr>
         <td>cipher_key_length:</td><td>512 bits</td>
      </tr>
      <tr>
         <td>cipher_salt_length:</td><td>192 bits</td>
      </tr>
      <tr>
         <td>aead_auth_tag_length:</td><td>256 bits</td>
      </tr>
      <tr>
         <td>auth_function:</td><td>NULL</td>
      </tr>
      <tr>
         <td>auth_key_length:</td><td>N/A</td>
      </tr>
      <tr>
         <td>auth_tag_length:</td><td>N/A</td>
      </tr>
      <tr>
         <td>maximum lifetime:</td><td>at most 2^31 2<sup>31</sup> SRTCP packets and at most 2^48 2<sup>48</sup> SRTP packets
</artwork></figure> packets</td>
      </tr>
   </tbody>
</table>
        <t>The first half of the key and salt is used for the inner (end-to-end)
algorithm and the second half is used for the outer (hop-by-hop)
algorithm.
</t>
      </section>
    </section>

<section anchor="acknowledgments" title="Acknowledgments">
<t>Thank you for reviews and improvements to this specification from Alex
Gouaillard, David Benham, Magnus Westerlund, Nils Ohlmeier, Paul
Jones, Roni Even, and Suhas Nandakumar. In addition, thank you to
Sergio Garcia Murillo proposed the change

  </middle>
  <back>
    <displayreference target="I-D.ietf-perc-dtls-tunnel" to="DTLS-TUNNEL"/>
    <displayreference target="I-D.ietf-perc-private-media-framework" to="PRIVATE-MEDIA-FRAMEWORK"/>
    <displayreference target="I-D.ietf-perc-srtp-ekt-diet" to="EKT-SRTP"/>
    <displayreference target="I-D.ietf-rtcweb-fec" to="WEBRTC-FEC"/>
    <references>
      <name>References</name>
      <references>
        <name>Normative References</name>
        <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.3711.xml"/>
        <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.6188.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.6904.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.7714.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"/>
        <xi:include href="https://xml2rfc.ietf.org/public/rfc/bibxml/reference.RFC.8285.xml"/>
      </references>
      <references>
        <name>Informative References</name>

        <!-- draft-ietf-payload-flexible-fec-scheme-20 is now RFC 8627 -->
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8627.xml"/>
        <!-- draft-ietf-perc-dtls-tunnel-06 is an active WG draft as of transporting the OHB
information 19 Dec 12-->
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-ietf-perc-dtls-tunnel-06.xml"/>
        <!-- draft-ietf-perc-private-media-framework-12 is in the RTP payload instead MISSREF*R as of the RTP header.
</t>
</section>

</middle>
<back>
<references title="Normative References">
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2119.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.3711.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5764.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6188.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.6904.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.7714.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8174.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.8285.xml"?> 19 Dec 12 -->
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-ietf-perc-private-media-framework-12.xml"/>
        <!-- draft-ietf-perc-srtp-ekt-diet-10 is in IESG Evaluation::AD Followup as of 19 Dec 12 -->
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-ietf-perc-srtp-ekt-diet-10.xml"/>
        <!-- draft-ietf-rtcweb-fec-10 is REF state as of 19 Dec 12  -->
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.draft-ietf-rtcweb-fec-10.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2198.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4588.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4733.xml"/>
        <xi:include href="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5234.xml"/>
      </references>
<references title="Informative References">
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-payload-flexible-fec-scheme.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-perc-dtls-tunnel.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-perc-private-media-framework.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-perc-srtp-ekt-diet.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml3/reference.I-D.ietf-rtcweb-fec.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.2198.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4588.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.4733.xml"?>
<?rfc include="https://xml2rfc.tools.ietf.org/public/rfc/bibxml/reference.RFC.5234.xml"?>
    </references>
    <section anchor="encryption-overview" title="Encryption Overview"> numbered="true" toc="default">
      <name>Encryption Overview</name>
      <t>The following figure shows a double encrypted double-encrypted SRTP packet. The sides
indicate the parts of the packet that are encrypted and authenticated
by the hop-by-hop and end-to-end operations.
</t>

<figure align="center"><artwork align="center">
<!-- [rfced]  XMLv3 imposes a 69-character limit on artwork.
Please help us adjust the figure in Appendix A to fit within
69 characters.
-->
      <artset>
        <artwork alt="" type="svg" src="diag-2.svg"/>
        <artwork alt="" type="ascii-art"><![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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    RTP extension (OPTIONAL) ...               | |O
+&gt;+&gt;+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+&lt;+O
+>+>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
O I |                          payload ...                          | IO
O I |                               +-------------------------------+ IO
O I |                               | RTP padding   | RTP pad count | IO
O +&gt;+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+&lt;+O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
O | |                    E2E authentication tag                     | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | |                            OHB ...                            | |O
+&gt;|
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | |                    HBH authentication tag                     | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| |                                                                   ||
| +- E2E Encrypted Portion               E2E Authenticated Portion ---+|
|
|
+--- HBH Encrypted Portion

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+<+
|V=2|P|X|  CC   |M|     PT      |       sequence number         | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I O
|                           timestamp                           | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ I O
|           synchronization source (SSRC) identifier            | I O
+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ I O
|            contributing source (CSRC) identifiers             | I O
|                               ....                            | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ O
|                    RTP extension (OPTIONAL) ...               | | O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ O
|                           payload ...                         | I O
|                               +-------------------------------+ I O
|                               | RTP padding   | RTP pad count | I O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+ O
|                    E2E authentication tag                     | | O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | O
|                            OHB ...                            | | O
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |<+
|                    HBH authentication tag                     | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | |
                                                                  | |
                                     E2E Authenticated Portion ---+ |
                                                                    |
                                     HBH Authenticated Portion ----+
</artwork></figure> -----+
]]></artwork>
</artset>
 </section>
    <section anchor="acknowledgments" numbered="false" toc="default">
      <name>Acknowledgments</name>
      <t>Thank you to <contact fullname="Alex Gouaillard"/>,
<contact fullname="David Benham"/>, <contact fullname="Magnus Westerlund"/>,
<contact fullname="Nils Ohlmeier"/>, <contact fullname="Paul Jones"/>,
<contact fullname="Roni Even"/>, and <contact fullname="Suhas Nandakumar"/>
for reviews and improvements to this specification. In addition, thank you to
<contact fullname="Sergio Garcia Murillo"/>, who proposed the change of transporting the OHB
information in the RTP payload instead of the RTP header.
</t>
    </section>
  </back>
<!--[rfced] We will update to follow the guidance at
https://www.rfc-editor.org/styleguide/part2/ and expand
abbreviations only on first use (aftward using the abbreviation)
(for example, MD instead of Media Distributor).  Please let us
know any objections.  -->

</rfc>