Network Working Group                                        C. Jennings
Internet-Draft                                                  P. Jones
Intended status: Standards Track                               R. Barnes
Expires: September 6, November 4, 2018                                  Cisco Systems
                                                                A. Roach
                                                           March 5,
                                                             May 3, 2018

                   SRTP Double Encryption Procedures


   In some conferencing scenarios, it is desirable for an intermediary
   to be able to manipulate some RTP parameters, while still providing
   strong end-to-end security guarantees.  This document defines SRTP
   procedures that use 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 scheme or take
   advantage of future SRTP transforms with different properties.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 6, November 4, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Cryptographic Context . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Key Derivation  . . . . . . . . . . . . . . . . . . . . .   5
   4.  Original Header Block . . . . . . . . . . . . . . . . . . . .   5
   5.  RTP Operations  . . . . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Encrypting a Packet . . . . . . . . . . . . . . . . . . .   6
     5.2.  Relaying a Packet . . . . . . . . . . . . . . . . . . . .   7
     5.3.  Decrypting a Packet . . . . . . . . . . . . . . . . . . .   8
   6.  RTCP Operations . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Use with Other RTP Mechanisms . . . . . . . . . . . . . . . .   9
     7.1.  RTX . . . . . . . . . . . . . . . . . . . . . . . . . . .   9
     7.2.  RED . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.3.  FEC . . . . . . . . . . . . . . . . . . . . . . . . . . .  10
     7.4.  DTMF  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   8.  Recommended Inner and Outer Cryptographic Algorithms  . . . .  11
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  11  12
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12  13
     10.1.  DTLS-SRTP  . . . . . . . . . . . . . . . . . . . . . . .  13
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  14
     12.2.  Informative References . . . . . . . . . . . . . . . . .  14  15
   Appendix A.  Encryption Overview  . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16  17

1.  Introduction

   Cloud conferencing systems that are based on switched conferencing
   have a central Media Distributor 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 from the sending endpoint to
   the receiving endpoint that can encrypt and authenticate the media
   end-to-end while still allowing certain RTP header information 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 [I-D.ietf-perc-private-media-framework]
   describes this concept in more detail.

   This specification defines an SRTP transform that uses the AES-GCM
   algorithm [RFC7714] to provide encryption and integrity for an RTP
   packet for the end-to-end cryptographic key as well as 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
   change some of the RTP header information that would impact the end-
   to-end integrity.  The  In that case, the original value of any RTP header
   field that is changed is included in a new RTP header extension
   called the Original Header Block.  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.

   One can think of the double as a normal SRTP transform for encrypting
   the RTP in a way where things that only know half of the key, can
   decrypt and modify part of the RTP packet but not other parts of if parts,
   including the media payload.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

   Terms used throughout this document include:

   o  Media Distributor: media distribution device that routes media
      from one endpoint to other endpoints

   o  end-to-end: meaning the link from one endpoint through one or more
      Media Distributors to the endpoint at the other end.

   o  hop-by-hop: meaning the link from the endpoint to or from the
      Media Distributor.

   o  OHB: Original Header Block is an octet string that contains the
      original values from the RTP header that might have been changed
      by a Media Distributor.

3.  Cryptographic Context

   This specification uses a cryptographic context with two parts: an

   o  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, end-
      to-end, and an

   o  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

   The RECOMMENDED cipher for the hop-by-hop and end-to-end algorithm is
   AES-GCM.  Other combinations of SRTP ciphers that support the
   procedures in this document can be added to the IANA registry.

   The keys and salt for these algorithms are generated with the
   following steps:

   o  Generate key and salt values of the length required for the
      combined inner (end-to-end) and outer (hop-by-hop) algorithms.

   o  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.

   o  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 used is the part of the salt generated with
      that key.

   o  the SSRC is the same for both the inner and out outer algorithms
      as it can not be changed.

   o  The SEQ and ROC are tracked independently for the inner and outer

   Obviously, if the Media Distributor is to be able to modify header
   fields but not decrypt the payload, then it must have cryptographic
   key for the outer 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 [I-D.ietf-perc-dtls-tunnel].

3.1.  Key Derivation

   In order to allow the inner and outer keys to be managed
   independently via the master key, the transforms defined in this
   document MUST be used with the following PRF, which preserves the
   separation between the two halves of the key:

         PRF_double_n(k_master,x) = PRF_inner_(n/2)(k_master,x) ||

         PRF_inner_n(k_master,x)  = PRF_n(inner(k_master),x)
         PRF_outer_n(k_master,x)  = PRF_n(outer(k_master),x)

   Here "PRF_n(k, x)" represents the default SRTP AES_CM PRF [RFC3711], KDF [RFC3711] for
   DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM algorithm and AES_256_CM_PRF
   KDF [RFC6188] for DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM algorithm.
   "inner(key)" represents the first half of the key, and "outer(key)"
   represents the second half of the key.

4.  Original Header Block

   The Original Header Block (OHB) contains the original values of any
   modified RTP header fields.  In the encryption process, the OHB is
   appended to the RTP payload.  In the decryption process, the
   receiving endpoint uses it to reconstruct the original RTP header, so
   that it can pass the proper AAD value to the inner transform.

   The OHB can reflect modifications to the following fields in an RTP
   header: the payload type, the sequence number, and the marker bit.
   All other fields in the RTP header MUST remain unmodified; since the
   OHB cannot reflect their original values, the receiver will be unable
   to verify the E2E integrity of the packet.

   The OHB has the following syntax (in ABNF):

   BYTE ABNF [RFC5234]):

   OCTET = %x00-FF

   Config = BYTE OCTET
   OHB = ?PT ?SEQ [ PT ] [ SEQ ] Config

   If present, the PT and SEQ parts of the OHB contain the original
   payload type and sequence number fields, respectively.  The final
   "config" octet of the OHB specifies whether these fields are present,
   and the original value of the marker bit (if necessary):

   |R R R R B M P Q|

   o  P: PT is present

   o  Q: SEQ is present

   o  M: Marker bit is present

   o  B: Value of marker bit

   o  R: Reserved, MUST be set to 0

   In particular, an all-zero OHB config octet (0x00) indicates that
   there have been no modifications from the original header.

5.  RTP Operations

5.1.  Encrypting a Packet

   To encrypt a packet, the endpoint encrypts the packet 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:

   1.  Form an RTP packet.  If there are any header extensions, they
       MUST use [RFC8285].

   2.  If the packet is for repair mode data, skip to step 6.

   3.  Form a synthetic RTP packet with the following contents:

       *  Header: The RTP header of the original packet with the
          following modifications:

       *  The X bit is set to zero

       *  The header is truncated to remove any extensions (12 + 4 * CC

       *  Payload: The RTP payload of the original packet

   4.  Apply the inner cryptographic algorithm to the synthetic RTP
       packet from the previous step.

   5.  Replace the header of the protected RTP packet with the header of
       the original packet, and append an empty OHB (0x00) to the
       encrypted payload of the packet (1) (with the authentication tag tag) obtained from the original transform, and (2) an
       empty OHB (0x00).
       step 4.

   6.  Apply the outer cryptographic algorithm to the RTP packet.  If
       encrypting RTP header extensions hop-by-hop, then [RFC6904] MUST
       be used when encrypting the RTP packet using the outer
       cryptographic key.

   When using EKT [I-D.ietf-perc-srtp-ekt-diet], the EKT Field comes
   after the SRTP packet exactly like using EKT with any other SRTP

5.2.  Relaying a Packet

   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 (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.

   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
   cryptographic using the outer (hop-by-hop) key. cryptographic key before

   1.  Apply the outer (hop-by-hop) cryptographic algorithm to decrypt
       the packet.  If decrypting RTP header extensions hop-by-hop, then
       [RFC6904] MUST 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.

   2.  Change any parts of the RTP packet that the relay wishes to
       change and should be changed.

   3.  A Media Distributor can add information to the OHB, but MUST NOT
       change existing information in the OHB.  If RTP value is changed
       and not already in the OHB, then add it with its original value
       to the OHB.

   4.  If the Media Distributor resets a parameter to its original
       value, it MAY drop it from the OHB.  Note that this might result
       in a decrease in the size of the OHB.

   5.  Apply the outer (hop-by-hop) cryptographic algorithm to the
       packet.  If the RTP Sequence Number has been modified, SRTP
       processing happens as defined in SRTP and will end up using the
       new Sequence Number.  If encrypting RTP header extensions hop-by-
       hop, then [RFC6904] MUST be used.

5.3.  Decrypting a Packet

   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.

   1.  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 [RFC6904] MUST be used when
       decrypting the RTP packet using the outer cryptographic key.

   2.  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.

   3.  Remove the inner authentication tag and the OHB from the end of
       the payload of the outer SRTP packet.

   4.  Form a new synthetic SRTP packet with:

       *  Header = Received header, with the following modifications:

       *  Header fields replaced with values from OHB (if any)

       *  The X bit is set to zero

       *  The header is truncated to remove any extensions (12 + 4 * CC

       *  Payload is the encrypted payload from the outer SRTP packet
          (after the inner tag and OHB have been stripped).

       *  Authentication tag is the inner authentication tag from the
          outer SRTP packet.

   5.  Apply the inner cryptographic algorithm to this synthetic SRTP
       packet.  Note if the RTP Sequence Number was changed by the Media
       Distributor, the synthetic packet has the original Sequence
       Number.  If the integrity check does not pass, discard the

   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 use only the information
   found in the synthetic SRTP packet and MUST NOT use the other data
   that was in the outer SRTP packet with the following exceptions:

   o  The PT from the outer SRTP packet is used for normal matching to
      SDP and codec selection.

   o  The sequence number from the outer SRTP packet is used for normal
      RTP ordering.

   The PT and sequence number from the inner SRTP packet can be used for
   collection of various statistics.

   If any of the following RTP headers extensions are found in the outer
   SRTP packet, they MAY be used:

   o  Mixer-to-client audio level indicators (See [RFC6465])

6.  RTCP Operations

   Unlike RTP, which is encrypted both hop-by-hop and end-to-end using
   two separate cryptographic key, keys, RTCP is encrypted using only the
   outer (hop-by-hop) cryptographic key.  The procedures for RTCP
   encryption are specified in [RFC3711] and this document introduces no
   additional steps.

7.  Use with Other RTP Mechanisms

   There are some RTP related extensions that need special consideration
   to be used by a relay when using the double transform due to the end-
   to-end protection of the RTP.  The repair mechanism, when used with
   double, typically operate operates on the double encrypted data then take the
   results of theses operations and encrypted encrypts
   them using only the HBH key.  This results in three cryptography
   operation happening to the repair data sent over the wire.

7.1.  RTX

   When using RTX [RFC4588] with double, the cached payloads MUST be the
   encrypted packets with the bits that are sent over the wire to the
   other side.  When encrypting a retransmission packet, it MUST be
   encrypted in the packet in repair mode.

   A typical RTX receiver would decrypt the packet, undo the RTX
   transformation, then process the resulting packet using the normally by using
   the steps in Section 5.3.

7.2.  RED

   When using RED [RFC2198] with double, the primary encoding MAY
   contain RTP header extensions and CSRC identifiers but non primary
   encodings can not. cannot.

   The sender takes encrypted payloads payload from the cached packets to form
   the RED payload.  Any header extensions from the primary encoding are
   copied to the RTP packet that will cary carry the RED payload and the
   other RTP header information such as SSRC, SEQ, CSRC, etc are set to
   the same as the primary payload.  The RED RTP packet is then
   encrypted in repair mode and sent.

   The receiver decrypts the payload to find the encrypted RED payload.
   Note a media relay can do this decryption as the packet was sent in
   repair mode that only needs the hop-by-hop key.  The RTP headers and
   header extensions along with the primary payload and PT from inside
   the RED payload (for the primary encoding) are used to form the
   encrypted primary RTP packet which can then be decrypted with double.

   The RTP headers (but not header extensions or CSRC) along with PT
   from inside the RED payload corresponding to the redundant encoding
   are used for to from the non primary payloads.  The time offset and
   packet rate information in the RED data MUST be used to adjust the
   sequence number in the RTP
   header by using the timestamp offset and packet rate to find a
   sequence number offset to adjust by. header.  At this point the non primary
   packets can be decrypted with double.

   Note that Flex FEC [I-D.ietf-payload-flexible-fec-scheme] is a
   superset of the capabilities of RED.  For most applications, FlexFEC
   is a better choice than RED.

7.3.  FEC

   When using Flex FEC [I-D.ietf-payload-flexible-fec-scheme] with
   double, the negotiation of double for the crypto is the out of band
   signaling that indicates that the repair packets MUST use the order
   of operations of SRTP followed by FEC when encrypting.  This is to
   ensure that the original media is not revealed to the Media
   Distributor but at the same time allow the Media Distributor to
   repair media.  When encrypting a packet that contains the Flex FEC
   data, which is already encrypted, it MUST be encrypted in repair mode

   The algorithm recommend in [I-D.ietf-rtcweb-fec] for repair of video
   is Flex FEC [I-D.ietf-payload-flexible-fec-scheme].  Note that for
   interoperability with WebRTC, [I-D.ietf-rtcweb-fec] recommends not
   using additional FEC only m-line in SDP for the repair packets.

7.4.  DTMF

   When DTMF is sent with [RFC4733], it is end-to-end encrypted and the
   relay can not read it so it can not cannot be used to control the relay.
   Other out of band methods to control the relay need to be used

8.  Recommended Inner and Outer Cryptographic Algorithms

   This specification recommends and defines AES-GCM as both the inner
   and outer cryptographic algorithms, identified as
   DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM.  These algorithm 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 is thus viewed as an acceptable
   trade-off in processing efficiency.

   Note that names for the cryptographic transforms are of the form
   DOUBLE_(inner algorithm)_(outer algorithm).

   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 crypto algorithms in this same framework.  For
   example, if a new SRTP transform was 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 defined that provided only integrity, that would also
   be reasonable to use for the hop-by-hop as the payload data is
   already encrypted by the end-to-end.

   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.  If no other header extensions are present in the
   packet and the OHB is introduced, that will consume an additional 8
   octets.  If other extensions are already present, the OHB will
   consume up to 4 additional octets.  For packets in repair mode, the
   data they are caring is often already encrypted further increasing
   the size.

9.  Security Considerations

   To summarize what is encrypted and authenticated, we will refer to
   all the RTP fields except headers created by the sender and before
   the pay load payload as the initial envelope and the RTP payload information
   with the media as the payload.  Any additional headers added by the
   sender or Media Distributor are referred to as the extra envelope.
   The sender uses the end-to-end key to encrypts encrypt the payload and
   authenticate the payload + initial envelope envelope, which using an AEAD
   cipher results in a slight longer new payload.  Then the sender uses
   the hop-by-hop key to encrypt the new payload and authenticate the
   initial envelope envelope, extra envelope and the new payload.  Also to note,
   the "Associated Data" input (which excludes header extensions ) to
   the inner crypto differs from [RFC7714] construction.  This shouldn't
   typically impact the strength of e2e integrity given the fact that
   there doesn't exist header extensions defined today that needs e2e
   protection.  However, if future specifications define header
   extensions that needs e2e integrity protection, the input to inner
   transform may be modified to consider including the header

   The Media Distributor has the hop-by-hop key so it can check the
   authentication of the received packet across the initial envelope,
   extra envelope and payload data but it can't decrypt the payload as
   it does not have the end-to-end key.  It can add or change extra
   envelope information.  It then authenticates the initial plus extra
   envelope information plus payload with a hop-by-hop key.  This hop-
   by-hop  The hop-by-
   hop key for the outgoing packet is typically different than the hop-
   by-hop key for the incoming packet.

   The receiver can check the authentication of the initial and extra
   envelope information from the Media Distributor.  This, along with
   the OHB, is used to construct a synthetic packet that is which should be
   identical to the initial envelope plus payload to one the sender
   created and the receiver can check that it is identical and then
   decrypt the original payload.

   The end result is that if the authentications succeed, the receiver
   knows exactly what the payload and initial envelope the sender sent, as
   well as exactly which modifications were made by the Media
   Distributor and what extra envelope the Media Distributor send. sent.  The
   receiver does not know exactly what extra envelope the sender sent.

   It is obviously critical that the intermediary has only access to just the
   outer (hop-by-hop) algorithm key and not the half of the key for the
   the inner (end-to-end) algorithm.  We rely on an external key
   management protocol to assure ensure this property.

   Modifications by the intermediary result results in the recipient getting
   two values for changed parameters (original and modified).  The
   recipient will have to choose which to use; there is risk in using
   either that depends on the session setup.

   The security properties for both the inner (end-to-end) and outer
   (hop-by-hop) key holders are the same as the security properties of
   classic SRTP.

10.  IANA Considerations

10.1.  DTLS-SRTP

   We request IANA to add the following values to defines a DTLS-SRTP
   "SRTP Protection Profile" defined in [RFC5764].

   | Value      | Profile                                  | Reference |
   | {0x00,     | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFCXXXX   |
   | 0x09}      |                                          |           |
   | {0x00,     | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFCXXXX   |
   | 0x0A}      |                                          |           |

   Note to IANA: Please assign value RFCXXXX and update table to point
   at this RFC for these values.

   The SRTP transform parameters for each of these protection are:

       cipher:                 AES_128_GCM then AES_128_GCM
       cipher_key_length:      256 bits
       cipher_salt_length:     192 bits
       aead_auth_tag_length:   32 octets   256 bits
       auth_function:          NULL
       auth_key_length:        N/A
       auth_tag_length:        N/A
       maximum lifetime:       at most 2^31 SRTCP packets and
                               at most 2^48 SRTP packets

       cipher:                 AES_256_GCM then AES_256_GCM
       cipher_key_length:      512 bits
       cipher_salt_length:     192 bits
       aead_auth_tag_length:   32 octets   256 bits
       auth_function:          NULL
       auth_key_length:        N/A
       auth_tag_length:        N/A
       maximum lifetime:       at most 2^31 SRTCP packets and
                               at most 2^48 SRTP packets

   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)

11.  Acknowledgments

   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 of transporting the OHB
   information in the RTP payload instead of the RTP header.

12.  References

12.1.  Normative References

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

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,

   [RFC6188]  McGrew, D., "The Use of AES-192 and AES-256 in Secure
              RTP", RFC 6188, DOI 10.17487/RFC6188, March 2011,

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

   [RFC7714]  McGrew, D. and K. Igoe, "AES-GCM Authenticated Encryption
              in the Secure Real-time Transport Protocol (SRTP)",
              RFC 7714, DOI 10.17487/RFC7714, December 2015,

   [RFC8285]  Singer, D., Desineni, H., and R. Even, Ed., "A General
              Mechanism for RTP Header Extensions", RFC 8285,
              DOI 10.17487/RFC8285, October 2017,

12.2.  Informative References

              Zanaty, M., Singh, V., Begen, A., Zanaty, M., and G. Mandyam, "RTP
              Payload Format for Flexible Forward Error Correction
              (FEC)", draft-ietf-payload-flexible-fec-scheme-05 draft-ietf-payload-flexible-fec-scheme-07 (work in
              progress), July 2017. March 2018.

              Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel
              between a Media Distributor and Key Distributor to
              Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-02 draft-ietf-perc-dtls-tunnel-03
              (work in progress), October 2017. April 2018.

              Jones, P., Benham, D., and C. Groves, "A Solution
              Framework for Private Media in Privacy Enhanced RTP
              Conferencing", draft-ietf-perc-private-media-framework-05 draft-ietf-perc-private-media-framework-06
              (work in progress), October 2017. March 2018.

              Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", draft-ietf-perc-srtp-ekt-diet-06 draft-ietf-perc-srtp-ekt-diet-07 (work in progress),
              October 2017.
              March 2018.

              Uberti, J., "WebRTC Forward Error Correction
              Requirements", draft-ietf-rtcweb-fec-08 (work in
              progress), March 2018.

   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J., Vega-Garcia, A., and S. Fosse-
              Parisis, "RTP Payload for Redundant Audio Data", RFC 2198,
              DOI 10.17487/RFC2198, September 1997,

   [RFC4588]  Rey, J., Leon, D., Miyazaki, A., Varsa, V., and R.
              Hakenberg, "RTP Retransmission Payload Format", RFC 4588,
              DOI 10.17487/RFC4588, July 2006,

   [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", RFC 4733,
              DOI 10.17487/RFC4733, December 2006,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC6465]  Ivov, E., Ed., Marocco, E., Ed., and J. Lennox, "A Real-
              time Transport Protocol (RTP) Header Extension for Mixer-
              to-Client Audio Level Indication", RFC 6465,
              DOI 10.17487/RFC6465, December 2011,

Appendix A.  Encryption Overview

   The following figure shows a double encrypted SRTP packet.  The sides
   indicate the parts of the packet that are encrypted and authenticated
   by the hob-by-hop hop-by-hop and end-to-end operations.

       0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    |V=2|P|X|  CC   |M|     PT      |       sequence number         | IO
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
    |                           timestamp                           | IO
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ IO
    |           synchronization source (SSRC) identifier            | IO
    +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+ IO
    |            contributing source (CSRC) identifiers             | IO
    |                               ....                            | IO
    |                    RTP extension (OPTIONAL) ...               | |O
O I |                          payload  ...                         | IO
O I |                               +-------------------------------+ IO
O I |                               | RTP padding   | RTP pad count | IO
O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+<+O
O | |                    E2E authentication tag                     | |O
O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |O
O | |                            OHB ...                            | |O
+>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |+
| | |                    HBH authentication tag                     | ||
| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ||
| |                                                                   ||
| +- E2E Encrypted Portion               E2E Authenticated Portion ---+|
|                                                                      |
+--- HBH Encrypted Portion               HBH Authenticated Portion ----+

Authors' Addresses

   Cullen Jennings
   Cisco Systems


   Paul E. Jones
   Cisco Systems


   Richard Barnes
   Cisco Systems

   Adam Roach