Network Working Group                                        C. Jennings
Internet-Draft                                                  P. Jones
Intended status: Standards Track                               R. Barnes
Expires: April 20, 2019 January 9, 2020                                   Cisco Systems
                                                                A. Roach
                                                        October 17, 2018
                                                            July 8, 2019

                   SRTP Double Encryption Procedures


   In some conferencing scenarios, it is desirable for an intermediary
   to be able to manipulate some parameters in Real Time Protocol (RTP)
   packets, while still providing strong end-to-end security guarantees.
   This document defines a cryptographic transform for the Secure Real
   Time 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 scheme (AEAD)
   algorithm or take advantage of future SRTP transforms with different

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 20, 2019. January 9, 2020.

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   described in the Simplified BSD License.

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 . . . . . . . . . . . . . . . . . . .   7
     5.2.  Relaying a Packet . . . . . . . . . . . . . . . . . . . .   8
     5.3.  Decrypting a Packet . . . . . . . . . . . . . . . . . . .   9
   6.  RTCP Operations . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Use with Other RTP Mechanisms . . . . . . . . . . . . . . . .  10
     7.1.  RTP Retransmission (RTX)  . . . . . . . . . . . . . . . .  11
     7.2.  Redundant Audio Data (RED)  . . . . . . . . . . . . . . .  11
     7.3.  Forward Error Correction (FEC)  . . . . . . . . . . . . .  11  12
     7.4.  DTMF  . . . . . . . . . . . . . . . . . . . . . . . . . .  12
   8.  Recommended Inner and Outer Cryptographic Algorithms  . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  DTLS-SRTP  . . . . . . . . . . . . . . . . . . . . . . .  13  14
   11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  14  15
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  15
     12.2.  Informative References . . . . . . . . . . . . . . . . .  15  16
   Appendix A.  Encryption Overview  . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17  18

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 enables encryption and authenticate
   authentication of the media end-to-end while still allowing certain
   information in the header of a Real Time 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 [I-D.ietf-perc-private-media-framework]
   describes this concept in more detail.

   This specification defines a transform for the Secure Real Time
   Protocol (SRTP) 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.  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 an "Original Header
   Block. 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.

   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,
   including the media payload.

2.  Terminology

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

   Terms used throughout this document include:

   o  Media Distributor: 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

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

   o  hop-by-hop: The path from the endpoint to or from the Media

   o  Original Header Block (OHB): 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:

   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, and

   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

   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

3.1.  Key Derivation

   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 be
   used with the following pseudo-random function (PRF), which preserves
   the separation between the two halves of the key.  Given a positive
   integer "n" representing the desired output length, a master key
   "k_master", and an input "x":


      PRF\_double\_n(k\_master,x) = PRF_inner_(n/2)(k_master,x) PRF\_(n/2)(inner(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 AES_CM PRF KDF [RFC3711] (see Section 4.3.3
   of [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.
   included in an SRTP packet as described in Section 5.  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 [RFC5234]):

   OCTET = %x00-FF

   Config = OCTET
   OHB = [ 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.

   If the marker bit is not present (M=0), then B MUST be set to zero.
   That is, if "C" represents the value of the config octet, then the
   masked value "C & 0x0C" MUST NOT have the value "0x80".

5.  RTP Operations

   As implied by the use of the word "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 and confidentiality of a packet.

   The first, "inner" application of AES-GCM encrypts the SRTP payload
   and integrity-protects 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 holding only the
   "outer" key.

   The second, "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.

   A 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.

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 (i.e., keep
          only the first 12 + 4 * CC bytes of the header)

       *  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, packet (to restore any header extensions and reset
       the X bit), and append an empty OHB (0x00) to the encrypted
       payload (with the authentication tag) obtained from the 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 the outer (hop-by-hop) 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.  Make any desired changes to the fields are allowed to be changed,
       i.e., PT, SEQ, and M.

   3.  A  The Media Distributor can add information MAY also make
       modifications to header extensions, without the OHB, but MUST NOT
       change existing information need to reflect
       these changes in the OHB.

   3.  Reflect any changes to header fields in the OHB:

       *  If RTP value is Media Distributor changed
       and a field that is not already in
          the OHB, then add it with its MUST add the original value of the field to
          the OHB.

   4.  Note that this might result in an increase in the
          size of the OHB.

       *  If the Media Distributor resets took a parameter field that had previously been
          modified and reset to its original value, then it MAY SHOULD drop it
          the corresponding information from the OHB.  Note that this
          might result in a decrease in the size of the OHB.


       *  Otherwise, the Media Distributor MUST NOT modify the OHB.

   4.  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.

   In order to avoid nonce reuse, the cryptographic contexts used in
   step 1 and step 5 MUST use different, independent master keys and
   master salts. keys.  Note
   that this means that the key used for decryption by the MD MUST be
   different from the key used for re-encryption to the end recipient.

   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.

   A Media Distributor that decrypts, modifies, and re-encrypts packets
   in this way MUST use an independent key for each recipient, SHOULD
   use an independent salt for each recipient, and MUST
   NOT 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 GCM. AES-GCM.

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 (i.e., keep
          only the first 12 + 4 * CC bytes of the header)

       *  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 the RTP header of the outer packet contains extensions, they MAY
   be used.  However, because extensions are not protected end-to-end,
   implementations SHOULD reject an RTP packet containing headers that
   would require end-to-end protection.

6.  RTCP Operations

   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 [RFC3711] and this document introduces no
   additional steps.

7.  Use with Other RTP Mechanisms

   Media distributors Distributors sometimes interact with RTP media packets sent by
   endpoints, e.g., to provide recovery or receive commands via DTMF.
   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.)
   Repair mechanisms, in general, will need to perform recovery on
   encrypted packets (double-encrypted when using this transform).  When transform), since
   the Media Distributor does not have access to the plaintext of the
   packet, only an intermediate, E2E-encrypted form.

   When the recovery mechanism calls for the recovery packet itself to
   be encrypted, it is encrypted with only the outer, HBH hop-by-hop key.
   This allows a media distributor to generate recovery packets without
   having access to the inner, E2E 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.

7.1.  RTP Retransmission (RTX)

   When using RTX [RFC4588] with double, the cached payloads MUST 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
   be encrypted the packet in repair mode (i.e., with only the HBH hop-by-
   hop key).

   If the Media Distributor were to cache the inner, E2E-encrypted
   payload and retransmit that with an RTX OSN field prepended, then the
   modifications to the payload would cause the inner integrity check to
   fail at the receiver.

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

7.2.  Redundant Audio Data (RED)

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

   The sender takes encrypted 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 carry processing at the RED payload sender
   and the
   other RTP header information such as SSRC, SEQ, CSRC, etc are set to receiver is 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 when using 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 any other SRTP

   The main difference between double and PT from inside
   the any other transform is that in
   an intermediated environment, usage of RED payload (for the primary encoding) are used to form the
   encrypted primary RTP packet which can then must be decrypted with double.

   The RTP headers (but not header extensions or CSRC) along with PT
   from inside the end-to-end.  A
   Media Distributor cannot synthesize RED payload corresponding packets, because it lacks
   access to the redundant encoding plaintext media payloads that are used combined to from the non primary payloads.  The time offset and
   packet rate information in the form a
   RED data MUST be used to adjust the
   sequence number in the RTP header.  At this point the non primary
   packets can be decrypted with double. payload.

   Note that Flex FEC [I-D.ietf-payload-flexible-fec-scheme] is a
   superset of the FlexFEC may often provide similar or better repair
   capabilities of compared to RED.  For most applications, FlexFEC is a
   better choice than RED. RED; in particular, FlexFEC has modes in which the
   Media Distributor can synthesize recovery packets.

7.3.  Forward Error Correction (FEC)

   When using Flex FEC [I-D.ietf-payload-flexible-fec-scheme] with
   double, repair packets MUST 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 at the same time allows the Media
   Distributor to repair media.  When encrypting a packet that contains
   the Flex FEC data, which is already encrypted, it MUST be encrypted
   with only the outer, HBH hop-by-hop transform.

   The algorithm recommended 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 using the mechanism in [RFC4733], it is end-to-end
   encrypted and the relay can not read it, so it cannot be used to
   control the relay.  Other out of band methods to control the relay
   need to be used instead.

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 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 outer transform as the payload data is
   already encrypted by the inner transform.

   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  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 1-4 octets.
   Packets in repair mode will carry additional repair data, further
   increasing their size.

9.  Security Considerations

   This SRTP transform provides protection against two classes of
   attacker: An network attacker that knows neither the inner nor outer
   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.

   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.

   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.

   The main innovation of this transform relative to other SRTP
   transforms is that it allows a 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 use different,
   independent master keys and master salts. 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.

   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 the MD doesn't modify any header fields,
   then an MD that supports AES-GCM could be unused unmodified.

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:   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:   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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <>.

   [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., and G. Mandyam, "RTP
              Payload Format for Flexible Forward Error Correction
              (FEC)", draft-ietf-payload-flexible-fec-scheme-08 draft-ietf-payload-flexible-fec-scheme-20 (work in
              progress), July 2018. May 2019.

              Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel
              between a Media Distributor and Key Distributor to
              Facilitate Key Exchange", draft-ietf-perc-dtls-tunnel-03 draft-ietf-perc-dtls-tunnel-05
              (work in progress), April 2018. 2019.

              Jones, P., Benham, D., and C. Groves, "A Solution
              Framework for Private Media in Privacy Enhanced RTP
              Conferencing", draft-ietf-perc-private-media-framework-07
              Conferencing (PERC)", draft-ietf-perc-private-media-
              framework-12 (work in progress), September 2018. June 2019.

              Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", draft-ietf-perc-srtp-ekt-diet-08 draft-ietf-perc-srtp-ekt-diet-09 (work in progress),
              October 2018.

              Uberti, J., "WebRTC Forward Error Correction
              Requirements", draft-ietf-rtcweb-fec-08 draft-ietf-rtcweb-fec-09 (work in
              progress), March 2018. July 2019.

   [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,

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 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