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For this RFC, original HTML is available from the RFC-Editor: RFC8723


Internet Engineering Task Force (IETF)                       C. Jennings
Request for Comments: 8723                                      P. Jones
Category: Standards Track                                      R. Barnes
ISSN: 2070-1721                                            Cisco Systems
                                                              A.B. Roach
                                                              April 2020

Double Encryption Procedures for the Secure Real-Time Transport Protocol


   In some conferencing scenarios, it is desirable for an intermediary
   to be able to manipulate some parameters in 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 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.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Terminology
   3.  Cryptographic Context
     3.1.  Key Derivation
   4.  Original Header Block
   5.  RTP Operations
     5.1.  Encrypting a Packet
     5.2.  Relaying a Packet
     5.3.  Decrypting a Packet
   6.  RTCP Operations
   7.  Use with Other RTP Mechanisms
     7.1.  RTP Retransmission (RTX)
     7.2.  Redundant Audio Data (RED)
     7.3.  Forward Error Correction (FEC)
     7.4.  DTMF
   8.  Recommended Inner and Outer Cryptographic Algorithms
   9.  Security Considerations
   10. IANA Considerations
     10.1.  DTLS-SRTP
   11. References
     11.1.  Normative References
     11.2.  Informative References
   Appendix A.  Encryption Overview
   Authors' Addresses

1.  Introduction

   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 an RTP packet to be changed by the MD.
   At the same time, a separate cryptographic key provides integrity and
   optional confidentiality for the media flowing between the MD and the
   endpoints.  The framework document [PRIVATE-MEDIA-FRAMEWORK]
   describes this concept in more detail.

   This specification defines a transform for SRTP that uses 1) the AES
   Galois/Counter Mode (AES-GCM) algorithm [RFC7714] to provide
   encryption and integrity for an RTP packet for the end-to-end
   cryptographic key and 2) a hop-by-hop cryptographic encryption and
   integrity between the endpoint and the MD.  The MD decrypts and
   checks integrity of the hop-by-hop security.  The MD 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 an "Original Header 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 MD
   changed using the information in the Original Header Block before
   decrypting and checking the end-to-end integrity.

   One can think of the double transform as a normal SRTP transform for
   encrypting the RTP in a way 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.

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:

   Media Distributor (MD):  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

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

   hop-by-hop:  The path from the endpoint to or from the MD.

   Original Header Block (OHB):  An octet string that contains the
      original values from the RTP header that might have been changed
      by an MD.

3.  Cryptographic Context

   This specification uses a cryptographic context with two parts:

   *  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

   *  An outer (hop-by-hop) part that is used between endpoints and MDs
      to ensure the integrity of media over a single hop and to enable
      an MD to modify certain RTP header fields.  RTCP is also handled
      using the hop-by-hop cryptographic part.

   The RECOMMENDED cipher for the hop-by-hop and end-to-end algorithms
   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:

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

   *  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.

   *  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.

   *  the synchronization source (SSRC) is the same for both the inner
      and outer algorithms as it cannot be changed.

   *  The sequence number (SEQ) and rollover counter (ROC) are tracked
      independently for the inner and outer algorithms.

   If the MD is to be able to modify header fields but not decrypt the
   payload, then it must have a cryptographic key for the outer
   algorithm but not the inner (end-to-end) algorithm.  This document
   does not define how the MD should be provisioned with this
   information.  One possible way to provide keying material for the
   outer (hop-by-hop) algorithm is to use [DTLS-TUNNEL].

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 pseudorandom 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_(n/2)(inner(k_master),x) ||

   Here "PRF_double_n(k_master, x)" represents the AES_CM PRF Key
   Derivation Function (KDF) (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.
   The term "inner(k_master)" represents the first half of the key;
   "outer(k_master)" represents the second half of the key.

4.  Original Header Block

   The 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 Section 5.  In the decryption process, the
   receiving endpoint uses it to reconstruct the original RTP header so
   that it can pass the proper additional authenticated data (AAD) value
   to the inner transform.

   The OHB can reflect modifications to the following fields in an RTP
   header: the payload type (PT), the SEQ, 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 end-to-end 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|

   *  P: PT is present

   *  Q: SEQ is present

   *  M: Marker bit is present

   *  B: Value of marker bit

   *  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 of a packet.

   The first, "inner" application of AES-GCM encrypts the SRTP payload
   and 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 an MD holding only the outer

   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.

   An MD 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

   An endpoint encrypts a packet by using the inner (end-to-end)
   cryptographic key and then 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 * CSRC count (CC) bytes of the

       *  Payload: The RTP payload of the original packet (including
          padding when present).

   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 (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 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 Encrypted Key Transport (EKT) [EKT-SRTP], the EKTField
   comes after the SRTP packet, exactly like using EKT with any other
   SRTP transform.

5.2.  Relaying a Packet

   The MD 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.

   In order to modify a packet, the MD 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
   outer (hop-by-hop) cryptographic key before transmitting using the
   following steps:

   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 that are allowed to be
       changed, i.e., PT, SEQ, and M.  The MD MAY also make
       modifications to header extensions, without the need to reflect
       these changes in the OHB.

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

       *  If the MD changed a field that is not already in the OHB, then
          it MUST add the original value of the field to the OHB.  Note
          that this might result in an increase in the size of the OHB.

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

       *  Otherwise, the MD 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
   steps 1 and 4 MUST use different, independent master 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.

   An MD that decrypts, modifies, and re-encrypts packets in this way
   MUST use an independent key for each recipient, and MUST NOT re-
   encrypt the packet using the sender's keys.  If the MD 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.

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 using the following steps:

   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 MD,
       the synthetic packet has the original sequence number.  If the
       integrity check does not pass, discard the packet.

   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:

   *  The PT from the outer SRTP packet is used for normal matching to
      Session Description Protocol (SDP) and codec selection.

   *  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

   MDs sometimes interact with RTP media packets sent by endpoints,
   e.g., to provide recovery or receive commands via 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.)

   Repair mechanisms, in general, will need to perform recovery on
   encrypted packets (double-encrypted when using this transform), since
   the MD 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, hop-by-hop key.
   This allows an MD 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.

7.1.  RTP Retransmission (RTX)

   When using RTX [RFC4588] with the double transform, 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 like a packet in repair
   mode (i.e., with only the hop-by-hop key).

   If the MD were to cache the inner, E2E-encrypted payload and
   retransmit it with an RTX original sequence number 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 the double transform, the processing at
   the sender and receiver is the same as when using RED with any other
   SRTP transform.

   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.  An MD cannot synthesize RED packets, because it lacks
   access to the plaintext media payloads that are combined to form a
   RED payload.

   Note that Flexible Forward Error Correction (Flex FEC) may often
   provide similar or better repair capabilities compared to RED.  For
   most applications, Flex FEC is a better choice than RED; in
   particular, Flex FEC has modes in which the MD can synthesize
   recovery packets.

7.3.  Forward Error Correction (FEC)

   When using Flex FEC [RFC8627] with the double transform, 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 MD but, at the
   same time, allows the MD 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, hop-by-hop transform.

   The algorithm recommended in [WEBRTC-FEC] for repair of video is Flex
   FEC [RFC8627].  Note that for interoperability with WebRTC,
   [WEBRTC-FEC] recommends not using additional FEC-only "m=" lines in
   SDP for the repair packets.

7.4.  DTMF

   When DTMF is sent using the mechanism in [RFC4733], it is end-to-end
   encrypted; the relay cannot 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 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; thus, it is 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 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 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.

   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.

9.  Security Considerations

   This SRTP transform provides protection against two classes of
   attacker: a 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.  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.  This property allows the use of an MD that
   supports AES-GCM but does not modify any header fields, without
   requiring any modification to the MD.

10.  IANA Considerations

10.1.  DTLS-SRTP

   IANA has added the following protection profiles to the "DTLS-SRTP
   Protection Profiles" registry defined in [RFC5764].

     | Value  | Profile                                  | Reference |
     | {0x00, | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM | RFC 8723  |
     | 0x09}  |                                          |           |
     | {0x00, | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM | RFC 8723  |
     | 0x0A}  |                                          |           |

       Table 1: Updates to the DTLS-SRTP Protection Profiles Registry

   The SRTP transform parameters for each of these protection profiles

        | DOUBLE_AEAD_AES_128_GCM_AEAD_AES_128_GCM                |
        | 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 |

                   Table 2: SRTP Transform Parameters for

        | DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM                |
        | 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 |

                   Table 3: SRTP Transform Parameters for

   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.  References

11.1.  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, <https://www.rfc-editor.org/info/rfc8174>.

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

11.2.  Informative References

              Jones, P., Ellenbogen, P., and N. Ohlmeier, "DTLS Tunnel
              between a Media Distributor and Key Distributor to
              Facilitate Key Exchange", Work in Progress, Internet-
              Draft, draft-ietf-perc-dtls-tunnel-06, 16 October 2019,

   [EKT-SRTP] Jennings, C., Mattsson, J., McGrew, D., Wing, D., and F.
              Andreasen, "Encrypted Key Transport for DTLS and Secure
              RTP", Work in Progress, Internet-Draft, draft-ietf-perc-
              srtp-ekt-diet-10, 8 July 2019,

              Jones, P., Benham, D., and C. Groves, "A Solution
              Framework for Private Media in Privacy Enhanced RTP
              Conferencing (PERC)", Work in Progress, Internet-Draft,
              draft-ietf-perc-private-media-framework-12, 5 June 2019,

   [RFC2198]  Perkins, C., Kouvelas, I., Hodson, O., Hardman, V.,
              Handley, M., Bolot, J.C., 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,

   [RFC8627]  Zanaty, M., Singh, V., Begen, A., and G. Mandyam, "RTP
              Payload Format for Flexible Forward Error Correction
              (FEC)", RFC 8627, DOI 10.17487/RFC8627, July 2019,

              Uberti, J., "WebRTC Forward Error Correction
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-rtcweb-fec-10, 16 July 2019,

Appendix A.  Encryption Overview

   The following figures show 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         |
       |                           timestamp                           |
       |           synchronization source (SSRC) identifier            |
       |            contributing source (CSRC) identifiers             |
       |                               ....                            |
       |                    RTP extension (OPTIONAL) ...               |
   O I |                          payload ...                          |
   O I |                               +-------------------------------+
   O I |                               | RTP padding   | RTP pad count |
   O +>+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   O | |                    E2E authentication tag                     |
   O | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   O | |                            OHB ...                            |
   +>| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | | |                    HBH authentication tag                     |
   | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | |
   | +- E2E Encrypted 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 -----+


   Thank you to Alex Gouaillard, David Benham, Magnus Westerlund, Nils
   Ohlmeier, Roni Even, and Suhas Nandakumar for reviews and
   improvements to this specification.  In addition, thank you to Sergio
   Garcia Murillo, who proposed the change of transporting the OHB
   information in the RTP payload instead of the RTP header.

Authors' Addresses

   Cullen Jennings
   Cisco Systems

   Email: fluffy@iii.ca

   Paul E. Jones
   Cisco Systems

   Email: paulej@packetizer.com

   Richard Barnes
   Cisco Systems

   Email: rlb@ipv.sx

   Adam Roach

   Email: adam@nostrum.com

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