Network Working Group                                        C. Jennings
Internet-Draft                                                  P. Jones
Intended status: Standards Track                           Cisco Systems
Expires: January 9, May 4, 2017                                            A. Roach
                                                            July 8,
                                                        October 31, 2016

                   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 contexts
   to provide "hop-by-hop" and "end-to-end" security guarantees.  Both
   the end-to-end and hop-by-hop cryptographic transforms can utilize an
   authenticated encryption with associated data scheme or take
   advantage of future SRTP transforms with different properties.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, May 4, 2017.

Copyright Notice

   Copyright (c) 2016 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
   ( 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
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Cryptographic Contexts  . . . . . . . . . . . . . . . . . . .   3
   4.  Original Header Block . . . . . . . . . . . . . . . . . . . .   4
   5.  RTP Operations  . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Encrypting a Packet . . . . . . . . . . . . . . . . . . .   5
     5.2.  Modifying  Relaying a Packet . . . . . . . . . . . . . . . . . . . .   6
     5.3.  Decrypting a Packet . . . . . . . . . . . . . . . . . . .   7
   6.  RTCP Operations . . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Recommended Inner and Outer Cryptographic Transforms  . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  RTP Header Extension  . . . . . . . . . . . . . . . . . .  10
     9.2.  DTLS-SRTP . . . . . . . . . . . . . . . . . . . . . . . .  11  10
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  11
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  12  11
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  12  11
     11.2.  Informative References . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13  12

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 context 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 context provides integrity and optional confidentiality
   for the media flowing between the Media Distributor and the
   endpoints.  See the framework document that describes this concept in
   more detail in more detail in

   This specification RECOMMENDS the SRTP AES-GCM transform [RFC7714] to
   encrypt an RTP packet for the end-to-end cryptographic context.  The
   output of this is treated as an RTP packet and again encrypted with
   an SRTP transform used in the hop-by-hop cryptographic context
   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 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 transform before it is
   sent.  The receiving endpoint decrypts and checks integrity using the
   hop-by-hop cryptographic transform 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.

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  E2E: end-to-end, meaning the link from one endpoint through one or
      more Media Distributors to the endpoint at the other end.

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

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

3.  Cryptographic Contexts

   This specification uses two cryptographic contexts: an inner ("end-
   to-end") context 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") context 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 encrypted using the hop-by-hop cryptographic context.
   The RECOMMENDED cipher for the hop-by-hop and end-to-end contexts 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 contexts are generated with the following

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

   o  Assign the key and salt values generated for the inner (end-to-
      end) transform.

   o  Assign the key and salt values for the outer (hop-by-hop)

   Obviously, if the Media Distributor is to be able to modify header
   fields but not decrypt the payload, then it must have cryptographic
   context for the outer transform, but not the inner transform.  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") transform is to use

4.  Original Header Block

   Any SRTP packet processed following these procedures MAY contain an
   Original Header Block (OHB) RTP header extension.

   The OHB contains the original values of any modified header fields
   and MUST be placed after any already-existing RTP header extensions.
   Placement of the OHB after any original header extensions is
   important so that the receiving endpoint can properly authenticate
   the original packet and any originally included RTP header
   extensions.  The receiving endpoint will authenticate the original
   packet by restoring the modified RTP header field values and header
   extensions.  It does this by copying the original values from the OHB
   and then removing the OHB extension and any other RTP header
   extensions that appear after the OHB extension.

   The Media Distributor is only permitted to modify the extension (X)
   bit, payload type (PT) field, and the RTP sequence number field.

   The OHB extension is either one octet in length, two octets in
   length, or three octets in length.  The length of the OHB indicates
   what data is contained in the extension.

   If the OHB is one octet in length, it contains the original PT field
   value.  In this case, the OHB has this form:

    0 1 2 3 4 5 6 7
   |R|     PT      |
   Note that "R" indicates a reserved bit that MUST be set to zero when
   sending a packet and ignored upon receipt.

   If the OHB is two octets in length, it contains the original RTP
   packet sequence number.  In this case, the OHB has this form:

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   |        Sequence Number        |

   If the OHB is three octets in length, it contains the original PT
   field value and RTP packet sequence number.  In this case, the OHB
   has this form:

    0                   1                   2
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
   |R|     PT      |        Sequence Number        |

   If a Media Distributor modifies an original RTP header value, the
   Media Distributor MUST include the OHB extension to reflect the
   changed value, setting the X bit in the RTP header to 1 if no header
   extensions were originally present.  If another Media Distributor
   along the media path makes additional changes to the RTP header and
   any original value is not already present in the OHB, the Media
   Distributor must extend the OHB by adding the changed value to the
   OHB.  To properly preserve original RTP header values, a Media
   Distributor MUST NOT change a value already present in the OHB

5.  RTP Operations

5.1.  Encrypting a Packet

   To encrypt a packet, the endpoint encrypts the packet using the inner
   cryptographic context and then encrypts using the outer cryptographic
   context.  The processes is as follows:

   o  Form an RTP packet.  If there are any header extensions, they MUST
      use [RFC5285].

   o  Apply the inner cryptographic transform to the RTP packet.  If
      encrypting RTP header extensions end-to-end, then [RFC6904] MUST
      be used when encrypting the RTP packet using the inner
      cryptographic context.

   o  If the endpoint wishes to insert header extensions that can be
      modified by an Media Distributor, it MUST insert an OHB header
      extension at the end of any header extensions protected end-to-end
      (if any), then add any Media Distributor-modifiable header
      extensions.  The OHB MUST replicate the information found in the
      RTP header following the application of the inner cryptographic
      transform.  If not already set, the endpoint MUST set the X bit in
      the RTP header to 1 when introducing the OHB extension.

   o  Apply the outer cryptographic transform 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 context.

5.2.  Modifying  Relaying a Packet

   The Media Distributor does not have a notion of outer or inner
   cryptographic contexts.  Rather, the Media Distributor has a single
   cryptographic context.  The cryptographic transform 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 cryptographic context.

   In order to modify a packet, the Media Distributor decrypts the
   packet, modifies the packet, updates the OHB with any modifications
   not already present in the OHB, and re-encrypts the packet using the
   cryptographic context used for next hop.

   o  Apply the cryptographic transform to the packet.  If decrypting
      RTP header extensions hop-by-hop, then [RFC6904] MUST be used.

   o  Change any required parameters

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

   o  If the Media Distributor resets a parameter to its original value,
      it MAY drop it from the OHB as long as there are no other header
      extensions following the OHB.  Note that this might result in a
      decrease in the size of the OHB.  It is also possible for the
      Media Distributor to remove the OHB entirely if all parameters in
      the RTP header are reset to their original values and no other
      header extensions follow the OHB.  If the OHB is removed and no
      other extension is present, the X bit in the RTP header MUST be
      set to 0.

   o  The Media Distributor MUST NOT delete any header extensions before
      the OHB, but MAY add, delete, or modify any that follow the OHB.

      *  If the Media Distributor adds any header extensions, it must
         append them and it must maintain the order of the original
         header extensions in the [RFC5285] block.

      *  If the Media Distributor appends header extensions, then it
         MUST add the OHB header extension (if not present), even if the
         OHB merely replicates the original header field values, and
         append the new extensions following the OHB.  The OHB serves as
         a demarcation point between original RTP header extensions
         introduced by the endpoint and those introduced by a Media

   o  The Media Distributor MAY modify any header extension appearing
      after the OHB, but MUST NOT modify header extensions that are
      present before the OHB.

   o  Apply the cryptographic transform to the packet.  If the RTP
      Sequence Number has been modified, SRTP processing happens as
      defined in SRTP and which 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 cryptographic context, then uses the OHB to reconstruct the
   original packet, which it decrypts and verifies with the inner
   cryptographic context.

   o  Apply the outer cryptographic transform 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 context.

   o  Form a new synthetic SRTP packet with:

      *  Header = Received header, with header fields replaced with
         values from OHB (if present).

      *  Insert all header extensions up to the OHB extension, but
         exclude the OHB and any header extensions that follow the OHB.
         If there are no extensions remaining, then the X bit MUST bet
         set to 0.  If there are extensions remaining, then the
         remaining extensions MUST be padded to the first 32-bit
         boundary and the overall length of the header extensions
         adjusted accordingly.

      *  Payload is the original encrypted payload. payload from the outer SRTP packet.

   o  Apply the inner cryptographic transform to this synthetic SRTP
      packet.  Note if the RTP Sequence Number was changed by the Media
      Distributor, the syntetic synthetic packet has the original Sequence
      Number.  If the integrity check does not pass, discard the packet.
      If decrypting RTP header extensions end-to-end, then [RFC6904]
      MUST be used when decrypting the RTP packet using the inner
      cryptographic context.

   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.

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

   o  TBD  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 contexts, RTCP is encrypted using only the
   outer (HBH) cryptographic context.  The procedures for RTCP
   encryption are specified in [RFC3711] and this document introduces no
   additional steps.

7.  Recommended Inner and Outer Cryptographic Transforms

   This specification recommends and defines AES-GCM as both the inner
   and outer cryptographic transforms, identified as
   DOUBLE_AEAD_AES_256_GCM_AEAD_AES_256_GCM.  These transforms provide
   for authenticated encryption and will consume additional processing
   time double-encrypting for HBH and E2E.  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 transform)_(outer transform).

   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 transforms 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 transform.  Similarly, if a new
   transform was defined that provided only integrity, that would also
   be reasonable to use for the HBH as the payload data is already
   encrypted by the E2E.

   The AES-GCM cryptographic transform introduces an additional 16
   octets to the length of the packet.  When using AES-GCM for both the
   inner and outer cryptographic transforms, 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.

   Open Issue: For an audio confernce using opus in a narrowband
   configuration at TBD kbps with 20 ms packetizaton, the total
   bandwidth of the RTP would change from TBD to TBD.  Do we want to
   consider having some AES-GCM transfroms with reduced length
   authentication tags for the HBH.  Since the actual authentication is
   provided by the E2E check, and tampering with the the HBH can only
   result in the wrong packet being selected as the loudest speaker, it
   might be desirable to have 64 bits or even less of securiyt for the
   HBH portion of the authentication.

8.  Security Considerations

   To summarize what is encrypted and authenticated, we will refer to
   all the RTP fields and headers created by the sender and before the
   pay load as the initial envelope and the RTP payload information with
   the media as the payload.  Any additional headers added by the Media
   Distributor are referred to as the extra envelope.  The sender uses
   the E2E key to encrypts the payload and authenticate the payload +
   initial envelope which using an AEAD cipher results in a slight
   longer new payload.  Then the sender uses the HBH key to encrypt the
   new payload and authenticate the initial envelope and new payload.

   The Media Distributor has the HBH key so it can check the
   authentication of the received packet across the initial envelope and
   payload data but it can't decrypt the payload as it does not have the
   E2E key.  It can add extra envelope information.  It then
   authenticates the initial plus extra envelope information plus
   payload with a HBH key.  This HBH for the outgoing packet is
   typically different than the HBH key for the incoming packet.

   The receiver can check the authentication of the initial and extra
   envelope information.  This, along with the OBH, i is used to construct
   a synthetic packet that is should be identital identical 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 original sender sent, as well as exactly which
   modifications were made by the Media Distributor.

   It is obviously critical that the intermediary have has only the outer
   transform parameters and not the inner transform parameters.  We rely
   on an external key management protocol to assure this property.

   Modifications by the intermediary result 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 and outer key holders are
   the same as the security properties of classic SRTP.

9.  IANA Considerations

9.1.  RTP Header Extension

   This document defines a new extension URI in the RTP Compact Header
   Extensions part of the Real-Time Transport Protocol (RTP) Parameters
   registry, according to the following data:

   Extension URI: urn:ietf:params:rtp-hdrext:ohb

   Description: Original Header Block

   Contact: Cullen Jennings <>

   Reference: RFCXXXX

   Note to RFC Editor: Replace RFCXXXX with the RFC number of this


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

     | Value | Profile                                  | Reference |

   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
       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
       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 (E2E)
   transform and the second half is used for the outer (HBH) transform.

10.  Acknowledgments

   Many thanks to review from Suhas Nandakumar, David Benham, Magnus
   Westerlund and significant text from Richard Barnes.

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, 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,

   [RFC5285]  Singer, D. and H. Desineni, "A General Mechanism for RTP
              Header Extensions", RFC 5285, DOI 10.17487/RFC5285, July
              2008, <>.

   [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,

   [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,

11.2.  Informative References

              Jones, P., "DTLS "A DTLS Tunnel between Media Distribution Device Distributor and
              Key Management Function Distributor to Facilitate Key Exchange",
              draft-jones-perc-dtls-tunnel-02 draft-jones-
              perc-dtls-tunnel-03 (work in progress), March July 2016.

              Jones, P. and D. Benham, "A Solution Framework for Private
              Media in Privacy Enhanced RTP Conferencing", draft-jones-
              perc-private-media-framework-02 (work in progress), March

   [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,

Authors' Addresses

   Cullen Jennings
   Cisco Systems

   Paul E. Jones
   Cisco Systems


   Adam Roach