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Versions: (draft-mcgrew-srtp-ekt) 00 01 02 03 draft-ietf-avtcore-srtp-ekt

AVT Working Group                                              D. McGrew
Internet-Draft                                              F. Andreasen
Intended status: Standards Track                                 D. Wing
Expires: May 3, 2012                                               Cisco
                                                              K. Fischer
                                       Siemens Enterprise Communications
                                                        October 31, 2011


                 Encrypted Key Transport for Secure RTP
                       draft-ietf-avt-srtp-ekt-03

Abstract

   SRTP Encrypted Key Transport (EKT) is an extension to SRTP that
   provides for the secure transport of SRTP master keys, Rollover
   Counters, and other information, within SRTP or SRTCP.  This facility
   enables SRTP to work for decentralized conferences with minimal
   control, and to handle situations caused by early media.

   This note defines EKT, and also describes how to use it with SDP
   Security Descriptions, DTLS-SRTP, and MIKEY.  These other key
   management protocols provide an EKT key to everyone in a session, and
   EKT coordinates the keys within the session.

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 http://datatracker.ietf.org/drafts/current/.

   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 May 3, 2012.

Copyright Notice

   Copyright (c) 2011 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



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   Provisions Relating to IETF Documents
   (http://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 . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Conventions Used In This Document  . . . . . . . . . . . .  5
   2.  Encrypted Key Transport  . . . . . . . . . . . . . . . . . . .  6
     2.1.  Authentication Tag Formats . . . . . . . . . . . . . . . .  6
     2.2.  Packet Processing and State Machine  . . . . . . . . . . .  8
       2.2.1.  Outbound (Tag Generation)  . . . . . . . . . . . . . .  9
       2.2.2.  Inbound (Tag Verification) . . . . . . . . . . . . . . 11
     2.3.  Ciphers  . . . . . . . . . . . . . . . . . . . . . . . . . 14
       2.3.1.  The Default Cipher . . . . . . . . . . . . . . . . . . 15
       2.3.2.  AES ECB  . . . . . . . . . . . . . . . . . . . . . . . 15
       2.3.3.  Other EKT Ciphers  . . . . . . . . . . . . . . . . . . 16
     2.4.  Synchronizing Operation  . . . . . . . . . . . . . . . . . 16
     2.5.  Transport  . . . . . . . . . . . . . . . . . . . . . . . . 17
     2.6.  Timing and Reliability Consideration . . . . . . . . . . . 17
   3.  Use of EKT with SDP Security Descriptions  . . . . . . . . . . 19
     3.1.  SDP Security Descriptions Recap  . . . . . . . . . . . . . 19
     3.2.  Relationship between EKT and SDP Security Descriptions . . 20
     3.3.  Overview of Combined EKT and SDP Security Description
           Operation  . . . . . . . . . . . . . . . . . . . . . . . . 22
     3.4.  EKT Extensions to SDP Security Descriptions  . . . . . . . 22
       3.4.1.  EKT_Cipher . . . . . . . . . . . . . . . . . . . . . . 22
       3.4.2.  EKT_Key  . . . . . . . . . . . . . . . . . . . . . . . 23
       3.4.3.  EKT_SPI  . . . . . . . . . . . . . . . . . . . . . . . 23
     3.5.  Offer/Answer Procedures  . . . . . . . . . . . . . . . . . 23
       3.5.1.  Generating the Initial Offer - Unicast Streams . . . . 24
       3.5.2.  Generating the Initial Answer - Unicast Streams  . . . 25
       3.5.3.  Processing of the Initial Answer - Unicast Streams . . 26
     3.6.  SRTP-Specific Use Outside Offer/Answer . . . . . . . . . . 27
     3.7.  Modifying the Session  . . . . . . . . . . . . . . . . . . 27
     3.8.  Backwards Compatibility Considerations . . . . . . . . . . 28
     3.9.  Grammar  . . . . . . . . . . . . . . . . . . . . . . . . . 28
   4.  Use of EKT with DTLS-SRTP Key Transport  . . . . . . . . . . . 30
     4.1.  EKT Extensions to DTLS-SRTP  . . . . . . . . . . . . . . . 30
       4.1.1.  Scaling to Large Groups  . . . . . . . . . . . . . . . 32
     4.2.  Offer/Answer Considerations  . . . . . . . . . . . . . . . 33
       4.2.1.  Generating the Initial Offer . . . . . . . . . . . . . 33



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       4.2.2.  Generating the Initial Answer  . . . . . . . . . . . . 34
       4.2.3.  Processing the Initial Answer  . . . . . . . . . . . . 34
       4.2.4.  Modifying the Session  . . . . . . . . . . . . . . . . 35
   5.  Use of EKT with MIKEY  . . . . . . . . . . . . . . . . . . . . 36
     5.1.  EKT extensions to MIKEY  . . . . . . . . . . . . . . . . . 37
     5.2.  Offer/Answer considerations  . . . . . . . . . . . . . . . 39
       5.2.1.  Generating the Initial Offer . . . . . . . . . . . . . 39
       5.2.2.  Generating the Initial Answer  . . . . . . . . . . . . 40
       5.2.3.  Processing the Initial Answer  . . . . . . . . . . . . 40
       5.2.4.  Modifying the Session  . . . . . . . . . . . . . . . . 40
   6.  Design Rationale . . . . . . . . . . . . . . . . . . . . . . . 42
     6.1.  Alternatives . . . . . . . . . . . . . . . . . . . . . . . 43
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 45
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 46
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 47
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 48
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 48
     10.2. Informative References . . . . . . . . . . . . . . . . . . 49
   Appendix A.  Using EKT to Optimize Interworking DTLS-SRTP with
                Security Descriptions . . . . . . . . . . . . . . . . 50
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 53






























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1.  Introduction

   RTP is designed to allow decentralized groups with minimal control to
   establish sessions, such as for multimedia conferences.
   Unfortunately, Secure RTP (SRTP [RFC3711]) cannot be used in many
   minimal-control scenarios, because it requires that SSRC values and
   other data be coordinated among all of the participants in a session.
   For example, if a participant joins a session that is already in
   progress, the SRTP rollover counter (ROC) of each SRTP source in the
   session needs to be provided to that participant.

   The inability of SRTP to work in the absence of central control was
   well understood during the design of that protocol; that omission was
   considered less important than optimizations such as bandwidth
   conservation.  Additionally, in many situations SRTP is used in
   conjunction with a signaling system that can provide most of the
   central control needed by SRTP.  However, there are several cases in
   which conventional signaling systems cannot easily provide all of the
   coordination required.  It is also desirable to eliminate the layer
   violations that occur when signaling systems coordinate certain SRTP
   parameters, such as SSRC values and ROCs.

   This document defines Encrypted Key Transport (EKT) for SRTP, an
   extension to SRTP that fits within the SRTP framework and reduces the
   amount of signaling control that is needed in an SRTP session.  EKT
   securely distributes the SRTP master key and other information for
   each SRTP source, using SRTCP or SRTP to transport that information.
   With this method, SRTP entities are free to choose SSRC values as
   they see fit, and to start up new SRTP sources with new SRTP master
   keys (see Section 2.2) within a session without coordinating with
   other entities via signaling or other external means.  This fact
   allows to reinstate the RTP collision detection and repair mechanism,
   which is nullified by the current SRTP specification because of the
   need to control SSRC values closely.  An SRTP endpoint using EKT can
   generate new keys whenever an existing SRTP master key has been
   overused, or start up a new SRTP source to replace an old SRTP source
   that has reached the packet-count limit.  EKT also solves the problem
   in which the burst loss of the N initial SRTP packets can confuse an
   SRTP receiver, when the initial RTP sequence number is greater than
   or equal to 2^16 - N. These features simplify many architectures that
   implement SRTP.

   EKT provides a way for an SRTP session participant, either sender or
   receiver, to securely transport its SRTP master key and current SRTP
   rollover counter to the other participants in the session.  This
   data, possibly in conjunction with additional data provided by an
   external signaling protocol, furnishes the information needed by the
   receiver to instantiate an SRTP/SRTCP receiver context.



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   EKT does not control the manner in which the SSRC and master key are
   generated; it is concerned only with their secure transport.  Those
   values may be generated on demand by the SRTP endpoint, or may be
   dictated by an external mechanism such as a signaling agent or a
   secure group controller.

   EKT is not intended to replace external key establishment mechanisms
   such as SDP Security Descriptions [RFC4568], DTLS-SRTP [RFC5764], or
   MIKEY [RFC3830][RFC4563].  Instead, it is used in conjunction with
   those methods, and it relieves them of the burden of tightly
   coordinating every SRTP source among every SRTP participant.

   This document is organized as follows.  A complete normative
   definition of EKT is provided in Section 2.  It consists of packet
   processing algorithms (Section 2.2) and cryptographic definitions
   (Section 2.3) .  In Section 3, the use of EKT with SDP Security
   Descriptions is defined, and in Section 4 its use with DTLS-SRTP is
   defined.  In Section 5 we outline the use of EKT with MIKEY.
   Section 6 provides a design rationale.  Security Considerations are
   provided in Section 7, and IANA considerations are provided in
   Section 8.

1.1.  Conventions Used In This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].
























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2.  Encrypted Key Transport

   In EKT, an SRTP master key is encrypted with a Key Encrypting Key
   (KEK), and the resulting ciphertext is transported (using the EKT
   Base Authentication Tag) in selected SRTCP or in selected SRTP
   packets.  A single KEK suffices for a single SRTP session, regardless
   of the number of participants in the session.  However, there can be
   multiple KEKs used within a particular session.  We use terms "KEK"
   or "EKT key" to mean the same thing; the latter term is used when
   describing the relation of EKT to external key management.

   In order to convey the ciphertext of the SRTP master key, and other
   additional information, the Authentication Tag field is subdivided as
   defined in Section 2.1.  EKT defines new SRTP and SRTCP
   authentication functions, which use this format.  It incorporates a
   conventional authentication function, which is called the base
   authentication function in this specification.  Any authentication
   function, such as the default one of HMAC-SHA1 with a 160-bit key and
   an 80-bit authentication tag, can be used as a base authentication
   function.  EKT also defines a new method of providing SRTP master
   keys to an endpoint.

2.1.  Authentication Tag Formats

   The EKT Authentication Tag uses one of the two formats defined below.
   These two formats can always be unambiguously distinguished on
   receipt by examining the final bit of the EKT Authentication Tag,
   which is also the final bit of the SRTP packet.  The first format is
   the Full EKT Authentication Tag (Figure 1), and the second is the
   Abbreviated EKT Authentication Tag (Figure 2).

   The following figure shows the packet layout for the Full EKT
   Authentication Tag:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                   Base Authentication Tag                     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                     Encrypted Master Key                      :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                       Rollover Counter                        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |    Initial Sequence Number    |   Security Parameter Index  |1|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 1: Full EKT Authentication Tag format




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   The Full EKT Authentication Tag field contains the following sub-
   fields:

   Base Authentication Tag:  This field contains the authentication tag
      computed by the base authentication function.  The value of this
      field is used to check the authenticity of the packet.

   Encrypted Master Key:  The length of this field is variable, and is
      equal to the ciphertext size N defined in Section 2.3.  Note that
      the length of the field is inferable from the SPI field, since the
      particular EKT cipher used by the sender of a packet is inferable
      from that field.  The Encrypted Master Key field is included
      outside of the authenticated portion of the SRTCP packet,
      immediately following the Authentication Tag. It contains the
      ciphertext value resulting from the encryption of the SRTP master
      key corresponding to the SSRC contained in the packet.  The
      encryption and decryption of this value is done using a cipher as
      defined in Section 2.3.

   Rollover Counter:  The length of this field is fixed at 32 bits.
      This field is set to the current value of the SRTP rollover
      counter in the SRTP context associated with the SSRC in the SRTCP
      packet.  This field immediately follows the Encrypted Master Key
      field.

   Initial Sequence Number (ISN):  The length of this field is fixed at
      16 bits.  If this field is nonzero, then it indicates the RTP
      sequence number of the initial RTP packet that is protected using
      the SRTP master key conveyed (in encrypted form) by the Encrypted
      Master Key field of this packet.  If this field is zero, it
      indicates that the initial RTP packet protected using the SRTP
      master key conveyed in this packet preceded, or was concurrent
      with, the last roll-over of the RTP sequence number.

   Security Parameter Index (SPI):  The length of this field is fixed at
      15 bits.  This field indicates the appropriate Key Encrypting Key
      and other parameters for the receiver to use when processing the
      packet.  It is an "index" into a table of possibilities (which are
      established via signaling or some other out-of-band means), much
      like the IPsec Security Parameter Index [RFC4301].  The parameters
      that are identified by this field are:

      *  The Key Encrypting Key used to process the packet.

      *  The EKT cipher used to process the packet.

      *  The Secure RTP parameters associated with the SRTP Master Key
         carried by the packet and the SSRC value in the packet.



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         Section 8.2. of [RFC3711] summarizes the parameters defined by
         that specification.

      *  The Master Salt associated with the Master Key. (This value is
         part of the parameters mentioned above, but we call it out for
         emphasis.)  The Master Salt is communicated separately, via
         signaling, typically along with the EKT Key Encrypting Key.

      Together, these elements are called an EKT parameter set.  Within
      each SRTP session, each distinct EKT parameter set that may be
      used MUST be associated with a distinct SPI value, to avoid
      ambiguity.  The SPI field follows the Initial Sequence Number.
      Since it appears at the end of the packet, and has a fixed length,
      it is always possible to unambiguously parse this field.

   Final bit:  This MUST be 1.  This flag distinguishes the packet
      layout between Figure 2 or Figure 1.

   The following figure shows the packet layout of the Abbreviated EKT
   Authentication Tag:

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                   Base Authentication Tag                     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Reserved   |0|
     +-+-+-+-+-+-+-+-+

            Figure 2: Abbreviated EKT Authentication Tag format

   The Abbreviated EKT Authentication Tag field contains the following
   sub-fields:

   Base Authentication Tag:  same as described above.

   Reserved:  7 bits.  MUST be 0 on transmission and MUST be ignored on
      reception.

   Final Bit:  This MUST be 0.  This flag distinguishes the packet
      layout betweenFigure 1 or Figure 2.s

2.2.  Packet Processing and State Machine

   At any given time, each SRTP/SRTCP source has associated with it a
   single EKT parameter set.  This parameter set is used to process all
   outbound packets, and is called the outbound parameter set.  There
   may be other EKT parameter sets that are used by other SRTP/SRTCP



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   sources in the same session.  All of these EKT parameter sets SHOULD
   be stored by all of the participants in an SRTP session, for use in
   processing inbound SRTCP traffic.

   We next review SRTP authentication and show how the EKT
   authentication method is built on top of a base authentication
   method.  An SRTP or SRTCP authentication method consists of a tag-
   generation function and a verification function.  The tag-generation
   function takes as input a secret key, the data to be authenticated,
   and the packet index.  It provides an authentication tag as its sole
   output, and is used in the processing of outbound packets.  The
   verification function takes as input a secret key, the data to be
   authenticated, the packet index, and the authentication tag.  It
   returns an indication of whether or not the data, index, and tag are
   valid or not.  It is used in the processing of inbound packets.  EKT
   defines a tag-generation function in terms of the base tag-generation
   function, and defines a verification function in terms of the base
   verification function.  The tag-generation function is used to
   process outbound packets, and the verification function is used to
   process inbound packets.

2.2.1.  Outbound (Tag Generation)

   When an SRTP or SRTCP packet needs to be sent, the EKT tag generation
   function works as follows.  The Rollover Counter field in the packet
   is set to the current value of the SRTP rollover counter (represented
   as an unsigned integer in network byte order).

   The Initial Sequence Number field is set to zero, if the initial RTP
   packet protected using the current SRTP master key for this source
   preceded, or was concurrent with, the last roll-over of the RTP
   sequence number.  Otherwise, that field is set to the value of the
   RTP sequence number of the initial RTP packet that was or will be
   protected by that key.  When the SRTP master key corresponding to a
   source is changed, the new key SHOULD be communicated in advance via
   EKT.  (Note that the ISN field allows the receiver to know when it
   should start using the new key to process SRTP packets.)  This
   enables the rekeying event to be communicated before any RTP packets
   are protected with the new key.  The rekeying event MUST NOT change
   the value of ROC (otherwise, the current value of the ROC would not
   be known to late joiners of existing sessions).

   The Security Parameter Index field is set to the value of the
   Security Parameter Index that is associated with the outbound
   parameter set.

   The Encrypted Master Key field is set to the ciphertext created by
   encrypting the SRTP master key with the EKT cipher, using the KEK as



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   the encryption key.  The encryption process is detailed in
   Section 2.3.  Implementations MAY cache the value of this field to
   avoid recomputing it for each packet that is sent.

2.2.1.1.  Computing the Base Authentication Tag

   If using the Base Authentication Tag format, the field is computed
   using the base tag-generation function as follows.  It can only be
   computed after all of the other fields have been set.  The
   authenticated input consists of the following elements, in order:

   1.  the SRTP or SRTCP authenticated portion,

   2.  a string of zero bits whose length exactly matches that of the
       Base Authentication Tag field,

   3.  the Encrypted Master Key field,

   4.  the Rollover Counter field,

   5.  the Initial Sequence Number field, and

   6.  the Security Parameter Index field.

      Implementation note: the string of zero bits is included in the
      authenticated input in order to allow implementations to compute
      the base authentication tag using a single pass of the base
      authentication function.  Implementations MAY write zeros into the
      Base Authentication Tag field prior to computing that function, on
      the sending side.

2.2.1.2.  Computing the Abbreviated Authentication Tag

   If using the Abbreviated Authentication Tag format, the field is
   computed using the base tag-generation function as follows.  It can
   only be computed after all of the other fields have been set.  The
   authenticated input consists of the following elements, in order:

   1.  the SRTP or SRTCP authenticated portion,

   2.  a string of zero bits whose length exactly matches that of the
       Base Authentication Tag field.  Then for SRTP only, place the ROC
       (in network order) into the first 4 bytes of the "base
       authentication tag" field.

   3.  set reserved bits and final bit to zeros.





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2.2.2.  Inbound (Tag Verification)

   The EKT verification function proceeds as follows (see Figure 3), or
   uses an equivalent set of steps.  Recall that the verification
   function is a component of SRTP and SRTCP processing.  When a packet
   does not pass the verification step, the function indicates this fact
   to the SRTCP packet processing function when it returns control to
   that function.

   1.  The Security Parameter Index field is checked to determine which
       EKT parameter set should be used when processing the packet.  If
       multiple parameter sets been defined for the SRTP session, then
       the one that is associated with the Security Parameter Index
       value that matches the Security Parameter Index field in the
       packet is used.  This parameter set is called the matching
       parameter set below.  If there is no matching SPI, then the
       verification function MUST return an indication of authentication
       failure, and the steps described below are not performed.

   2.  If there is already an SRTP crypto context associated with the
       SSRC in the packet, and replay protection is in use, then the
       receiver performs the replay check described in Section 3.3.2 of
       [RFC3711].  If the EKT fields are conveyed in an RTCP packet,
       then the packet index used in that check is formed from the
       Rollover Counter and the Initial Sequence Number fields in that
       packet.  If the EKT fields are conveyed in an SRTP packet, then
       the packet index used in that check is formed from the EKT
       Rollover Counter field and the RTP Sequence Number in that
       packet.

   3.  The Encrypted Master Key field is decrypted using the EKT
       cipher's decryption function.  That field is used as the
       ciphertext input, and the Key Encrypting Key in the matching
       parameter set is used as the decryption key.  The decryption
       process is detailed in Section 2.3.  The plaintext resulting from
       this decryption is provisionally accepted as the SRTP master key
       corresponding to the SSRC in the packet.  If an SRTP master key
       identifier (MKI) is present in the packet, then the provisional
       key corresponds to the particular SSRC and MKI combination.  A
       provisional key MUST be used only to process one single packet.
       A provisional SRTP or SRTCP authentication key is generated from
       the provisional master key, and the SRTP master salt from the
       matching parameter set, using the SRTP key derivation algorithm
       (see Section 4.3 of [RFC3711]).

   4.  An authentication check is performed on the packet, using the
       provisional SRTP or SRTCP authentication key.  This key is
       provided to the base authentication function (see Figure 3),



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       which is evaluated as described in Section 2.2.1.1.  If the Base
       Authentication Tag field matches the tag computed by the base
       authentication function, then the packet passes the check.

          Implementation note: a receiver MAY copy the Base
          Authentication Tag field into temporary storage, then write
          zeros into that field, prior to computing the base
          authentication tag value.  This step allows the base
          authentication function to be computed in a single pass over
          the data in the packet.

   5.  If the base authentication check using the provisional key fails,
       then the provisional key MUST be discarded and it MUST NOT affect
       any subsequent processing.  The verification function MUST return
       an indication of authentication failure, and the steps described
       below are not performed.

   6.  Otherwise, if the base authentication check is passed, the
       provisional key is also accepted as the SRTP master key
       corresponding to the SRTP source that sent the packet.  If an MKI
       is present in the packet, then the master key corresponds to the
       particular SSRC and MKI combination.  If there is no SRTP crypto
       context corresponding to the SSRC in the packet, then a new
       crypto context is created.  The rollover counter in the context
       is set to the value of the Rollover Counter field.  If the crypto
       context is not new, then the rollover counter in the context MUST
       NOT be set to a value lower than its current value.  (If the
       replay protection step described above is performed, it ensures
       that this requirement is satisfied.)

   7.  If the Initial Sequence Number field is nonzero, then the initial
       sequence number for the SRTP master key is set to the packet
       index created by appending that field to the current rollover
       counter and treating the result as a 48-bit unsigned integer.
       The initial sequence number for the master key is equivalent to
       the "From" value of the <From, To> pair of indices (Section 8.1.1
       of [RFC3711]) that can be associated with a master key.

   8.  The newly accepted SRTP master key, the SRTP parameters from the
       matching parameter set, the SSRC from the packet, and the MKI
       from the packet, if one is present, are stored in the crypto
       context associated with the SRTP source.  The SRTP Key Derivation
       algorithm is run in order to compute the SRTP encryption and
       authentication keys, and those keys are stored for use in SRTP
       processing of inbound packets.  The Key Derivation algorithm
       takes as input the newly accepted SRTP master key, along with the
       Master Salt from the matching parameter set.




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          Implementation note: the receiver may want to retain old
          master keys for some brief period of time, so that out of
          order packets can be processed.

   9.  The verification function then returns an indication that the
       packet passed the verification.

          Implementation note: the value of the Encrypted Master Key
          field is identical in successive packets protected by the same
          KEK and SRTP master key.  This value MAY be cached by an SRTP
          receiver to minimize computational effort, by allowing it to
          recognize when the SRTP master key is unchanged, and thus
          avoid repeating Steps 2, 6, and 7.






































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                 +------- Encrypted Master Key
                 |
                 v
           +------------+
           | Decryption |
           |  Function  |<-------------------------- Key Encrypting Key
           +------------+
                 |                    +----------------+     EKT
        +--------+-- provisional ---->|     SRTCP      |<--  master
        |            master key       | Key Derivation |     salt
        |                             +----------------+
        |                                     |
        |                    provisional SRTCP authentication key
        |                                     |
        |                                     v
        |                             +----------------+
        |   authenticated portion --> |   Base SRTCP   |
        |   authentication tag -----> |  Verification  |
        |                             +----------------+
        |                                     |
        |        +----------------+         +---+
        |        |  return FAIL   |<- FAIL -| ? |
        |        +----------------+         +---+
        |                                     |
        |        +----------------+           |
        +------->| set master key,|<- PASS ---+
                 | ROC, and MKI   |
                 +----------------+
                         |
                         v
                 +----------------+
                 |  return PASS   |
                 +----------------+

                     Figure 3: EKT inbound processing.

2.3.  Ciphers

   EKT uses a cipher to encrypt the SRTP master keys.  We first specify
   the interface to the cipher, in order to abstract the interface away
   from the details of that function.  We then define the cipher that is
   used in EKT by default.  This cipher MUST be implemented, but another
   cipher that conforms to this interface MAY be used, in which case its
   use MUST be coordinated by external means (e.g., call signaling).

   An EKT cipher consists of an encryption function and a decryption
   function.  The encryption function E(K, P) takes the following
   inputs:



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   o  a secret key K with a length of L bytes, and

   o  a plaintext value P with a length of M bytes.

   The encryption function returns a ciphertext value C whose length is
   N bytes, where N is at least M. The decryption function D(K, C) takes
   the following inputs:

   o  a secret key K with a length of L bytes, and

   o  a ciphertext value C with a length of N bytes.

   The decryption function returns a plaintext value P that is M bytes
   long.  These functions have the property that D(K, E(K, P)) = P for
   all values of K and P. Each cipher also has a limit T on the number
   of times that it can be used with any fixed key value.  For each key,
   the encryption function MUST NOT be invoked on more than T distinct
   values of P, and the decryption function MUST NOT be invoked on more
   than T distinct values of C.

   An EKT cipher MUST resist attacks in which both ciphertexts and
   plaintexts can be adaptively chosen.  For each randomly chosen key,
   the encryption and decryption functions cannot be distinguished from
   a random permutation and its inverse with non-negligible advantage.
   This must be true even for adversaries that can query both the
   encryption and decryption functions adaptively.  The advantage is
   defined as the difference between the probability that the adversary
   will identify the cipher as such and the probability that the
   adversary will identify the random permutation as the cipher, when
   each case is equally likely.

2.3.1.  The Default Cipher

   The default EKT Cipher is the AES Key Wrap [RFC3394] algorithm, which
   can be used with plaintexts larger than 16 bytes in length, and is
   thus suitable for keys of any size.  It requires a plaintext length M
   that is a multiple of eight bytes, and it returns a ciphertext with a
   length of N = M + 8 bytes.  It can be used with key sizes of L = 16,
   24, and 32, and its use with those key sizes is indicated as
   AESKW_128, AESKW_192, and AESKW_256, respectively.  The key size
   determines the length of the AES key used by the Key Wrap algorithm.
   With this cipher, T=2^48.

2.3.2.  AES ECB

   The simplest EKT cipher is the Advanced Encryption Standard (AES)
   [FIPS197] with 128-bit keys, in Electronic Codebook (ECB) Mode.  Its
   use is indicated as AES_ECB, and its parameters are fixed at L=16,



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   M=16, and T=2^48.  Note that M matches the size of the SRTP master
   keys used by the default SRTP key derivation function; if an SRTP
   cipher with a different SRTP master key length is to be used with
   EKT, then another EKT cipher must be used.  ECB is the simplest mode
   of operation of a block cipher, in which the block cipher is used in
   its raw form.

2.3.3.  Other EKT Ciphers

   Other specification MAY extend this one by defining other EKT ciphers
   per Section 8.  This section defines how those ciphers interact with
   this specification.

   An EKT cipher determines how the Encrypted Master Key field is
   written, and how it is processed when it is read.  This field is
   opaque to the other aspects of EKT processing.  EKT ciphers are free
   to use this field in any way, but they SHOULD NOT use other EKT or
   SRTP fields as an input.  The values of the parameters L, M, N, and T
   MUST be defined by each EKT cipher, and those values MUST be
   inferable from the EKT parameter set.

2.4.  Synchronizing Operation

   A participant in a session MAY opt to use a particular EKT key to
   protect outbound packets after it accepts that EKT key for protecting
   inbound traffic.  In this case, the fact that one participant has
   changed to using a new EKT key for outbound traffic can trigger other
   participants to switch to using the same key.

   An SRTP/SRTCP source SHOULD change its SRTP master key after its EKT
   key has been changed.  This will ensure that the set of participants
   able to decrypt the traffic will be limited to those who know the
   current EKT key.

   EKT can be transported over SRTCP, but some of the information that
   it conveys is used for SRTP processing; some elements of the EKT
   parameter set apply to both SRTP and SRTCP.  Furthermore, SRTCP
   packets can be lost and both SRTP and SRTCP packets may be delivered
   out of order.  This can lead to various race conditions, which we
   review below.

   When joining an SRTP session, SRTP packets may be received before any
   EKT over SRTCP packets, which implies the crypto context has not been
   established, unless other external signaling mechanism has done so.
   Rather than automatically discarding such SRTP packets, the receiver
   MAY want to provisionally place them in a jitter buffer and delay
   discarding them until playout time.




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   When an SRTP source using EKT over SRTCP performs a rekeying
   operation, there is a race between the actual rekeying signaled via
   SRTCP and the SRTP packets secured by the new keying material.  If
   the SRTP packets are received first, they will fail authentication;
   alternatively, if authentication is not being used, they will decrypt
   to unintelligible random-looking plaintext.  (Note, however, that
   [RFC3711] says that SRTP "SHOULD NOT be used without message
   authentication".)  In order to address this problem, the rekeying
   event can be sent before packets using the new SRTP master key are
   sent (by use of the ISN field).  Another solution involves using an
   MKI at the expense of added overhead in each SRTP packet.
   Alternatively, receivers MAY want to delay discarding packets from
   known SSRCs that fail authentication in anticipation of receiving a
   rekeying event via EKT (SRTCP) shortly.

   The ROC signaled via EKT over SRTCP may be off by one when it is
   received by the other party(ies) in the session.  In order to deal
   with this, receivers should simply follow the SRTP packet index
   estimation procedures defined in Section 3.3.1 [RFC3711].

2.5.  Transport

   EKT MUST be used over SRTCP, whenever RTCP is in use.  EKT MAY be
   used over SRTP.  When EKT over SRTP is used in an SRTP session in
   which SRTCP is available, then EKT MUST be used for both SRTP and
   SRTCP.

   The packet processing, state machine, and Authentication Tag format
   for EKT over SRTP are nearly identical to that for EKT over SRTCP.
   Differences are highlighted in Section 2.2.1 and Section 2.2.2.

2.6.  Timing and Reliability Consideration

   SRTCP communicates the master key and ROC for the SRTP session.
   Thus, as explained above, if SRTP packets are received prior to the
   corresponding SRTCP (EKT) packet, a race condition occurs.  From an
   EKT point of view, it is therefore desirable for an SRTP sender to
   send an EKT packet containing the Base Authentication Tag as soon as
   possible, and in no case any later than when the initial SRTP packet
   is sent.  It is RECOMMENDED that the Base Authentication Tag be
   transmitted 3 times (to accomodate packet loss) and to provide a
   reliable indication to the receiver that the sender is now using the
   EKT key.  If the Base Authentication Tag sent in SRTCP, the SRTCP
   timing rules associated with the profile under which it runs (e.g.,
   RTP/SAVP or RTP/SAVPF) MUST be obeyed.  Subject to that constraint,
   SRTP senders using EKT over SRTCP SHOULD send an SRTCP packet as soon
   as possible after joining a session.  Note that there is no need for
   SRTP receivers to do so.  Also note, that per RFC 3550, Section 6.2,



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   it is permissible to send a compound RTCP packet immediately after
   joining a unicast session (but not a multicast session).

   SRTCP is not reliable and hence SRTCP packets may be lost.  This is
   obviously a problem for endpoints joining an SRTP session and
   receiving SRTP traffic (as opposed to SRTCP), or for endpoints
   receiving SRTP traffic following a rekeying event.  To reduce the
   impact of lost packets, SRTP senders using EKT over SRTCP SHOULD send
   SRTCP packets as often as allowed by the profile under which they
   operate.









































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3.  Use of EKT with SDP Security Descriptions

   The SDP Security Descriptions (SDESC) [RFC4568] specification defines
   a generic framework for negotiating security parameters for media
   streams negotiated via the Session Description Protocol by use of a
   new SDP "crypto" attribute and the Offer/Answer procedures defined in
   [RFC3264].  In addition to the general framework, SDES also defines
   how to use that framework specifically to negotiate security
   parameters for Secure RTP.  Below, we first provide a brief recap of
   the crypto attribute when used for SRTP and we then explain how it is
   complementary to EKT.  In the rest of this Section, we provide
   extensions to the crypto attribute and associated offer/answer
   procedures to define its use with EKT.

3.1.  SDP Security Descriptions Recap

   The SRTP crypto attribute defined for SDESC contains a tag followed
   by three types of parameters (refer to [RFC4568] for details):

   o  Crypto-suite.  Identifies the encryption and authentication
      transform

   o  Key parameters.  SRTP keying material and parameters.

   o  Session parameters.  Additional (optional) SRTP parameters such as
      Key Derivation Rate, Forward Error Correction Order, use of
      unencrypted SRTP, and other parameters defined by SDESC.

   The crypto attributes in the example SDP in Figure 4 illustrate these
   parameters.





















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        v=0
        o=sam 2890844526 2890842807 IN IP4 192.0.2.5
        s=SRTP Discussion
        i=A discussion of Secure RTP
        u=http://www.example.com/seminars/srtp.pdf
        e=marge@example.com (Marge Simpson)
        c=IN IP4 192.0.2.12
        t=2873397496 2873404696
        m=audio 49170 RTP/SAVP 0
        a=crypto:1 AES_CM_128_HMAC_SHA1_80
           inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
           FEC_ORDER=FEC_SRTP
        a=crypto:2 F8_128_HMAC_SHA1_80
           inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4;
           inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4
           FEC_ORDER=FEC_SRTP

                Figure 4: SDP Security Descriptions example

   For legibility the SDP shows line breaks that are not present on the
   wire.

   The first crypto attribute has the tag "1" and uses the crypto-suite
   AES_CM_128_HMAC_SHA1_80.  The "inline" parameter provides the SRTP
   master key and salt, the master key lifetime (number of packets), and
   the (optional) Master Key Identifier (MKI) whose value is "1" and has
   a byte length of "4" in the SRTP packets.  Finally, the FEC_ORDER
   session parameter indicates the order of Forward Error Correction
   used (FEC is applied before SRTP processing by the sender of the SRTP
   media).

   The second crypto attribute has the tag "2" and uses the crypto-suite
   F8_128_HMAC_SHA1_80.  It includes two SRTP master keys and associated
   salts.  The first one is used with the MKI value 1, whereas the
   second one is used with the MKI value 2.  Finally, the FEC_ORDER
   session parameter indicates the order of Forward Error Correction
   used.

3.2.  Relationship between EKT and SDP Security Descriptions

   SDP Security Descriptions [RFC4568] define a generic framework for
   negotiating security parameters for media streams negotiated via the
   Session Description Protocol by use of the Offer/Answer procedures
   defined in [RFC3264].  In addition to the general framework, SDESC
   also defines how to use it specifically to negotiate security
   parameters for Secure RTP.

   EKT and SDESC are complementary.  SDESC can negotiate several of the



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   SRTP security parameters (e.g., cipher and use of Master Key
   Identifier/MKI) as well as SRTP master keys.  SDESC, however, does
   not negotiate SSRCs and their associated Rollover Counter (ROC).
   Instead, SDESC relies on a so-called "late binding", where a newly
   observed SSRC will have its crypto context initialized to a ROC value
   of zero.  Clearly, this does not work for participants joining an
   SRTP session that has been established for a while and hence has a
   non-zero ROC.  It is impossible to use SDESC to join an SRTP session
   that is already in progress.  In this case, EKT on the endpoint
   running SDP Security can provide the additional signaling necessary
   to communicate the ROC (Section 6.4.1 of [RFC4568]).  The use of EKT
   solves this problem by communicating the ROC associated with the SSRC
   in the media plane.

   SDP Security Descriptions negotiates different SRTP master keys in
   the send and receive direction.  The offer contains the master key
   used by the offerer to send media, and the answer contains the master
   key used by the answerer to send media.  Consequently, if media is
   received by the offerer prior to the answer being received, the
   offerer does not know the master key being used.  Use of SDP security
   preconditions can solve this problem, however it requires an
   additional round-trip as well as a more complicated state machine.
   EKT solves this problem by simply sending the master key used in the
   media plane thereby avoiding the need for security preconditions.

   If multiple crypto-suites were offered, the offerer also will not
   know which of the crypto-suites offered was selected until the answer
   is received.  EKT solves this problem by using a correlator, the
   Security Parameter Index (SPI), which uniquely identifies each crypto
   attribute in the offer.

   One of the primary call signaling protocols using offer/answer is the
   Session Initiation Protocol (SIP) [RFC3261].  SIP uses the INVITE
   message to initiate a media session and typically includes an offer
   SDP in the INVITE.  An INVITE may be "forked" to multiple recipients
   which potentially can lead to multiple answers being received.
   SDESC, however, does not properly support this scenario, mainly
   because SDP and RTP/RTCP does not contain sufficient information to
   allow for correlation of an incoming RTP/RTCP packet with a
   particular answer SDP.  Note that extensions providing this
   correlation do exist (e.g., Interactive Connectivity Establishment
   (ICE)).  SDESC addresses this point-to-multipoint problem by moving
   each answer to a separate RTP transport address thereby turning a
   point-to-multipoint scenario into multiple point-to-point scenarios.
   There are however significant disadvantages to doing so.  As long as
   the crypto attribute in the answer does not contain any declarative
   parameters that differ from those in the offer, EKT solves this
   problem by use of the SPI correlator and communication of the



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   answerer's SRTP master key in EKT.

   As can be seen from the above, the combination of EKT and SDESC
   provides a better solution to SRTP negotiation for offer/answer than
   either of them alone.  SDESC negotiates the various SRTP crypto
   parameters (which EKT does not), whereas EKT addresses the
   shortcomings of SDESC.

3.3.  Overview of Combined EKT and SDP Security Description Operation

   We define three session extension parameters to SDESC to communicate
   the EKT cipher, EKT key, and Security Parameter Index to the peer.
   The original SDESC parameters are used as defined in [RFC4568],
   however the procedures associated with the SRTP master key differ
   slightly, since both SDESC and EKT communicate an SRTP master key.
   In particular, the SRTP master key communicated via SDESC is used
   only if there is currently no crypto context established for the SSRC
   in question.  This will be the case when an entity has received only
   the offer or answer, but has yet to receive a valid EKT message from
   the peer.  Once a valid EKT message is received for the SSRC, the
   crypto context is initialized accordingly, and the SRTP master key
   will then be derived from the EKT message.  Subsequent offer/answer
   exchanges do not change this: The most recent SRTP master key
   negotiated via EKT will be used, or, if none is available for the
   SSRC in question, the most recent SRTP master key negotiated via
   offer/answer will be used.  Note that with these rules, once a valid
   EKT message has been received for a given SSRC, rekeying for that
   SSRC can only be done via EKT.  The associated SRTP crypto parameters
   however can be changed via SDESC.

3.4.  EKT Extensions to SDP Security Descriptions

   In order to use EKT and SDESC in conjunction with each other, the
   following new SDES session parameters are defined.  These MUST NOT
   appear more than once in a given crypto attribute:

   EKT_Cipher:  The EKT cipher used to encrypt the SRTP Master Key

   EKT_Key:  The EKT key used to encrypt the SRTP Master Key

   EKT_SPI:  The EKT Security Parameter Index

   Below are details on each of these attributes.

3.4.1.  EKT_Cipher

   The (optional) EKT_Cipher parameter defines the EKT cipher used to
   encrypt the EKT key with in SRTCP packets.  The default value is



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   "AESKW_128" in accordance with Section 2.3.1.  For the AES Key Wrap
   cipher, the values "AESKW_128", "AESKW_192", and "AESKW_256" are
   defined for values of L=16, 24, and 32 respectively.  For the AES ECB
   cipher, "AES_ECB" is defined.  In the Offer/Answer model, the
   EKT_Cipher parameter is a negotiated parameter.

3.4.2.  EKT_Key

   The (mandatory) EKT_Key parameter is the key K used to encrypt the
   SRTP Master Key in SRTCP packets.  The value is base64 encoded as
   described in Section 4 [RFC4648].  When base64 decoding the key,
   padding characters (i.e., one or two "=" at the end of the base64
   encoded data) are discarded (see [RFC4648] for details).  Base64
   encoding assumes that the base64 encoding input is an integral number
   of octets.  If a given EKT cipher requires the use of a key with a
   length that is not an integral number of octets, said cipher MUST
   define a padding scheme that results in the base64 input being an
   integral number of octets.  For example, if the length defined was
   250 bits, then 6 padding bits would be needed, which could be defined
   to be the last 6 bits in a 256 bit input.  In the Offer/Answer model,
   the EKT_Key parameter is a negotiated parameter.

3.4.3.  EKT_SPI

   The (mandatory) EKT_SPI parameter is the Security Parameter Index.
   It is encoded as an ASCII string representing the hexadecimal value
   of the Security Parameter Index.  The SPI identifies the *offer*
   crypto attribute (including the EKT Key and Cipher) being used for
   the associated SRTP session.  A crypto attribute corresponds to an
   EKT Parameter Set and hence the SPI effectively identifies a
   particular EKT parameter set.  Note that the scope of the SPI is the
   SRTP session, which may or may not be limited to the scope of the
   associated SIP dialog.  In particular, if one of the participants in
   an SRTP session is an SRTP translator, the scope of the SRTP session
   is not limited to the scope of a single SIP dialog.  However, if all
   of the participants in the session are endpoints or mixers, the scope
   of the SRTP session will correspond to a single SIP dialog.  In the
   Offer/Answer model, the EKT_SPI parameter is a negotiated parameter.

3.5.  Offer/Answer Procedures

   In this section, we provide the offer/answer procedures associated
   with use of the three new SDESC parameters defined in Section
   Section 3.4.  Since SDESC is defined only for unicast streams, we
   provide only offer/answer procedures for unicast streams here as
   well.





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3.5.1.  Generating the Initial Offer - Unicast Streams

   When the initial offer is generated, the offerer MUST follow the
   steps defined in [RFC4568] Section 7.1.1 as well as the following
   steps.

   For each unicast media line using SDESC and where use of EKT is
   desired, the offerer MUST include one EKT_Key parameter and one
   EKT_SPI parameter in at least one "crypto" attribute (see [RFC4568]).
   The EKT_SPI parameter serves to identify the EKT parameter set used
   for a particular SRTCP packet.  Consequently, within a single media
   line, a given EKT_SPI value MUST NOT be used with multiple crypto
   attributes.  Note that the EKT parameter set to use for the session
   is not yet established at this point; each offered crypto attribute
   contains a candidate EKT parameter set.  Furthermore, if the media
   line refers to an existing SRTP session, then any SPI values used for
   EKT parameter sets in that session MUST NOT be remapped to any
   different EKT parameter sets.  When an offer describes an SRTP
   session that is already in progress, the offer SHOULD use an EKT
   parameter set (incl.  EKT_SPI and EKT_KEY) that is already in use.

   If an EKT_Cipher other than the default cipher is to be used, then
   the EKT_Cipher parameter MUST be included as well.

   If a given crypto attribute includes more than one set of SRTP key
   parameters (SRTP master key, salt, lifetime, MKI), they MUST all use
   the same salt.  (EKT requires a single shared salt between all the
   participants in the direct SRTP session).

   Important Note:  The scope of the offer/answer exchange is the SIP
      dialog(s) established as a result of the INVITE, however the scope
      of EKT is the direct SRTP session, i.e., all the participants that
      are able to receive SRTP and SRTCP packets directly.  If an SRTP
      session spans multiple SIP dialogs, the EKT parameter sets MUST be
      synchronized between all the SIP dialogs where SRTP and SRTCP
      packets can be exchanged.  In the case where the SIP entity
      operates as an RTP mixer (and hence re-originates SRTP and SRTCP
      packets with its own SSRC), this is not an issue, unless the mixer
      receives traffic from the various participants on the same
      destination IP address and port, in which case further
      coordination of SPI values and crypto parameters may be needed
      between the SIP dialogs (note that SIP forking with multiple early
      media senders is an example of this).  However if it operates as
      an RTP translator, synchronized negotiation of the EKT parameter
      sets on *all* the involved SIP dialogs will be needed.  This is
      non-trivial in a variety of use cases, and hence use of the
      combined SDES/EKT mechanism with RTP translators should be
      considered very carefully.  It should be noted, that use of SRTP



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      with RTP translators in general should be considered very
      carefully as well.

   The EKT session parameters can either be included as optional or
   mandatory parameters, however within a given crypto attribute, they
   MUST all be either optional or mandatory.

3.5.2.  Generating the Initial Answer - Unicast Streams

   When the initial answer is generated, the answerer MUST follow the
   steps defined in [RFC4568] Section 7.1.2 as well as the following
   steps.

   For each unicast media line using SDESC, the answerer examines the
   associated crypto attribute(s) for the presence of EKT parameters.
   If mandatory EKT parameters are included with a "crypto" attribute,
   the answerer MUST support those parameters in order to accept that
   offered crypto attribute.  If optional EKT parameters are included
   instead, the answerer MAY accept the offered crypto attribute without
   using EKT.  However, doing so will prevent the offerer from
   processing any packets received before the answer.  If neither
   optional nor mandatory EKT parameters are included with a crypto
   attribute, and that crypto attribute is accepted in the answer, EKT
   MUST NOT be used.  If a given a crypto attribute includes a mixture
   of optional and mandatory EKT parameters, or an incomplete set of
   mandatory EKT parameters, that crypto attribute MUST be considered
   invalid.

   When EKT is used with SDESC, the offerer and answerer MUST use the
   same SRTP master salt.  Thus, the SRTP key parameter(s) in the answer
   crypto attribute MUST use the same master salt as the one accepted
   from the offer.

   When the answerer accepts the offered media line and EKT is being
   used, the crypto attribute included in the answer MUST include the
   same EKT parameter values as found in the accepted crypto attribute
   from the offer (however, if the default EKT cipher is being used, it
   may be omitted).  Furthermore, the EKT parameters included MUST be
   mandatory (i.e., no "-" prefix).

   Acceptance of a crypto attribute with EKT parameters leads to
   establishment of the EKT parameter set for the corresponding SRTP
   session.  Consequently, the answerer MUST send packets in accordance
   with that particular EKT parameter set only.  If the answerer wants
   to enable the offerer to process SRTP packets received by the offerer
   before it receives the answer, the answerer MUST NOT include any
   declarative session parameters that either were not present in the
   offered crypto attribute, or were present but with a different value.



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   Otherwise, the offerer's view of the EKT parameter set would differ
   from the answerer's until the answer is received.  Similarly, unless
   the offerer and answerer has other means for correlating an answer
   with a particular SRTP session, the answer SHOULD NOT include any
   declarative session parameters that either were not present in the
   offered crypto attribute, or were present but with a different value.
   If this recommendation is not followed and the offerer receives
   multiple answers (e.g., due to SIP forking), the offerer may not be
   able to process incoming media stream packets correctly.

3.5.3.  Processing of the Initial Answer - Unicast Streams

   When the offerer receives the answer, it MUST perform the steps in
   [RFC4568] Section 7.1.3 as well as the following steps for each SRTP
   media stream it offered with one or more crypto lines containing EKT
   parameters in it.

   If the answer crypto line contains EKT parameters, and the
   corresponding crypto line from the offer contained the same EKT
   values, use of EKT has been negotiated successfully and MUST be used
   for the media stream.  When determining whether the values match,
   optional and mandatory parameters MUST be considered equal.
   Furthermore, if the default EKT cipher is being used, it MAY be
   either present or absent in the offer and/or answer.

   If the answer crypto line does not contain EKT parameters, then EKT
   MUST NOT be used for the corresponding SRTP session.  Note that if
   the accepted crypto attribute contained mandatory EKT parameters in
   the offer, and the crypto attribute in the answer does not contain
   EKT parameters, then negotiation has failed (Section 5.1.3 of
   [RFC4568]).

   If the answer crypto line contains EKT parameters but the
   corresponding offered crypto line did not, or if the parameters don't
   match or are invalid, then the offerer MUST consider the crypto line
   invalid (see Section 7.1.3 of [RFC4568] for further operation).

   The EKT parameter set is established when the answer is received,
   however there are a couple of special cases to consider here.  First
   of all, if an SRTCP packet is received prior to the answer, then the
   EKT parameter set is established provisionally based on the SPI
   included.  Once the answer (which may include declarative session
   parameters) is received, the EKT parameter set is fully established.
   The second case involves receipt of multiple answers due to SIP
   forking.  In this case, there will be multiple EKT parameter sets;
   one for each SRTP session.  As mentioned earlier, reliable
   correlation of SIP dialogs to SRTP sessions requires extensions, and
   hence if one or more of the answers include declarative session



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   parameters, it may be difficult to fully establish the EKT parameter
   set for each SRTP session.  In the absence of a specific correlation
   mechanism, it is RECOMMENDED, that such correlation be done based on
   the signaled receive IP-address in the SDP and the observed source
   IP-address in incoming SRTP/SRTCP packets, and, if necessary, the
   signaled receive UDP port and the observed source UDP port.

3.6.  SRTP-Specific Use Outside Offer/Answer

   Security Descriptions use for SRTP is not defined outside offer/
   answer and hence neither does Security Descriptions with EKT.

3.7.  Modifying the Session

   When a media stream using the SRTP security descriptions has been
   established, and a new offer/answer exchange is performed, the
   offerer and answerer MUST follow the steps in Section 7.1.4 of
   [RFC4568] as well as the following steps.  SDESC allows for all
   parameters of the session to be modified, and the EKT session
   parameters are no exception to that, however, there are a few
   additional rules to be adhered to when using EKT.

   It is permissible to start a session without the use of EKT, and then
   subsequently start using EKT, however the converse is not.  Thus,
   once use of EKT has been negotiated on a particular media stream, EKT
   MUST continue to be used on that media stream in all subsequent
   offer/answer exchanges.

   The reason for this is that both SDESC and EKT communicate the SRTP
   Master Key with EKT Master Keys taking precedence.  Reverting back to
   an SDESC-controlled master key in a synchronized manner is difficult.

   Once EKT is being used, the salt for the direct SRTP session MUST NOT
   be changed.  Thus, a new offer/answer which does not create a new
   SRTP session (e.g., because it reuses the same IP address and port)
   MUST use the same salt for all crypto attributes as is currently used
   for the direct SRTP session.

   Finally, subsequent offer/answer exchanges MUST NOT remap a given SPI
   value to a different EKT parameter set until 2^32 other mappings have
   been used within the SRTP session.  In practice, this requirements is
   most easily met by using a monotonically increasing SPI value (modulo
   2^32 and starting with zero) per direct SRTP session.  Note that a
   direct SRTP session may span multiple SIP dialogs, and in such cases
   coordination of SPI values across those SIP dialogs will be required.
   In the simple point-to-point unicast case without translators, the
   requirement simply applies within each media line in the SDP.  In the
   point-to-multipoint case, the requirement applies across all the



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   associated SIP dialogs.

3.8.  Backwards Compatibility Considerations

   Backwards compatibility can be achieved in a couple of ways.  First
   of all, SDESC allows for session parameters to be prefixed with "-"
   to indicate that they are optional.  If the answerer does not support
   the EKT session parameters, such optional parameters will simply be
   ignored.  When the answer is received, absence of the parameters will
   indicate that EKT is not being used.  Receipt of SRTCP packets prior
   to receipt of such an answer will obviously be problematic (as is
   normally the case for SDESC without EKT).

   Alternatively, SDESC allows for multiple crypto lines to be included
   for a particular media stream.  Thus, two crypto lines that differ in
   their use of EKT parameters (presence in one, absence in the other)
   can be used as a way to negotiate use of EKT.  When the answer is
   received, the accepted crypto attribute will indicate whether EKT is
   being used or not.

3.9.  Grammar

   The ABNF [RFC5234] syntax for the three new SDP Security Descriptions
   session parameters is shown in Figure 5.

     EKT = EKT_Cipher "|" EKT_Key "|" EKT_SPI
     EKT_Cipher = "EKT=" EKT_Cipher_Name
     EKT_Cipher_Name = 1*(ALPHA / DIGIT / "_") ; "AES_128", "AESKW_128"
                                               ; "AESKW_192" and
                                               ; "AESKW_256" defined in
                                               ; this document.
     EKT_Key = 1*(base64)    ; See Section 4 of [RFC4648]
     base64  =  ALPHA / DIGIT / "+" / "/" / "="
     EKT_SPI = 4HEXDIG   ; See [RFC5234]

               Figure 5: ABNF for the EKT session parameters

   Using the example from Figure 5 with the EKT extensions to SDP
   Security Descriptions results in the following example SDP:












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        v=0
        o=sam 2890844526 2890842807 IN IP4 192.0.2.5
        s=SRTP Discussion
        i=A discussion of Secure RTP
        u=http://www.example.com/seminars/srtp.pdf
        e=marge@example.com (Marge Simpson)
        c=IN IP4 192.0.2.12
        t=2873397496 2873404696
        m=audio 49170 RTP/SAVP 0
        a=crypto:1 AES_CM_128_HMAC_SHA1_80
          inline:WVNfX19zZW1jdGwgKCkgewkyMjA7fQp9CnVubGVz|2^20|1:4
          FEC_ORDER=FEC_SRTP EKT=AES_128|FE9C|AAE0
        a=crypto:2 F8_128_HMAC_SHA1_80
          inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjZGVm|2^20|1:4;
          inline:QUJjZGVmMTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5|2^20|2:4
          FEC_ORDER=FEC_SRTP EKT=AES_128|FE9C|AAE0

   For legibility the SDP shows line breaks that are not present on the
   wire.
































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4.  Use of EKT with DTLS-SRTP Key Transport

   This document defines an extension to DTLS-SRTP called Key Transport.
   Using EKT with the DTLS-SRTP Key Transport extensions allows securely
   transporting SRTP keying material from one DTLS-SRTP peer to another,
   so the same SRTP keying material can be used by those peers and so
   those peers can process EKT keys.  This combination of protocols is
   valuable because it combines the advantages of DTLS (strong
   authentication of the endpoint and flexibility) with the advantages
   of EKT (allowing secure multiparty RTP with loose coordination and
   efficient communication of per-source keys).

4.1.  EKT Extensions to DTLS-SRTP

   This document adds a new TLS negotiated extension called "ekt".  This
   adds a new TLS content type, EKT, and a new negotiated extension EKT.
   The negotiated extension MUST only be requested in conjunction with
   the "use_srtp" extension (Section 3.2 of [RFC5764]).  The DTLS server
   indicates its support for EKT by including "dtls-srtp-ekt" in its SDP
   and "ekt" in its TLS ServerHello message.  If a DTLS client includes
   "ekt" in its ClientHello, but does not receive "ekt" in the
   ServerHello, the DTLS client MUST NOT send DTLS packets with the
   "ekt" content-type.




























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   Using the syntax described in DTLS [I-D.ietf-tls-rfc4347-bis], the
   following structures are used:

                   enum {
                     ekt_key(0),
                     ekt_key_ack(1),
                     ekt_key_error(254),
                     (255)
                   } SRTPKeyTransportType;

                   struct {
                     SRTPKeyTransportType keytrans_type;
                     uint24 length;
                     uint16 message_seq;
                     uint24 fragment_offset;
                     uint24 fragment_length;
                     select (SRTPKeyTransportType) {
                        case ekt_key:
                           EKTkey;
                      };
                   } KeyTransport;

                   enum {
                    AES_128(0),
                    AESKW_128(1),
                    AESKW_192(2),
                    AESKW_256(3),
                   } ektcipher;

                   struct {
                     ektcipher EKT_Cipher;
                     uint EKT_Key_Value<1..256>;
                     uint EKT_Master_Salt<1..256>;
                     uint16 EKT_SPI;
                   } EKTkey;

                 Figure 6: Additional TLS Data Structures

   The diagram below shows a message flow of DTLS client and DTLS server
   using the DTLS-SRTP Key Transport extension.  SRTP packets exchanged
   prior to the ekt_message are encrypted using the SRTP master key
   derived from the normal DTLS-SRTP key derivation function.  After the
   ekt_key message, they can be encrypted using the EKT key.

      Editor's note: do we need reliability for the ekt_key messages?






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        Client                                               Server

        ClientHello + use_srtp + EKT
                                    -------->
                                      ServerHello + use_srtp + EKT
                                                      Certificate*
                                                ServerKeyExchange*
                                               CertificateRequest*
                                    <--------      ServerHelloDone
        Certificate*
        ClientKeyExchange
        CertificateVerify*
        [ChangeCipherSpec]
        Finished                     -------->
                                                [ChangeCipherSpec]
                                    <--------             Finished
        SRTP packets                 <------->      SRTP packets
        SRTP packets                 <------->      SRTP packets
        ekt_key                      -------->
        SRTP packets                 <------->      SRTP packets
        SRTP packets                 <------->      SRTP packets

                     Figure 7: Handshake Message Flow

4.1.1.  Scaling to Large Groups

   In certain scenarios it is useful to perform DTLS-SRTP with a device
   that is not the RTP peer.  A common scenario is multicast, where it
   is necessary to distribute the DTLS-SRTP (and EKT distribution) to
   several devices.  To allow for this, a new SDP attribute, dtls-srtp-
   host, is defined which follows the general syntax specified in
   Section 5.13 of [RFC4566].  When signaled, it indicates this host
   controls the EKT keying for all group members.  For the dtls-srtp-
   host attribute:

   o  the name is the ASCII string "dtls-srtp-host" (lowercase)

   o  the value is the IP address and port number used for DTLS-SRTP

   o  This is a media-level attribute and MUST NOT appear at the session
      level

   The formal description of the attribute is defined by the following
   ABNF [RFC5234] syntax:

   attribute = "a=dtls-srtp-host:"
               dtls-srtp-host-info *(SP dtls-srtp-host-info)
   host-info = nettype space addrtype space



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               connection-address space port CRLF

   Multiple IP/port pairs are provided for IPv6/IPv4 interworking, and
   to allow failover.  The receiving host SHOULD attempt to use them in
   the order provided.

   An example of SDP containing the dtls-srtp-host attribute:

       v=0
       o=sam 2890844526 2890842807 IN IP4 192.0.2.5
       s=SRTP Discussion
       i=A discussion of Secure RTP
       u=http://www.example.com/seminars/srtp.pdf
       e=marge@example.com (Marge Simpson)
       c=IN IP4 192.0.2.12
       t=2873397496 2873404696
       m=audio 49170 UDP/TLS/RTP/SAVP 0
       a=fingerprint:SHA-1
         4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
       a=dtls-srtp-ekt
       a=dtls-srtp-host:IN IP4 192.0.2.13 56789

   For legibility the SDP shows line breaks that are not present on the
   wire.

4.2.  Offer/Answer Considerations

   This section describes Offer/Answer considerations for the use of EKT
   together with DTLS-SRTP for unicast and multicast streams.  The
   offerer and answerer MUST follow the procedures specified in
   [RFC5764] as well as the following ones.

   As most DTLS-SRTP processing is performed on the media channel,
   rather than in SDP, there is little processing performed in SDP other
   than informational and to redirect DTLS-SRTP to an alternate host.
   Advertising support for the extension is necessary in SDP because in
   some cases it is required to establish an SRTP call.  For example, a
   mixer may be able to only support SRTP listeners if those listeners
   implement DTLS Key Transport (because it lacks the CPU cycles
   necessary to encrypt SRTP uniquely for each listener).

4.2.1.  Generating the Initial Offer

   The initial offer contains a new SDP attribute, "dtls-srtp-ekt",
   which contains no value.  This indicates the offerer is capable of
   supporting DTLS-SRTP with EKT extensions, and indicates the desire to
   use the "ekt" extension during the DTLS-SRTP handshake.  If the
   offerer wants another host to perform DTLS-SRTP-EKT processing, it



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   also includes the dtls-srtp-host attribute in its offer
   (Section 4.1).

   An example of SDP containing the dtls-srtp-ekt attribute::

       v=0
       o=sam 2890844526 2890842807 IN IP4 192.0.2.5
       s=SRTP Discussion
       i=A discussion of Secure RTP
       u=http://www.example.com/seminars/srtp.pdf
       e=marge@example.com (Marge Simpson)
       c=IN IP4 192.0.2.12
       t=2873397496 2873404696
       m=audio 49170 UDP/TLS/RTP/SAVP 0
       a=fingerprint:SHA-1
         4A:AD:B9:B1:3F:82:18:3B:54:02:12:DF:3E:5D:49:6B:19:E5:7C:AB
       a=dtls-srtp-ekt

   For legibility the SDP shows line breaks that are not present on the
   wire.

4.2.2.  Generating the Initial Answer

   Upon receiving the initial offer, the presence of the dtls-srtp-ekt
   attribute indicates a desire to receive the EKT extension in the
   DTLS-SRTP handshake.  The presence of the dtls-srtp-host attribute
   indicates an alternate host to send the DTLS-SRTP handshake (instead
   of the host on the c/m lines).  DTLS messages should be constructed
   according to those two attributes.

   The SDP answer SHOULD contain the dtls-srtp-ekt attribute to indicate
   the answerer understands dtls-srtp.  It should only contain the dtls-
   srtp-host attribute if the answerer also wishes to offload its DTLS-
   SRTP processing to another host.

4.2.3.  Processing the Initial Answer

   The presence of the dtls-srtp-ekt attribute indicates a desire by the
   answerer to perform DTLS-SRTP with EKT extensions, and the dtls-srtp-
   host attribute indicates an alternate host for DTLS-SRTP processing.

   After successful negotiation of the key_transport extension, the DTLS
   client and server MAY exchange SRTP packets, encrypted using the KDF
   described in [RFC5764].  This is normal and expected, even if Key
   Transport was negotiated by both sides, as neither side may (yet)
   have a need to alter the SRTP key.  However, it is also possible that
   one (or both) peers will immediately send new_srtp_key message before
   sending any SRTP, and also possible that SRTP, encrypted with an



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   unknown key, may be received before the new_srtp_key message is
   received.

4.2.4.  Modifying the Session

   As DTLS-SRTP-EKT processing is done on the DTLS-SRTP channel (media
   channel) rather than signaling, no special processing for modifying
   the session is necessary.











































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5.  Use of EKT with MIKEY

   The advantages outlined in Section 1 are useful in some scenarios in
   which MIKEY is used to establish SRTP sessions.  In this section, we
   briefly review MIKEY and related work, and discuss these scenarios.

   An SRTP sender or a group controller can use MIKEY to establish a
   SRTP cryptographic context.  This capability includes the
   distribution of a TEK generation key (TGK) or the TEK itself,
   security policy payload, crypto session bundle ID (CSB_ID) and a
   crypto session ID (CS_ID).  The TEK directly maps to an SRTP master
   key, whereas the TGK is used along with the CSB_ID and a CS_ID to
   generate a TEK.  The CS_ID is used to generate multiple TEKs (SRTP
   master keys) from a single TGK.  For a media stream in SDP, MIKEY
   allocates two consecutive numbers for the crypto session IDs, so that
   each direction uses a different SRTP master key (see [RFC4567]).

   The MIKEY specification [RFC3830] defines three modes to exchange
   keys, associated parameters and to protect the MIKEY message: pre-
   shared key, public-key encryption and Diffie-Hellman key exchange.
   In the first two modes the MIKEY initiator only chooses and
   distributes the TGK or TEK, whereas in the third mode both MIKEY
   entities (the initiator and responder) contribute to the keys.  All
   three MIKEY modes have in common that for establishing a SRTP session
   the exchanged key is valid for the send and receive direction.
   Especially for group communications it is desirable to update the
   SRTP master key individually per direction.  EKT provides this
   property by distributing the SRTP master key within the SRTP/SRTCP
   packet.

   MIKEY already supports synchronization of ROC values between the
   MIKEY initiator and responder.  The SSRC / ROC value pair is part of
   the MIKEY Common Header payload.  This allows providing the current
   ROC value to late joiners of a session.  However, in some scenarios a
   key management based ROC synchronization is not sufficient.  For
   example, in mobile and wireless environments, members may go in and
   out of coverage and may miss a sequence number overrun.  In point-to-
   multipoint translator scenarios it is desirable to not require the
   group controller to track the ROC values of each member, but to
   provide the ROC value by the originator of the SRTP packet.  A better
   alternative to synchronize the ROC values is to send them directly
   via SRTP/SRTCP, as EKT does.  A separate SRTP extension is being
   proposed [RFC4771] to include the ROC as part of a modified
   authentication tag.  Unlike EKT, this extension uses only SRTP and
   not SRTCP as its transport and does not allow updating the SRTP
   master key.

   Besides the ROC, MIKEY synchronizes also the SSRC values of the SRTP



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   streams.  Each sender of a stream sends the associated SSRC within
   the MIKEY message to the other party.  If a SRTP session participant
   starts a new SRTP source or a new participant is added to a group,
   subsequent SDP offer/answer and MIKEY exchanges are necessary to
   update the SSRC values.  EKT improves these scenarios by updating the
   keys and SSRC values without coordination on the signaling channel.
   With EKT, SRTP can handle early media, since the EKT SPI allows the
   receiver to identify the cryptographic keys and parameters used by
   the source.

   The MIKEY specification [RFC3830] suggests the use of unicast for
   rekeying.  This method does not scale well to large groups or
   interactive groups.  The EKT extension of SRTP/SRTCP provides a
   solution for rekeying the SRTP master key and for ROC/SSRC
   synchronization.  EKT is not a substitution for MIKEY, but rather a
   complementary addition to address the above described limitations of
   MIKEY.

   In the next section we provide an extension to MIKEY for support of
   EKT.  EKT can be used only with the pre-shared key or public-key
   encryption MIKEY mode of [RFC3830].  The Diffie-Hellman exchange mode
   is not suitable in conjunction with EKT, because it is not possible
   to establish one common EKT key over multiple EKT entities.
   Additional MIKEY modes specified in separate documents are not
   considered for EKT.

5.1.  EKT extensions to MIKEY

   In order to use EKT with MIKEY, the EKT cipher, EKT key and EKT SPI
   must be negotiated in the MIKEY message exchange.

   For EKT we specify a new SRTP Policy Type in the Security Policy (SP)
   payload of MIKEY (see Section 6.10 of [RFC3830]).  The SP payload
   contains a set of policies.  Each policy consists of a number Policy
   Param TLVs.


               Prot type  | Value
               -------------------
               EKT        | TBD (will be requested from IANA)

   For legibility the SDP shows line breaks that are not present on the
   wire.

                       Figure 8: EKT Security Policy

   The EKT Security Policy has one parameter representing the EKT
   cipher.



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            Type | Meaning                     | Possible values
            ----------------------------------------------------
               0 | EKT cipher                  | see below

                 Figure 9: EKT Security Policy Parameters


                            EKT cipher | Value
                            -------------------
                            AES_128    |  0
                            AESKW_128  |  1
                            AESKW_192  |  2
                            AESKW_256  |  3

                     Figure 10: EKT Cipher Parameters

   AES_128 is the default value for the EKT cipher.

   The two mandatory EKT parameters (EKT_Key and EKT_SPI) are
   transported in the MIKEY KEMAC payload within one separate Key Data
   sub-payload.  As specified in Section 6.2 of [RFC3830], the KEMAC
   payload carries the TEK Generation Key (TGK) or the Traffic
   Encryption Key (TEK).  One or more TGKs or TEKs are carried in
   individual Key Data sub-payloads within the KEMAC payload.  The KEMAC
   payload is encrypted as part of MIKEY.  The Key Data sub- payload,
   specified in Section 6.13 of [RFC3830], has the following format:


                          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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Next Payload  | Type  |  KV   | Key data length               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                         Key data                              :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     : Salt length (optional)        ! Salt data (optional)          :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                        KV data (optional)                     :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                 Figure 11: Key Data Sub-Payload of MIKEY

   These fields are described below:

   Type:  4 bits in length, indicates the type of key included in the
      payload.  We define Type = TBD (will be requested from IANA) to
      indicate transport of the EKT key.




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   KV:  (4 bits): indicates the type of key validity period specified.
      KV=1 is currently specified as an SPI.  We use that value to
      indicate the KV_data contains the ETK_SPI for the key type
      EKT_Key. KV_data would be 16 bits in length, but it is also
      possible to interpret the length from the 'Key data len' field.
      KV data MUST NOT be optional for the key type EKT_Key when KV = 1.

   Salt length, Salt Data:  These optional fields SHOULD be omitted for
      the key type EKT_Key, if the SRTP master salt is already present
      in the TGK or TEK Key Data sub-payload.  The EKT_Key sub-payload
      MUST contain a SRTP master salt, if the SRTP master salt is not
      already present in the TGK or TEK Key Data sub-payload.

   KV Data:  length determined by Key Data Length field.

5.2.  Offer/Answer considerations

   This section describes Offer/Answer considerations for the use of EKT
   together with MIKEY for unicast streams.  The offerer and answerer
   MUST follow the procedures specified in [RFC3830] and [RFC4567] as
   well as the following ones.

5.2.1.  Generating the Initial Offer

   If it is intended to use MIKEY together with EKT, the offerer MUST
   include at least one MIKEY key-mgmt attribute with one EKT_Key Key
   Data sub-payload and the EKT_Cipher Security Policy payload.  MIKEY
   can be used on session or media level.  On session level, MIKEY
   provides the keys for multiple SRTP sessions in the SDP offer.  The
   EKT SPI references a EKT parameter set including the Secure RTP
   parameters as specified in Section 8.2 in [RFC3711].  If MIKEY is
   used on session level, it is only possible to use one EKT SPI value.
   Therefore, the session-level MIKEY message MUST contain one SRTP
   Security Policy payload only, which is valid for all related SRTP
   media lines.  If MIKEY is used on media level, different SRTP
   Security Policy parameters (and consequently different EKT SPI
   values) can be used for each media line.  If MIKEY is used on session
   and media level, the medial level content overrides the session level
   content.

   EKT requires a single shared SRTP master salt between all
   participants in the direct SRTP session.  If a MIKEY key-mgmt
   attribute contains more than one TGK or TEK Key Data sub-payload, all
   the sub-payloads MUST contain the same master salt value.
   Consequently, the EKT_Key Key Data sub-payload MAY also contain the
   same salt or MAY omit the salt value.  If the SRTP master salt is not
   present in the TGK and TEK Key Data sub-payloads, the EKT_Key sub-
   payload MUST contain a master salt.



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5.2.2.  Generating the Initial Answer

   For each media line in the offer using MIKEY, provided on session or/
   and on media level, the answerer examines the related MIKEY key-mgmt
   attributes for the presence of EKT parameters.  In order to accept
   the offered key-mgmt attribute, the MIKEY message MUST contain one
   EKT_Key Key Data sub-payload and the EKT_Cipher Security Policy
   payload.  The answerer examines also the existence of a SRTP master
   salt in the TGK/TEK and/or the EKT_Key sub-payloads.  If multiple
   salts are available, all values MUST be equal.  If the salt values
   differ or no salt is present, the key-mgmt attribute MUST be
   considered as invalid.

   The MIKEY responder message in the SDP answer does not contain a
   MIKEY KEMAC or Security Policy payload and consequently does not
   contain any EKT parameters.  If the key-mgmt attribute for a media
   line was accepted by the answerer, the EKT parameter set of the
   offerer is valid for both directions of the SRTP session.

5.2.3.  Processing the Initial Answer

   On reception of the answer, the offerer examines if EKT has been
   accepted for the offered media lines.  If a MIKEY key-mgmt attribute
   is received containing a valid MIKEY responder message, EKT has been
   successfully negotiated.  On receipt of a MIKEY error message, EKT
   negotiation has failed.  For example, this may happen if an EKT
   extended MIKEY initiator message is sent to a MIKEY entity not
   supporting EKT.  A MIKEY error code 'Invalid SP' or 'Invalid DT' is
   returned to indicate that the EKT_Cipher Security Policy payload or
   the EKT_Key sub-payload is not supported.  In this case, the offerer
   may send a second SDP offer with a MIKEY key-mgmt attribute without
   the additional EKT extensions.

   This behavior can be improved by defining an additional key-mgmt
   prtcl-id value 'mikeyekt' and offering two key-mgmt SDP attributes.
   One attribute offers MIKEY together with EKT and the other one offers
   MIKEY without EKT.  This is for further discussion.

5.2.4.  Modifying the Session

   Once a SRTP stream has been established, a new offer/answer exchange
   can modify the session including the EKT parameters.  If the EKT key
   or EKT cipher is modified (i.e., a new EKT parameter set is created)
   the offerer MUST also provide a new EKT SPI value.  The offerer MUST
   NOT remap an existing EKT SPI value to a new EKT parameter set.
   Similar, a modification of the SRTP Security Policy leads to a new
   EKT parameter set and requires a fresh EKT SPI, even the EKT key or
   cipher did not change.



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   Once EKT is being used, the SRTP master salt for the SRTP session
   MUST NOT be changed.  The salt in the Key Data sub-payloads within
   the subsequent offers MUST be the same as the one already used.

   After EKT has been successfully negotiated for a session and a SRTP
   master key has been transported by EKT, it is difficult to switch
   back to a pure MIKEY based key exchange in a synchronized way.
   Therefore, once EKT is being used for a session, EKT MUST be used
   also in all subsequent offer/answer exchanges for that session.










































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6.  Design Rationale

   From [RFC3550], a primary function of RTCP is to carry the CNAME, a
   "persistent transport-level identifier for an RTP source" since
   "receivers require the CNAME to keep track of each participant."  EKT
   works in much the same way, using SRTCP to carry information needed
   for the proper processing of the SRTP traffic.

   With EKT, SRTP gains the ability to synchronize the creation of
   cryptographic contexts across all of the participants in a single
   session.  This feature provides some, but not all, of the
   functionality that is present in IKE phase two (but not phase one).
   Importantly, EKT does not provide a way to indicate SRTP options.

   With EKT, external signaling mechanisms provide the SRTP options and
   the EKT Key, but need not provide the key(s) for each individual SRTP
   source.  EKT provides a separation between the signaling mechanisms
   and the details of SRTP.  The signaling system need not coordinate
   all SRTP streams, nor predict in advance how many streams will be
   present, nor communicate SRTP-level information (e.g., rollover
   counters) of current sessions.

   EKT is especially useful for multi-party sessions, and for the case
   where multiple RTP sessions are sent to the same destination
   transport address (see the example in the definition of "RTP session"
   in [RFC3550]).  A SIP offer that is forked in parallel (sent to
   multiple endpoints at the same time) can cause multiple RTP sessions
   to be sent to the same transport address, making EKT useful for use
   with SIP.

   EKT can also be used in conjunction with a scalable group-key
   management system like GDOI [RFC3547].  Such a system provides a
   secure entity authentication method and a way to revoke group
   membership, both of which are out of scope of EKT.

   It is natural to use SRTCP to transport encrypted keying material for
   SRTP, as it provides a secure control channel for (S)RTP.  However,
   there are several different places in SRTCP in which the encrypted
   SRTP master key and ROC could be conveyed.  We briefly review some of
   the alternatives in order to motivate the particular choice used in
   this specification.  One alternative is to have those values carried
   as a new SDESC item or RTCP packet.  This would require that the
   normal SRTCP encryption be turned off for the packets containing that
   SDESC item, since on the receiver's side, SRTCP processing completes
   before the RTCP processing starts.  This tension between encryption
   and the desire for RTCP privacy is highly undesirable.  Additionally,
   this alternative makes SRTCP dependent upon the parsing of the RTCP
   compound packet, which adds complexity.  It is simpler to carry the



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   encrypted key in a new SRTCP field.  One way to do this and to be
   backwards compatible with the existing specification is to define a
   new crypto function that incorporates the encrypted key.  We define a
   new authentication transform because EKT relies on the normal SRTCP
   authentication to provide implicit authentication of the encrypted
   key.

   An SRTP packet containing an SSRC that has not been seen will be
   discarded.  This practice may induce a burst of packet loss at the
   outset of an SRTP stream, due to the loss or reorder of the first
   SRTCP packet with the EKT containing the key and rollover counter for
   that stream.  However, this practice matches the conservative RTP
   memory-allocation strategy; many existing applications accept this
   risk of initial packet loss.  Alternatively, implementations may wish
   to delay discarding such packets for a short period of time as
   described in Section 2.4.

   When EKT is carried in SRTCP, it adds eight additional bytes to each
   SRTCP packet, plus the length of the Encrypted Master Key field.
   Using the SRTP and EKT defaults, the total overhead is 24 bytes.
   This overhead does not detract from the total bandwidth used by SRTP,
   since it is included in the RTCP bandwidth computation.  However, it
   will cause the control protocol to issue packets less frequently.

   The main motivation for the use of the variable-length format is
   bandwidth conservation.  If EKT is used of SRTP, there will be a loss
   of bandwidth due to the additional 24 bytes in each RTP packet.  For
   some applications, this bandwidth loss is significant.

6.1.  Alternatives

   In its current design, EKT requires that the Master Salt be
   established out of band.  That requirement is undesirable.  In an
   offer/answer environment, it forces the answerer to re-use the same
   Master Salt value used by the offerer.  The Master Salt value could
   be carried in EKT packets though that would consume yet more
   bandwidth.

   In some scenarios, two SRTP sessions may be combined into a single
   session.  When using EKT in such sessions, it is desirable to have an
   SPI value that is larger than 15 bits, so that collisions between SPI
   values in use in the two different sessions are unlikely (since each
   collision would confuse the members of one of the sessions.)

   An alternative that addresses both of these needs is as follows: the
   SPI value can be lengthed from 15 bits to 63 bits, and the Master
   Salt can be identical to, or constructed from, the SPI value.  SRTP
   conventionally uses a 14-byte Master Salt, but shorter values are



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   acceptable.  This alternative would add six bytes to each EKT packet;
   that overhead may be a reasonable tradeoff for addressing the
   problems outlined above.
















































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7.  Security Considerations

   With EKT, each SRTP sender and receiver can generate distinct SRTP
   master keys.  This property avoids any security concern over the re-
   use of keys, by empowering the SRTP layer to create keys on demand.
   Note that the inputs of EKT are the same as for SRTP with key-
   sharing: a single key is provided to protect an entire SRTP session.
   However, EKT provides complete security, even in the absence of
   further out-of-band coordination of SSRCs, and even when SSRC values
   collide.

   EKT uses encrypted key transport with implicit authentication.  A
   strong cipher is used to ensure the confidentiality of the master
   keys as they are transported.  The authenticity of the master keys is
   ensured by the base authentication check, which uses the plaintext
   form of that key.  If the base authentication function and the cipher
   cannot be defeated by a particular attacker, then that attacker will
   be unable to defeat the implicit authentication.

   In order to avoid potential security issues, the SRTP authentication
   tag length used by the base authentication method MUST be at least
   ten octets.





























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8.  IANA Considerations

   This section registers with IANA the following SRTP session
   parameters for SDP Security Descriptions [RFC4568]:

   o  EKT_KEY

   o  EKT_CIPHER

   o  EKT_SPI

   The definition of these parameters is provided in Section 3.4.

   We request the following IANA assignments from existing MIKEY IANA
   tables:

   o  From the Key Data payload name spaces, a value to indicate the
      type as the 'EKT_Key'.

   o  From the Security Policy table name space, a new value to be
      assigned for 'EKT' (see Figure 8).

   Furthermore, we need the following two new IANA registries created,
   populated with the initial values in this document.  New values for
   both of these registries can be defined via Specification Required
   [RFC5226].

   o  EKT parameter type (initially populated with the list from
      Figure 9)

   o  EKT cipher (initially populated with the list from Figure 10)




















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9.  Acknowledgements

   Thanks to Lakshminath Dondeti for assistance with earlier versions of
   this document.  Thanks to Nermeen Ismail, Eddy Lem, and Rob Raymond
   for fruitful discussions and comments.  Thanks to Romain Biehlmann
   for his encouragement to add support DTLS-SRTP-EKT key servers for
   multicast.  Thanks to Felix Wyss for his review and comments
   regarding ciphers.











































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10.  References

10.1.  Normative References

   [FIPS197]  "The Advanced Encryption Standard (AES)", FIPS-197 Federal
              Information          Processing Standard.

   [I-D.ietf-tls-rfc4347-bis]
              Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security version 1.2", draft-ietf-tls-rfc4347-bis-06 (work
              in progress), July 2011.

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

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              June 2002.

   [RFC3394]  Schaad, J. and R. Housley, "Advanced Encryption Standard
              (AES) Key Wrap Algorithm", RFC 3394, September 2002.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

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

   [RFC4563]  Carrara, E., Lehtovirta, V., and K. Norrman, "The Key ID
              Information Type for the General Extension Payload in
              Multimedia Internet KEYing (MIKEY)", RFC 4563, June 2006.

   [RFC4566]  Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
              Description Protocol", RFC 4566, July 2006.

   [RFC4567]  Arkko, J., Lindholm, F., Naslund, M., Norrman, K., and E.
              Carrara, "Key Management Extensions for Session
              Description Protocol (SDP) and Real Time Streaming
              Protocol (RTSP)", RFC 4567, July 2006.

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session



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              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

   [RFC4648]  Josefsson, S., "The Base16, Base32, and Base64 Data
              Encodings", RFC 4648, October 2006.

   [RFC4771]  Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
              Transform Carrying Roll-Over Counter for the Secure Real-
              time Transport Protocol (SRTP)", RFC 4771, January 2007.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 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, May 2010.

10.2.  Informative References

   [RFC3547]  Baugher, M., Weis, B., Hardjono, T., and H. Harney, "The
              Group Domain of Interpretation", RFC 3547, July 2003.

   [RFC3830]  Arkko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing", RFC 3830,
              August 2004.

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.



















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Appendix A.  Using EKT to Optimize Interworking DTLS-SRTP with Security
             Descriptions

   Today, SDP Security Descriptions [RFC4568] is used for distributing
   SRTP keys in several different IP PBX systems and is expected to be
   used by 3GPP's Long Term Evolution (LTE).  The IP PBX systems are
   typically used within a single enterprise, and LTE is used within the
   confines of a mobile operator's network.  A Session Border Controller
   is a reasonable solution to interwork between Security Descriptions
   in one network and DTLS-SRTP in another network.  For example, a
   mobile operator (or an Enterprise) could operate Security
   Descriptions within their network and DTLS-SRTP towards the Internet.

   However, due to the way Security Descriptions and DTLS-SRTP manage
   their SRTP keys, such an SBC has to authenticate, decrypt, re-
   encrypt, and re-authenticate the SRTP (and SRTCP) packets in one
   direction, as shown in Figure 12, below.  This is computationally
   expensive.


       RFC4568 endpoint            SBC            DTLS-SRTP endpoint
             |                      |                       |
        1.   |---key=A------------->|                       |
        2.   |                      |<-DTLS-SRTP handshake->|
        3.   |<--key=B--------------|                       |
        4.   |                      |<--SRTP, encrypted w/B-|
        5.   |<-SRTP, encrypted w/B-|                       |
        6.   |-SRTP, encrypted w/A->|                       |
        7.   |            (decrypt, re-encrypt)             |
        8.   |                      |-SRTP, encrypted w/C-->|
             |                      |                       |

        Figure 12: Interworking Security Descriptions and DTLS-SRTP

   The message flow is as follows (similar steps occur with SRTCP):

   1.  The Security Descriptions [RFC4568] endpoint discloses its SRTP
       key to the SBC, using a=crypto in its SDP.

   2.  SBC completes DTLS-SRTP handshake.  From this handshake, the SBC
       derives the SRTP key for traffic from the DTLS-SRTP endpoint (key
       B) and to the DTLS-SRTP endpoint (key C).

   3.  The SBC communicates the SRTP encryption key (key B) to the
       Security Descriptions endpoint (using a=crypto).  (There is no
       way, with DTLS-SRTP, to communicate the Security Descriptions key
       to the DTLS-SRTP key endpoint.)




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   4.  The DTLS-SRTP endpoint sends an SRTP key, encrypted with its key
       B. This is received by the SBC.

   5.  The received SRTP packet is simply forwarded; the SBC does not
       need to do anything with this packet as its key (key B) was
       already communicated in step 3.

   6.  The Security Descriptions endpoint sends an SRTP packet,
       encrypted with its key A.

   7.  The SBC has to authenticate and decrypt the SRTP packet (using
       key A), and re-encrypt it and generate an HMAC (using key C).

   8.  The SBC sends the new SRTP packet.

   If EKT is deployed on the DTLS-SRTP endpoints, EKT helps to avoid the
   computationally expensive operation so the SBC does not need not
   perform any per-packet operations on the SRTP (or SRTCP) packets in
   either direction.  With EKT the SBC can simply forward the SRTP (and
   SRTCP) packets in both directions without per-packet HMAC or
   cryptographic operations.

   To accomplish this interworking, DTLS-SRTP EKT must be supported on
   the DTLS-SRTP endpoint, which allows the SBC to transport the
   Security Description key to the EKT endpoint and send the DTLS-SRTP
   key to the Security Descriptions endpoint.  This works equally well
   for both incoming and outgoing calls.  An abbreviated message flow is
   shown in Figure 13, below.


        RFC4568 endpoint            SBC          DTLS-SRTP endpoint
              |                      |                       |
         1.   |---key=A------------->|                       |
         2.   |                      |<-DTLS-SRTP handshake->|
         3.   |<--key=B--------------|                       |
         4.   |                      |--new_srtp_key:A------>|
         5.   |                      |<--SRTP, encrypted w/B-|
         5.   |<-SRTP, encrypted w/B-|                       |
         6.   |-SRTP, encrypted w/A->|                       |
         7.   |                      |-SRTP, encrypted w/A-->|
              |                      |                       |

           Figure 13: Interworking Security Descriptions and EKT

   The message flow is as follows (similar steps occur with SRTCP):

   1.  Security Descriptions endpoint discloses its SRTP key to the SBC
       (a=crypto).



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   2.  SBC completes DTLS-SRTP handshake.  From this handshake, the SBC
       derives the SRTP key for traffic from the DTLS-SRTP endpoint (key
       B) and to the DTLS-SRTP endpoint (key C).

   3.  The SBC communicates the SRTP encryption key (key B) to the
       Security Descriptions endpoint.

   4.  The SBC uses the EKT to indicate that SRTP packets will be
       encrypted with 'key A' towards the DTLS-SRTP endpoint.

   5.  The DTLS-SRTP endpoint sends an SRTP key, encrypted with its key
       B. This is received by the SBC.

   6.  The received SRTP packet is simply forwarded; the SBC does not
       need to do anything with this packet as its key (key B) was
       communicated in step 3.

   7.  The Security Descriptions endpoint sends an SRTP packet,
       encrypted with its key A.

   8.  The received SRTP packet is simply forwarded; the SBC does not
       need to do anything with this packet as its key (key A) was
       communicated in step 4.




























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Authors' Addresses

   David A. McGrew
   Cisco Systems, Inc.
   510 McCarthy Blvd.
   Milpitas, CA  95035
   US

   Phone: (408) 525 8651
   Email: mcgrew@cisco.com
   URI:   http://www.mindspring.com/~dmcgrew/dam.htm


   Flemming Andreason
   Cisco Systems, Inc.
   499 Thornall Street
   Edison, NJ  08837
   US

   Email: fandreas@cisco.com


   Dan Wing
   Cisco Systems, Inc.
   510 McCarthy Blvd.
   Milpitas, CA  95035
   US

   Phone: (408) 853 4197
   Email: dwing@cisco.com


   Kai Fischer
   Siemens Enterprise Communications GmbH & Co. KG
   Hofmannstr. 51
   Munich, Bavaria  81739
   Germany

   Email: kai.fischer@siemens-enterprise.com












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