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

AVTCORE Working Group                                          D. McGrew
Internet-Draft                                                   D. Wing
Intended status: Standards Track                            F. Andreasen
Expires: August 18, 2014                                           Cisco
                                                       February 14, 2014


                 Encrypted Key Transport for Secure RTP
                     draft-ietf-avtcore-srtp-ekt-02

Abstract

   Encrypted Key Transport (EKT) is an extension to Secure Real-time
   Transport Protocol (SRTP) that provides for the secure transport of
   SRTP master keys, Rollover Counters, and other information.  This
   facility enables SRTP to work for decentralized conferences with
   minimal control.

   This note defines EKT, and also describes how to use it with SDP
   Security Descriptions, DTLS-SRTP, and MIKEY.  With EKT, these other
   key management protocols provide an EKT key to everyone in a session,
   and EKT coordinates the SRTP 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 August 18, 2014.

Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  History . . . . . . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Conventions Used In This Document . . . . . . . . . . . .   5
   2.  Encrypted Key Transport . . . . . . . . . . . . . . . . . . .   5
     2.1.  EKT Field Formats . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Packet Processing and State Machine . . . . . . . . . . .   8
       2.2.1.  Outbound Processing . . . . . . . . . . . . . . . . .   8
       2.2.2.  Inbound Processing  . . . . . . . . . . . . . . . . .   9
     2.3.  Ciphers . . . . . . . . . . . . . . . . . . . . . . . . .  11
       2.3.1.  The Default Cipher  . . . . . . . . . . . . . . . . .  12
       2.3.2.  Other EKT Ciphers . . . . . . . . . . . . . . . . . .  12
     2.4.  Synchronizing Operation . . . . . . . . . . . . . . . . .  13
     2.5.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  13
     2.6.  Timing and Reliability Consideration  . . . . . . . . . .  14
   3.  Use of EKT with SDP Security Descriptions . . . . . . . . . .  15
     3.1.  SDP Security Descriptions Recap . . . . . . . . . . . . .  15
     3.2.  Relationship between EKT and SDP Security Descriptions  .  16
     3.3.  Overview of Combined EKT and SDP Security Description
           Operation . . . . . . . . . . . . . . . . . . . . . . . .  18
     3.4.  EKT Extensions to SDP Security Descriptions . . . . . . .  18
       3.4.1.  EKT_Cipher  . . . . . . . . . . . . . . . . . . . . .  18
       3.4.2.  EKT_Key . . . . . . . . . . . . . . . . . . . . . . .  19
       3.4.3.  EKT_SPI . . . . . . . . . . . . . . . . . . . . . . .  19
     3.5.  Offer/Answer Procedures . . . . . . . . . . . . . . . . .  19
       3.5.1.  Generating the Initial Offer - Unicast Streams  . . .  19
       3.5.2.  Generating the Initial Answer - Unicast Streams . . .  20
       3.5.3.  Processing of the Initial Answer - Unicast Streams  .  21
     3.6.  SRTP-Specific Use Outside Offer/Answer  . . . . . . . . .  22
     3.7.  Modifying the Session . . . . . . . . . . . . . . . . . .  23
     3.8.  Backwards Compatibility Considerations  . . . . . . . . .  23
     3.9.  Grammar . . . . . . . . . . . . . . . . . . . . . . . . .  24
   4.  Use of EKT with DTLS-SRTP Key Transport . . . . . . . . . . .  25
     4.1.  DTLS-SRTP Recap . . . . . . . . . . . . . . . . . . . . .  25
     4.2.  EKT Extensions to DTLS-SRTP . . . . . . . . . . . . . . .  25
     4.3.  Offer/Answer Considerations . . . . . . . . . . . . . . .  27
       4.3.1.  Generating the Initial Offer  . . . . . . . . . . . .  27
       4.3.2.  Generating the Initial Answer . . . . . . . . . . . .  28
       4.3.3.  Processing the Initial Answer . . . . . . . . . . . .  28
       4.3.4.  Sending DTLS EKT Key Reliably . . . . . . . . . . . .  29



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       4.3.5.  Modifying the Session . . . . . . . . . . . . . . . .  29
   5.  Use of EKT with MIKEY . . . . . . . . . . . . . . . . . . . .  29
     5.1.  EKT extensions to MIKEY . . . . . . . . . . . . . . . . .  31
     5.2.  Offer/Answer considerations . . . . . . . . . . . . . . .  32
       5.2.1.  Generating the Initial Offer  . . . . . . . . . . . .  32
       5.2.2.  Generating the Initial Answer . . . . . . . . . . . .  33
       5.2.3.  Processing the Initial Answer . . . . . . . . . . . .  33
       5.2.4.  Modifying the Session . . . . . . . . . . . . . . . .  34
   6.  Using EKT for interoperability between key management systems  34
   7.  Design Rationale  . . . . . . . . . . . . . . . . . . . . . .  35
     7.1.  Alternatives  . . . . . . . . . . . . . . . . . . . . . .  36
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  36
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  37
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  38
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  39
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  39
     11.2.  Informative References . . . . . . . . . . . . . . . . .  40
   Appendix A.  Using EKT to Optimize Interworking DTLS-SRTP with
                Security Descriptions  . . . . . . . . . . . . . . .  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

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, that participant needs to be told the SRTP keys (and SSRC,
   ROC and other details) of the other SRTP sources.

   The inability of SRTP to work in the absence of central control was
   well understood during the design of the protocol; the 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 external signaling control that is needed in an SRTP
   session.  EKT securely distributes the SRTP master key and other
   information for each SRTP source (SSRC), using SRTCP or SRTP to



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   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 (SSRC) with new SRTP master keys (see Section 2.2) within a
   session without coordinating with other entities via external
   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 (SSRC) to replace an old SRTP source that has reached
   the packet-count limit.  However, EKT does not allow SRTP's ROC to
   rollover; that requires re-keying outside of EKT (e.g., using MIKEY
   or DTLS-SRTP).  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 can simplify many architectures that implement SRTP.

   EKT provides a way for an SRTP session participant, either a 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.

   EKT does not control the manner in which the SSRC is generated; it is
   only concerned 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 (SSRC) among every SRTP participant.

1.1.  History

   [[RFC Editor Note: please remove this section prior to publication as
   an RFC.]]

   A substantial change occurred between the EKT documents draft-ietf-
   avt-srtp-ekt-03 and draft-ietf-avtcore-srtp-ekt-00.  The change makes
   it possible for the EKT data to be removed from a packet without
   affecting the ability of the receiver to correctly process the data
   that is present in that packet.  This capability facilitates
   interoperability between SRTP implementations with different SRTP key
   management methods.  The changes also greatly simplify the EKT



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   processing rules, and makes the EKT data that must be carried in SRTP
   and/or SRTCP packets somewhat larger.

   In draft-ietf-avtcore-srtp-ekt-02 (this document), SRTP master keys
   have to be always generated randomly and never shared, MKI is no
   longer allowed with EKT (as MKI duplicates some of EKT's functions),
   and text clarifies that EKT must be negotiated during call setup.
   Some text was consolidated and re-written, notably Section 2.6
   ("Timing and Reliability").

1.2.  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].

2.  Encrypted Key Transport

   In EKT, an SRTP master key is encrypted with a key encrypting key and
   the resulting ciphertext is transported in selected SRTCP packets or
   in selected SRTP packets.  The key encrypting key is called an EKT
   key.  A single such key suffices for a single SRTP session,
   regardless of the number of participants in that session.  However,
   there can be multiple EKT keys used within a particular session.

   EKT defines a new method of providing SRTP master keys to an
   endpoint.  In order to convey the ciphertext of the SRTP master key,
   and other additional information, an additional EKT field is added to
   SRTP or SRTCP packets.  When added to SRTCP, the EKT field appears at
   the end of the packet, after the authentication tag, if that tag is
   present, or after the SRTCP index otherwise.  When added to SRTP, The
   EKT field appears at the end of the SRTP packet, after the
   authentication tag (if that tag is present), or after the ciphertext
   of the encrypted portion of the packet otherwise.

   EKT MUST NOT be used in conjunction with SRTP's MKI (Master Key
   Identifier) or with SRTP's <From, To> [RFC3711], as those SRTP
   features duplicate some of the functions of EKT.

2.1.  EKT Field Formats

   The EKT Field 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 Field, which is also the final bit
   of the SRTP or SRTCP packet.  The first format is the Full EKT Field
   (or Full_EKT_Field), and the second is the Short EKT Field (or
   Short_EKT_Field).  The formats are defined as




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        EKT_Plaintext = SRTP_Master_Key || SSRC || ROC || ISN

        EKT_Ciphertext = EKT_Encrypt(EKT_Key, EKT_Plaintext)

        Full_EKT_Field = EKT_Ciphertext || SPI || '1'

        Short_EKT_Field = Reserved || '0'

                        Figure 1: EKT data formats

   Here || denotes concatenation, and '1' and '0' denote single one and
   zero bits, respectively.  These fields and data elements are defined
   as follows:

   EKT_Plaintext:  The data that is input to the EKT encryption
      operation.  This data never appears on the wire, and is used only
      in computations internal to EKT.

   EKT_Ciphertext:  The data that is output from the EKT encryption
      operation, described in Section 2.3.  This field is included in
      SRTP and SRTCP packets when EKT is in use.  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 can be inferred from that field.

   SRTP_Master_Key:  On the sender side, the SRTP Master Key associated
      with the indicated SSRC.  The length of this field depends on the
      cipher suite negotiated during call setup for SRTP or SRTCP.

   SSRC:  On the sender side, this field is the SSRC for this SRTP
      source.  The length of this field is fixed at 32 bits.

   Rollover Counter (ROC):  On the sender side, this field is set to the
      current value of the SRTP rollover counter in the SRTP context
      associated with the SSRC in the SRTP or SRTCP packet.  The length
      of this field is fixed at 32 bits.

   Initial Sequence Number (ISN):  If this field is nonzero, 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 EKT Ciphertext field of this packet.  When this field
      is present in an RTCP packet it indicates the RTP sequence number
      of the first RTP packet encrypted by this master key.  If the ISN
      field is zero, it indicates that the initial RTP/RTCP 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, and thus should be used as the current master key



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      for processing this packet.  The length of this field is fixed at
      16 bits.

   Security Parameter Index (SPI):  This field is included in SRTP and
      SRTCP packets when EKT is in use.  It indicates the appropriate
      EKT 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 length of this field is fixed at 15 bits.  The
      parameters identified by this field are:

      *  The EKT 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.
         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.

      Together, these data 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.

   Reserved:  The length of this field is 7 bits.  MUST be all zeros on
      transmission, and MUST be ignored on reception.

   The Full_EKT_Field and Short_EKT_Field formats are shown in Figure 2
   and Figure 3, respectively.  These figures show the on-the-wire data.
   The Ciphertext field holds encrypted data, and thus has no apparent
   inner structure.

      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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     :                                                               :
     :                        EKT Ciphertext                         :
     :                                                               :
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Security Parameter Index  |1|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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                      Figure 2: Full EKT Field format

                             0 1 2 3 4 5 6 7 8
                            +-+-+-+-+-+-+-+-+-+
                            |   Reserved    |0|
                            +-+-+-+-+-+-+-+-+-+

                     Figure 3: Short EKT Field format

2.2.  Packet Processing and State Machine

   At any given time, each SRTP/SRTCP source (SSRC) 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 sources in the same session, including other SRTP/SRTCP sources
   on the same endpoint (e.g., one endpoint with voice and video might
   have two EKT parameter sets, or there might be multiple video sources
   on an endpoint each with their own EKT parameter set).  All of these
   EKT parameter sets SHOULD be stored by all of the participants in an
   SRTP session, for use in processing inbound SRTP and SRTCP traffic.

   All SRTP master keys MUST NOT be re-used, MUST be randomly generated
   according to [RFC4086], MUST NOT be equal to or derived from other
   SRTP master keys.

2.2.1.  Outbound Processing

   See Section 2.6 which describes when to send an EKT packet and
   describes if a Full EKT Field or Short EKT Field is sent.

   When an SRTP or SRTCP packet is to be sent, the EKT field for that
   packet is created as follows, or uses an equivalent set of steps.
   The creation of the EKT field MUST precede the normal SRTP or SRTCP
   packet processing.  The ROC used in EKT processing MUST be the same
   as the one used in the SRTP processing.

   If the Short format is used, an all-zero octet is appended to the
   packet.  Otherwise, processing continues 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



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   RTP sequence number of the initial RTP packet that was or will be
   protected by that key.  See "rekey" in Section 2.6.  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).
   This means rekeying near the end of sequence number space (e.g., 100
   packets before sequence number 65535) is not possible because ROC
   needs to roll over.

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

   The EKT_Plaintext field is computed from the SRTP Master Key, SSRC,
   ROC, and ISN fields, as shown in Figure 1.

   The EKT_Ciphertext field is set to the ciphertext created by
   encrypting the EKT_Plaintext with the EKT cipher, using the EKT Key
   as 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.

   Implementation note: Because of the format of the Full EKT Field, a
   packet containing the Full EKT Field MUST be sent when the ROC
   changes (i.e., every 2^32 packets).

2.2.2.  Inbound Processing

   When an SRTP or SRTCP packet containing a Full EKT Field or Short EKT
   Field is received, it is processed as follows or using an equivalent
   set of steps.  Inbound EKT processing MUST take place prior to the
   usual SRTP or SRTCP processing.  Implementation note: the receiver
   may want to have a sliding window to retain old master keys for some
   brief period of time, so that out of order packets can be processed.
   The following steps show processing as packets are received in order.

   1.  The final bit is checked to determine which EKT format is in use.
       If the packet contains a Short EKT Field then the Short EKT Field
       is removed and normal SRTP or SRTCP processing is applied.  If
       the packet contains a Full EKT Field, then processing continues
       as described below.

   2.  The Security Parameter Index (SPI) field is checked to determine
       which EKT parameter set should be used when processing the
       packet.  If multiple parameter sets have been defined for the
       SRTP session, then the one that is associated with the value of
       the SPI 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



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       authentication failure, and the steps described below are not
       performed.

   3.  The EKT_Ciphertext is decrypted using the EKT_Key and EKT_Cipher
       in the matching parameter set, as described in Section 2.3.  If
       the EKT decryption operation returns an authentication failure,
       then the packet processing halts with an indication of failure.
       Otherwise, the resulting EKT_Plaintext is parsed as described in
       Figure 1, to recover the SRTP Master Key, SSRC, ROC, and ISN
       fields.

   4.  The SSRC field output from the decryption operation is compared
       to the SSRC field from the SRTP header.  If the values of the two
       fields do not match, then packet processing halts with an
       indication of failure.  Otherwise, it continues as follows.

   5.  If an SRTP context associated with the SSRC in the previous step
       already exists and the ROC from the EKT_Plaintext is less than
       the ROC in the SRTP context, then EKT processing halts and the
       packet is processed as an out-of-order packet (if within the
       implementation's sliding window) or dropped (as it is a replay).
       Otherwise, the ROC in the SRTP context is set to the value of the
       ROC from the EKT_Plaintext, and the SRTP Master Key from the
       EKT_Plaintext is accepted as the SRTP master key corresponding to
       the SSRC indicated in the EKT_Plaintext, beginning at the
       sequence number indicated by the ISN (see next step).

   6.  If the ISN from the EKT_Plaintext is less than the RTP sequence
       number of an authenticated received SRTP packet, then EKT
       processing halts (as this is a replay).  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.

   7.  The newly accepted SRTP master key, the SRTP parameters from the
       matching parameter set, and the SSRC from the packet are stored
       in the crypto context associated with the SRTP source (SSRC).
       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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   8.  At this point, EKT processing has successfully completed, and the
       normal SRTP or SRTCP processing takes place.

          Implementation note: the value of the EKT Ciphertext field is
          identical in successive packets protected by the same EKT
          parameter set and the same SRTP master key, ROC, and ISN.
          This ciphertext value MAY be cached by an SRTP receiver to
          minimize computational effort by noting when the SRTP master
          key is unchanged and avoiding repeating Steps 2 through 6.

2.3.  Ciphers

   EKT uses an authenticated cipher to encrypt the EKT Plaintext, which
   is comprised of the SRTP master keys, SSRC, ROC, and ISN.  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.  The default cipher described
   in Section 2.3.1 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., key management).

   The master salt length for the offered cipher suites MUST be the
   same.  In practice the easiest way to achieve this is by offering the
   same crypto suite.

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

   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, or returns an indication that the decryption operation failed
   because the ciphertext was invalid (i.e. it was not generated by the
   encryption of plaintext with the key K).

   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



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   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.

   The length of the EKT Plaintext is ten bytes, plus the length of the
   SRTP Master Key.

   Security requirements for EKT ciphers are discussed in Section 8.

2.3.1.  The Default Cipher

   The default EKT Cipher is the Advanced Encryption Standard (AES)
   [FIPS197] Key Wrap with Padding [RFC5649] algorithm.  It requires a
   plaintext length M that is at least one octet, and it returns a
   ciphertext with a length of N = M + 8 octets.  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.

                               length of  length of
        SRTP         EKT          EKT        EKT        length of
      transform   transform    plaintext  ciphertext  Full EKT Field
      ---------  ------------  ---------  ----------  --------------
      AES-128    AESKW_128 (m)    26          40            42
      AES-192    AESKW_192        34          48            50
      AES-256    AESKW_256        42          56            58
      F8-128     AESKW_128        26          40            42
      SEED-128   AESKW_128        26          40            42

                           Figure 4: AESKW Table

   The mandatory to implement transform is AESKW_128, indicated by (m).

   As AES-128 is the mandatory to implement transform in SRTP [RFC3711],
   AESKW_128 MUST be implemented for EKT.

   For all the SRTP transforms listed in the table, the corresponding
   EKT transform MUST be used, unless a stronger EKT transform is
   negotiated by key management.

2.3.2.  Other EKT Ciphers

   Other specifications may extend this one by defining other EKT
   ciphers per Section 9.  This section defines how those ciphers
   interact with this specification.




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   An EKT cipher determines how the EKT Ciphertext 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 parameter
   set to protect outbound packets after it accepts that EKT parameter
   set 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.

   If a source has its EKT key changed by key management, it MUST also
   change its SRTP master key, which will cause it to send out a new
   Full EKT Field.  This ensures that if key management thought the EKT
   key needs changing (due to a participant leaving or joining) and
   communicated that in key management to a source, the source will also
   change its SRTP master key, so that traffic can be decrypted only by
   those who know the current EKT key.

   The use of EKT MUST be negotiated during key management or call setup
   (e.g., using DTLS-SRTP, Security Descriptions, MIKEY, or similar).

2.5.  Transport

   EKT SHOULD be used over SRTP, and MAY be used over SRTCP.  SRTP is
   preferred because it shares fate with transmitted media and because
   SRTP rekeying can occur without concern for RTCP transmission limits.

   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.

   The Full EKT Field is appended to the SRTP or SRTCP payload and is
   42, 50, or 58 octets long for AES-128, AES-192, or AES-192,
   respectively.  This length impacts the maximum payload size of the
   SRTP (or SRTCP) packet itself.  To remain below the network path MTU,
   senders SHOULD constrain the SRTP (or SRTCP) payload size by this
   length of the Full EKT Field.

   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



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   out of order.  This can lead to various race conditions if EKT is
   transported over SRTCP but not SRTP, which we review below.

   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].

2.6.  Timing and Reliability Consideration

   A system using EKT has the SRTP keys distributed with EKT, rather
   than with call signaling.  This means a receiver can immediately
   decrypt an SRTP (or SRTCP packet) using that new key, provided the
   SRTP packet (or SRTCP packet) also contains the EKT key.  However, a
   receiver cannot immediately authenticate or decrypt an SRTCP packets
   without the EKT key.  Fortunately, the inability to authenticate or
   decrypt SRTCP has little immediate impact on the end user.

   This section describes how to reliably and expediently deliver EKT
   keys to receivers.

   There are three cases to consider.  The first case is a new sender
   joining a session which needs to communicate its key to all the
   receivers.  The second case is a sender changing its key which needs
   to be communicated to all the receivers.  The third case is a new
   receiver joining a session already in progress which needs to know
   the sender's key.

   New sender: A new sender SHOULD send a packet containing the Full EKT
   Field as soon as possible, always before or coincident with sending
   its initial SRTP packet.  To accommodate packet loss, it is
   RECOMMENDED that three consectutive packets contain the Full EKT
   Field be transmitted.  Inclusion of that Full EKT Field can be
   stopped early if the sender determines all receivers have received
   the new EKT key by receipt of an SRTCP receiver report or explicit
   ACK for a sequence number with the new key.

   Rekey: By sending EKT over SRTP, the rekeying event shares fate with
   the SRTP packets protected with that new key.  To avoid sending large
   SRTP packets (such as video key frames) with the Full EKT Field, it
   can be advantageous to send smaller SRTP packets with the Full EKT
   Field with the Initial Sequence Number prior to the actual rekey
   event, but this does eliminate the benefits of fate-sharing EKT with
   the SRTP packets with the new key, which increases the chance a new
   receiver won't have seen the new key.

   New receiver: When a new receiver joins a session it does not need to
   communicate its sending key (because it is a receiver).  When a new



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   receiver joins a session the sender is generally unaware of the
   receiver joining the session.  Thus, senders SHOULD periodically
   transmit the Full EKT Field.  That interval depends on how frequently
   new receivers join the session, the acceptable delay before those
   receivers can start processing SRTP packets, and the acceptable
   overhead of sending the Full EKT Field.  The RECOMMENDED frequency is
   the same as the key frame frequency if sending video or every 5
   seconds.  When joining a session it is likely that SRTP or SRTCP
   packets might be received before a packet containing the Full EKT
   Field is received.  Thus, to avoid doubling the authentication
   effort, an implementation joining an EKT session SHOULD buffer
   received SRTP and SRTCP packets until it receives the Full EKT Field
   packet and use the information in that packet to authenticate and
   decrypt the received SRTP/SRTCP packets.

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 with the
   "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 5 illustrate these
   parameters.

   v=0
   o=sam 2890844526 2890842807 IN IP4 192.0.2.5



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   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
      FEC_ORDER=FEC_SRTP
   a=crypto:2 F8_128_HMAC_SHA1_80
      inline:MTIzNDU2Nzg5QUJDREUwMTIzNDU2Nzg5QUJjiiKt|2^20
      FEC_ORDER=FEC_SRTP

                Figure 5: 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 and the master key lifetime (number of 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", the crypto-suite
   F8_128_HMAC_SHA1_80, two SRTP master keys and associated salts.
   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 SDP Security Descriptions are complementary.  SDP Security
   Descriptions can negotiate several of the SRTP security parameters
   (e.g., cipher and use of Master Key Identifier) 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



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   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 SDESC 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
   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.



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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 packet from
   the peer.  Once a valid EKT packet is received for the SSRC, the
   crypto context is initialized accordingly, and the SRTP master key
   will then be derived from the EKT packet.  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.  This is done to avoid race conditions
   between the offer/answer exchange and EKT, even though this breaks
   some offer/answer rules.  Note that with the rules described in this
   paragraph, once a valid EKT packet 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 SDESC 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 within SRTP and SRTCP packets.  The default value
   is "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.  In the Offer/
   Answer model, the EKT_Cipher parameter is a negotiated parameter.





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3.4.2.  EKT_Key

   The (mandatory) EKT_Key parameter is the key K used to encrypt the
   SRTP Master Key within SRTP and SRTCP packets.  The value is base64
   encoded with "=" padding if padding is necessary (see Section 3.2 and
   4 of [RFC4648]).  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 3.4.
   Since SDESC is defined only for unicast streams, we provide only
   offer/answer procedures for unicast streams here as well.

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.

   [[Editor's Note: following paragraph would benefit from rewording.]]

   For each unicast media line using Security Descriptions 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



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   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 (including EKT_SPI and EKT_KEY) that
   is already in use.

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

   As EKT is not defined for use with MKI, the offer and the answer MUST
   NOT contain MKI.

   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
      a transport translator (relay) then 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 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.




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   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.
   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





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   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.

   [[Editor's Note: following paragraph would benefit from rewording.]]

   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 SRTP packet containing a Full EKT Field 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 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




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   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.

   [[Editor's Note: following paragraph would benefit from re-arranging
   into earlier-described steps.]]

   Finally, subsequent offer/answer exchanges MUST NOT remap a given SPI
   value to a different EKT parameter set until 2^15 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
   2^15 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
   associated SIP dialogs.

3.8.  Backwards Compatibility Considerations

   Backwards compatibility can be achieved in a couple of ways.  First
   of all, Security Descriptions allows for session parameters to be



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   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 SRTP or SRTCP packets prior to receipt of such an answer
   will obviously be problematic (as is normally the case for Security
   Descriptions without EKT).

   Alternatively, Security Descriptions 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 one new SDP Security Descriptions
   session parameter, EKT, comprising three parts is shown in Figure 6.

     ekt        = "EKT=" cipher "|" key "|" spi
     cipher     = cipher-ext / "AESKW_128" / "AESKW_192" / "AESKW_256"
     cipher-ext = 1*64(ALPHA / DIGIT / "_")
     key        = 1*(base64)    ; See Section 4 of [RFC4648]
     base64     = ALPHA / DIGIT / "+" / "/" / "="
     spi        = 4HEXDIG   ; See [RFC5234]

               Figure 6: ABNF for the EKT session parameters

   Using the example from Figure 6 with the EKT extensions to SDP
   Security Descriptions results in the following example SDP:

   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=AESKW_128|WWVzQUxvdmVseUVLVGtleQ|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=AESKW_128|VHdvTG92ZWx5RUtUa2V5cw|AAE0



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


4.  Use of EKT with DTLS-SRTP Key Transport

   This document defines an extension to DTLS-SRTP called Key Transport.
   The EKT with the DTLS-SRTP Key Transport enables secure transport of
   EKT keying material from one DTLS-SRTP peer to another.  This enables
   those peers to process EKT keying material in SRTP (or SRTCP) and
   retrieve the embedded SRTP keying material.  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.  DTLS-SRTP Recap

   DTLS-SRTP [RFC5764] uses an extended DTLS exchange between two peers
   to exchange keying material, algorithms, and parapeters for SRTP.
   The SRTP flow operates over the same transport as the DTLS-SRTP
   exchange (i.e., the same 5-tuple).  DTLS-SRTP combines the
   performance and encryption flexibility benefits of SRTP with the
   flexibility and convenience of DTLS-integrated key and association
   management.  DTLS-SRTP can be viewed in two equivalent ways: as a new
   key management method for SRTP, and a new RTP-specific data format
   for DTLS.

4.2.  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
   MUST include "dtls-srtp-ekt" in its SDP (as a session or media level
   attribute) 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.

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

   attribute = "a=dtls-srtp-ekt"


   Using the syntax described in DTLS [RFC6347], the following
   structures are used:



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   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 {
    RESERVED(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 7: 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 SRTP key carried by
   EKT.

   Client                                               Server

   ClientHello + use_srtp + EKT
                                -------->
                                 ServerHello + use_srtp + EKT
                                                 Certificate*



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                                           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 8: Handshake Message Flow

4.3.  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).

4.3.1.  Generating the Initial Offer

   The initial offer contains a new SDP attribute, "dtls-srtp-ekt",
   which contains no value.  This attribute MUST only appear at the
   media level.  This attribute 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.

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








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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 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.


4.3.2.  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.  DTLS messages should be constructed according
   to those two attributes.

   If the answerer does not wish to perform EKT, it MUST NOT include a
   =dtls-srtp-ekt in the SDP answer, and it MUST NOT negotiate EKT
   during its DTLS-SRTP exchange.

   Otherwise, the dtls-srtp-ekt attribute SHOULD be included in the
   answer, and EKT SHOULD be negotiated in the DTLS-SRTP handshake.

4.3.3.  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.  There are two
   indications the remote peer does not want to do EKT: the dtls-srtp-
   ekt attribute is not present in the answer, or the DTLS-SRTP exchange
   fails to negotiate the EKT extension.  If the dtls-srtp-ekt attribute
   is not present in the answer, the DTLS-SRTP exchange MUST NOT attempt
   to negotiate the EKT extension.  If the dtls-srtp-ekt attribute is
   present in the answer but the DTLS-SRTP exchange fails to negotiate
   the EKT extension, EKT MUST NOT be used with that media stream.

   After successful DTLS negotiation of the EKT 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



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   one (or both) peers will immediately send an EKT packet before
   sending any SRTP, and also possible that SRTP, encrypted with an
   unknown key, may be received before the EKT packet is received.

4.3.4.  Sending DTLS EKT Key Reliably

   In the absence of a round trip time estimate, the DTLS ekt_key
   message is sent using an exponential backoff initialized to 250ms, so
   that if the first message is sent at time 0, the next transmissions
   are at 250ms, 500ms, 1000ms, and so on.  If a recent round trip time
   estimate is available, exponential backoff is used with the first
   transmission at 1.5 times the round trip time estimate.  In either
   case, re-transmission stops when ekt_key_ack or ekt_key_error message
   is received for the matching message_seq.

4.3.5.  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.

   If the initial offer and initial answer both contained EKT attributes
   (indicating the answerer desired to perform EKT), a subsequent offer/
   answer exchange MUST also contain those same EKT attributes.  If not,
   operation is undefined and the sesion MAY be terminated.  If the
   initial offer and answer failed to negotiate EKT (that is, the answer
   did not contain EKT attributes), EKT negotiation failed and a
   subsequent offer SHOULD NOT include EKT attributes.

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-



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   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 [RFC4771]
   includes the ROC in a modified authentication tag but that extension
   does not support updating the SRTP master key.

   Besides the ROC, MIKEY synchronizes also the SSRC values of the SRTP
   streams.  Each sender of a stream sends the associated SSRC within
   the MIKEY message to the other party.  If an SRTP session participant
   starts a new SRTP source (SSRC) 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



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   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 of
   Policy Param TLVs.

   Prot type  | Value
   -------------------
   EKT        | TBD (will be assigned by IANA)  NOTE TO RFC EDITOR

                       Figure 9: EKT Security Policy

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

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

                 Figure 10: EKT Security Policy Parameters

   EKT cipher | Value
   -------------------
   (reserved) |  0
   AESKW_128  |  1
   AESKW_192  |  2
   AESKW_256  |  3

                     Figure 11: EKT Cipher Parameters

   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:




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                        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 12: 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 [NOTE
      TO RFC EDITOR]) to indicate transport of the EKT key.

   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 EKT_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 be present 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



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   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 media 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.

5.2.2.  Generating the Initial Answer

   For each media line in the offer using MIKEY, provided on session and
   /or 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 a 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



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   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 with SRTP and EKT and the other attribute
   offers MIKEY with SRTP without EKT.

5.2.4.  Modifying the Session

   Once an 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 if the EKT key
   or cipher did not change.

   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 an 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.

6.  Using EKT for interoperability between key management systems

   A media gateway (MGW) can provide interoperability between an SRTP-
   EKT endpoint and a non-EKT SRTP endpoint.  When doing this function,
   the MGW can perform non-cryptographic transformations on SRTP packets
   outlined above.  However, there are some uses of cryptography that
   will be required for that gateway.  If a new SRTP master key is
   communicated to the MGW (via EKT from the EKT leg, or via Security
   Descriptions without EKT from the Security Descriptions leg), the MGW
   needs to convert that information for the other leg, and that process
   will incur some cryptographic operations.  Specifically, if the new
   key arrived via EKT, the key must be decrypted and then sent in
   Security Descriptions (e.g., as a SIP re-INVITE); likewise, if a new
   key arrives via Security Descriptions that must be encrypted via EKT
   and sent in SRTP/SRTCP.




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   Additional non-normative information can be found in Appendix A.

7.  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 but uses SRTP 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 sources 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 [RFC6407].  In such a combination GDOI
   would provide a secure entity authentication method for group
   members, and a scalable way to revoke group membership; by itself,
   EKT does not attempt to provide either capability.

   EKT carries the encrypted key in a new SRTP field (at the end of the
   SRTP packet).  This maintains compatibility with the existing SRTP
   specification by defining a new crypto function that incorporates the
   encrypted key, and a new authentication transform to provide implicit
   authentication of the encrypted key.








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

7.1.  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
   acceptable.  This alternative would add six bytes to each EKT packet;
   that overhead may be a reasonable tradeoff for addressing the
   problems outlined above.  This is considered too high a bandwidth
   penalty.

8.  Security Considerations

   EKT inherits the security properties of the SRTP keying it uses:
   Security Descriptions, DTLS-SRTP, or MIKEY.

   With EKT, each SRTP sender and receiver MUST 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 remains secure even in the absence of out-of-band
   coordination of SSRCs, and even when SSRC values collide.

   The EKT Cipher includes its own authentication/integrity check.  For
   an attacker to successfully forge a full EKT packet, it would need to
   defeat the authentication mechanisms of both the EKT Cipher and the
   SRTP authentication mechanism.




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   The presence of the SSRC in the EKT_Plaintext ensures that an
   attacker cannot substitute an EKT_Ciphertext from one SRTP stream
   into another SRTP stream.

   An attacker who strips a Full_EKT_Field from an SRTP packet may
   prevent the intended receiver of that packet from being able to
   decrypt it.  This is a minor denial of service vulnerability.
   Similarly, an attacker who adds a Full_EKT_Field can disrupt service.

   An attacker could send packets containing either Short EKT Field or
   Full EKT Field, in an attempt to consume additional CPU resources of
   the receiving system.  In the case of the Short EKT Field, this field
   is stripped and normal SRTP or SRTCP processing is performed.  In the
   case of the Full EKT Field, the attacker would have to have guessed
   or otherwise determined the SPI being used by the receiving system.
   If an invalid SPI is provided by the attacker, processing stops.  If
   a valid SPI is provided by the attacker, the receiving system will
   decrypt the EKT ciphertext and return an authentication failure (Step
   3 of Section 2.2.2).

   EKT can rekey an SRTP stream until the SRTP rollover counter (ROC)
   needs to roll over.  EKT does not extend SRTP's rollover counter
   (ROC), and like SRTP itself EKT cannot properly handle a ROC
   rollover.  Thus even if using EKT, new (master or session) keys need
   to be established after 2^48 packets are transmitted in a single SRTP
   stream as described in Section 3.3.1 of [RFC3711].  Due to the
   relatively low packet rates of typical RTP sessions, this is not
   expected to be a burden.

   The confidentiality, integrity, and authentication of the EKT cipher
   MUST be at least as strong as the SRTP cipher.

   Part of the EKT_Plaintext is known, or easily guessable to an
   attacker.  Thus, the EKT Cipher MUST resist known plaintext attacks.
   In practice, this requirement does not impose any restrictions on our
   choices, since the ciphers in use provide high security even when
   much plaintext is known.

   An EKT cipher MUST resist attacks in which both ciphertexts and
   plaintexts can be adaptively chosen.  An EKT cipher MUST resist
   attacks in which both ciphertexts and plaintexts can be adaptively
   chosen and adversaries that can query both the encryption and
   decryption functions adaptively.

9.  IANA Considerations






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   IANA is requested to register EKT from Section 3.9 into the Session
   Description Protocol (SDP) Security Descriptions [iana-sdp-sdesc]
   registry for "SRTP Session Parameters".

   IANA is requested to register the following new attributes into the
   SDP Attributes registry [iana-sdp-attr].

   Attribute name:  dtls-srtp-ekt

   Long form name:  DTLS-SRTP with EKT

   Type of attribute:  Media-level

   Subject to charset:  No

   Purpose:    Indicates support for DTLS-SRTP with EKT

   Appropriate values:  No values

   Contact name:  Dan Wing, dwing@cisco.com

   We request the following IANA assignments from the existing
   [iana-mikey] name spaces in the IETF consensus range (0-240)
   [RFC3830]:

   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 9).

   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
      10

   o  EKT cipher, initially populated with the list from Figure 11

10.  Acknowledgements

   Thanks to Lakshminath Dondeti for assistance with earlier versions of
   this document.  Thanks to Kai Fischer for writing the MIKEY section.

   Thanks to Nermeen Ismail, Eddy Lem, and Rob Raymond for fruitful
   discussions and comments.  Thanks to Felix Wyss for his review and



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   comments regarding ciphers.  Thanks to Michael Peck for his review.
   Thanks to John Mattsson for his detailed security review highlighting
   the duplicative interaction between SRTP MKI with EKT ISN and
   encourged the EKT specification to prohibit sharing SRTP master keys.
   Thanks to Magnus Westerlund for his review.

11.  References

11.1.  Normative References

   [FIPS197]  National Institute of Standards and Technology (NIST),
              "The Advanced Encryption Standard (AES)", FIPS-197 Federal
              Information Processing Standard, November 2001.

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

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

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [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.

   [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
              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.




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   [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.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012.

11.2.  Informative References

   [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.

   [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.

   [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.

   [RFC5649]  Housley, R. and M. Dworkin, "Advanced Encryption Standard
              (AES) Key Wrap with Padding Algorithm", RFC 5649,
              September 2009.

   [RFC6407]  Weis, B., Rowles, S., and T. Hardjono, "The Group Domain
              of Interpretation", RFC 6407, October 2011.

   [iana-mikey]
              IANA, ., "Multimedia Internet KEYing (Mikey) Payload Name
              Spaces", 2011, <http://www.iana.org/assignments/mikey-
              payloads/mikey-payloads.xhtml>.

   [iana-sdp-attr]
              IANA, ., "SDP Parameters", 2011, <http://www.iana.org/
              assignments/sdp-parameters/sdp-parameters.xml>.




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   [iana-sdp-sdesc]
              IANA, ., "Session Description Protocol (SDP) Security
              Descriptions: SRTP Session Parameters", 2011, <http://
              www.iana.org/assignments/sdp-security-descriptions/sdp-
              security-descriptions.xml#sdp-security-descriptions-4>.

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.  The IP PBX systems
   are typically used within a single enterprise.  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 13, 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 13: 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



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       way, with DTLS-SRTP, to communicate the Security Descriptions key
       to the DTLS-SRTP key endpoint.)

   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 to
   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 14, below.

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

           Figure 14: Interworking Security Descriptions and EKT

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




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

   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 sends an EKT packet indicating that SRTP 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.

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


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

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



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

   Email: fandreas@cisco.com












































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