PERC Working Group                                           C. Jennings
Internet-Draft                                                     Cisco
Intended status: Standards Track                        J. Mattsson, Ed.
Expires: October 30, December 31, 2017                                      Ericsson
                                                               D. McGrew
                                                                 D. Wing
                                                            F. Andreasen
                                                          April 28,
                                                           June 29, 2017

            Encrypted Key Transport for DTLS and Secure RTP


   Encrypted Key Transport (EKT) is an extension to DTLS and Secure
   Real-time Transport Protocol (SRTP) that provides for the secure
   transport of SRTP master keys, rollover counters, and other
   information within SRTP.  This facility enables SRTP for
   decentralized conferences by distributing a common key to all of the
   conference endpoints.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Conventions Used In This Document . . . . . . . . . . . . . .   4
   4.  Encrypted Key Transport . . . . . . . . . . . . . . . . . . .   4
     4.1.  EKT Field Formats . . . . . . . . . . . . . . . . . . . .   4
     4.2.  Packet Processing and State Machine . . . . . . . . . . .   7
       4.2.1.  Outbound Processing . . . . . . . . . . . . . . . . .   8
       4.2.2.  Inbound Processing  . . . . . . . . . . . . . . . . .   9
     4.3.  Implementation Notes  . . . . . . . . . . . . . . . . . .  10
     4.4.  Ciphers . . . . . . . . . . . . . . . . . . . . . . . . .  10
       4.4.1.  Ciphers . . . . . . . . . . . . . . . . . . . . . . .  11
       4.4.2.  Defining New EKT Ciphers  . . . . . . . . . . . . . .  12
     4.5.  Synchronizing Operation . . . . . . . . . . . . . . . . .  12
     4.6.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  12
     4.7.  Timing and Reliability Consideration  . . . . . . . . . .  13
   5.  Use of EKT with DTLS-SRTP . . . . . . . . . . . . . . . . . .  13
     5.1.  DTLS-SRTP Recap . . . . . . . . . . . . . . . . . . . . .  14
     5.2.  SRTP EKT Key Transport Extensions to DTLS-SRTP  . . . . .  14
     5.3.  Offer/Answer Considerations . . . . . . . . . . . . . . .  17
     5.4.  Sending the DTLS EKT_Key Reliably . . . . . . . . . . . .  17
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17  18
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  EKT Message Types . . . . . . . . . . . . . . . . . . . .  19
     7.2.  EKT Ciphers . . . . . . . . . . . . . . . . . . . . . . .  20
     7.3.  TLS Extensions  . . . . . . . . . . . . . . . . . . . . .  20
     7.4.  TLS Content Type  . . . . . . . . . . . . . . . . . . . .  20  21
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  21
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  21
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22  23

1.  Introduction

   Real-time Transport Protocol (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 synchronization source (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 along with the SSRC,
   rollover counter (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 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 and
   reduces the amount of external signaling control that is needed in a
   SRTP session with multiple receivers.  EKT securely distributes the
   SRTP master key and other information for each SRTP source.  With
   this method, SRTP entities are free to choose SSRC values as they see
   fit, and to start up new SRTP sources with new SRTP master keys (see
   Section 2.2) within a session without coordinating with other
   entities via external signaling or other external means.

   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 furnishes the information needed by the receiver to instantiate
   an SRTP/SRTCP receiver context.

   EKT can be used in conferences where the central media distributor or
   conference bridge can not decrypt the media, such as the type defined
   for [I-D.ietf-perc-private-media-framework].  It can also be used for
   large scale conferences where the conference bridge or media
   distributor can decrypt all the media but wishes to encrypt the media
   it is sending just once then send the same encrypted media to a large
   number of participants.  This reduces the amount of CPU time needed
   for encryption and can be used for some optimization to media sending
   that use source specific multicast.

   EKT does not control the manner in which the SSRC is generated; it is
   only concerned with their secure transport.

   EKT is not intended to replace external key establishment mechanisms.
   Instead, it is used in conjunction with those methods, and it
   relieves those methods of the burden to deliver the context for each
   SRTP source to every SRTP participant.

2.  Overview

   This specification defines a way for the server in a DTLS-SRTP
   negotiation to provide an ekt_key to the client during the DTLS
   handshake.  This ekt_key can be used to encrypt the SRTP master key
   used to encrypt the media the endpoint sends.  This specification
   also defines a way to send the encrypted SRTP master key along with
   the SRTP packet.  Endpoints that receive this and know the ekt_key
   can use the ekt_key to decrypt the SRTP master key then use the SRTP
   master key to decrypt the SRTP packet.

   One way to use this is used is described in the architecture defined
   by [I-D.ietf-perc-private-media-framework].  Each participants in the
   conference call forms a DTLS-SRTP connection to a common key
   distributor that gives all the endpoints the same ekt_key.  Then each
   endpoint picks there own SRTP master key for the media they send.
   When sending media, the endpoint also includes the SRTP master key
   encrypted with the ekt_key.  This allows all the endpoints to decrypt
   the media.

3.  Conventions Used In This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

4.  Encrypted Key Transport

   EKT defines a new method of providing SRTP master keys to an
   endpoint.  In order to convey the ciphertext corresponding to the
   SRTP master key, and other additional information, an additional EKT
   field is added to SRTP packets.  The EKT field appears at the end of
   the SRTP packet.  The EKT field appears after the optional
   authentication tag if one is present, otherwise the EKT field appears
   after the ciphertext portion of the packet.

   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.  Senders MUST not
   include MKI when using EKT.  Receivers SHOULD simply ignore any MKI
   field received if EKT is in use.

4.1.  EKT Field Formats

   The EKT Field uses the format defined below for the FullEKTField and

      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    | Length                        |
     |0 0 0 0 0 0 1 0|

                      Figure 1: Full EKT Field format

                              0 1 2 3 4 5 6 7
                             |0 0 0 0 0 0 0 0|

                     Figure 2: Short EKT Field format

   The following shows the syntax of the EKTField expressed in ABNF
   [RFC5234].  The EKTField is added to the end of an SRTP or SRTCP
   packet.  The EKTPlaintext is the concatenation of
   SRTPMasterKeyLength, SRTPMasterKey, SSRC, and ROC in that order.  The
   EKTCiphertext is computed by encrypting the EKTPlaintext using the
   EKTKey.  Future extensions to the EKTField MUST conform to the syntax
   of ExtensionEKTField.

   BYTE = %x00-FF

   EKTMsgTypeFull = %x02
   EKTMsgTypeShort = %x00
   EKTMsgTypeExtension = %x03-FF

   EKTMsgLength = 2BYTE;

   SRTPMasterKeyLength = BYTE
   SRTPMasterKey = 1*256BYTE
   SSRC = 4BYTE; SSRC from RTP

   EKTPlaintext = SRTPMasterKeyLength SRTPMasterKey SSRC ROC

   EKTCiphertext = 1*256BYTE ; EKTEncrypt(EKTKey, EKTPlaintext)
   SPI = 2BYTE

   FullEKTField = EKTCiphertext SPI EKTMsgLength EKTMsgTypeFull

   ShortEKTField = EKTMsgTypeShort

   ExtensionData = 1*1024BYTE
   ExtensionEKTField = ExtensionData EKTMsgLength EKTMsgTypeExtension

   EKTField = FullEKTField / ShortEKTField / ExtensionEKTField

                         Figure 3: EKTField Syntax

   These fields and data elements are defined as follows:

   EKTPlaintext:  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.  This is the concatenation of the
      SRTP Master Key, the SSRC, and the ROC.

   EKTCiphertext:  The data that is output from the EKT encryption
      operation, described in Section 4.4.  This field is included in
      SRTP packets when EKT is in use.  The length of EKTCiphertext can
      be larger than the length of the EKTPlaintext that was encrypted.

   SRTPMasterKey:  On the sender side, the SRTP Master Key associated
      with the indicated SSRC.

   SRTPMasterKeyLength:  The length of the SRTPMasterKey in bytes.  This
      depends on the cipher suite negotiated for SRTP using [RFC3264]
      SDP Offer/Answer for the SRTP.

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

   Security Parameter Index (SPI):  This field indicates the appropriate
      EKT Key and other parameters for the receiver to use when
      processing the packet.  The length of this field is 16 bits.  The
      parameters identified by this field are:

      *  The EKT cipher used to process the packet.

      *  The EKT Key used to process the packet.

      *  The SRTP Master Salt associated with any Master Key encrypted
         with this EKT Key. The Master Salt is communicated separately,
         via signaling, typically along with the EKTKey.

      Together, these data elements are called an EKT parameter set.
      Each distinct EKT parameter set that is used MUST be associated
      with a distinct SPI value to avoid ambiguity.

   EKTMsgLength:  All EKT message other that ShortEKTField have a length
      as second from the last element.  This is the length in octets of
      either the FullEKTField/ExtensionEKTField including this length
      field and the following message type.

   Message Type:  The last byte is used to indicate the type of the
      EKTField.  This MUST be 2 in the FullEKTField format and 0 in
      ShortEKTField format.  Values less than 64 are mandatory to
      understand while other values are optional to understand.  A
      receiver SHOULD discard the whole EKTField if it contains any
      message type value that is less than 64 and that is not
      understood.  Message type values that are 64 or greater but not
      implemented or understood can simply be ignored.

4.2.  Packet Processing and State Machine

   At any given time, each SRTP/SRTCP source has associated with it a
   single EKT parameter set.  This parameter set is used to process all
   outbound packets, and is called the outbound parameter set for that
   SSRC.  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 the received EKT parameter sets SHOULD be stored by all of the
   participants in an SRTP session, for use in processing inbound SRTP
   and SRTCP traffic.

   Either the FullEKTField or ShortEKTField is appended at the tail end
   of all SRTP packets.  The decision on which to send is specified in
   Section 4.7.

4.2.1.  Outbound Processing

   See Section 4.7 which describes when to send an SRTP packet with a
   FullEKTField.  If a FullEKTField is not being sent, then a
   ShortEKTField is sent so the receiver can correctly determine how to
   process the packet.

   When an SRTP packet is sent with a FullEKTField, the EKTField for
   that packet is created as follows, or uses an equivalent set of
   steps.  The creation of the EKTField MUST precede the normal SRTP
   packet processing.

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

   2.  The EKTPlaintext field is computed from the SRTP Master Key,
       SSRC, and ROC fields, as shown in Section 4.1.  The ROC, SRTP
       Master Key, and SSRC used in EKT processing SHOULD be the same as
       the one used in the SRTP processing.

   3.  The EKTCiphertext field is set to the ciphertext created by
       encrypting the EKTPlaintext with the EKT cipher, using the EKTKey
       as the encryption key.  The encryption process is detailed in
       Section 4.4.

   4.  Then the FullEKTField is formed using the EKTCiphertext and the
       SPI associated with the EKTKey used above.  Also appended are the
       Length and Message Type using the FullEKTField format.

          Note: the value of the EKTCiphertext field is identical in
          successive packets protected by the same EKTKey and SRTP
          master key.  This value MAY be cached by an SRTP sender to
          minimize computational effort.

       The computed value of the FullEKTField is written into the

   When a packet is sent with the Short EKT Field, the ShortEKFField is
   simply appended to the packet.

4.2.2.  Inbound Processing

   When receiving a packet on a RTP stream, the following steps are
   applied for each received packet.

   1.  The final byte is checked to determine which EKT format is in
       use.  When an SRTP or SRTCP packet contains a ShortEKTField, the
       ShortEKTField is removed from the packet then normal SRTP or
       SRTCP processing occurs.  If the packet contains a FullEKTField,
       then processing continues as described below.  The reason for
       using the last byte of the packet to indicate the type is that
       the length of the SRTP or SRTCP part is not known until the
       decryption has occurred.  At this point in the processing, there
       is no easy way to know where the EKT field would start.  However,
       the whole UDP packet has been received so instead of the starting
       at the front of the packet, the parsing works backwards off the
       end of the packet and thus the type is put at the very end of the

   2.  The Security Parameter Index (SPI) field is used to find which
       EKT parameter set to be used when processing the packet.  If
       there is no matching SPI, then the verification function MUST
       return an indication of authentication failure, and the steps
       described below are not performed.  The EKT parameter set
       contains the EKTKey, EKTCipher, and SRTP Master Salt.

   3.  The EKTCiphertext authentication is checked and it is decrypted,
       as described in Section 4.4, using the EKTKey and EKTCipher found
       in the previous step.  If the EKT decryption operation returns an
       authentication failure, then the packet processing stops.

   4.  The resulting EKTPlaintext is parsed as described in Section 4.1,
       to recover the SRTP Master Key, SSRC, and ROC fields.  The SRTP
       Master Salt that is associated with the EKTKey is also retrieved.
       If the value of the srtp_master_salt sent as part of the EKTkey
       is longer than needed by SRTP, then it is truncated by taking the
       first N bytes from the srtp_master_salt field.

   5.  If the SSRC in the EKTPlaintext does not match the SSRC of the
       SRTP packet, then all the information from this EKTPlaintext MUST
       be discarded and the following steps in this list are not done.

   6.  The SRTP Master Key, ROC, and SRTP Master Salt from the previous
       step are saved in a map indexed by the SSRC found in the
       EKTPlaintext and can be used for any future crypto operations on
       the inbound packets with that SSRC.  If the SRTP Master Key
       recovered from the EKTPlaintext is longer than needed by SRTP
       transform in use, the first bytes are used.  If the SRTP Master
       Key recovered from the EKTPlaintext is shorter than needed by
       SRTP transform in use, then the bytes received replace the first
       bytes in the existing key but the other bytes after that remain
       the same as the old key.  This allows for replacing just half the
       key for transforms such as [I-D.ietf-perc-double].  Outbound
       packets SHOULD continue to use the old SRTP Master Key for 250 ms
       after sending any new key.  This gives all the receivers in the
       system time to get the new key before they start receiving media
       encrypted with the new key.

   7.  At this point, EKT processing has successfully completed, and the
       normal SRTP or SRTCP processing takes place including replay

4.3.  Implementation Notes

   The value of the EKTCiphertext field is identical in successive
   packets protected by the same EKT parameter set and the same SRTP
   master key, and ROC.  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 the above steps.

   The receiver may want to have a sliding window to retain old SRTP
   master keys (and related context) for some brief period of time, so
   that out of order packets can be processed as well as packets sent
   during the time keys are changing.

   When receiving a new EKTKey, implementations need to use the ekt_ttl
   to create a time after which this key cannot be used and they also
   need to create a counter that keeps track of how many times the keys
   has been used to encrypt data to ensure it does not exceed the T
   value for that cipher.  If either of these limits are exceeded, the
   key can no longer be used for encryption.  At this point
   implementation need to either use the call signaling to renegotiation
   a new session or need to terminate the existing session.  Terminating
   the session is a reasonable implementation choice because these
   limits should not be exceeded except under an attack or error

4.4.  Ciphers

   EKT uses an authenticated cipher to encrypt and authenticate the
   EKTPlaintext.  This specification defines the interface to the
   cipher, in order to abstract the interface away from the details of
   that function.  This specification also defines the default cipher
   that is used in EKT.  The default cipher described in Section 4.4.1
   MUST be implemented, but another cipher that conforms to this
   interface MAY be used.

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

   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 may be larger than 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
   times that it can be used with any fixed key value.  The EKTKey MUST
   NOT be used for encryption more that T times.  Note that if the same
   FullEKTField is retransmitted 3 times, that only counts as 1

   Security requirements for EKT ciphers are discussed in Section 6.

4.4.1.  Ciphers

   The default EKT Cipher is the Advanced Encryption Standard (AES) 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 + (M mod 8) + 8 octets.  It can be used with key
   sizes of L = 16, and L = 32 octets, and its use with those key sizes
   is indicated as AESKW128, or AESKW256, respectively.  The key size
   determines the length of the AES key used by the Key Wrap algorithm.
   With this cipher, T=2^48.

                         | Cipher   |  L |    T |
                         | AESKW128 | 16 | 2^48 |
                         |          |    |      |
                         | AESKW256 | 32 | 2^48 |

                           Table 1: EKT Ciphers

   As AES-128 is the mandatory to implement transform in SRTP [RFC3711],
   AESKW128 MUST be implemented for EKT and AESKW256 MAY be implemented.

4.4.2.  Defining New EKT Ciphers

   Other specifications may extend this document by defining other
   EKTCiphers as described in Section 7.  This section defines how those
   ciphers interact with this specification.

   An EKTCipher determines how the EKTCiphertext 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, and T MUST be defined by
   each EKTCipher.  The cipher MUST provide integrity protection.

4.5.  Synchronizing Operation

   If a source has its EKTKey changed by the key management, it MUST
   also change its SRTP master key, which will cause it to send out a
   new FullEKTField.  This ensures that if key management thought the
   EKTKey needs changing (due to a participant leaving or joining) and
   communicated that 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 EKTKey.

4.6.  Transport

   EKT SHOULD be used over SRTP, and other specification MAY define how
   to use it over SRTCP.  SRTP is preferred because it shares fate with
   transmitted media, because SRTP rekeying can occur without concern
   for RTCP transmission limits, and to avoid SRTCP compound packets
   with RTP translators and mixers.

4.7.  Timing and Reliability Consideration

   A system using EKT learns the SRTP master keys distributed with
   FullEKTFields sent with the SRTP, rather than with call signaling.  A
   receiver can immediately decrypt an SRTP packet, provided the SRTP
   packet contains a Full EKT Field.

   This section describes how to reliably and expediently deliver new
   SRTP master keys to receivers.

   There are three cases to consider.  The first case is a new sender
   joining a session which needs to communicate its SRTP master key to
   all the receivers.  The second case is a sender changing its SRTP
   master 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 SRTP master key.

   The three cases are:

   New sender:  A new sender SHOULD send a packet containing the
      FullEKTField as soon as possible, always before or coincident with
      sending its initial SRTP packet.  To accommodate packet loss, it
      is RECOMMENDED that three consecutive packets contain the Full EKT
      Field be transmitted.

   Rekey:  By sending EKT over SRTP, the rekeying event shares fate with
      the SRTP packets protected with that new SRTP master key.  To
      accommodate packet loss, it is RECOMMENDED that three consecutive
      packets contain the FullEKTField be transmitted.

   New receiver:  When a new receiver joins a session it does not need
      to communicate its sending SRTP master key (because it is a
      receiver).  When a new receiver joins a session the sender is
      generally unaware of the receiver joining the session.  Thus,
      senders SHOULD periodically transmit the FullEKTField.  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 FullEKT
      Field.  If sending audio and video, the RECOMMENDED frequency is
      the same as the rate of intra coded video frames.  If only sending
      audio, the RECOMMENDED frequency is every 100ms.

5.  Use of EKT with DTLS-SRTP

   This document defines an extension to DTLS-SRTP called SRTP EKT Key
   Transport which enables secure transport of EKT keying material from
   one DTLS-SRTP peer to another.  This allows 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, which has strong authentication
   of the endpoint and flexibility, along with allowing secure
   multiparty RTP with loose coordination and efficient communication of
   per-source keys.

5.1.  DTLS-SRTP Recap

   DTLS-SRTP [RFC5764] uses an extended DTLS exchange between two peers
   to exchange keying material, algorithms, and parameters 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.

5.2.  SRTP EKT Key Transport Extensions to DTLS-SRTP

   This document defines a new TLS negotiated extension called
   "srtp_ekt_key_transport"and a new TLS content type called EKTMessage.

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

                 enum {
                 } EKTCipherType;

                 struct {
                   EKTCipherType ekt_ciphers<1..255>;
                 } SupportedEKTCiphers;

                 struct {
                   EKTCipherType ekt_cipher;
                   uint ekt_key_value<1..256>;
                   uint srtp_master_salt<1..256>;
                   uint16 ekt_spi;
                   uint24 ekt_ttl;
                 } EKTkey;

                 enum {
                 } EKTMessageType;

                 struct {
                   EKTMessageType ekt_message_type;
                   select (EKTMessage.ekt_message_type) {
                   case ekt_key:
                   } message;
                 } EKTMessage;

                 Figure 4: Additional TLS Data Structures

   If a DTLS client includes "srtp_ekt_key_transport" in its
   ClientHello, then a DTLS server that supports this extensions will
   includes "srtp_ekt_key_transport" in its ServerHello message.  If a
   DTLS client includes "srtp_ekt_key_transport" in its ClientHello, but
   does not receive "srtp_ekt_key_transport" in the ServerHello, the
   DTLS client MUST NOT send DTLS EKTMessage messages.  Also, the
   "srtp_ekt_key_transport" in the ServerHello MUST select one and only
   one EKTCipherType from the list provided by the client in the
   "srtp_ekt_key_transport" in the ClientHello.

   When a DTLS client sends the "srtp_ekt_key_transport" in its
   ClientHello message, it MUST include the SupportedEKTCiphers as the
   extension_data for the extension, listing the EKTCipherTypes the
   client is willing to use in preference order, with the most preferred
   version first.  When the server responds in the
   "srtp_ekt_key_transport" in its ServerHello message, it MUST include
   a SupportedEKTCiphers list that selects a single EKTCipherType to use
   (selected from the list provided by the client) or it returns an
   empty list to indicate there is no matching EKTCipherType in the
   clients list that the server is also willing to use.  The value to be
   used in the EKTCipherType for future extensions that define new
   ciphers is the value from the "EKT Ciphers Type" IANA registry
   defined in Section 7.2.

   The figure above defines the contents for a new TLS content type
   called EKTMessage which is registered in Section 7.4.  The EKTMessage
   above is used as the opaque fragment in the TLSPlaintext structure
   defined in Section 6.2.1 of [RFC5246] and the "srtp_ekt_message" as
   the content type.  The "srtp_ekt_message" content type is defined and
   registered in Section 7.3.

   ekt_ttl:  The maximum amount of time, in seconds, that this
      ekt_key_value can be used.  The ekt_key_value in this message MUST
      NOT be used for encrypting or decrypting information after the TTL

   When the Server wishes to provide a new EKT Key, it can send
   EKTMessage containing an EKTKey with the new key information.  The
   client MUST respond with an EKTMessage of type ekt_key_ack, if the
   EKTKey was successfully processed and stored or respond with the the
   ekt_key_error EKTMessage otherwise.

   The diagram below shows a message flow of DTLS client and DTLS server
   using the DTLS-SRTP Key Transport extension.

   Client                                               Server

   ClientHello + use_srtp + srtp_ekt_key_trans
                                <--------     ServerHelloDone
   Finished                     -------->
                                <--------            Finished
   ekt_key                      <--------
   ekt_key_ack                  -------->
   SRTP packets                 <------->      SRTP packets
   SRTP packets                 <------->      SRTP packets
   ekt_key (rekey)              <-------
   ekt_key_ack                  -------->
   SRTP packets                 <------->      SRTP packets
   SRTP packets                 <------->      SRTP packets

                     Figure 5: DTLS/SRTP Message Flow

   Note that when used in PERC [I-D.ietf-perc-private-media-framework],
   the Server is actually split between the Media Distrbutor and Key
   Distributor.  The messages in the above figure that are "SRTP
   packets" will not got to the Key Distributor but the oter packets
   will be relayed by the Media Distributor to the Key Distributor.

5.3.  Offer/Answer Considerations

   When using EKT with DTLS-SRTP, the negotiation to use EKT is done at
   the DTLS handshake level and does not change the [RFC3264] Offer /
   Answer messaging.

5.4.  Sending the DTLS EKT_Key Reliably

   The DTLS ekt_key is sent using the retransmissions specified in
   Section 4.2.4. of DTLS [RFC6347].

6.  Security Considerations

   EKT inherits the security properties of the DTLS-SRTP (or other)
   keying it uses.

   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 when SSRC values collide.

   SRTP master keys MUST be randomly generated, and [RFC4086] offers
   some guidance about random number generation.  SRTP master keys MUST
   NOT be re-used for any other purpose, and SRTP master keys MUST NOT
   be derived from other SRTP master keys.

   The EKT Cipher includes its own authentication/integrity check.  For
   an attacker to successfully forge a FullEKTField, it would need to
   defeat the authentication mechanisms of the EKT Cipher authentication

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

   An attacker who tampers with the bits in FullEKTField can prevent the
   intended receiver of that packet from being able to decrypt it.  This
   is a minor denial of service vulnerability.  Similarly the attacker
   could take an old FullEKTField from the same session and attach it to
   the packet.  The FullEKTField would correctly decode and pass
   integrity but the key extracted from the FullEKTField , when used to
   decrypt the SRTP payload, would be wrong and the SRTP integrity check
   would fail.  Note that the FullEKTField only changes the decryption
   key and does not change the encryption key.  None of these are
   considered significant attacks as any attacker that can modify the
   packets in transit and cause the integrity check to fail.

   An attacker could send packets containing a Full EKT Field, in an
   attempt to consume additional CPU resources of the receiving system
   by causing the receiving system will decrypt the EKT ciphertext and
   detect an authentication failure.  In some cases, caching the
   previous values of the Ciphertext as described in Section 4.3 helps
   mitigate this issue.

   Each EKT cipher specifies a value T that is the maximum number of
   times a given key can be used.  An endpoint MUST NOT encrypt more
   than T different Full EKT Field using the same EKTKey.  In addition,
   the EKTKey MUST NOT be used beyond the lifetime provided by the TTL
   described in Section 5.2.

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

   Part of the EKTPlaintext 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 and adversaries that can query
   both the encryption and decryption functions adaptively.

   In some systems, when a member of a conference leaves the
   conferences, the conferences is rekeyed so that member no longer has
   the key.  When changing to a new EKTKey, it is possible that the
   attacker could block the EKTKey message getting to a particular
   endpoint and that endpoint would keep sending media encrypted using
   the old key.  To mitigate that risk, the lifetime of the EKTKey
   SHOULD be limited using the ekt_ttl.

7.  IANA Considerations

7.1.  EKT Message Types

   IANA is requested to create a new table for "EKT Messages Types" in
   the "Real-Time Transport Protocol (RTP) Parameters" registry.  The
   initial values in this registry are:

                 | Message Type | Value | Specification |
                 | Short        |     0 | RFCAAAA       |
                 |              |       |               |
                 | Full         |     2 | RFCAAAA       |
                 |              |       |               |
                 | Reserved     |    63 | RFCAAAA       |
                 |              |       |               |
                 | Reserved     |   255 | RFCAAAA       |

                        Table 2: EKT Messages Types

   Note to RFC Editor: Please replace RFCAAAA with the RFC number for
   this specification.

   New entries to this table can be added via "Specification Required"
   as defined in [RFC5226].  When requesting a new value, the requestor
   needs to indicate if it is mandatory to understand or not.  If it is
   mandatory to understand, IANA needs to allocate a value less than 64,
   if it is not mandatory to understand, a value greater than or equal
   to 64 needs to be allocated.  IANA SHOULD prefer allocation of even
   values over odd ones until the even code points are consumed to avoid
   conflicts with pre standard versions of EKT that have been deployed.

   All new EKT messages MUST be defined to have a length as second from
   the last element.

7.2.  EKT Ciphers

   IANA is requested to create a new table for "EKT Ciphers" in the
   "Real-Time Transport Protocol (RTP) Parameters" registry.  The
   initial values in this registry are:

                   | Name     | Value | Specification |
                   | AESKW128 |     1 | RFCAAAA       |
                   |          |       |               |
                   | AESKW256 |     2 | RFCAAAA       |
                   |          |       |               |
                   | Reserved |   255 | RFCAAAA       |

                         Table 3: EKT Cipher Types

   Note to RFC Editor: Please replace RFCAAAA with the RFC number for
   this specification.

   New entries to this table can be added via "Specification Required"
   as defined in [RFC5226].  The expert SHOULD ensure the specification
   defines the values for L and T as required in Section 4.4 of RFCAAA.
   Allocated values MUST be in the range of 1 to 254.

7.3.  TLS Extensions

   IANA is requested to add "srtp_ekt_key_transport" as an new extension
   name to the "ExtensionType Values" table of the "Transport Layer
   Security (TLS) Extensions" registry with a reference to this
   specification and allocate a value of TBD to for this.  Note to RFC
   Editor: TBD will be allocated by IANA.

   Considerations for this type of extension are described in Section 5
   of [RFC4366] and requires "IETF Consensus".

7.4.  TLS Content Type

   IANA is requested to add "srtp_ekt_message" as an new descriptions
   name to the "TLS ContentType Registry" table of the "Transport Layer
   Security (TLS) Extensions" registry with a reference to this
   specification, a DTLS-OK value of "Y", and allocate a value of TBD to
   for this content type.  Note to RFC Editor: TBD will be allocated by

   This registry was defined in Section 12 of [RFC5246] and requires
   "Standards Action".

8.  Acknowledgements

   Thank you to Russ Housley provided detailed review and significant
   help with crafting text for this document.  Thanks to David Benham,
   Yi Cheng, Lakshminath Dondeti, Kai Fischer, Nermeen Ismail, Paul
   Jones, Eddy Lem, Jonathan Lennox, Michael Peck, Rob Raymond, Sean
   Turner, Magnus Westerlund, and Felix Wyss for fruitful discussions,
   comments, and contributions to this document.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3711]  Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, DOI 10.17487/RFC3711, March 2004,

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,

   [RFC5649]  Housley, R. and M. Dworkin, "Advanced Encryption Standard
              (AES) Key Wrap with Padding Algorithm", RFC 5649,
              DOI 10.17487/RFC5649, September 2009,

   [RFC5764]  McGrew, D. and E. Rescorla, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for the Secure
              Real-time Transport Protocol (SRTP)", RFC 5764,
              DOI 10.17487/RFC5764, May 2010,

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

9.2.  Informative References

              Jennings, C., Jones, P., and A. Roach, "SRTP Double
              Encryption Procedures", draft-ietf-perc-double-02 (work in
              progress), October 2016.

              Jones, P., Benham, D., and C. Groves, "A Solution
              Framework for Private Media in Privacy Enhanced RTP
              Conferencing", draft-ietf-perc-private-media-framework-02
              (work in progress), October 2016.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
              with Session Description Protocol (SDP)", RFC 3264,
              DOI 10.17487/RFC3264, June 2002,

   [RFC4366]  Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J.,
              and T. Wright, "Transport Layer Security (TLS)
              Extensions", RFC 4366, DOI 10.17487/RFC4366, April 2006,

Authors' Addresses

   Cullen Jennings
   Cisco Systems
   Calgary, AB


   John Mattsson (editor)
   Ericsson AB
   SE-164 80 Stockholm

   Phone: +46 10 71 43 501

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

   Phone: (408) 525 8651

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

   Phone: (408) 853 4197

   Flemming Andreason
   Cisco Systems
   499 Thornall Street
   Edison, NJ  08837