PERC Working Group                                      J. Mattsson, Ed.
Internet-Draft                                                  Ericsson
Intended status: Standards Track                               D. McGrew
Expires: November 10, 2016                                       D. Wing
                                                            F. Andreasen
                                                             C. Jennings
                                                             May 9, 2016

                 Encrypted Key Transport for Secure RTP


   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 within
   SRTP.  This facility enables SRTP to work for decentralized
   conferences with minimal control by allowing a common key to be used
   across multiple endpoints.

Status of This Memo

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

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   This Internet-Draft will expire on November 10, 2016.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents

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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Conventions Used In This Document . . . . . . . . . . . .   3
   2.  Encrypted Key Transport . . . . . . . . . . . . . . . . . . .   3
     2.1.  EKT Field Formats . . . . . . . . . . . . . . . . . . . .   4
     2.2.  Packet Processing and State Machine . . . . . . . . . . .   6
       2.2.1.  Outbound Processing . . . . . . . . . . . . . . . . .   7
       2.2.2.  Inbound Processing  . . . . . . . . . . . . . . . . .   7
     2.3.  Ciphers . . . . . . . . . . . . . . . . . . . . . . . . .   8
       2.3.1.  The Default Cipher  . . . . . . . . . . . . . . . . .   9
       2.3.2.  Other EKT Ciphers . . . . . . . . . . . . . . . . . .  10
     2.4.  Synchronizing
           Operation . . . . . . . . . . . . . . . . . . . . . . . .  10
     2.5.  Transport . . . . . . . . . . . . . . . . . . . . . . . .  10
     2.6.  Timing and Reliability Consideration  . . . . . . . . . .  11
   3.  Use of EKT with DTLS-SRTP . . . . . . . . . . . . . . . . . .  11
     3.1.  DTLS-SRTP Recap . . . . . . . . . . . . . . . . . . . . .  12
     3.2.  EKT Extensions to DTLS-SRTP . . . . . . . . . . . . . . .  12
     3.3.  Offer/Answer Considerations . . . . . . . . . . . . . . .  14
   4.  Sending the DTLS EKT_Key Reliably . . . . . . . . . . . . . .  14
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   6.  Open Issues . . . . . . . . . . . . . . . . . . . . . . . . .  15
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  16
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  16
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  16
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  16
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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.

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   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 and
   reduces the amount of external signaling control that is needed in a
   SRTP session that is shared 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 (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.

   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 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 them of the burden of tightly coordinating every SRTP source
   (SSRC) among every SRTP participant.

1.1.  Conventions Used In This Document

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

2.  Encrypted Key Transport

   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 packets.  When added to SRTP, the EKT field appears at the end
   of the SRTP packet, after the authentication tag (if that tag is

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   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 the format defined below for the Full_EKT_Field
   and Short_EKT_Field.

     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                        | 2

                      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

   TODO: move following syntax to ABNF.  Move name of Short to Empty.
   Move name of Full to EncryptedKey.  Drop extra EKT.

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   EKT_Msg_Type_Full = 2 ;  8 bits
   EKT_Msg_Length ; 16 bits. length in octets including type and length

   EKT_Plaintext = SRTP_Master_Key || SSRC || ROC || TTL

   EKT_Ciphertext = EKT_Encrypt(EKT_Key, EKT_Plaintext)

   Full_EKT_Field = EKT_Ciphertext || SPI ||
                              EKT_Msg_Length || EKT_Msg_Type_Full

   Short_EKT_Field = '00000000'

                        Figure 3: EKT data formats

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

   EKT_Ciphertext:  The data that is output from the EKT encryption
      operation, described in Section 2.3.  This field is included in
      SRTP 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 SPI will indicate the cipher being used
      and thus the size.

   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 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.

   Time to Live (TTL):  The maximum amount of time that this key can be
      used.  A unsigned 16 bit integer representing duration in seconds.
      The SRTP Master key in this message MUST NOT be used for
      encrypting or decrypting information after this time.  Open Issue:
      does this need to be absolute time not duration?  TODO: discuss in
      security section.

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   Security Parameter Index (SPI):  This field indicates the appropriate
      EKT Key and other parameters for the receiver to use when
      processing the packet.  Each time a different EKT Key is received,
      it will have a larger SPI than the previos key.  The length of
      this field is 16 bits.  The parameters identified by this field

      *  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.

      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

   EKT_Msg_Length  All EKT message other that Short EKT Field must have
      a length as the second from last elements.  This is the length in
      octets of the full EKT message including this length field and the
      following message type.

   Message Type  The last byte is used to indicate the type of the
      Field.  This MUST be 2 in the Full EKT Field format and 0 in Short
      EKT Field.  Future specifications that define new types SHOULD use
      even values until all the even code points are consumed to avoid
      conflicts with pre standards version of EKT that have been
      deployed.  Values less than 64 are mandatory to understand the
      whole EKT field SHOULD be discarded if it contains value message
      type that is less than 64 and not implemented.

      TODO - add IANA registry for Message Type.

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 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.

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

   Either the Full_EKT_Field or Short_EKT_Field is appended at the tail
   end of all the SRTP packet.

2.2.1.  Outbound Processing

   See Section 2.6 which describes when to send an EKT packet with a
   Full EKT Field.  If a Full EKT Field is not being sent, then a Short
   EKT Field needs to be sent so the receiver can correctly determine
   how to process the packet.

   When an SRTP packet is to be sent with a Full EKT Field, 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 packet processing.

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

   2.  The EKT_Plaintext field is computed from the SRTP Master Key,
       SSRC, and ROC fields, as shown in Section 2.1.  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 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.

   4.  Then the Full EKT Field is formed using the EKT Ciphertext and
       the SPI associated with the EKT Key used above.  The computed
       value of the Full EKT Field is written into the packet.

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

2.2.2.  Inbound Processing

   Inbound EKT processing MUST take place prior to the usual SRTP or
   SRTCP processing.  The following steps show processing as packets are
   received in order.

   1.  The final byte is checked to determine which EKT format is in
       use.  When an SRTP or SRTCP packet contains a Short EKT Field,

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       the Short EKT Field is removed from the packet then normal SRTP
       or SRTCP processing occurs.  If the packet contains a Full EKT
       Field, then processing continues as described below.

   2.  The combination of the SSRC and the Security Parameter Index
       (SPI) field is used to find which EKT parameter set should 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.  EKT parameter set contains the EKT Key, EKT Cipher,
       and SRTP Master Salt.

   3.  The EKT Ciphertext authentication is checked and it is decrypted,
       as described in Section 2.3, using the EKT Key and EKT Cipher
       found in the previous step.  If the EKT decryption operation
       returns an authentication failure, then the packet processing

   4.  The resulting EKT Plaintext is parsed as described in
       Section 2.1, to recover the SRTP Master Key, SSRC, and ROC
       fields.  The Master Salt that is assocted with the EKT Keys used
       to do the decription is also retreived.

   5.  The SRTP Master Key, ROC, and Master Salt from the prevous step
       are saved in a map indexed by the SSRC found in the EKT Plaintext
       and can be used for any future crypto operations for inbound or
       outbound packets with the that SSRC.  Outbound packets SHOULD
       continue to use the old key for 250 ms after receipt of the 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.  The key MUST NOT be used beyond the lifetime found in the
       TTL field.

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

   Implementation note: 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.

2.3.  Ciphers

   EKT uses an authenticated cipher to encrypt and authenticate the EKT
   Plaintext.  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

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   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).

   An EKT cipher 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 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
   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.

   Security requirements for EKT ciphers are discussed in Section 5.

2.3.1.  The Default Cipher

   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 + 8 octets.  It can be used with key sizes of L =
   16, and 32 octets, and its use with those key sizes is indicated as
   AESKW_128, or 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.

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       length of  length of
       SRTP         EKT          EKT        EKT        length of
      transform   transform    plaintext  ciphertext  Full EKT Field
       ---------  ------------  ---------  ----------  --------------
      AES-128    AESKW_128        26          40            42
      AES-256    AESKW_256        42          56            58

                           Figure 4: AESKW Table

   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 7.  This section defines how those ciphers
   interact with this specification.

   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

   If a source has its EKT key changed by the 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.

2.5.  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.

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2.6.  Timing and Reliability Consideration

   A system using EKT learns the SRTP master keys distributed with Full
   EKT Fields send with the SRTP, rather than with call signaling.  A
   receiver can immediately decrypt an SRTP 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.

   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 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.

   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 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 and
   every 100ms for audio.

3.  Use of EKT with DTLS-SRTP

   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).

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3.1.  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.

3.2.  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 DTLS server includes "ekt" in its TLS ServerHello message.  If a
   DTLS client includes "ekt" in its ClientHello, but does not receive
   "ekt" in the ServerHello, the DTLS client MUST NOT send DTLS packets
   with the "ekt" content-type.

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

                   enum {
                   } SRTPKeyTransportType;

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

                   enum {
                   } ektcipher;

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

                 Figure 5: Additional TLS Data Structures

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

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        Client                                               Server

        ClientHello + use_srtp + EKT
                                      ServerHello + use_srtp + EKT
                                     <--------     ServerHelloDone
        Finished                     -------->
                                     <--------            Finished
        ekt_key                      <--------
        ACK                          -------->
        SRTP packets                 <------->      SRTP packets
        SRTP packets                 <------->      SRTP packets
        ekt_key (rekey)              <-------
        ACK                          -------->
        SRTP packets                 <------->      SRTP packets
        SRTP packets                 <------->      SRTP packets

                     Figure 6: Handshake Message Flow

3.3.  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.

4.  Sending the DTLS EKT_Key Reliably

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

5.  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-

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   sharing: a single key is provided to protect an entire SRTP session.
   However, EKT remains secure 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 the EKT Cipher authentication

   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 tampers with the bits in Full_EKT_Field can prevent
   the intended receiver of that packet from being able to decrypt it.
   This is a minor denial of service vulnerability.

   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

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

6.  Open Issues

   What length should the SPI be?

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   Should we limit the number of saved SPI for a given SSRC?  Or limit
   the lifetime of old ones after a new one is received?  At some level
   this may not matter because even if the a SRTP packet is injected
   with an old value, it will be discards by the RTP stack for being
   old.  It is more important that new things are encrypted with the
   most recent EKT Key.

   How many bits to differentiate different types of packets and allow
   for extensibility?

   Given the amount of old EKT deployed, should the Full EKT use a a
   different code point than the "1" at the end?

   Do we need AES-192?

7.  IANA Considerations

   No IANA actions are required.

8.  Acknowledgements

   Thanks to David Benham, Eddy Lem, Felix Wyss, Jonathan Lennox, Kai
   Fischer, Lakshminath Dondeti, Magnus Westerlund, Michael Peck,
   Nermeen Ismail, Paul Jones, Rob Raymond, and Yi Cheng for fruitful
   discussions, comments, and contributions to this document.

   This draft is a cut down version of draft-ietf-avtcore-srtp-ekt-03
   and most of the text here came from that draft.

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,

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   [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

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

Authors' Addresses

   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

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

   Phone: (408) 853 4197

   Flemming Andreason
   Cisco Systems
   499 Thornall Street
   Edison, NJ  08837


   Cullen Jennings
   Cisco Systems
   Calgary, AB


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