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RFC 6267 - MIKEY-IBAKE: Identity-Based Authenticated Key Exchang


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Internet Engineering Task Force (IETF)                        V. Cakulev
Request for Comments: 6267                                   G. Sundaram
Category: Informational                                   Alcatel Lucent
ISSN: 2070-1721                                                June 2011

 MIKEY-IBAKE: Identity-Based Authenticated Key Exchange (IBAKE) Mode of
         Key Distribution in Multimedia Internet KEYing (MIKEY)

Abstract

   This document describes a key management protocol variant for the
   Multimedia Internet KEYing (MIKEY) protocol that relies on a trusted
   key management service.  In particular, this variant utilizes
   Identity-Based Authenticated Key Exchange (IBAKE) framework that
   allows the participating clients to perform mutual authentication and
   derive a session key in an asymmetric Identity-Based Encryption (IBE)
   framework.  This protocol, in addition to providing mutual
   authentication, eliminates the key escrow problem that is common in
   standard IBE and provides perfect forward and backward secrecy.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6267.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must

   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
     2.2.  Definitions and Notation . . . . . . . . . . . . . . . . .  4
     2.3.  Abbreviations  . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Use Case Scenarios . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Forking  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Retargeting  . . . . . . . . . . . . . . . . . . . . . . .  6
     3.3.  Deferred Delivery  . . . . . . . . . . . . . . . . . . . .  7
   4.  MIKEY-IBAKE Protocol Description . . . . . . . . . . . . . . .  7
     4.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Message Exchanges and Processing . . . . . . . . . . . . . 10
       4.2.1.  REQUEST_KEY_INIT/REQUEST_KEY_RESP Message Exchange . . 10
       4.2.2.  I_MESSAGE/R_MESSAGE Message Exchanges  . . . . . . . . 12
   5.  Key Management . . . . . . . . . . . . . . . . . . . . . . . . 16
     5.1.  Generating Keys from the Session Key . . . . . . . . . . . 17
     5.2.  Generating Keys for MIKEY Messages . . . . . . . . . . . . 17
     5.3.  CSB Update . . . . . . . . . . . . . . . . . . . . . . . . 18
     5.4.  Generating MAC and Verification Message  . . . . . . . . . 18
   6.  Payload Encoding . . . . . . . . . . . . . . . . . . . . . . . 19
     6.1.  Common Header Payload (HDR)  . . . . . . . . . . . . . . . 19
       6.1.1.  IBAKE Payload  . . . . . . . . . . . . . . . . . . . . 20
       6.1.2.  Encrypted Secret Key (ESK) Payload . . . . . . . . . . 21
       6.1.3.  Key Data Sub-Payload . . . . . . . . . . . . . . . . . 21
       6.1.4.  EC Diffie-Hellman Sub-Payload  . . . . . . . . . . . . 22
       6.1.5.  Secret Key Sub-Payload . . . . . . . . . . . . . . . . 23
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 24
     7.1.  General Security Considerations  . . . . . . . . . . . . . 24
     7.2.  IBAKE Protocol Security Considerations . . . . . . . . . . 25
     7.3.  Forking  . . . . . . . . . . . . . . . . . . . . . . . . . 26
     7.4.  Retargeting  . . . . . . . . . . . . . . . . . . . . . . . 26
     7.5.  Deferred Delivery  . . . . . . . . . . . . . . . . . . . . 26
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 28
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 28
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 29

1.  Introduction

   The Multimedia Internet Keying (MIKEY) [RFC3830] specification
   describes several modes of key distribution solution that address
   multimedia scenarios using pre-shared keys, Public Keys, and
   optionally a Diffie-Hellman key exchange.  Multiple extensions of
   MIKEY have been specified, such as HMAC-Authenticated (Hashed Message
   Authentication Code) Diffie-Hellman [RFC4650] and MIKEY-RSA-R
   [RFC4738].

   To address deployment scenarios in which security systems serve a
   large number of users, a key management service is often preferred.
   With such a service in place, it would be possible for a user to
   request credentials for any other user when they are needed.  Some
   proposed solutions [RFC6043] rely on Key Management Services (KMSs)
   in the network that create, distribute, and manage keys in a real
   time.  Due to this broad functionality, key management services would
   have to be online, maintain high availability, and be networked
   across operator boundaries.

   This document describes a solution in which KMSs are low-availability
   servers that communicate with end-user clients periodically (e.g.,
   once a month).  The online transactions between the end-user clients
   (for media plane security) are based on Identity-Based Encryption
   (IBE) [BF].  These online transactions between the end-user clients
   allow them to perform mutual authentication and derive a session key
   not known to any external entity (including KMSs).  This protocol, in
   addition to providing keys not known to any external entity and
   allowing for end-user clients to mutually authenticate each other (at
   the media plane layer), provides perfect forward and backward
   secrecy.  In this protocol, the KMS-to-client exchange is used
   sparingly (e.g., once a month); hence, the KMS is no longer required
   to be a high-availability server, and in particular different KMSs
   don't have to communicate with each other (across operator
   boundaries).  Moreover, given that an IBE is used, the need for
   costly Public Key Infrastructure (PKI) and all the operational costs
   of certificate management and revocation are eliminated.  This is
   achieved by concatenating Public Keys with a date field, thereby
   ensuring corresponding Private Keys change with the date and, more
   importantly, limiting the damage due to loss of a Private Key to just
   that date while not requiring endpoints involved in communication to
   be time synchronized.  The granularity in the date field is a matter
   of security policy and deployment scenario.  For instance, an
   operator may choose to use one key per day and hence the KMS may
   issue Private Keys for a whole subscription cycle at the beginning of
   a subscription cycle.  Therefore, unlike in the PKI systems, where
   issued certificate is typically valid for period of time thereby
   requiring revocation procedures to limit their validity, the scheme

   described in this document uses time-bound public identities, which
   automatically expire at the end of a time span indicated in the
   identity itself.  With the self-expiration of the public identities,
   the traditional real-time validity verification and revocation is not
   required.  For example, if the public identity is bound to one day,
   then, at the end of the day, the Public/Private Key pair issued to
   this peer will simply not be valid anymore.  Nevertheless, just like
   with Public-Key-based certificate systems, if there is a need to
   revoke keys before the designated expiry time, communication with a
   third party will be needed.

   Additionally, various call scenarios are securely supported -- this
   includes secure forking, retargeting, deferred delivery and pre-
   encoded content.

   MIKEY is widely used in the 3GPP community.  This specification is
   intended primarily for use with 3GPP media security, but it may also
   be applicable in Internet applications.

2.  Terminology

2.1.  Requirements Language

   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].

2.2.  Definitions and Notation

   IBE Encryption: Identity-Based Encryption (IBE) is a Public-Key
   encryption technology that allows a Public Key to be calculated from
   an identity, and the corresponding Private Key to be calculated from
   the Public Key. [RFC5091], [RFC5408], and [RFC5409] describe
   algorithms required to implement the IBE.

   (Media) session: The communication session intended to be secured by
   the MIKEY-IBAKE provided key(s).

      E(k, x)  Encryption of x with the key k
      [x]P     Point multiplication on an elliptic curve, i.e., adding
               a point P to itself total of x times
      K_PUBx   Public Key of x
      [x]      x is optional
      {x}      Zero or more occurrences of x
      (x)      One or more occurrences of x
      ||       Concatenation
      |        OR (selection operator)

2.3.  Abbreviations

   EC        Elliptic Curve

   ESK       Encrypted Secret Key

   HMAC      Hashed Message Authentication Code

   IBE       Identity-Based Encryption

   I         Initiator

   IBAKE     Identity-Based Authenticated Key Exchange

   IDRi      Initiator's Identity

   IDRr      Responder's Identity

   KMS       Key Management Service

   K_PR      Private Key

   K_PUB     Public Key

   K_SESSION Session Key

   MAC       Message Authentication Code

   MIKEY     Multimedia Internet KEYing

   MKI       Master Key Identifier

   MPK       MIKEY Protection Key

   PKI       Public Key Infrastructure

   PRF       Pseudorandom Function

   R         Responder

   SK        Secret Key

   SIP       Session Initiation Protocol

   SPI       Security Parameter Index

   SRTP      Secure Realtime Transport Protocol

   TEK       Traffic Encryption Key

   TGK       TEK Generation Key

3.  Use Case Scenarios

   This section describes some of the use case scenarios supported by
   MIKEY-IBAKE, in addition to regular two-party communication.

3.1.  Forking

   Forking is the delivery of a request (e.g., SIP INVITE message) to
   multiple endpoints.  This happens when a single user is registered
   more than once.  An example of forking is when a user has a desk
   phone, PC client, and mobile handset all registered with the same
   public identity.

         +---+             +-------+             +---+             +---+
         | A |             | PROXY |             | B |             | C |
         +---+             +-------+             +---+             +---+
               Request
           -------------------->
                                      Request
                               -------------------->
                                      Request
                               ------------------------------------->

                             Figure 1: Forking

3.2.  Retargeting

   Retargeting is a scenario in which a functional element decides to
   redirect the session to a different destination.  This decision to
   redirect a session may be made for different reasons by a number of
   different functional elements and at different points in the
   establishment of the session.

   There are two basic scenarios of session redirection.  In scenario
   one, a functional element (e.g., Proxy) decides to redirect the
   session by passing the new destination information to the originator.
   As a result, the originator initiates a new session to the redirected
   destination provided by the Proxy.  For the case of MIKEY-IBAKE, this
   means that the originator will initiate a new session with the
   identity of the redirected destination.  This scenario is depicted in
   Figure 2 below.

         +---+             +-------+             +---+             +---+
         | A |             | PROXY |             | B |             | C |
         +---+             +-------+             +---+             +---+
               Request
           -------------------->
                                      Request
                               -------------------->
                                      Redirect
                               <--------------------
               Redirect
           <-------------------
                                      Request
           ---------------------------------------------------------->

                           Figure 2: Retargeting

   In the second scenario, a proxy decides to redirect the session
   without informing the originator.  This is a common scenario
   specified in SIP [RFC3261].

3.3.  Deferred Delivery

   Deferred delivery is a type of service such that the session content
   cannot be delivered to the destination at the time that it is being
   sent (e.g., the destination user is not currently online).
   Nevertheless, the sender expects the network to deliver the message
   as soon as the recipient becomes available.  A typical example of
   deferred delivery is voicemail.

4.  MIKEY-IBAKE Protocol Description

4.1.  Overview

   Most of the previously defined MIKEY modes consist of a single (or
   half) roundtrip between two peers.  MIKEY-IBAKE consists of up to
   three roundtrips.  In the first roundtrip, users (Initiator and
   Responder) obtain their Private Key(s) (K_PR) from the KMS.  This
   roundtrip can be performed at anytime and, as explained earlier,
   takes place, for example, once a month (or once per subscription
   cycle).  The second and the third roundtrips are between the
   Initiator and the Responder.  Observe that the Key Management Service
   is only involved in the first roundtrip.  In Figure 3, a conceptual
   signaling diagram for the MIKEY-IBAKE mode is depicted.

      +---+             +------+         +------+                 +---+
      | I |             | KMS1 |         | KMS2 |                 | R |
      +---+             +------+         +------+                 +---+
          REQUEST_KEY_INIT                       REQUEST_KEY_INIT
        ------------------>                  <----------------------
          REQUEST_KEY_RESP                       REQUEST_KEY_RESP
        <------------------                  ---------------------->
                                  I_MESSAGE_1
        ----------------------------------------------------------->
                                  R_MESSAGE_1
        <-----------------------------------------------------------
                                  I_MESSAGE_2
        ----------------------------------------------------------->
                                  R_MESSAGE_2
        <-----------------------------------------------------------

                    Figure 3: Example Message Exchange

   The Initiator (I) wants to establish a secure media session with the
   Responder (R).  The Initiator and the Responder trust a third party,
   the Key Management Service (KMS), with which they both have, or can
   establish, shared credentials.  These pre-established trust relations
   are used by a user (i.e., Initiator and Responder) to obtain Private
   Keys.  Rather than a single KMS, several different KMSs may be
   involved, e.g., one for the Initiator and one for the Responder as
   shown in Figure 3.  The Initiator and the Responder do not share any
   credentials; however, the Initiator knows the Responder's public
   identity.  The assumed trust model is illustrated in Figure 4.

      +---+             +------+         +------+                 +---+
      | I |             | KMS1 |         | KMS2 |                 | R |
      +---+             +------+         +------+                 +---+
          Pre-established                         Pre-established
           trust relation                         trust relation
        <----------------->                  <--------------------->

            Security association based on mutual authentication
                   performed during MIKEY-IBAKE exchange
        <---------------------------------------------------------->

                           Figure 4: Trust Model

   Below, a description of how Private Keys are obtained using MIKEY
   messages is provided.  An alternative way for obtaining Private Keys
   using HTTP is described in [RFC5408].

   The Initiator obtains Private Key(s) from the KMS by sending a
   REQUEST_KEY_INIT message.  The REQUEST_KEY_INIT message includes
   Initiator's public identity(s) (if the Initiator has more than one
   public identity, it may request Private Keys for every identity
   registered) and is protected via a MAC based on a pre-shared key or
   via a signature (similar to the MIKEY-PSK and MIKEY-RSA modes).  If
   the request is authorized, the KMS generates the requested keys,
   encodes them, and returns them in a REQUEST_KEY_RESP message.  This
   exchange takes place periodically and does not need to be performed
   every time an Initiator needs to establish a secure connection with a
   Responder.

   The Initiator next chooses a random x and computes [x]P, where P is a
   point on elliptic curve E known to all users.  The Initiator uses the
   Responder's public identity to generate the Responder's Public Key
   (e.g., K_PUBr=H1(IDRr||date)), where Hi is hash function known to all
   users, and the granularity in date is a matter of security policy and
   known publicly.  Then the Initiator uses this generated Public Key to
   encrypt [x]P, IDRi and IDRr and includes this encrypted information
   in an I_MESSAGE_1 message, which is sent to the Responder.  The
   encryption is Identity-Based Encryption (IBE) as specified in
   [RFC5091] and [RFC5408].  In turn, the Responder IBE-decrypts the
   received message using its Private Key for that date, chooses random
   y and computes [y]P.  Next, the Responder uses Initiator's identity
   obtained from I_MESSAGE_1 to generate Initiator's Public Key (e.g.,
   K_PUBi=H1(IDRi||date)) and IBE-encrypts (IDRi, IDRr, [x]P, [y]P)
   using K_PUBi, and includes it in R_MESSAGE_1 message sent to the
   Initiator.  At this point, the Responder is able to generate the
   session key as [x][y]P.  This session key is then used to generate
   TGK as specified in Section 5.1.

   Upon receiving and IBE-decrypting an R_MESSAGE_1 message, the
   Initiator verifies the received [x]P.  At this point, the Initiator
   is able to generate the same session key as [x][y]P.  Upon successful
   verification, the Initiator sends I_MESSAGE_2 message to the
   Responder, including IBE-encrypted IDRi, IDRr and previously received
   [y]P.  The Responder sends a R_MESSAGE_2 message to the Initiator as
   verification.

   The above described is the most typical use case; in Section 3, some
   alternative use cases are discussed.

   MIKEY-IBAKE is based on [RFC3830]; therefore, the same terminology,
   processing, and considerations still apply unless otherwise stated.
   Payloads containing EC Diffie-Hellman values and keys exchanged in
   I_MESSAGE/R_MESSAGE are IBE encrypted as specified in [RFC5091] and
   [RFC5408], while the keys exchanged in KEY_REQUES_INIT/
   KEY_REQUEST_RESPONSE are encrypted as specified in [RFC3830].  In all
   exchanges, encryption is only applied to the payloads containing keys
   and EC Diffie-Hellman values and not to the entire messages.

4.2.  Message Exchanges and Processing

4.2.1.  REQUEST_KEY_INIT/REQUEST_KEY_RESP Message Exchange

   This exchange is used by a user (e.g., Initiator or Responder) to
   request Private Keys from a trusted Key Management Service, with
   which the user has pre-shared credentials.  A full roundtrip is
   required for a user to receive keys.  As this message must ensure the
   identity of the user to the KMS, it is protected via a MAC based on a
   pre-shared key or via a signature.  The initiation message
   REQUEST_KEY_INIT comes in two variants corresponding to the pre-
   shared key (PSK) and Public-Key encryption (PKE) methods of
   [RFC3830].  The response message REQUEST_KEY_RESP is the same for the
   two variants and SHALL be protected by using the pre-shared/envelope
   key indicated in the REQUEST_KEY_INIT message.

    Initiator/Responder                    KMS

    REQUEST_KEY_INIT_PSK =          ---->
    HDR, T, RAND, (IDRi/r),
    IDRkms, [IDRpsk], [KEMAC], V    <----  REQUEST_KEY_RESP =
                                             HDR, T, [IDRi/r], [IDRkms],
                                             KEMAC, V

    REQUEST_KEY_INIT_PKE =          ---->
    HDR, T, RAND, (IDRi/r),
       {CERTi/r}, IDRkms,           <----  REQUEST_KEY_RESP =
       [KEMAC], [CHASH],                     HDR, T, [IDRi/r], [IDRkms],
       PKE, SIGNi/r                          KEMAC, V

4.2.1.1.  Components of the REQUEST_KEY_INIT Message

   The main objective of the REQUEST_KEY_INIT message is for a user to
   request one or more Private Keys (K_PR) from the KMS.  The user may
   request a K_PR for each public identity it possesses, as well as for
   multiple dates.

   The REQUEST_KEY_INIT message MUST always include the Header (HDR),
   Timestamp (T), and RAND payloads.  The CSB ID (Crypto Session Bundle
   ID) SHALL be assigned as in [RFC3830].  The user SHALL include it in
   the CSB ID field of the Header.  The user SHALL set the #CS field to
   '0' since CS (Crypto Session(s)) SHALL NOT be handled.  The CS ID map
   type SHALL be the "Empty map" as defined in [RFC4563].

   IDRi/r contains the identity of the user.  Since the user may have
   multiple identities, multiple IDRi/r fields may appear in the
   message.

   IDRkms SHALL be included.

   The KEMAC payload SHALL be used only when the user needs to use
   specific keys.  Otherwise, this payload SHALL NOT be used.

4.2.1.1.1.  Components of the REQUEST_KEY_INIT_PSK Message

   The IDRpsk payload MAY be used to indicate the pre-shared key used.

   The last payload SHALL be a Verification (V) payload where the
   authentication key (auth_key) is derived from the pre-shared key (see
   Section 4.1.4 of [RFC3830] for key derivation specification).

4.2.1.1.2.  Components of the REQUEST_KEY_INIT_PKE Message

   The certificate (CERT) payload SHOULD be included.  If a certificate
   chain is to be provided, each certificate in the chain MUST be
   included in a separate CERT payload.

   The PKE payload contains the encrypted envelope key: PKE = E(PKkms,
   env_key).  It is encrypted using the KMS's Public Key (PKkms).  If
   the KMS possesses several Public Keys, the user can indicate the key
   used in the CHASH payload.

   SIGNi/r is a signature covering the entire MIKEY message, using the
   Initiator's signature key.

4.2.1.2.  Processing of the REQUEST_KEY_INIT Message

   If the KMS can verify the integrity of the received message and the
   message can be correctly parsed, the KMS MUST check the Initiator's
   authorization.  If the Initiator is authorized to receive the
   requested Private Key(s), the KMS MUST send a REQUEST_KEY_RESP
   message.  Unexpected payloads in the REQUEST_KEY_INIT message SHOULD
   be ignored.  Errors are handled as described in [RFC3830].

4.2.1.3.  Components of the REQUEST_KEY_RESP Message

   The version, PRF func and CSB ID, #CS, and CS ID map type fields in
   the HDR payload SHALL be identical to the corresponding fields in the
   REQUEST_KEY_INIT message.  The KMS SHALL set the V flag to 0 and the
   user receiving it SHALL ignore it as it has no meaning in this
   context.

   The Timestamp type and value SHALL be identical to the one used in
   the REQUEST_KEY_INIT message.

                      KEMAC = E(encr_key, (ID || K_PR))

   The KEMAC payload SHOULD use the NULL authentication algorithm, as a
   MAC is included in the V payload.  Depending on the type of
   REQUEST_KEY_INIT message, either the pre-shared key or the envelope
   key SHALL be used to derive the encr_key.

   The last payload SHALL be a Verification (V) payload.  Depending on
   the type of REQUEST_KEY_INIT message, either the pre-shared key or
   the envelope key SHALL be used to derive the auth_key.

4.2.1.4.  Processing of the REQUEST_KEY_RESP Message

   If the Initiator/Responder can correctly parse the received message,
   the received session information SHOULD be stored.  Otherwise, the
   Initiator/Responder SHOULD silently discard the message and abort the
   protocol.

4.2.2.  I_MESSAGE/R_MESSAGE Message Exchanges

   This exchange is used for Initiator and Responder to mutually
   authenticate each other and to exchange EC Diffie-Hellman values used
   to generate TGK.  These exchanges are modeled after the pre-shared
   key mode, with the exception that the Elliptic Curve Diffie-Hellman
   values and Secret Keys (SKs) are encoded in IBAKE and ESK payloads
   instead of a KEMAC payload.  Two full roundtrips are required for
   this exchange to successfully complete.  The messages are preferably
   included in the session setup signaling (e.g., SIP INVITE).

   Initiator                               Responder

      I_MESSAGE_1 =                    ---->
      HDR, T, RAND, IDRi, IDRr,
         IBAKE, [ESK]                  <----  R_MESSAGE_1 =
                                                HDR, T, IDRi,
                                                IDRr, IBAKE

      I_MESSAGE_2 =                    ---->
      HDR, T, RAND, IDRi, IDRr,
         IBAKE, [ESK]                  <----  R_MESSAGE_2 =
                                              HDR, T, [IDRi], [IDRr],
                                              [IBAKE], V

4.2.2.1.  Components of the I_MESSAGE_1 Message

   The I_MESSAGE_1 message MUST always include the Header (HDR),
   Timestamp (T), and RAND payloads.  The CSB ID (Crypto Session Bundle
   ID) SHALL be randomly selected by the Initiator.  As the R_MESSAGE_1
   message is mandatory, the Initiator indicates with the V flag that a
   verification message is expected.

   The IDRi and IDRr payloads SHALL be included.

   The IBAKE payload contains Initiator's Identity and EC Diffie-Hellman
   values (ECCPTi), and Responder's Identity all encrypted using
   Responder's Public Key (i.e., encr_key = K_PUBr) as follows:

                      IBAKE = E(encr_key, IDRi || ECCPTi || IDRr)

   Optionally, Encrypted Secret Key (ESK) payload MAY be included.  If
   included, ESK contains an identity and a Secret Key (SK) encrypted
   using intended Responder's Public Key (i.e., encr_key = K_PUBr).

                      ESK = E(encr_key, ID || SK)

4.2.2.2.  Processing of the I_MESSAGE_1 Message

   The parsing of I_MESSAGE_1 message SHALL be done as in [RFC3830].  If
   the received message is correctly parsed, the Responder SHALL use the
   Private Key (K_PRr) corresponding to the received IDRr to decrypt the
   IBAKE payload.  If the message contains ESK payload, the Responder
   SHALL decrypt the SK and use it to decrypt the received IBAKE
   payload.  Otherwise, if the Responder is not able to decrypt the

   IBAKE payload, the Responder SHALL indicate it to the Initiator by
   including only its own EC Diffie-Hellman value (ECCPTr) in the next
   message (i.e., R_MESSAGE_1) it sends to the Initiator.

   If the received message cannot be correctly parsed, the Responder
   SHOULD silently discard the message and abort the protocol.

4.2.2.3.  Components of the R_MESSAGE_1 Message

   The version, PRF func, CSB ID, #CS, and CS ID map type fields in the
   HDR payload SHALL be identical to the corresponding fields in the
   I_MESSAGE_1 message.  The V flag SHALL be set to 1 as I_MESSAGE_2
   message is mandatory.

   The Timestamp type and value SHALL be identical to the one used in
   the I_MESSAGE_1 message.

   The IDRi and IDRr payloads SHALL be included.  The IDRi payload SHALL
   be as received in the I_MESSAGE_1.  In the IDRr payload, the
   Responder SHALL include its own identity.  Note that this identity
   might be different from the identity contained in the IDRr payload
   received in I_MESSAGE_1 message.  The IDRr payloads of I_MESSAGE_1
   and R_MESSAGE_1 will be different in the case of forking,
   retargeting, and deferred delivery.

   The Responder's IBAKE payload contains the Initiator's EC Diffie-
   Hellman value (ECCPTi) received in I_MESSAGE_1 (if successfully
   decrypted), and the Initiator's EC Diffie-Hellman value generated by
   the Responder (ECCPTr), as well as corresponding Initiator and
   Responder's identities.  If the Responder is unable to decrypt the
   IBAKE payload received in I_MESSAGE_1 (e.g., the message is received
   by the intended Responder's mailbox), the Responder SHALL include
   only its own EC Diffie-Hellman value (ECCPTr).  The IBAKE payload in
   R_MESSAGE_1 is encrypted using Initiator's Public Key (i.e., encr_key
   = P_PUBi) as follows:

           IBAKE = E(encr_key, IDRi || {ECCPTi} || IDRr || ECCPTr)

4.2.2.4.  Processing of the R_MESSAGE_1 Message

   The parsing of R_MESSAGE_1 message SHALL be done as in [RFC3830].  If
   the received message is correctly parsed, the Initiator shall use the
   Private Key corresponding to the received IDRi to decrypt the IBAKE
   payload.  If the ECCPTi sent in I_MESSAGE_1 is not present in the
   received IBAKE payload (e.g., the Responder is currently offline and
   the R_MESSAGE_1 is received from Responder's mailbox), the Initiator

   SHALL include ECCPTi again in the next message, I_MESSAGE_2.  In this
   case, I_MESSAGE_2 SHALL also contain an ESK payload encrypted using
   the intended recipient's K_PUB.

   If the received message cannot be correctly parsed, the Initiator
   SHOULD silently discard the message and abort the protocol.

4.2.2.5.  Components of the I_MESSAGE_2 Message

   The I_MESSAGE_2 message MUST always include the Header (HDR),
   Timestamp (T), and RAND payloads.  The version, PRF func, CSB ID,
   #CS, and CS ID map type fields in the HDR payload SHALL be identical
   to the corresponding fields in the R_MESSAGE_2 message.  As the
   R_MESSAGE_2 message is mandatory, the Initiator indicates with the V
   flag that a verification message is expected.

   The IDRi and IDRr payloads SHALL be included.  The IDRr payload SHALL
   be the same as the IDRr payload received in the R_MESSAGE_1.

   The Initiator's IBAKE payload SHALL contain the Initiator's EC
   Diffie-Hellman value (ECCPTi) if the ECCPTi was not received in
   R_MESSAGE_1.  Otherwise, ECCPTi SHALL NOT be included.  The IBAKE
   payload in I_MESSAGE_2 SHALL contain the Initiator's and Responder's
   identities as well as Responder's EC Diffie-Hellman value received in
   message R_MESSAGE_1.  IBAKE payload SHALL be encrypted using
   Responder's Public Key (i.e., encr_key = K_PUBr) as follows:

             IBAKE = E(encr_key, IDRi || {ECCPTi} || IDRr || ECCPTr)

   Optionally, Encrypted Secret Key (ESK) payload can be included.  ESK
   SHALL be included in case R_MESSAGE_1 did not contain Initiator's EC
   Diffie-Hellman value (ECCPTi) (e.g., in the case of deferred
   delivery).  If included, it contains an Initiator's identity and
   Initiator-generated Secret Key (SK) encrypted using intended
   recipient Public Key (i.e., encr_key = K_PUB) as follows:

                      ESK = E(encr_key, ID || SK)

4.2.2.6.  Processing of the I_MESSAGE_2 Message

   The parsing of the I_MESSAGE_2 message SHALL be done as in [RFC3830].
   If the received message is correctly parsed, the Responder shall use
   the K_PRr corresponding to the received IDRr to decrypt the IBAKE
   payload.  If an ESK is received, the Responder SHALL store it for
   future use (e.g., the Responder is a mailbox and will forward the key
   to the user once the user is online).

   If the received message cannot be correctly parsed, the Responder
   SHOULD silently discard the message and abort the protocol.

4.2.2.7.  Components of the R_MESSAGE_2 Message

   The version, PRF func, CSB ID, #CS, and CS ID map type fields in the
   HDR payload SHALL be identical to the corresponding fields in the
   I_MESSAGE_2 message.  The V flag SHALL be set to 0 by the Responder
   and ignored by the Initiator.

   The Timestamp type and value SHALL be identical to the one used in
   the I_MESSAGE_2 message.

   The IDRi and IDRr payloads SHOULD be included.

   If Initiator's EC Diffie-Hellman value (ECCPTi) was received in
   I_MESSAGE_2, the Responder SHALL also include the IBAKE payload.  If
   included, the IBAKE payload SHALL contain Initiator's EC Diffie-
   Hellman value (ECCPTi), and the Initiator's identity previously
   received in I_MESSAGE_2, encrypted using Initiator's Public Key
   (i.e., encr_key = K_PUBi) as follows:

                    IBAKE = E(encr_key, IDRi || ECCPTi)

   The last payload SHALL be a Verification (V) payload where the
   authentication key (auth_key) is derived as specified in Section 5.2.

4.2.2.8.  Processing of the R_MESSAGE_2 Message

   The parsing of R_MESSAGE_2 message SHALL be done as in [RFC3830].  If
   the received message is correctly parsed, and if it contains the
   IBAKE payload, the Initiator SHALL use the K_PRi corresponding to the
   received IDRi to decrypt the IBAKE payload.

   If the received message cannot be correctly parsed, the Initiator
   SHOULD silently discard the message and abort the protocol.

5.  Key Management

   The keys used in REQUEST_KEY_INIT/REQUEST_KEY_RESP exchange are
   derived from the pre-shared key or the envelope key as specified in
   [RFC3830].  As crypto sessions are not handled in this exchange,
   further keying material (i.e., TEKs) for this message exchange SHALL
   NOT be derived.

5.1.  Generating Keys from the Session Key

   As stated above, the session key [x][y]P is generated using exchanged
   EC Diffie-Hellman values, where x and y are randomly chosen by the
   Initiator and Responder.  The session key, as a point on an elliptic
   curve, is then converted into octet string as specified in [SEC1].
   This octet string K_SESSION is then used to generate MPK and TGK.
   Finally, the traffic encryption keys (e.g., TEK) are generated from
   TGK as specified in [RFC3830].

   The MPK and TGK are generated from K_SESSION as follows.

      inkey      : K_SESSION
      inkey_len  : bit length of the MPK
      label      : constant || 0xFF || 0xFFFFFFFF || RAND
      outkey_len : desired bit length of the output key (MPK or TGK)

   The constant depends on the derived key type as summarized below.

                       +-------------+------------+
                       | Derived Key |  Constant  |
                       +-------------+------------+
                       |     MPK     | 0x220E99A2 |
                       |     TGK     | 0x1F4D675B |
                       +-------------+------------+

                   Table 1: Constants for Key Derivation

   The constants are taken from the decimal digits of e as described in
   [RFC3830].

5.2.  Generating Keys for MIKEY Messages

   The keys for MIKEY messages are used to protect the MIKEY messages
   exchanged between the Initiator and Responder (i.e., I_MESSAGE and
   R_MESSAGE).  In the REQUEST_KEY_INIT/REQUEST_KEY_RESP exchange, the
   key derivation SHALL be done exactly as in [RFC3830].

   MIKEY Protection Key (MPK) for I_MESSAGE/R_MESSAGE exchange is
   generated as described in Section 5.1.  This MPK is then used to
   derive keys to protect R_MESSAGE_2 message.

      inkey      : MPK
      inkey_len  : bit length of the MPK
      label      : constant || 0xFF || csb_id || RAND
      outkey_len : desired bit length of the output key

   where the constants are as defined in [RFC3830].

5.3.  CSB Update

   Similar to [RFC3830], MIKEY-IBAKE provides means for updating the CSB
   (Crypto Session Bundle), e.g., transporting new EC Diffe-Hellman
   values or adding new crypto sessions.  The CSB updating is done by
   executing the exchange of I_MESSAGE_1/R_MESSAGE_1.  The CSB updating
   MAY be started by either the Initiator or the Responder.

      Initiator                               Responder

      I_MESSAGE_1 =                 ---->
      HDR, T, [IDRi], [IDRr],
         [IBAKE]                    <----     R_MESSAGE_1 =
                                              HDR, T, [IDRi], [IDRr],
                                              [IBAKE], V

      Responder                               Initiator

      I_MESSAGE_1 =                 ---->
      HDR, T, [IDRr], [IDRi],
         [IBAKE]                    <----     R_MESSAGE_1 =
                                              HDR, T, [IDRi], [IDRr],
                                              [IBAKE], V

   The new message exchange MUST use the same CSB ID as the initial
   exchange, but MUST use a new Timestamp.  Other payloads that were
   provided in the initial exchange SHOULD NOT be included.  New RANDs
   MUST NOT be included in the message exchange (the RANDs will only
   have effect in the initial exchange).

   IBAKE payload with new EC Diffie-Hellman values SHOULD be included.
   If new EC Diffie-Hellman values are being exchanged during CSB
   updating, messages SHALL be protected with keys derived from EC
   Diffie-Hellman values exchanged as specified in Section 5.2.
   Otherwise, if new EC Diffie-Hellman values are not being exchanged
   during CSB update exchange, messages SHALL be protected with the keys
   that protected the I_MESSAGE/R_MESSAGE messages in the initial
   exchange.

5.4.  Generating MAC and Verification Message

   The authentication tag in all MIKEY-IBAKE messages is generated as
   described in [RFC3830].  As described above, the MPK is used to
   derive the auth_key.  The MAC/Signature in the V/SIGN payloads covers
   the entire MIKEY message, except the MAC/Signature field itself and
   if there is an ESK payload in the massage it SHALL be omitted from
   MAC/Signature calculation.  The identities (not whole payloads) of

   the involved parties MUST directly follow the MIKEY message in the
   Verification MAC/Signature calculation.  Note that in the I_MESSAGE/
   R_MESSAGE exchange, IDRr in R_MESSAGE_1 MAY not be the same as that
   appearing in I_MESSAGE_1.

6.  Payload Encoding

   This section does not describe all the payloads that are used in the
   new message types.  It describes in detail the new IBAKE and ESK
   payloads and in less detail the payloads for which changes has been
   made compared to [RFC3830].  For a detailed description of the MIKEY
   payloads (e.g., Timestamp (T) payload, RAND payload, etc.), see
   [RFC3830].  For the description of IDR payload as well as for the
   definition of additional PRF functions and encryption algorithms not
   defined in [RFC3830], see [RFC6043].

6.1.  Common Header Payload (HDR)

   For the Common Header Payload, new values are added to the data type
   and the next payload namespaces.

   o  Data type (8 bits): describes the type of message.

     +------------------+-------+------------------------------------+
     |     Data Type    | Value |               Comment              |
     +------------------+-------+------------------------------------+
     |  REQUEST_KEY_PSK |   19  | Request Private Keys message (PSK) |
     |  REQUEST_KEY_PKE |   20  | Request Private Keys message (PKE) |
     | REQUEST_KEY_RESP |   21  |    Response Private Keys message   |
     |    I_MESSAGE_1   |   22  |      First Initiator's message     |
     |    R_MESSAGE_1   |   23  |      First Responder's message     |
     |    I_MESSAGE_2   |   24  |     Second Initiator's message     |
     |    R_MESSAGE_2   |   25  |     Second Responder's message     |
     +------------------+-------+------------------------------------+

                      Table 2: Data Type (Additions)

   o  Next payload (8 bits): identifies the payload that is added after
      this payload.

                 +--------------+-------+---------------+
                 | Next Payload | Value |    Section    |
                 +--------------+-------+---------------+
                 |     IBAKE    |   22  | Section 6.1.1 |
                 |      ESK     |   23  | Section 6.1.2 |
                 |      SK      |   24  | Section 6.1.5 |
                 |     ECCPT    |   25  | Section 6.1.4 |
                 +--------------+-------+---------------+

                     Table 3: Next Payload (Additions)

   o  V (1 bits): flag to indicate whether or not a response message is
      expected (this only has meaning when it is set in an initiation
      message).  If a response is required, the V flag SHALL always be
      set to 1 in the initiation messages and the receiver of the
      initiation message (Responder or KMS) SHALL ignore it.

   o  #CS (8 bits): indicates the number of crypto sessions that will be
      handled within the CSB.  It SHALL be set to 0 in the Request Key
      exchange, as crypto sessions SHALL NOT be handled.

   o  CS ID map type (8 bits): specifies the method of uniquely mapping
      crypto sessions to the security protocol sessions.  In the Request
      Key exchange, the CS ID map type SHALL be the "Empty map" (defined
      in [RFC4563]) as crypto sessions SHALL NOT be handled.

6.1.1.  IBAKE Payload

   The IBAKE payload contains IBE encrypted (see [RFC5091] and [RFC5408]
   for details about IBE) Initiator and Responder's Identities and EC
   Diffie-Hellman Sub-Payloads (see Section 6.1.4 for the definition of
   EC Diffie-Hellman Sub-Payload).  It may contain one or more EC
   Diffie-Hellman Sub-Payloads and their associated identities.  The
   last EC Diffie-Hellman or Identity Sub-Payload has its Next payload
   field set to Last payload.

                           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  ! Encr data len                 !  Encr data    !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                        Encr data                              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Next payload (8 bits): identifies the payload that is added after
      this payload.

   o  Encr data len (16 bits): length of Encr data (in bytes).

   o  Encr data (variable length): the IBE encrypted EC Diffie-Hellman
      Sub-Payloads (see Section 6.1.4) and their associated Identity
      payloads.

6.1.2.  Encrypted Secret Key (ESK) Payload

   The Encrypted Secret Key payload contains IBE encrypted (see
   [RFC5091] and [RFC5408] for details about IBE) Secret Key Sub-Payload
   and its associated identity (see Section 6.1.5 for the definition of
   the Secret Key Sub-Payload).

                           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  ! Encr data len                 !  Encr data    !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                        Encr data                              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Next payload (8 bits): identifies the payload that is added after
      this payload.

   o  Encr data len (16 bits): length of Encr data (in bytes).

   o  Encr data (variable length): the encrypted secret key Sub-Payloads
      (see Section 6.1.5).

6.1.3.  Key Data Sub-Payload

   For the key data Sub-Payload, a new type of key is defined.  The
   Private Key (K_PR) is used to decrypt the content encrypted using the
   corresponding Public Key (K_PUB).  KEMAC in the REQUEST_KEY_RESP
   SHALL contain one or more Private Keys.

   o  Type (4 bits): indicates the type of key included in the payload.

                      +------+-------+-------------+
                      | Type | Value |   Comments  |
                      +------+-------+-------------+
                      | K_PR |   7   | Private Key |
                      +------+-------+-------------+

                    Table 4: Key Data Type (Additions)

6.1.4.  EC Diffie-Hellman Sub-Payload

   The EC Diffie-Hellman (ECCPT) Sub-Payload uses the format defined
   below.  The EC Diffie-Hellman Sub-Payload in MIKEY-IBAKE is never
   included in clear, but as an encrypted part of the IBAKE payload.

                           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  ! ECC Curve     ! ECC Point                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! Auth alg      ! TGK len                       ! Reserv! KV    !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      ! KV data (optional)                                            ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Next payload (8 bits): identifies the payload that is added after
      this payload.  See Section 6.1 of [RFC3830] for values.

   o  ECC curve (8 bits): identifies the ECC curve used.

             +--------------------------------------+-------+
             |               ECC Curve              | Value |
             +--------------------------------------+-------+
             |  ECPRGF192Random / P-192 / secp192r1 |   1   |
             |  EC2NGF163Random / B-163 / sect163r2 |   2   |
             | EC2NGF163Koblitz / K-163 / sect163k1 |   3   |
             |  EC2NGF163Random2 / none / sect163r1 |   4   |
             |  ECPRGF224Random / P-224 / secp224r1 |   5   |
             |  EC2NGF233Random / B-233 / sect233r1 |   6   |
             | EC2NGF233Koblitz / K-233 / sect233k1 |   7   |
             |  ECPRGF256Random / P-256 / secp256r1 |   8   |
             |  EC2NGF283Random / B-283 / sect283r1 |   9   |
             | EC2NGF283Koblitz / K-283 / sect283k1 |   10  |
             |  ECPRGF384Random / P-384 / secp384r1 |   11  |
             |  EC2NGF409Random / B-409 / sect409r1 |   12  |
             | EC2NGF409Koblitz / K-409 / sect409k1 |   13  |
             |  ECPRGF521Random / P-521 / secp521r1 |   14  |
             |  EC2NGF571Random / B-571 / sect571r1 |   15  |
             | EC2NGF571Koblitz / K-571 / sect571k1 |   16  |
             +--------------------------------------+-------+

                         Table 5: Elliptic Curves

   o  ECC point (variable length): ECC point data, padded to end on a
      32-bit boundary, encoded in octet string representation.

   o  Auth alg (8 bits): specifies the MAC algorithm used for the
      verification message.  For MIKEY-IBAKE this field is ignored.

   o  TGK len (16 bits): the length of the TGK (in bytes).  For MIKEY-
      IBAKE this field is ignored.

   o  KV (4 bits): indicates the type of key validity period specified.
      This may be done by using an SPI (alternatively an MKI in SRTP) or
      by providing an interval in which the key is valid (e.g., in the
      latter case, for SRTP this will be the index range where the key
      is valid).  See Section 6.13 of [RFC3830] for pre-defined values.

   o  KV data (variable length): This includes either the SPI/MKI or an
      interval (see Section 6.14 of [RFC3830]).  If KV is NULL, this
      field is not included.

6.1.5.  Secret Key Sub-Payload

   Secret Key payload is included as a Sub-Payload in Encrypted Secret
   Key payload.  Similar to EC Diffie-Hellman Sub-Payload, it is never
   included in clear, but as an encrypted part of the ESK payload.

                           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 len                  !
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                         Key data                              ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      !                        KV data (optional)                     ~
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   o  Next payload (8 bits): identifies the payload that is added after
      this payload.

   o  Type (4 bits): indicates the type of the key included in the
      payload.

                             +------+-------+
                             | Type | Value |
                             +------+-------+
                             |  SK  |   1   |
                             +------+-------+

                         Table 6: Secret Key Types

   o  KV (4 bits): indicates the type of key validity period specified.
      This may be done by using an SPI (or MKI in the case of [RFC3711])
      or by providing an interval in which the key is valid (e.g., in
      the latter case, for SRTP this will be the index range where the
      key is valid).  KV values are the same as in Section 6.13 of
      [RFC3830]

   o  Key data len (16 bits): the length of the Key data field (in
      bytes).

   o  Key data (variable length): The SK data.

   o  KV data (variable length): This includes either the SPI or an
      interval.  If KV is NULL, this field is not included.

7.  Security Considerations

   Unless explicitly stated, the security properties of the MIKEY
   protocol as described in [RFC3830] apply to MIKEY-IBAKE as well.  In
   addition, MIKEY-IBAKE is based on the basic Identity-Based Encryption
   protocol, as specified in [RFC5091], [RFC5408], and [RFC5409], and as
   such inherits some properties of that protocol.  For instance, by
   concatenating the "date" with the identity (to derive the Public
   Key), the need for any key revocation mechanisms is virtually
   eliminated.  Moreover, by allowing the participants to acquire
   multiple Private Keys (e.g., for duration of contract) the
   availability requirements on the KMS are also reduced without any
   reduction in security.

7.1.  General Security Considerations

   The MIKEY-IBAKE protocol relies on the use of Identity-Based
   Encryption.  [RFC5091] describes attacks on the cryptographic
   algorithms used in Identity-Based Encryption.  In addition, [RFC5091]
   provides recommendations for security parameters for described IBE
   algorithms.

   It is assumed that the Key Management Services are secure, not
   compromised, trusted, and will not engage in launching active attacks
   independently or in a collaborative environment.  However, any
   malicious insider could potentially launch passive attacks (by
   decryption of one or more message exchanges offline).  While it is in
   the best interest of administrators to prevent such attacks, it is
   hard to eliminate this problem.  Hence, it is assumed that such
   problems will persist, and hence the protocols are designed to
   protect participants from passive adversaries.

7.2.  IBAKE Protocol Security Considerations

   For the basic IBAKE protocol, from a cryptographic perspective, the
   following security considerations apply.

   In every step, Identity-Based Encryption (IBE) is used with the
   recipient's Public Key.  This guarantees that only the intended
   recipient of the message can decrypt the message [BF].

   Next, the use of identities within the encrypted payload is intended
   to eliminate some basic reflection attacks.  For instance, suppose
   identities were not used as part of the encrypted payload, in the
   first step of the IBAKE protocol (i.e., I_MESSAGE_1 of Figure 3 in
   Section 4.1).  Furthermore, assume an adversary who has access to the
   conversation between Initiator and Responder and can actively snoop
   into packets and drop/modify them before routing them to the
   destination.  For instance, assume that the IP source address and
   destination address can be modified by the adversary.  After the
   first message is sent by the Initiator (to the Responder), the
   adversary can take over and trap the packet.  Next, the adversary can
   modify the IP source address to include adversary's IP address,
   before routing it onto the Responder.  The Responder will assume the
   request for an IBAKE session came from the adversary and will execute
   step 2 of the IBAKE protocol (i.e., R_MESSAGE_1 of Figure 3 in
   Section 4.1) but encrypt it using the adversary's Public Key.  The
   above message can be decrypted by the adversary (and only by the
   adversary).  In particular, since the second message includes the
   challenge sent by the Initiator to the Responder, the adversary will
   now learn the challenge sent by the Initiator.  Following this, the
   adversary can carry on a conversation with the Initiator "pretending"
   to be the Responder.  This attack will be eliminated if identities
   are used as part of the encrypted payload.  In summary, at the end of
   the exchange both Initiator and Responder can mutually authenticate
   each other and agree on a session key.

   Recall that Identity-Based Encryption guarantees that only the
   recipient of the message can decrypt the message using the Private
   Key.  The caveat being, the KMS that generated the Private Key of
   recipient of message can decrypt the message as well.  However, the
   KMS cannot learn the session key [x][y]P given [x]P and [y]P based on
   the Elliptic Curve Diffie-Hellman problem.  This property of
   resistance to passive key escrow from the KMS is not applicable to
   the basic IBE protocols proposed in [RFC5091], [RFC5408], and
   [RFC5409].

   Observe that the protocol works even if the Initiator and Responder
   belong to two different Key Management Services.  In particular, the
   parameters used for encryption to the Responder and parameters used

   for encryption to the Initiator can be completely different and
   independent of each other.  Moreover, the Elliptic Curve used to
   generate the session key [x][y]P can be completely different.  If
   such flexibility is desired, then it would be advantageous to add
   optional extra data to the protocol to exchange the algebraic
   primitives used in deriving the session key.

   In addition to mutual authentication, and resistance to passive
   escrow, the Diffie-Hellman property of the session key exchange
   guarantees perfect secrecy of keys.  In others, accidental leakage of
   one session key does not compromise past or future session keys
   between the same Initiator and Responder.

7.3.  Forking

   In the Forking feature, given that there are multiple potential
   Responders, it is important to observe that there is one "common
   Responder" identity (and corresponding Public and Private Keys) and
   each Responder has a unique identity (and corresponding Public and
   Private Keys).  Observe that, in this framework, if one Responder
   responds to the invite from the Initiator, it uses its unique
   identity such that the protocol guarantees that no other Responder
   learns the session key.

7.4.  Retargeting

   In the Retargeting feature, the forwarding server does not learn the
   Private Key of the intended Responder since it is encrypted using the
   retargeted Responder's Public Key.  Additionally, the Initiator will
   learn that the retargeted Responder answered the phone (and not the
   intended Responder) since the retargeted Responder includes its own
   identity in the message sent to the Initiator.  This will allow the
   Initiator to decide whether or not to carry on the conversation.
   Finally, the session key cannot be discovered by the intended
   Responder since the random number chosen by the retargeted Responder
   is not known to the intended Responder.

7.5.  Deferred Delivery

   In the Deferred Delivery feature, the Initiator and the Responder's
   mailbox will mutually authenticate each other thereby preventing
   server side "phishing" attacks and conversely guarantees to the
   server (and eventually to the Responder) the identity of the
   Initiator.  Moreover, the key used by Initiator to encrypt the
   contents of the message is completely independent from the session
   key derived between the Initiator and the server.  Finally, the key

   used to encrypt the message is encrypted using the Responder's Public
   Key, which allows the contents of the message to remain unknown to
   the mailbox server.

8.  IANA Considerations

   This document defines several new values for the namespaces Data
   Type, Next Payload, and Key Data Type defined in [RFC3830].  The
   following IANA assignments have been added to the MIKEY Payload
   registry (in bracket is a reference to the table containing the
   registered values):

   o  Data Type (see Table 2)

   o  Next Payload (see Table 3)

   o  Key Data Type (see Table 4)

   The ECCPT payload defines an 8-bit ECC Curve field for which IANA has
   created and will maintain a new namespace in the MIKEY Payload
   registry.  Assignments consist of an ECC curve and its associated
   value.  Values in the range 1-239 SHOULD be approved by the process
   of Specification Required, values in the range 240-254 are for
   Private Use, and the values 0 and 255 are Reserved according to
   [RFC5226].  The initial contents of the registry are as follows:

           Value    ECC curve
           -------  ------------------------------------
           0        Reserved
           1        ECPRGF192Random  / P-192 / secp192r1
           2        EC2NGF163Random  / B-163 / sect163r2
           3        EC2NGF163Koblitz / K-163 / sect163k1
           4        EC2NGF163Random2 / none  / sect163r1
           5        ECPRGF224Random  / P-224 / secp224r1
           6        EC2NGF233Random  / B-233 / sect233r1
           7        EC2NGF233Koblitz / K-233 / sect233k1
           8        ECPRGF256Random  / P-256 / secp256r1
           9        EC2NGF283Random  / B-283 / sect283r1
           10       EC2NGF283Koblitz / K-283 / sect283k1
           11       ECPRGF384Random  / P-384 / secp384r1
           12       EC2NGF409Random  / B-409 / sect409r1
           13       EC2NGF409Koblitz / K-409 / sect409k1
           14       ECPRGF521Random  / P-521 / secp521r1
           15       EC2NGF571Random  / B-571 / sect571r1
           16       EC2NGF571Koblitz / K-571 / sect571k1
           17-239   Unassigned
           240-254  Private Use
           255      Reserved

   The SK Sub-Payload defines a 4-bit Type field for which IANA has
   created and will maintain a new namespace in the MIKEY Payload
   registry.  Assignments consist of a type of key and its associated
   value.  Values in the range 2-15 SHOULD be approved by the process of
   Specification Required.  The initial contents of the registry are as
   follows:

                     Value    Type
                     -------  ---------------
                     0        Reserved
                     1        Secret Key (SK)
                     2-15     Unassigned

9.  References

9.1.  Normative References

   [BF]       Boneh, D. and M. Franklin, "Identity-Based Encryption from
              the Weil Pairing", in SIAM J. of Computing, Vol. 32,
              No. 3, pp. 586-615, 2003.

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

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

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

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

   [RFC6043]  Mattsson, J. and T. Tian, "MIKEY-TICKET: Ticket-Based
              Modes of Key Distribution in Multimedia Internet KEYing
              (MIKEY)", RFC 6043, March 2011.

   [SEC1]     Standards for Efficient Cryptography Group, "Elliptic
              Curve Cryptography", September 2000.

9.2.  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.

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

   [RFC4650]  Euchner, M., "HMAC-Authenticated Diffie-Hellman for
              Multimedia Internet KEYing (MIKEY)", RFC 4650,
              September 2006.

   [RFC4738]  Ignjatic, D., Dondeti, L., Audet, F., and P. Lin, "MIKEY-
              RSA-R: An Additional Mode of Key Distribution in
              Multimedia Internet KEYing (MIKEY)", RFC 4738,
              November 2006.

   [RFC5091]  Boyen, X. and L. Martin, "Identity-Based Cryptography
              Standard (IBCS) #1: Supersingular Curve Implementations of
              the BF and BB1 Cryptosystems", RFC 5091, December 2007.

   [RFC5408]  Appenzeller, G., Martin, L., and M. Schertler, "Identity-
              Based Encryption Architecture and Supporting Data
              Structures", RFC 5408, January 2009.

   [RFC5409]  Martin, L. and M. Schertler, "Using the Boneh-Franklin and
              Boneh-Boyen Identity-Based Encryption Algorithms with the
              Cryptographic Message Syntax (CMS)", RFC 5409,
              January 2009.

Authors' Addresses

   Violeta Cakulev
   Alcatel Lucent
   600 Mountain Ave.
   3D-517
   Murray Hill, NJ  07974
   US

   Phone: +1 908 582 3207
   EMail: violeta.cakulev@alcatel-lucent.com

   Ganapathy Sundaram
   Alcatel Lucent
   600 Mountain Ave.
   3D-517
   Murray Hill, NJ  07974
   US

   Phone: +1 908 582 3209
   EMail: ganesh.sundaram@alcatel-lucent.com

 

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