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Versions: (draft-fries-msec-mikey-applicability) 00 01 02 03 04 05 06 07 08 09 RFC 5197

MSEC                                                            S. Fries
Internet-Draft                                                   Siemens
Intended status: Informational                               D. Ignjatic
Expires: May 20, 2007                                            Polycom
                                                       November 16, 2006


       On the applicability of various MIKEY modes and extensions
               draft-ietf-msec-mikey-applicability-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).













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Abstract

   Multimedia Internet Keying - MIKEY - is a key management protocol
   that can be used for real-time applications.  In particular, it has
   been defined focusing on the support of the Secure Real-time
   Transport Protocol.  MIKEY itself defines four key distribution
   methods.  Moreover, it is defined to allow extensions of the
   protocol.  As MIKEY becomes more and more accepted, extensions to the
   base protocol arose, especially in terms of additional key
   distribution methods, but also in terms of payload enhancements.

   This document provides an overview about MIKEY in general as well as
   the existing extensions in MIKEY, which have been defined or are in
   the process of definition.  It is intended as additional source of
   information for developers or architects to provide more insight in
   use case scenarios and motivations as well as advantages and
   disadvantages for the different key distribution schemes.  The use
   cases discussed in this document are strongly related to dedicated
   SIP call scenarios providing challenges for key management in general
   beyond them media before SDP answer, forking, and shared key
   conferencing.






























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Table of Contents

   1   Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2   Terminology and Definitions  . . . . . . . . . . . . . . . . .  6
   3   MIKEY Overview . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.1  Pre-shared key protected distribution . . . . . . . . . . .  8
     3.2  Public Key encrypted key distribution . . . . . . . . . . .  9
     3.3  Diffie-Hellman key agreement protected with digital
          signatures  . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.4  Unprotected key distribution  . . . . . . . . . . . . . . . 10
     3.5  Diffie-Hellman key agreement protected with pre-shared
          secrets . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.6  SAML assisted DH-key agreement  . . . . . . . . . . . . . . 11
     3.7  Asymmetric key distribution with in-band certificate
          exchange  . . . . . . . . . . . . . . . . . . . . . . . . . 13
   4   Further MIKEY Extensions . . . . . . . . . . . . . . . . . . . 15
     4.1  ECC algorithms support  . . . . . . . . . . . . . . . . . . 15
       4.1.1  Elliptic Curve Integrated Encryption Scheme
              application in MIKEY  . . . . . . . . . . . . . . . . . 16
       4.1.2  Elliptic Curve Menezes-Qu-Vanstone Scheme
              application in MIKEY  . . . . . . . . . . . . . . . . . 16
     4.2  New Payload for bootstrapping TESLA . . . . . . . . . . . . 16
     4.3  MBMS extensions to the Key ID information type  . . . . . . 17
     4.4  OMA BCAST MIKEY General Extension Payload Specification . . 17
     4.5  Supporting Integrity Transform carrying the Rollover
          Counter . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   5   Selection and interworking of MIKEY modes  . . . . . . . . . . 19
     5.1  MIKEY and Early Media . . . . . . . . . . . . . . . . . . . 20
     5.2  MIKEY and Forking . . . . . . . . . . . . . . . . . . . . . 21
     5.3  MIKEY and Call Transfer/Redirect/Retarget . . . . . . . . . 22
     5.4  MIKEY and Shared Key Conferencing . . . . . . . . . . . . . 22
   6   Transport of MIKEY messages  . . . . . . . . . . . . . . . . . 24
   7   MIKEY alternatives for SRTP security parameter negotiation . . 25
   8   Summary of MIKEY related IANA Registrations  . . . . . . . . . 27
   9   Security Considerations  . . . . . . . . . . . . . . . . . . . 28
   10  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 29
   11  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 30
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 31
     12.1 Normative References  . . . . . . . . . . . . . . . . . . . 31
     12.2 Informative References  . . . . . . . . . . . . . . . . . . 31
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 34
   Intellectual Property and Copyright Statements . . . . . . . . . . 35









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1  Introduction

   Key distribution describes the process of delivering cryptographic
   keys to the required parties.  MIKEY [RFC3830], the Multimedia
   Internet Keying, has been defined focusing on support for the
   establishment of security context for the Secure Real-time Transport
   Protocol [RFC3711].  Note that MIKEY is not restricted to be used for
   SRTP only, as it features a generic approach and allows for
   extensions to the key distribution schemes.Thus, it may also be used
   for security parameter negotiation for other protocols.

   For MIKEY meanwhile seven key distribution methods are described as
   there are:

   o  Symmetric key distribution as defined in [RFC3830] (MIKEY-PSK)

   o  Asymmetric key distribution as defined in [RFC3830] (MIKEY-RSA)

   o  Diffie-Hellman key agreement protected by digital signatures as
      defined in [RFC3830] (MIKEY-DHSIGN)

   o  Unprotected key distribution (MIKEY-NULL)

   o  Diffie-Hellman key agreement protected by symmetric pre-shared
      keys as defined in [RFC4650] (MIKEY-DHHMAC)

   o  SAML assisted Diffie-Hellman key agreement as defined [Reference
      to draft-moskowitz-MIKEY-SAML-DH] (MIKEY-DHSAML)

   o  Asymmetric key distribution (based on asymmetric encryption) with
      in-band certificate provision as defined in
      [I-D.ietf-msec-mikey-rsa-r] (MIKEY-RSA-R)

   Note that the latter three modes are extensions to MIKEY as there
   have been scenarios where none of the first four modes defined in
   [RFC3830] fits perfectly.  There are further extensions to MIKEY
   comprising algorithm enhancements and a new payload definition
   supporting other protocols than SRTP.

   Algorithm extensions are defined in the following document:

   o  ECC algorithms for MIKEY as defined in [I-D.ietf-msec-mikey-ecc]

   Payload extensions are defined in the following documents:

   o  Bootstrapping TESLA, defining a new payload for the Timed
      Efficient Stream Loss-tolerant Authentication protocol [RFC4082]
      as defined in [RFC4442]



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   o  The Key ID information type for the general extension payload as
      defined in [RFC4563]

   o  OMA BCAST MIKEY General Extension Payload Specification, as
      defined in [I-D.dondeti-msec-mikey-genext-oma]

   o  Integrity Transform Carrying Roll-over Counter for SRTP, as
      defined in [I-D.lehtovirta-srtp-rcc].  Note that this is rather an
      extension to SRTP and requires MIKEY to carry a new parameter, but
      is stated here for completeness.

   This document provides an overview about MIKEY and the relations to
   the different extensions to provide a framework when using MIKEY.  It
   is intended as additional source of information for developers or
   architects to provide more insight in use case scenarios and
   motivations as well as advantages and disadvantages for the different
   key distribution schemes.  The use cases discussed in this document
   are strongly related to dedicated SIP call scenarios providing
   challenges for key management in general, as there are:

   o  Early Media res.  Media before SDP answer

   o  Forking

   o  Call Transfer/Redirect/Retarget

   o  Shared Key Conferencing
























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2  Terminology and Definitions

   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 RFC 2119 [RFC2119].

   The following definitions have been taken from [RFC3830]:

   (Data) Security Protocol: the security protocol used to protect the
   actual data traffic.  Examples of security protocols are IPsec and
   SRTP.

   Data Security Association (Data SA): information for the security
   protocol, including a TEK and a set of parameters/policies.

   Crypto Session (CS): uni- or bi-directional data stream(s), protected
   by a single instance of a security protocol.

   Crypto Session Bundle (CSB): collection of one or more Crypto
   Sessions, which can have common TGKs (see below) and security
   parameters.

   Crypto Session ID: unique identifier for the CS within a CSB.

   Crypto Session Bundle ID (CSB ID): unique identifier for the CSB.

   TEK Generation Key (TGK): a bit-string agreed upon by two or more
   parties, associated with CSB.  From the TGK, Traffic-encrypting Keys
   can then be generated without needing further communication.

   Traffic-Encrypting Key (TEK): the key used by the security protocol
   to protect the CS (this key may be used directly by the security
   protocol or may be used to derive further keys depending on the
   security protocol).  The TEKs are derived from the CSB's TGK.

   TGK re-keying: the process of re-negotiating/updating the TGK (and
   consequently future TEK(s)).

   Initiator: the initiator of the key management protocol, not
   necessarily the initiator of the communication.

   Responder: the responder in the key management protocol.

   Salting key: a random or pseudo-random (see [RAND, HAC]) string used
   to protect against some off-line pre-computation attacks on the
   underlying security protocol.

   HDR: denotes the protocol header



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   PRF(k,x): a keyed pseudo-random function

   E(k,m): encryption of m with the key k

   RAND: Random value

   T: Timestamp

   CERTx: the certificate of x

   SIGNx: the signature from x using the private key of x

   PKx: the public key of x

   IDx: the identity of x

   [] an optional piece of information

   {} denotes zero or more occurrences

   || concatenation

   | OR (selection operator)

   ^ exponentiation

   XOR exclusive or

   The following definitions have been added additionally to the ones
   from [RFC3830]:

   SSRC Synchronization Source Identifier



















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3  MIKEY Overview

   This section will provide an overview about MIKEY.  The focus lies
   here on the key distribution methods as well as the discussion about
   advantages and disadvantages of the different schemes.  Note that the
   MIKEY key distribution schemes rely on loosely synchronized clocks.
   A secure network clock synchronization protocol should realize this.
   MIKEY recommends the ISO time synchronization protocol
   [ISO_sec_time].  The format applied to the timestamps submitted in
   the MIKEY have to match the NTP format described in [RFC1305].  In
   other cases, such as of a SIP endpoint clock synchronization by
   deriving time from a trusted outbound proxy may be appropriate.

   If MIKEY is used for SRTP [RFC3711] bootstrapping, it also uses the
   SSRC to associate security policies with actual sessions.  The SSRC
   identifies the synchronization source.  The value is chosen randomly,
   with the intent that no two synchronization sources within the same
   SRTP session will have the same SSRC.  Although the probability of
   multiple sources choosing the same identifier is low, all (S)RTP
   implementations must be prepared to detect and resolve collisions.
   Nevertheless in multimedia communication scenarios supporting forking
   Section 5.2, collisions may occur leading to so-called two-time pads,
   i.e., the same key is used for media streams to different
   destinations.  Note that two time pads may also occur for media
   streams to the same destination.

3.1  Pre-shared key protected distribution

   This option of the key management uses a pre-shared secret key to
   derive key material for integrity protection and encryption to
   protect the actual exchange of key material.  Note that the pre-
   shared secret is agreed upon before the session, e.g., by out-of-band
   means.  The response message is optional and may be used for mutual
   authentication or error signaling.

   Initiator                                  Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi],[IDr],
       {SP}, KEMAC                --->
                                              R_MESSAGE =
                                 [<---]       HDR, T, [IDr], V

   The advantages of this approach lay in the fact that there is no
   dependency on a PKI (Public Key Infrastructure), the solution
   consumes low bandwidth and enables high performance, and is all in
   all a simple straightforward master key provisioning.  The
   disadvantages are that no perfect forward secrecy is provided and key



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   generation is just performed by the initiator.  Furthermore, the
   approach is not scalable to larger configurations but is acceptable
   in small-sized groups.  Note that according to [RFC3830] this option
   is mandatory to implement.

3.2  Public Key encrypted key distribution

   Using the asymmetric option of the key management, the initiator
   generates the key material (TGK's) to be transmitted and sends it
   encrypted with a so-called envelope key, which in turn is encrypted
   with the receiver's public key.  The envelope key, env-key, which is
   a random number, is used to derive the auth-key and the enc-key.
   Moreover, the envelope key may be used as a pre-shared key to
   establish further crypto sessions.  The response message is optional
   and may be used for mutual authentication or error signaling.

   Initiator                                    Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi],
     [IDr], {SP}, KEMAC, [CHASH],
     PKE, SIGNi                   --->
                                               R_MESSAGE =
                                 [<---]         HDR, T, [IDr], V

   An advantage of this approach are that the usage of self-signed
   certificates can avoid PKI.  Note that using self-signed certificates
   may result in limited scalability.  The disadvantages comprise the
   necessity of a PKI for fully scalability, the performance of the key
   generation just by the initiator, and no provision of perfect forward
   secrecy.  Additionally, the responder certificate needs to be
   available in advance at the sender's side.  Furthermore, the
   verification of certificates may not be done in real-time.  This
   could be the case in scenarios where the revocation status of
   certificates is checked through a further component.  Note, according
   to [RFC3830] this option is mandatory to implement.

3.3  Diffie-Hellman key agreement protected with digital signatures

   The Diffie-Hellman option of the key management enables a shared
   secret establishment between initiator and responder in a way where
   both parties contribute to the shared secret.  The Diffie-Hellman key
   agreement is authenticated (and integrity protected) using digital
   signatures.







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   Initiator                                 Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi],
        [IDr], {SP}, DHi, SIGNi   --->
                                             R_MESSAGE =
                                  <---        HDR, T, [IDr|CERTr],
                                               IDi, DHr, DHi, SIGNr

   [RFC3830] does mandate the support of RSA as specific asymmetric
   algorithm for the signature generation.  Additionally the algorithm
   used for signature or public key encryption is defined by, and
   dependent on the certificate used.  Besides the use of X.509v3
   certificates it is mandatory to support the Diffie-Hellmann group
   "OAKLEY5" [RFC2412].  The advantages of this approach are a fair,
   mutual key agreement (both parties provide to the key), perfect
   forward secrecy, and the absence of the need to fetch a certificate
   in advance as needed for the MIKEY-RSA method depicted above.
   Moreover, it provides also the option to use self-signed certificates
   to avoid PKI (would result in limited scalability and more complex
   provisioning).  Note that, depending on the security policy, self-
   signed certificates may not be suitable for every use case.
   Negatively to remark is that this approach scales mainly to point-to-
   point groups and depends on PKI for full scalability.  Multiparty
   conferencing is not supported using just MIKEY-DHSIGN.  Nevertheless,
   the established Diffie-Hellman-Secret may serve as a pre-shared key
   to bootstrap group-related security parameter.  Furthermore, as for
   the MIKEY-RSA mode described above, the verification of certificates
   may not be necessarily done in real-time.  This could be the case in
   scenarios where the revocation status of certificates is checked
   through a further component.

3.4  Unprotected key distribution

   MIKEY also supports a mode to provide a key in an unprotected manner
   (MIKEY-NULL).  This is based on the symmetric key encryption option
   depicted in Section 3.1 but is used with the NULL encryption and the
   NULL authentication algorithm.  It may be compared with the plain
   approach in sdescriptions [RFC4568].  MIKEY-NULL completely relies on
   the security of the underlying layer, e.g., provided by TLS.  This
   option should be used with caution as it does not protect the key
   management.

3.5  Diffie-Hellman key agreement protected with pre-shared secrets

   This is an additional option which has been defined in [RFC4650].  In
   contrast to the method described in Section 3.3 here the Diffie-
   Hellmann key agreement is authenticated (and integrity protected)



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   using a pre-shared secret and keyed hash function.

   Initiator                                  Responder

   I_MESSAGE =
   3D HDR, T, RAND, [IDi],
       IDr, {SP}, DHi, KEMAC      --->
                                             R_MESSAGE =
                                  <---       3D HDR, T,[IDr], IDi,
                                                 DHr, DHi, KEMAC

   TGK =3D g^(xi * yi)                        TGK =3D g^(xi * yi)

   For the integrity protection of the Diffie-Hellman key agreement
   [RFC4650] mandates the use of HMAC SHA-1.  Regarding Diffie-Hellman
   groups [RFC3830] is referenced.  Thus, it is mandatory to support the
   Diffie-Hellman group "OAKLEY5" [RFC2412].  This option has also
   several advantages, as there are the fair mutual key agreement, the
   perfect forward secrecy, and no dependency on a PKI and PKI
   standards.  Moreover, this scheme has a sound performance and reduced
   bandwidth requirements and provides a simple and straightforward
   master key provisioning.  The scalability of this approach comprising
   only point-to-point communication is a disadvantage.

   This mode of operation provides an efficient scheme in deployments
   where there is a central trusted server that is provisioned with
   shared secrets for many clients.  Such setups could for example be
   enterprise PBXs, service provider proxies, etc.  In contrast to the
   plain pre-shared key encryption based mode, described in Section 3.1,
   this mode offers perfect forward secrecy.

3.6  SAML assisted DH-key agreement

   There has been a longer discussion during meetings and the MSEC
   mailing about a SAML assisted DH approach, which have not been
   submitted as a draft.[Reference to draft-moskowitz-MIKEY-SAML-DH].
   Nevertheless, the discussed is targeted to fulfill general
   requirements on key management approaches and is therefore stated
   here:

   1.  Mutual authentication of involved parties

   2.  Both parties involved contribute to the session key generation

   3.  Provide perfect forward secrecy

   4.  Support distribution of group session keys




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   5.  Provide liveliness tests when involved parties do not have a
       reliable clock

   6.  Support of limited parties involved

   To fulfill all of the requirements, the document proposes the use of
   a classic Diffie-Hellman key agreement protocol for key establishment
   in conjunction with UA's SIP server signed element authenticating the
   Diffie-Hellman key and the ID using the SAML (Security Association
   Markup Language, [SAML_overview]) approach.  Here the client's public
   Diffie-Hellman-credentials are signed by the server to form a SAML
   assertion [CRED], which may be used for later sessions with other
   clients.  This assertion needs at least to convey the ID, public DH
   key, expiry, and the signature from the server.  This provides the
   involved clients with mutual authentication and message integrity of
   the key management messages exchanged.

   Initiator                             Responder

   I_MESSAGE =
   HDR, T, RAND1, [CREDi],
   IDr, {SP}                      --->
                                         R_MESSAGE =
                                  <---   HDR, T, [CREDr], IDi, DHr,
                                         RAND2, (SP)
          TGK = HMACx(RAND1|RAND2), where x = g^(xi * xr).

   Additionally the document proposes a second roundtrip to avoid the
   dependence on synchronized clocks and provide liveliness checks.
   This is achieved by exchanging nonces, protected with the session
   key.  This second roundtrip can also be used for distribution of
   group keys or for the leverage of a weak DH key for a stronger
   session key.  The trigger for the second round trip would be handled
   via SP, the Security Policy communicated via MIKEY.

   Initiator                             Responder

   I_MESSAGE =
   HDR, SIGN(ENC(RAND3))          --->
                                         R_MESSAGE =
                                  <---   SIGN(ENC(RAND4))

   Note if group keys are to be provided RAND would be substituted by
   that group key.

   With the second roundtrip, this approach also provides an option for
   all of the other key distribution methods, when liveliness checks are
   needed.  The drawback of the second roundtrip is that these messages



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   need to be integrated into the call flow of the signaling protocol.
   In straight forward call one roundtrip may be enough to setup a
   session.  Thus this second roundtrip would require additional
   messages to be exchanged.

3.7  Asymmetric key distribution with in-band certificate exchange

   This is an additional option which has been defined in
   [I-D.ietf-msec-mikey-rsa-r].  It describes the asymmetric key
   distribution with optional in-band certificate exchange.

   Initiator                             Responder

   I_MESSAGE =
   HDR, T, [IDi|CERTi], [IDr],
         {SP}, [RAND], SIGNi      --->
                                         R_MESSAGE =
                                  <---   HDR, [GenExt(CSB-ID)], T,
                                           RAND, [IDr|CERTr], [SP],
                                           KEMAC, SIGNr

   This option has some advantages compared to the asymmetric key
   distribution stated in Section 3.2.  Here, the sender and receiver do
   not need to know the certificate of the other peer in advance as it
   may be sent in the MIKEY initiator message.  Thus, the receiver of
   this message can utilize the received key material to encrypt the
   session parameter and send them back as part of the MIKEY response
   message.  The certificate check may be done depending on the signing
   authority.  If the certificate is signed by an publicly accepted
   authority the certificate validation is done on the common base.  In
   the other case additional steps may be necessary.  The disadvantage
   is that no perfect forward secrecy is provided.

   This mode is meant to provide an easy option for certificate
   provisioning when PKI is present and/or required.  Specifically in
   SIP, session invitations can be retargeted or forked.  MIKEY modes
   that require the Initiator to target a single well known Responder
   may be impractical here as they may require multiple roundtrips to do
   key negotiation.  By allowing the Responder to generate secret
   material used for key derivation this mode allows for an efficient
   key delivery scheme.  Note that the Initiator can contribute to the
   material the key is derived from through CSB-ID and RAND payloads in
   unicast use cases.  This mode is also useful in multicast scenarios
   where multiple clients are contacting a known server and are
   downloading the key.  Server workload is significantly reduced in
   these scenarios compared to MIKEY in public key mode.  Examples of
   deployments where this mode can be used are enterprises with PKI,
   service provider setups where the service provider decides to



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   provision certificates to its users, etc.


















































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4  Further MIKEY Extensions

   This section will provide an overview about further MIKEY extensions
   for crypto algorithms, generic payload enhancements, as well as
   enhancements to support the negotiation of security parameters for
   other security protocols than SRTP.  These extensions have been
   defined in several additional documents.

4.1  ECC algorithms support

   [I-D.ietf-msec-mikey-ecc] proposes extensions to the authentication,
   encryption and digital signature methods described for use in MIKEY,
   employing elliptic-curve cryptography (ECC).  These extensions are
   defined to align MIKEY with other ECC implementations and standards.

   The motivation for supporting ECC within the MIKEY stems from the
   following advantages:

   o  ECC support is generally added to security protocols

   o  ECC support requires considerably smaller keys by keeping the same
      security level compared to other asymmetric techniques (like RSA).
      Elliptic curve algorithms are capable of providing security
      consistent with AES keys of 128, 192, and 256 bits without
      extensive growth in asymmetric key sizes.

   o  As stated in [I-D.ietf-msec-mikey-ecc] implementations have shown
      that elliptic curve algorithms can significantly improve
      performance and security-per-bit over other recommended
      algorithms.

   These advantages make the usage of ECC especially interesting for
   embedded devices, which may have only limited performance and storage
   capabilities.

   [I-D.ietf-msec-mikey-ecc] proposes several ECC based mechanisms to
   enhance the MIKEY key distribution schemes, as there are:

   o  Use of ECC methods extending the Diffie-Hellman key exchange:
      MIKEY-DHSIGN with ECDSA

   o  Use of ECC methods extending the Diffie-Hellman key exchange:
      MIKEY-DHSIGN with ECDH

   o  Use of Elliptic Curve Integrated Encryption Scheme (MIKEY-ECIES)

   o  Use of Elliptic Curve Scheme Menezes-Qu-Vanstone (MIKEY-ECMQV)




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   The following subsections will provide more detailed information
   about the message exchanges for MIKEY-ECIES and MIKEY-ECMQV.

4.1.1  Elliptic Curve Integrated Encryption Scheme application in MIKEY

   The following figure shows the message exchange for the MIKEY-ECIES
   scheme:

   Initiator                                       Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi],
       [IDr], {SP}, ECCPT, KEMAC,
       [CHASH], SIGNi             --->
                                                   R_MESSAGE =
                                 [<---]            HDR, T, [IDr], V

4.1.2  Elliptic Curve Menezes-Qu-Vanstone Scheme application in MIKEY

   The following figure shows the message exchange for the MIKEY-ECMQV
   scheme:

   Initiator                                      Responder

   I_MESSAGE =
   HDR, T, RAND, [IDi|CERTi],
      [IDr], {SP}, ECCPT, KEMAC,
      [CHASH], SIGNi               --->
                                                  R_MESSAGE =
                                  [<---]          HDR, T, [IDr], V

4.2  New Payload for bootstrapping TESLA

   TESLA [RFC4082] is a protocol for providing source authentication in
   multicast scenarios.  TESLA is an efficient protocol with low
   communication and computation overhead, which scales to large numbers
   of receivers, and also tolerates packet loss.  TESLA is based on
   loose time synchronization between the sender and the receivers.
   Source authentication is realized in TESLA by using Message
   Authentication Code (MAC) chaining.  The use of TESLA within the
   Secure Real-time Transport Protocol (SRTP) has been published in
   [RFC4383] targeting multicast authentication in scenarios, where SRTP
   is applied to protect the multimedia data.  This solution assumes
   that TESLA parameters are made available by out-of-band mechanisms.

   [RFC4442] specifies payloads for MIKEY to bootstrap TESLA for source
   authentication of secure group communications using SRTP.  TESLA may
   be bootstrapped using one of the MIKEY key management approaches



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   described above by sending the MIKEY message via unicast, multicast
   or broadcast.  This approach provides the necessary parameter payload
   extensions for the usage of TESLA in SRTP.  Nevertheless, if the
   parameter set is also sufficient for other TESLA use cases, it can be
   applied as well.

4.3  MBMS extensions to the Key ID information type

   This extension specifies a new Type (the Key ID Information Type) for
   the General Extension Payload.  This is used in, e.g., the Multimedia
   Broadcast/Multicast Service (MBMS) specified in the 3rd Generation
   Partnership Project (3GPP).  MBMS requires the use of MIKEY to convey
   the keys and related security parameters needed to secure the
   multimedia that is multicast or broadcast.

   One of the requirements that MBMS puts on security is the ability to
   perform frequent updates of the keys.  The rationale behind this is
   that it will be costly for subscribers to re-distribute the
   decryption keys to non-subscribers.  The cost for re-distributing the
   keys using the unicast channel should be higher than the cost of
   purchasing the keys for this scheme to have an effect.  To achieve
   this, MBMS uses a three-level key management, to distribute group
   keys to the clients, and be able to re-key by pushing down a new
   group key.  MBMS has the need to identify, which types of keys are
   involved in the MIKEY message and their identity.

   [RFC4563] specifies a new Type for the General Extension Payload in
   MIKEY, to identify the type and identity of involved keys.  Moreover,
   as MBMS uses MIKEY both as a registration protocol and a re-key
   protocol, this RFC specifies the necessary additions that allow MIKEY
   to function both as a unicast and multicast re-key protocol in the
   MBMS setting.

4.4  OMA BCAST MIKEY General Extension Payload Specification

   The document [I-D.dondeti-msec-mikey-genext-oma] specifies a new
   general extension payload type for use in the Open Mobile Alliance's
   (OMA) Browser and Content Broadcast (BCAST) group.  OMA BCAST's
   service and content protection specification uses short term key
   message and long term key message payloads that in certain broadcast
   distribution systems are carried in MIKEY.  The document defines a
   general extensions payload to allow possible extensions to MIKEY
   without defining a new payload.  The general extension payload can be
   used in any MIKEY message and is part of the authenticated or signed
   data part.  Note, that only a parameter description is included, but
   no key information.





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4.5  Supporting Integrity Transform carrying the Rollover Counter

   The document [I-D.lehtovirta-srtp-rcc] defines a new integrity
   transform for SRTP [RFC3711] providing the option to also transmit
   the Roll Over Counter (ROC) as part of dedicated SRTP packets.  This
   extension has been defined for the use in the 3GPP multicast/
   broadcast service.  While the communicating parties did agree on a
   starting ROC, in some cases the receiver will not be able to
   synchronize his ROC with the one used by the sender even if it is
   signaled to him out of band.  Here the new extension provides the
   possibility for the receiver to re-synchronize to the sender's ROC.
   To signal the use of the new integrity transform new definitions for
   certain MIKEY payloads need to be done.  These MIKEY new definition
   comprise the integrity transform s and new integrity transform
   parameter.  Moreover, the document specifies integrity parameter, to
   enable the usage of different integrity transforms for SRTP and
   SRTCP.


































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5  Selection and interworking of MIKEY modes

   While MIKEY and its extensions provide plenty of choice in terms of
   modes of operation an implementation may choose to simplify its
   behavior.  This can be achieved by operating in a single mode of
   operation when in Initiator's role.  Where PKI is available and/or
   required an implementation may choose for example to start all
   sessions in RSA-R mode but it would be trivial for it to act as a
   Responder in public key mode.  If envelope keys are cached it can
   then also choose to do re-keying in shared key mode.  In general,
   modes of operation where the Initiator generates keying material are
   useful when two peers are aware of each other before the MIKEY
   communication takes place.  If an implementation chooses not to
   operate in shared key mode its behavior may be identical to a peer
   that does but lacks the shared key.  Similarly, if a peer chooses not
   to operate in the public key mode it may reject the certificate of
   the Initiator.  The same applies to peers that choose to operate in
   one of the DH modes exclusively.

   Forward MIKEY modes like public key or shared key mode when used in
   SIP/SDP may lead to complications in some calls scenarios, for
   example forking scenarios were key derivation material gets
   distributed to multiple parties.  As mentioned earlier this may be
   impractical as some of the destinations may not have the resources to
   validate the message and may cause the initiator to drop the session
   invitation.  Even in the case all parties involved have all the
   prerequisites for interpreting the MIKEY message received there is a
   possible problem with multiple responders starting media sessions
   using the same key.  While the SSRCs will be different in most of the
   cases they are only sixteen bits long and there is a high probability
   of a two-time pad problem.  As suggested earlier forward modes are
   most useful when the two peers are aware of each other before the
   communication takes place (as is the case in key renewal scenarios
   when costly public key operations can be avoided by using the
   envelope key).

   The following list may give an idea, how the different MIKEY modes
   may be used or combined, depending on available key material at the
   initiator side.

   1.  If the Initiator has a PSK with the Responder, it uses the PSK
       mode.

   2.  If the Initiator has a PSK with the Responder, but needs PFS or
       knows that the responder has a policy that both parties should
       provide entropy to the key, then it uses the DH-HMAC mode.





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   3.  If the Initiator has the RSA key of the Responder, it uses the
       RSA mode to establish the TGK.  Note that the TGK may be used as
       PSK together with Option 1 in the future.

   4.  The Initiator uses RSA-R when he does expect the receiver not
       having his certificate.  Using RSA-R he can provide his
       certificate information in-band to the receiver.  Moreover, the
       initiator may also provide a random number which can be used by
       the receiver for key generation.  Thus both parties can be
       involved in the key management.  But as the inclusion of the
       random number cannot be forced by the initiator, true PFS cannot
       be provided.  Note that in this mode, after establishing the TGK,
       it may be used as PSK with other MIKEY options.

   5.  The Initiator uses DH-SIGN when PFS is required by his policy and
       he knows that the responder has a policy that both parties should
       provide entropy.  Note that also in this mode, after establishing
       the TGK, it may be used as PSK with other MIKEY options.

   6.  If no PSK or certificate is available at the initiators side (and
       likewise at the receivers side) but lower level security (like
       TLS ot IPSec) is in place the user may use the unprotected mode
       of MIKEY.

   Besides the available key material choosing between the different
   modes of MIKEY depends strongly on the use case.  This document will
   discuss further scenarios to argue for preferred modes.  The
   following call scenarios provide a list of potential call scenarios
   and are matter of discussion:

   o  Early Media

   o  Forking

   o  Call Transfer/Redirect/Retarget

   o  Shared key conferencing

5.1  MIKEY and Early Media

   In early media scenarios, SRTP data may be received before the answer
   over the SIP signaling arrives.  The two MIKEY modes, which only
   require one message to be transported (Section 3.1 and Section 3.2),
   work nicely in early media situations, as both, sender and receiver
   have all the necessary parameters in place before actually sending/
   receiving encrypted data.  The other modes, featuring either Diffie-
   Hellman key agreement (Section 3.3, Section 3.5, and Section 3.6) or
   the enhanced asymmetric variant (Section 3.7) suffer from the



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   requirements that the initiator has to wait for the response before
   being able to decrypt the incoming SRTP media.  In fact, even if
   early media is not used, in other words if media is not sent before
   the SDP answer a similar problem may arise from the fact that SIP/SDP
   signaling has to traverse multiple proxies on its way back and media
   may arrive before the SDP answer.  It is expected that this delay
   would be significantly shorter than in the case of early media
   though.

   It is worth mentioning here that security descriptions ([RFC4568])
   have the same problem as the initiating end needs the SDP answer
   before it can start decrypting SRTP media.

   To cope with the early media problem there are further approaches to
   describe security preconditions
   [I-D.ietf-mmusic-securityprecondition], i.e., certain preconditions
   need to be met to enable voice data encryption.  One example is for
   instance that a scenario where a provisional response, containing the
   required MIKEY parameter, is sent before encrypted media is
   processed.

5.2  MIKEY and Forking

   In SIP forking scenarios a SIP proxy server sends an INVITE request
   to more than one location.  This means that also the MIKEY payload,
   which is part of the SDP is sent to several (different) locations.
   MIKEY modes supporting signatures may be used in forking scenarios
   (Section 3.3 and Section 3.7) as here the receiver can validate the
   signature.  There are limitations with the symmetric key encryption
   as well as the asymmetric key encryption modes (Section 3.1 and
   Section 3.2).  This is due to the fact that in symmetric encryption
   the recipient needs to possess the symmetric key before handling the
   MIKEY data.  For asymmetric MIKEY modes, if the sender is aware of
   the forking he may not know in advance to which location the INVITE
   is forked and thus may not use the right receiver certificate to
   encrypt the MIKEY envelope key.  Note, the sender may include several
   MIKEY containers into the same INVITE message to cope with forking,
   but this requires the knowledge of all forking targets in advance and
   also requires the possession of the target certificates.  It is out
   of the scope of MIKEY to specify behavior in such a case.  DH modes
   or the Section 3.7 do not have this problem.  In scenarios, where the
   sender is not aware of forking, only the intended receiver is able to
   decrypt the MIKEY container.

   If forking is combined with early media the situation gets
   aggravated.  If MIKEY modes requiring full roundtrip are used, like
   the signed Diffie-Hellman, multiple responses may overload the end
   device.  An example is forking to 30 destinations (group pickup),



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   while MIKEY is used with the signed Diffie-Hellman mode together with
   security preconditions.  Here, every target would answer with a
   provisional response, leading to 30 signature validations and Diffie-
   Hellman calculations at the senders site.  This may lead to a
   prolonged media setup delay.

   Moreover, depending on the MIKEY mode chosen, a two-time pad may
   occur in dependence of the negotiated key material and the SSRC.  For
   the non Diffie-Hellman modes, a two-time pad may occur when multiple
   receivers pick the same SSRC.  For the MIKEY Diffie-Hellman modes
   this can only happen, when multiple receiver pick the same SSRC and
   the same Diffie-Hellman half key.

5.3  MIKEY and Call Transfer/Redirect/Retarget

   In a SIP environment MIKEY exchange is tied to SDP offer/answer and
   irrespective of the implementation model used for call transfer the
   same properties and limitations of MIKEY modes apply as in a normal
   call setup scenarios.

   In certain SIP scenarios the functionality of redirect is supported.
   In redirect scenarios the call initiator gets a response that the
   called party for instance has temporarily moved and may be reached at
   a different destination.  The caller can now perform a call
   establishment with the new destination.  Depending on the originally
   chosen MIKEY mode, the caller may not be able to perform this mode
   with the new destination.  To be more precise MIKEY-PSK, and MIKEY-
   DHHMAC require a pre-shared secret in advance.  MIKEY-RSA requires
   the knowledge about the target's certificate.  Thus, these modes may
   influence the ability of the caller to initiate a session.

   Another functionality, which may be supported in SIP is retargeting.
   In contrast to redirect, the call initiator does not get a response
   about the different target.  The SIP proxy sends the request to a
   different target about receiving a redirect response from the
   originally called target.  This most likely will lead to problems
   when using MIKEY modes requiring a pre-shared key (MIKEY-PSK, MIKEY-
   DHHMAC) or were the caller used asymmetric key encryption (MIKEY-RSA)
   because the key management was originally targeted to a different
   destination.

5.4  MIKEY and Shared Key Conferencing

   First of all, not all modes of MIKEY support shared key conferencing.
   Mainly the Diffie Hellman modes cannot be used straight forward for
   conferencing as this mechanism results in a pairwise shared secret
   key.  All other modes can be applied in conferencing scenarios by
   obeying the initiator and responder role, i.e., the half roundtrip



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   modes need to be initiated by the conferencing unit, to be able to
   distribute the conferencing key.  The remaining full roundtrip mode,
   MIKEY RSA-R will be initiated by the client, while the conferencing
   unit provides the conferencing key based on the received certificate.

   An example conferencing architecture is defined in the IETF's XCON
   WG.  The scope of this working group relates to mechanism for
   membership and authorization control, a mechanism to manipulate and
   describe media "mixing" or "topology" for multiple media types
   (audio, video, text), a mechanism for notification of conference
   related events/changes (for example a floor change), and a basic
   floor control protocol.  A docuemnt describing possible use case
   scenarios is available in [I-D.ietf-xcon-conference-scenarios].






































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6  Transport of MIKEY messages

   MIKEY defines message formats to transport key information and
   security policies between communicating entities.  It does not define
   the embedding of these messages into the used signaling protocol.
   This definition is provided in separate documents, depending on the
   used signaling protocol.  Nevertheless, MIKEY can also be transported
   over plain UDP or TCP to port 2269.

   Several IETF defined protocols utilize the Session Description
   Protocol (SDP, [RFC2327]) to transport the session parameters.
   Examples are the Session Initiation Protocol (SIP, [RFC3261] or the
   Gateway Control Protocol (GCP, [RFC3525]).  The transport of MIKEY
   messages as part of SDP is described in [RFC4567].  Here, the
   complete MIKEY message is base64 encoded and transmitted as part of
   the SDP part of the signaling protocol message.  Note, as several key
   distribution messages may be transported within one SDP container,
   [RFC4567] also comprises an integrity protection regarding all
   supplied key distribution attempts.  Thus, bidding down attacks will
   be recognized.

   MIKEY is also applied in ITU-T protocols like H.323, which is used to
   establish communication sessions similar to SIP.  For H.323 a
   security framework exists, which is defined in H.235.  Within this
   framework H.235.7 [H.235.7] describes the usage of MIKEY and SRTP in
   the context of H.323.  In contrast to SIP H.323 uses ASN.1 (Abstract
   Syntax Notation).  Thus there is no need to encode the MIKEY
   container as base64.  Within H.323 the MIKEY container is binary
   encoded.






















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7  MIKEY alternatives for SRTP security parameter negotiation

   Besides MIKEY there exists several approaches to handle the security
   parameter establishment.  This is due to the fact, that some
   limitations in certain scenarios have been seen.  Examples are early
   media and forking situations as described in Section 5.  The
   following list provides a short summary about possible alternatives:

   o  sdescription - [RFC4568] describes a key management scheme, which
      uses SDP for transport and completely relies on underlying
      protocol security.  For transport the documents defines a SDP
      attribute transmitting all necessary SRTP parameter in clear.  For
      security it references TLS and S/MIME.In contrast to MIKEY in the
      message from the initiator to the responder the SRTP parameter for
      the direction initiator to responder is sent rather than vice
      versa.  This may lead to problems in early media scenarios.

   o  sdescription with early media support -
      [I-D.wing-mmusic-sdes-early-media] enhances the above scheme with
      the possibility to also be usable in early media scenarios, when
      security preconditions is not used.

   o  Encrypted Key Transport for Secure RTP - [I-D.mcgrew-srtp-ekt] is
      an extension to SRTP that provides for the secure transport of
      SRTP master keys, Rollover Counters, and other information, within
      SRTCP.  This facility enables SRTP to work for decentralized
      conferences with minimal control, and to handle situations caused
      by SIP forking and early media.

   o  Diffie Hellman support in SDP - [I-D.baugher-mmusic-sdp-dh]
      defines a new SDP attribute for exchanging Diffie-Hellman public
      keys.  The attribute is an SDP session-level attribute for
      describing DH keys, and there is a new media-level parameter for
      describing public keying material for SRTP key generation.

   o  DTLS/SRTP compatibility mode - is described as part of
      [I-D.tschofenig-avt-rtp-dtls] and provides for using DTLS as key
      management approach in conjunction with partial encryption
      targeted for low bandwidth connections.

   o  SRTP extensions for DTLS - [Reference to I-D.mcgrew-dtls-srtp]
      describes a method of using DTLS key management for SRTP by using
      a new extension that indicates that SRTP is to be used for data
      protection, and which establishes SRTP keys.

   o  ZRTP - [I-D.zimmermann-avt-zrtp] This document defines ZRTP as RTP
      header extensions for a Diffie-Hellman exchange to agree on a
      session key and parameters for establishing SRTP sessions.  The



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      ZRTP protocol is completely self-contained in RTP and does not
      require support in the signaling protocol or assume a PKI.

















































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8  Summary of MIKEY related IANA Registrations

   For MIKEY and the extensions to MIKEY IANA registrations have been
   made.  Here only a link to the appropriate IANA registration is
   provided to avoid inconsistencies.  The IANA registrations for MIKEY
   payloads can be found under
   http://www.iana.org/assignments/mikey-payloads These registrations
   comprise the MIKEY base registrations as well as registrations made
   by MIKEY extensions regarding the payload.

   The IANA registrations for MIKEY port numbers can be found under
   http://www.iana.org/assignments/port-numbers (search for MIKEY).







































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9  Security Considerations

   This document does not define extensions to existing protocols.  It
   rather provides an overview about the set of MIKEY and available
   extensions.  Thus, the reader is referred to the original documents
   defining the base protocol and the extensions for the security
   considerations.












































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10  IANA Considerations

   This document does not require any IANA registration.
















































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11  Acknowledgments

   The authors would like to thank Lakshminath Dondeti for his document
   reviews and for his guidance.















































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12.  References

12.1.  Normative References

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

12.2.  Informative References

   [H.235.7]  ""ITU-T Recommendation H.235.7: Usage of the MIKEY Key
              Management Protocol for the Secure Real Time Transport
              Protocol (SRTP) within H.235"", 2005.

   [I-D.baugher-mmusic-sdp-dh]
              Baugher, M. and D. McGrew, "Diffie-Hellman Exchanges for
              Multimedia Sessions", draft-baugher-mmusic-sdp-dh-00 (work
              in progress), February 2006.

   [I-D.dondeti-msec-mikey-genext-oma]
              Dondeti, L., "OMA BCAST MIKEY General Extension Payload
              Specification", draft-dondeti-msec-mikey-genext-oma-02
              (work in progress), September 2006.

   [I-D.ietf-mmusic-securityprecondition]
              Andreasen, F. and D. Wing, "Security Preconditions for
              Session Description Protocol (SDP) Media  Streams",
              draft-ietf-mmusic-securityprecondition-03 (work in
              progress), October 2006.

   [I-D.ietf-msec-mikey-ecc]
              Milne, A., "ECC Algorithms for MIKEY",
              draft-ietf-msec-mikey-ecc-01 (work in progress),
              October 2006.

   [I-D.ietf-msec-mikey-rsa-r]
              Ignjatic, D., "An additional mode of key distribution in
              MIKEY: MIKEY-RSA-R", draft-ietf-msec-mikey-rsa-r-07 (work
              in progress), August 2006.

   [I-D.ietf-xcon-conference-scenarios]
              Even, R. and N. Ismail, "Conferencing Scenarios",
              draft-ietf-xcon-conference-scenarios-05 (work in
              progress), September 2005.

   [I-D.lehtovirta-srtp-rcc]
              Lehtovirta, V., "Integrity Transform Carrying Roll-over
              Counter", draft-lehtovirta-srtp-rcc-06 (work in progress),



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              October 2006.

   [I-D.mcgrew-srtp-ekt]
              McGrew, D., "Encrypted Key Transport for Secure RTP",
              draft-mcgrew-srtp-ekt-01 (work in progress), June 2006.

   [I-D.tschofenig-avt-rtp-dtls]
              Tschofenig, H. and E. Rescorla, "Real-Time Transport
              Protocol (RTP) over Datagram Transport Layer Security
              (DTLS)", draft-tschofenig-avt-rtp-dtls-00 (work in
              progress), March 2006.

   [I-D.wing-mmusic-sdes-early-media]
              Raymond, R. and D. Wing, "Security Descriptions Extension
              for Early Media", draft-wing-mmusic-sdes-early-media-00
              (work in progress), October 2005.

   [I-D.zimmermann-avt-zrtp]
              Zimmermann, P., "ZRTP: Extensions to RTP for Diffie-
              Hellman Key Agreement for SRTP",
              draft-zimmermann-avt-zrtp-02 (work in progress),
              October 2006.

   [ISO_sec_time]
              ""ISO/IEC 18014 Information technology - Security
              techniques - Time-stamping services, Part 1-3."", 2002.

   [RFC1305]  Mills, D., "Network Time Protocol (Version 3)
              Specification, Implementation", RFC 1305, March 1992.

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

   [RFC2327]  Handley, M. and V. Jacobson, "SDP: Session Description
              Protocol", RFC 2327, April 1998.

   [RFC2412]  Orman, H., "The OAKLEY Key Determination Protocol",
              RFC 2412, November 1998.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [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.




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   [RFC3525]  Groves, C., Pantaleo, M., Anderson, T., and T. Taylor,
              "Gateway Control Protocol Version 1", RFC 3525, June 2003.

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

   [RFC4082]  Perrig, A., Song, D., Canetti, R., Tygar, J., and B.
              Briscoe, "Timed Efficient Stream Loss-Tolerant
              Authentication (TESLA): Multicast Source Authentication
              Transform Introduction", RFC 4082, June 2005.

   [RFC4383]  Baugher, M. and E. Carrara, "The Use of Timed Efficient
              Stream Loss-Tolerant Authentication (TESLA) in the Secure
              Real-time Transport Protocol (SRTP)", RFC 4383,
              February 2006.

   [RFC4442]  Fries, S. and H. Tschofenig, "Bootstrapping Timed
              Efficient Stream Loss-Tolerant Authentication (TESLA)",
              RFC 4442, March 2006.

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

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

   [RFC4568]  Andreasen, F., Baugher, M., and D. Wing, "Session
              Description Protocol (SDP) Security Descriptions for Media
              Streams", RFC 4568, July 2006.

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

   [SAML_overview]
              Huges, J. and E. Maler, ""Security Assertion Markup
              Language (SAML) 2.0 Technical Overview, Working Draft"",
              2005.









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Authors' Addresses

   Steffen Fries
   Siemens
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: steffen.fries@siemens.com


   Dragan Ignjatic
   Polycom
   1000 W. 14th Street
   North Vancouver, BC  V7P 3P3
   Canada

   Email: dignjatic@polycom.com

































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Full Copyright Statement

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