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Versions: (draft-wing-rtpsec-keying-eval) 00 01 02 03 04 05 06 07 08 09 RFC 5479

SIP                                                              D. Wing
Internet-Draft                                                     Cisco
Intended status: Informational                                  S. Fries
Expires: March 31, 2008                                       Siemens AG
                                                           H. Tschofenig
                                                  Nokia Siemens Networks
                                                                F. Audet
                                                                  Nortel
                                                      September 28, 2007


  Requirements and Analysis of Media Security Key Management Protocols
           draft-ietf-sip-media-security-requirements-00.txt

Status of this Memo

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   This Internet-Draft will expire on March 31, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   A number of proposals have been published to address the need of
   securing media traffic.  A summary of the proposals available at that
   time is available in the appendix of this document.  Different



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   assumptions, requirements, and usage environments justify every one
   of them.  This document aims to summarize the discussed media
   security requirements.  A comparison of the requirements against the
   individual proposals is provided.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Call Scenarios . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  Clipping Media Before Signaling Answer . . . . . . . . . .  5
     3.2.  Retargeting and Forking  . . . . . . . . . . . . . . . . .  6
     3.3.  Shared Key Conferencing  . . . . . . . . . . . . . . . . .  8
   4.  Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Requirements Classification  . . . . . . . . . . . . . . . . . 14
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Appendix A.  Overview of Keying Mechanisms . . . . . . . . . . . . 20
     A.1.  Signaling Path Keying Techniques . . . . . . . . . . . . . 21
       A.1.1.  MIKEY-NULL . . . . . . . . . . . . . . . . . . . . . . 21
       A.1.2.  MIKEY-PSK  . . . . . . . . . . . . . . . . . . . . . . 21
       A.1.3.  MIKEY-RSA  . . . . . . . . . . . . . . . . . . . . . . 21
       A.1.4.  MIKEY-RSA-R  . . . . . . . . . . . . . . . . . . . . . 21
       A.1.5.  MIKEY-DHSIGN . . . . . . . . . . . . . . . . . . . . . 22
       A.1.6.  MIKEY-DHHMAC . . . . . . . . . . . . . . . . . . . . . 22
       A.1.7.  MIKEY-ECIES and MIKEY-ECMQV (MIKEY-ECC)  . . . . . . . 22
       A.1.8.  Security Descriptions with SIPS  . . . . . . . . . . . 22
       A.1.9.  Security Descriptions with S/MIME  . . . . . . . . . . 23
       A.1.10. SDP-DH (expired) . . . . . . . . . . . . . . . . . . . 23
       A.1.11. MIKEYv2 in SDP (expired) . . . . . . . . . . . . . . . 23
     A.2.  Media Path Keying Technique  . . . . . . . . . . . . . . . 23
       A.2.1.  ZRTP . . . . . . . . . . . . . . . . . . . . . . . . . 23
     A.3.  Signaling and Media Path Keying Techniques . . . . . . . . 24
       A.3.1.  EKT  . . . . . . . . . . . . . . . . . . . . . . . . . 24
       A.3.2.  DTLS-SRTP  . . . . . . . . . . . . . . . . . . . . . . 24
       A.3.3.  MIKEYv2 Inband (expired) . . . . . . . . . . . . . . . 25
   Appendix B.  Evaluation Criteria - SIP . . . . . . . . . . . . . . 25
     B.1.  Secure Retargeting and Secure Forking  . . . . . . . . . . 25
     B.2.  Clipping Media Before SDP Answer . . . . . . . . . . . . . 27
     B.3.  Centralized Keying . . . . . . . . . . . . . . . . . . . . 29
     B.4.  SSRC and ROC . . . . . . . . . . . . . . . . . . . . . . . 31
   Appendix C.  Evaluation Criteria - Security  . . . . . . . . . . . 33
     C.1.  Public Key Infrastructure  . . . . . . . . . . . . . . . . 33



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     C.2.  Perfect Forward Secrecy  . . . . . . . . . . . . . . . . . 35
     C.3.  Best Effort Encryption . . . . . . . . . . . . . . . . . . 36
     C.4.  Upgrading Algorithms . . . . . . . . . . . . . . . . . . . 39
   Appendix D.  Out-of-Scope  . . . . . . . . . . . . . . . . . . . . 40
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 40
   Intellectual Property and Copyright Statements . . . . . . . . . . 42













































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1.  Introduction

   The work on media security started a long time ago where the
   capability of the Session Initiation Protocol (SIP) was still at its
   infancy.  With the increased SIP deployment and the availability of
   new SIP extensions and related protocols the need for end-to-end
   security was re-evaluated.  The procedure of re-evaluating prior
   protocol work and design decisions is not an uncommon strategy and,
   to some extend, considered necessary protocol work to ensure that the
   developed protocols indeed meet the previously envisioned needs for
   the users in the Internet.

   This document aims to summarize the discussed media security
   requirements, i.e., requirements for mechanisms that negotiate keys
   and parameters for SRTP.  The organization of this document is as
   follows: Section 2 introduces terminology, Section 3 provides an
   overview about possible call scenarios, Section 4 lists requirements
   for media security, Section 5 will provide a clustering of
   requirements to certain deployment environments to adress the problem
   that there might not be a single solution with universal
   applicability.  The main part of the document concludes with the
   security considerations Section 6, IANA considerations Section 7 and
   an acknowledgement section in Section 8.  The appendix contains an
   overview of the Appendix A lists the available solution proposals and
   compares them to the requirements.  Appendix D lists non-goals for
   this document.


2.  Terminology

   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], with the
   important qualification that, unless otherwise stated, these terms
   apply to the design of the media security key management protocol,
   not its implementation or application.

   Additionally, the following items are used in this document:

   AOR (Address-of-Record):   A SIP or SIPS URI that points to a domain
      with a location service that can map the URI to another URI where
      the user might be available.  Typically, the location service is
      populated through registrations.  An AOR is frequently thought of
      as the "public address" of the user.







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   SSRC:   The 32-bit value that defines the synchronization source,
      used in RTP.  These are generally unique, but collisions can
      occur.

   two-time pad:   The use of the same key and the same key index to
      encrypt different data.  For SRTP, a two-time pad occurs if two
      senders are using the same key and the same RTP SSRC value.

   PKI  Public Key Infrastructure.  Throughout this paper, the term PKI
      refers to a global PKI.


3.  Call Scenarios

   The following subsections describe call scenarios with relevance for
   media security.  These call scenarios pose the most challenge to the
   key management for media data in cooperation with SIP signaling.

3.1.  Clipping Media Before Signaling Answer

   Per the SDP Offer/Answer Model [RFC3264],

      "Once the offerer has sent the offer, it MUST be prepared to
      receive media for any recvonly streams described by that offer.
      It MUST be prepared to send and receive media for any sendrecv
      streams in the offer, and send media for any sendonly streams in
      the offer (of course, it cannot actually send until the peer
      provides an answer with the needed address and port information)."

   To meet this requirement with SRTP, the offerer needs to know the
   SRTP key for arriving media.  If encrypted SRTP media arrives before
   the associated SRTP key, the offerer cannot play the media -- causing
   clipping.

   For key exchange mechanisms that send the answerer's key in SDP, a
   SIP provisional response [RFC3261], such as 183 (session progress),
   is useful.  However, the 183 messages are not reliable unless both
   the calling and called end point support PRACK [RFC3262], use TCP
   across all SIP proxies, implement Security Preconditions
   [I-D.ietf-mmusic-securityprecondition], or the both ends implement
   ICE [I-D.ietf-mmusic-ice] and the answerer implements the reliable
   provisional response mechanism described in ICE.  Unfortunately,
   there is not wide deployment of any of these techniques and there is
   industry reluctance to set requirements regarding these techniques to
   avoid the problem described in this section.

   Note that the receipt of an SDP answer is not always sufficient to
   allow media to be played to the offerer.  Sometimes, the offerer must



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   send media in order to open up firewall holes or NAT bindings before
   media can be received.  In this case a solution that makes the key
   available before the SDP answer arrives will not help.

   Requirements are created due to early media, in the sense of pre-
   offer/answer media, as described in [I-D.barnes-sip-em-ps-req-sol].
   Fixes to early media might make the requirements to become obsolete,
   but at the time of writing no progress has been accomplished.

3.2.  Retargeting and Forking

   In SIP, a request sent to a specific AOR but delivered to a different
   AOR is called a "retarget".  A typical scenario is a "call
   forwarding" feature.  In Figure 1 Alice sends an Invite in step 1
   that is sent to Bob in step 2.  Bob responds with a redirect (SIP
   response code 3xx) pointing to Carol in step 3.  This redirect
   typically does not propagate back to Alice but only goes to a proxy
   (i.e., the retargeting proxy) that sends the original Invite to Carol
   in step 4.


                                    +-----+
                                    |Alice|
                                    +--+--+
                                       |
                                       | Invite (1)
                                       V
                                  +----+----+
                                  |  proxy  |
                                  ++-+-----++
                                   | ^     |
                        Invite (2) | |     | Invite (4)
                    & redirect (3) | |     |
                                   V |     V
                                  ++-++   ++----+
                                  |Bob|   |Carol|
                                  +---+   +-----+

                           Figure 1: Retargeting

   The mechanism used by SIP for identifying the calling party is SIP
   Identity [RFC3261].  However, due to SIP retargeting issues
   [I-D.peterson-sipping-retarget], SIP Identity can only identify the
   calling party (that is, the party that initiated the SIP request).
   Some key exchange mechanisms predate SIP Identity and include their
   own identity mechanism.  However, those built-in identity mechanism
   suffer from the same SIP retargeting problem described in the above
   draft.  Going forward, Connected Identity [RFC4916] allows



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   identifying the called party.  This is also described as the
   'retargeting identity' problem.

   In SIP, 'forking' is the delivery of a request to multiple locations.
   This happens when a single AOR 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 AOR.


                                  +-----+
                                  |Alice|
                                  +--+--+
                                     |
                                     | Invite
                                     V
                               +-----+-----+
                               |   proxy   |
                               ++---------++
                                |         |
                         Invite |         | Invite
                                V         V
                             +--+--+   +--+--+
                             |Bob-1|   |Bob-2|
                             +-----+   +-----+

                             Figure 2: Forking

   With forking, both Bob-1 and Bob-2 might send back SDP answers in SIP
   responses.  Alice will see those intermediate (18x) and final (200)
   responses.  It is useful for Alice to be able to associate the SIP
   response with the incoming media stream.  Although this association
   can be done with ICE [I-D.ietf-mmusic-ice], and ICE is useful to make
   this association with RTP, it is not desirable to require ICE to
   accomplish this association.

   Forking and retargeting are often used together.  For example, a boss
   and secretary might have both phones ring and rollover to voice mail
   if neither phone is answered.

   To maintain security of the media traffic, only the end point that
   answers the call should know the SRTP keys for the session.  This is
   only an issue with public key encryption and not with DH-based
   approaches.  For key exchange mechanisms that do not provide secure
   forking or secure retargeting, one workaround is to re-key
   immediately after forking or retargeting (that is, perform a re-
   Invite).  However, because the originator may not be aware that the
   call forked this mechanism requires rekeying immediately after every
   session is established.  This doubles the Invite messages processed



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   by the network.

   Retargeting securely introduces a more significant problem.  With
   retargeting, the actual recipient of the request is not the original
   recipient.  This means that if the offerer encrypted material (such
   as the session key or the SDP) using the original recipient's public
   key, the actual recipient will not be able to decrypt that material
   because the recipient won't have the original recipient's private
   key.  In some cases, this is the intended behavior, i.e., you wanted
   to establish a secure connection with a specific individual.  In
   other cases, it is not intended behavior (you want all voice media to
   be encrypted, regardless of who answers).

   For some forms of key management the calling party needs to know in
   advance the certificate or shared secret of the called party, and
   retargeting can interfere with this.

   Further compounding this problem is a particularity of SIP that when
   forking is used, there is always only one final error response
   delivered to the sender of the request: the forking proxy is
   responsible for choosing which final response to choose in the event
   where forking results in multiple final error responses being
   received by the forking proxy.  This means that if a request is
   rejected, say with information that the keying information was
   rejected and providing the far end's credentials, it is very possible
   that the rejection will never reach the sender.  This problem, called
   the Heterogeneous Error Response Forking Problem (HERFP)
   [I-D.mahy-sipping-herfp-fix], is difficult to solve in SIP.

3.3.  Shared Key Conferencing

   For efficient scaling, large audio and video conference bridges
   operate most efficiently by encrypting the current speaker once and
   distributing that stream to the conference attendees.  Typically,
   inactive participants receive the same streams -- they hear (or see)
   the active speaker(s), and the active speakers receive distinct
   streams that don't include themselves.  In order to maintain
   confidentiality of such conferences where listeners share a common
   key, all listeners must rekeyed when a listener joins or leaves a
   conference.

   An important use case for mixers/translators is a conference bridge:









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                                         +----+
                             A --- 1 --->|    |
                               <-- 2 ----| M  |
                                         | I  |
                             B --- 3 --->| X  |
                               <-- 4 ----| E  |
                                         | R  |
                             C --- 5 --->|    |
                               <-- 6 ----|    |
                                         +----+

                       Figure 3: Centralized Keying

   In the figure above, 1, 3, and 5 are RTP media contributions from
   Alice, Bob, and Carol, and 2, 4, and 6 are the RTP flows to those
   devices carrying the 'mixed' media.

   Several scenarios are possible:

   a.  Multiple inbound sessions: 1, 3, and 5 are distinct RTP sessions,

   b.  Multiple outbound sessions: 2, 4, and 6 are distinct RTP
       sessions,

   c.  Single inbound session: 1, 3, and 5 are just different sources
       within the same RTP session,

   d.  Single outbound session: 2, 4, and 6 are different flows of the
       same (multi-unicast) RTP session

   If there are multiple inbound sessions and multiple outbound sessions
   (scenarios a and b), then every keying mechanism behaves as if the
   mixer were an end point and can set up a point-to-point secure
   session between the participant and the mixer.  This is the simplest
   situation, but is computationally wasteful, since SRTP processing has
   to be done independently for each participant.  The use of multiple
   inbound sessions (scenario a) doesn't waste computational resources,
   though it does consume additional cryptographic context on the mixer
   for each participant and has the advantage of non-repudiation of the
   originator of the incoming stream.

   To support a single outbound session (scenario d), the mixer has to
   dictate its encryption key to the participants.  Some keying
   mechanisms allow the transmitter to determine its own key, and others
   allow the offerer to determine the key for the offerer and answerer.
   Depending on how the call is established, the offerer might be a
   participant (such as a participant dialing into a conference bridge)
   or the offerer might be the mixer (such as a conference bridge



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   calling a participant).  The use of offerless Invites may help some
   keying mechanisms reverse the role of offerer/answerer.  A
   difficulty, however, is knowing a priori if the role should be
   reversed for a particular call.


4.  Requirements

   R1:    Negotiation of SRTP keys MUST NOT cause the call setup to fail
          in forked and retargeted calls where all end points are
          willing to use SRTP, unless the execution of the
          authentication and key exchange protocol leads to a failure
          (e.g., an unsuccessful authentication attempt).

   R2:    Even when some end points of a forked or retargeted call are
          incapable of using SRTP, the key management protocol MUST
          allow the establishment of SRTP associations with SRTP-capable
          endpoints and / or RTP associations with non-SRTP-capable
          endpoints.

   R3:    Forked end points MUST NOT know the SRTP key of any call
          established with another forked end point.

   R4:    The media security key management protocol MAY support the
          ability to utilize an initially established security context
          that was established as part of the first call setup with a
          remote end point.

          Specialized devices may need to avoid public key operations or
          Diffie-Hellman operations as much as possible because of the
          computational cost or because of the additional call setup
          delay.  For example, it can take a second or two to perform a
          Diffie-Hellman operation in certain devices.  Examples of
          these specialized devices would include some handsets,
          intelligent SIMs, and PSTN gateways.  For the typical case
          because a phone call has not yet been established, ancillary
          processing cycles can be utilized to perform the PK or DH
          operation; for example, in a PSTN gateway the DSP, which is
          not yet involved with typical DSP operations, could be used to
          perform the calculation, so as to avoid having the central
          host processor perform the calculation.  Some devices, such as
          handsets, and intelligent SIMs do not have such ancillary
          processing capability.

   R5:    The media security key management protocol SHOULD avoid
          clipping media before SDP answer without requiring PRACK
          [RFC3262].




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   R6:    The media security key management protocol MUST provide
          protection against passive attacks on the media path.

   R7:    The media security key management protocol MUST provide
          protection against passive attacks of a SIP proxy that is
          legitimately routing SIP messages.

   R8:    The media security key management protocol MUST be able to
          support perfect forward secrecy (or PFS).  The term PFS is the
          property that disclosure of the long-term secret keying
          material that is used to derive an agreed ephemeral key does
          not compromise the secrecy of agreed keys from earlier runs.

   R9:    The media security key management protocol MUST support
          negotiation of SRTP cipher suites without incurring per-
          algorithm computational expense.  This allows an offer to be
          built without incurring computational expense for each
          algorithm.

   R10:   If SRTP keying is performed over the media path, the keying
          packets MUST NOT pass the RTP validity check defined in
          Appendix A.1 of [RFC3550].

   R11:   The media security key management protocol that utilizes
          expensive cryptographic computations SHOULD offer the ability
          to resume previous sessions in an efficient way.

   R12:   The media security key management protocol MUST NOT require
          3rd parties to sign certificates.

          This requirement points to the fact that a global PKI cannot
          be assumed and opportunistic security approaches should be
          considered as part of the solution.

   R13:   The media security key management protocol SHOULD use
          algorithms that allow FIPS 140-2 [FIPS-140-2] certification.

          Note that the United States Government can only purchase and
          use crypto implementations that have been validated by the
          FIPS-140 [FIPS-140-2] process:

          "The FIPS-140 standard is applicable to all Federal agencies
          that use cryptographic-based security systems to protect
          sensitive information in computer and telecommunication
          systems, including voice systems.  The adoption and use of
          this standard is available to private and commercial
          organizations."[cryptval]




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          Some commercial organizations, such as banks and defense
          contractors, also require or prefer equipment which has
          validated by the FIPS-140 process.

   R14:   The media security key management protocol SHOULD be able to
          associate the signaling exchange with the media traffic.

          For example, if using a Diffie-Hellman keying technique with
          security preconditions that forks to 20 end points, the call
          initiator would get 20 provisional responses containing 20
          signed Diffie-Hellman key pairs.  Calculating 20 DH secrets
          and validating signatures can be a difficult task depending on
          the device capabilities.  Hence, in the case of forking, it is
          not desirable to perform a DH or PK operation with every
          party, but rather only with the party that answers the call
          (and incur some media clipping).  To do this, the signaling
          and media need to be associated so the calling party knows
          which key management needs to be completed.  This might be
          done by using the transport address indicated in the SDP,
          although NATs can complicate this association.

          Allowing such an association also allows the SDP offerer to
          avoid performing CPU-consuming operations (e.g., DH or public
          key operations) with attackers that have not seen the
          signaling messages.

   R15:   The media security key management protocol SHOULD allow to
          start with RTP and then upgrade to SRTP.

   R16:   The media security key management protocol SHOULD NOT
          introduce new denial of service vulnerabilities.

   R17:   The media security key management protocol SHOULD require the
          adversary to have access to the data as well as the signaling
          path for a successful attack to be launched.  An adversary
          that is located only along the data or only along the
          signaling path MUST NOT be able to successfully mount an
          attack.  A successful attack refers to the ability for the
          adversary to obtain keying material to decrypt the SRTP
          encrypted media traffic.

   R18:   If two parties share an authentication infrastructure that has
          3rd parties signing certificates, they SHOULD be able to make
          use of it.







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   R19:   The media security key management protocol MUST provide
          crypto-agility.

   R20:   The media security key management protocol MUST protect cipher
          suite negotiation against downgrading attacks.

   R21:   The media security key management protocol MUST allow a SIP
          User Agent to negotiate media security parameters for each
          individual session.

   R22:   The media security key management protocol SHOULD allow
          establishing SRTP keying between different call signaling
          protocols (e.g., between Jabber, SIP, H.323, MGCP)

   R23:   The media security key management protocol SHOULD support
          recording of decrypted media.

          Media recording may be realized by an intermediate nodes.  An
          example for those intermediate nodes are devices, which could
          be used in banking applications or for quality monitoring in
          call centers.  Here, it must be ensured, that the media
          security is ensured by the intermediate nodes on the
          connections to the involved endpoints as originally
          negotiated.  The endpoints need to be informed that there is
          an intermediate device and need to cooperate.  A solution
          approach for this is described in [I-D.wing-sipping-srtp-key].

   R24:   The media security key management protocol SHOULD NOT allow
          end users to determine whether their end-to-end interaction is
          subject to lawful interception.

   R25:   The media security key management protocol MUST work when
          there are intermediate nodes, terminating or processing media,
          between the end points.

   R26:   The media security key management protocol MUST consider
          termination of media security in a PSTN gateway.

          A typical case of using media security is the one where two
          entities are having a VoIP conversation over IP capable
          networks.  However, there are cases where the other end of the
          communication is not connected to an IP capable network.  In
          this kind of setting, there needs to be some kind of gateway
          at the edge of the IP network which converts the VoIP
          conversation to format understood by the other network.  An
          example of such gateway is a PSTN gateway sitting at the edge
          of IP and PSTN networks.




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          If media security (e.g., SRTP protection) is employed in this
          kind of gateway-setting, then media security and the related
          key management needs to be terminated at the gateway.  The
          other network (e.g., PSTN) may have its own measures to
          protect the communication, but this means that from media
          security point of view the media security is not employed end-
          to-end between the communicating entities.


5.  Requirements Classification

   An adversary might be located along

   1.  the media path,

   2.  the signaling path,

   3.  the media and the signaling path.

   An attacker that can solely be located along the signaling path, and
   does not have access to media, is not considered (ref item 2).

   Furthermore, it is reasonable to consider the capabilities of the
   adversary.  We also have different types of adversaries, namely

   a.  active adversary

   b.  passive adversary

   Note that the adversary model for (a) and (b) also assumes the
   attacker being able to control SIP signaling entities.

   With respect to item (a) an adversary may need to be active with
   regard to the key exchange relevant information traveling along the
   data or the signaling path.

   Some of the deployment variants of the media security key management
   proposals under considerations do not provide protection against man-
   in-the-middle adversaries under certain conditions, for example when
   SIP signaling entities are compromised, when a global PKI is missing
   or pre-shared secrets are not exchanged between the end points prior
   to the protocol exchange.

   Based on the above-mentioned considerations the following
   classifications can be made:






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   Class I:

      Passive attack on the signaling and the data path sufficient to
      reveal the content of the media traffic.


   Class II:

      Active attack on the signaling path and passive attack on the data
      path to reveal the content of the media traffic.


   Class III:

      Active attack on the signaling and the data path necessary to
      reveal the content of the media traffic.


   Class IV:

      Active attack is required and will be detected by the end points
      when adversary tampers with the messages.

   For example, SDES falls into Class I since the adversary needs to
   learn the SDES key by progressing a signaling message at a SIP proxy
   (assuming that the adversary is in control of the SIP proxy).
   Subsequent media traffic can be decrypted with the help of the
   learned key.

   As another example, DTLS-RTP falls into Class III when DTLS is used a
   public key based ciphersuite with self-signed certificates and
   without SIP Identity.  An adversary would have to modify the
   fingerprint that is sent along the signaling path and subsequently to
   modify the certificates carried in the DTLS handshake that travel
   along the media path.

   An attack is not successful when SIP Identity is used, the adversary
   is not between the SIP UA and its Authentication Service (or at the
   Authentication Service), both end points are able to verify the
   digital signature (of the SIP Identity) and are able to validate the
   corresponding certificates.


6.  Security Considerations

   This document lists requirements for securing media traffic.  As
   such, it addresses security throughout the document.




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7.  IANA Considerations

   This document does not require actions by IANA.


8.  Acknowledgements

   For contributions to the requirements portion of this document, the
   authors would like to thank the active participants of the RTPSEC BoF
   and on the RTPSEC mailing list.  The authors would furthermore like
   to thank Wolfgang Buecker, Guenther Horn, Peter Howard, Hans-Heinrich
   Grusdt, Srinath Thiruvengadam, Martin Euchner, Eric Rescorla, Matt
   Lepinski, Dan York, Werner Dittmann, Richard Barnes, Vesa Lehtovirta,
   Colin Perkins, Peter Schneider, and Christer Holmberg for their
   feedback to this document.

   For contributions to the analysis portion of this document, the
   authors would like to thank Special thanks to Steffen Fries and
   Dragan Ignjatic for their excellent MIKEY comparison document
   [I-D.ietf-msec-mikey-applicability].  The authors would furthermore
   like to thank Cullen Jennings, David Oran, David McGrew, Mark
   Baugher, Flemming Andreasen, Eric Raymond, Dave Ward, Leo Huang, Eric
   Rescorla, Lakshminath Dondeti, Steffen Fries, Alan Johnston, Dragan
   Ignjatic and John Elwell for their feedback to this document.


9.  References

9.1.  Normative References

   [FIPS-140-2]
              NIST, "Security Requirements for Cryptographic Modules",
              June 2005, <http://csrc.nist.gov/publications/fips/
              fips140-2/fips1402.pdf>.

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

   [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.

   [RFC3262]  Rosenberg, J. and H. Schulzrinne, "Reliability of
              Provisional Responses in Session Initiation Protocol
              (SIP)", RFC 3262, June 2002.

   [RFC3264]  Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model



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              with Session Description Protocol (SDP)", RFC 3264,
              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.

   [cryptval]
              NIST, "Cryptographic Module Validation Program",
              December 2006,
              <http://csrc.nist.gov/cryptval/140-2APP.htm>.

9.2.  Informative References

   [I-D.barnes-sip-em-ps-req-sol]
              Barnes, R. and M. Lepinski, "Early Media in SIP: Problem
              Statement, Requirements, and Analysis of  Solutions",
              draft-barnes-sip-em-ps-req-sol-00 (work in progress),
              February 2007.

   [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-rtpsec-mikeyv2]
              Dondeti, L., "MIKEYv2: SRTP Key Management using MIKEY,
              revisited", draft-dondeti-msec-rtpsec-mikeyv2-01 (work in
              progress), March 2007.

   [I-D.fischl-sipping-media-dtls]
              Fischl, J., "Datagram Transport Layer Security (DTLS)
              Protocol for Protection of Media  Traffic Established with
              the Session Initiation Protocol",
              draft-fischl-sipping-media-dtls-03 (work in progress),
              July 2007.

   [I-D.ietf-mmusic-ice]
              Rosenberg, J., "Interactive Connectivity Establishment
              (ICE): A Protocol for Network Address  Translator (NAT)
              Traversal for Offer/Answer Protocols",
              draft-ietf-mmusic-ice-18 (work in progress),
              September 2007.

   [I-D.ietf-mmusic-sdp-capability-negotiation]
              Andreasen, F., "SDP Capability Negotiation",
              draft-ietf-mmusic-sdp-capability-negotiation-06 (work in
              progress), July 2007.



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   [I-D.ietf-mmusic-securityprecondition]
              Andreasen, F. and D. Wing, "Security Preconditions for
              Session Description Protocol (SDP) Media  Streams",
              draft-ietf-mmusic-securityprecondition-04 (work in
              progress), July 2007.

   [I-D.ietf-msec-mikey-applicability]
              Fries, S. and D. Ignjatic, "On the applicability of
              various MIKEY modes and extensions",
              draft-ietf-msec-mikey-applicability-06 (work in progress),
              July 2007.

   [I-D.ietf-msec-mikey-ecc]
              Milne, A., "ECC Algorithms for MIKEY",
              draft-ietf-msec-mikey-ecc-03 (work in progress),
              June 2007.

   [I-D.ietf-sip-certs]
              Jennings, C., "Certificate Management Service for The
              Session Initiation Protocol (SIP)",
              draft-ietf-sip-certs-04 (work in progress), July 2007.

   [I-D.jennings-sipping-multipart]
              Wing, D. and C. Jennings, "Session Initiation Protocol
              (SIP) Offer/Answer with Multipart Alternative",
              draft-jennings-sipping-multipart-02 (work in progress),
              March 2006.

   [I-D.mahy-sipping-herfp-fix]
              Mahy, R., "A Solution to the Heterogeneous Error Response
              Forking Problem (HERFP) in  the Session Initiation
              Protocol (SIP)", draft-mahy-sipping-herfp-fix-01 (work in
              progress), March 2006.

   [I-D.mcgrew-srtp-ekt]
              McGrew, D., "Encrypted Key Transport for Secure RTP",
              draft-mcgrew-srtp-ekt-03 (work in progress), July 2007.

   [I-D.mcgrew-tls-srtp]
              Rescorla, E. and D. McGrew, "Datagram Transport Layer
              Security (DTLS) Extension to Establish Keys for  Secure
              Real-time Transport Protocol (SRTP)",
              draft-mcgrew-tls-srtp-02 (work in progress), March 2007.

   [I-D.peterson-sipping-retarget]
              Peterson, J., "Retargeting and Security in SIP: A
              Framework and Requirements",
              draft-peterson-sipping-retarget-00 (work in progress),



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              February 2005.

   [I-D.wing-sipping-srtp-key]
              Wing, D., "Disclosing Secure RTP (SRTP) Session Keys with
              a SIP Event Package", draft-wing-sipping-srtp-key-01 (work
              in progress), July 2007.

   [I-D.zimmermann-avt-zrtp]
              Zimmermann, P., "ZRTP: Media Path Key Agreement for Secure
              RTP", draft-zimmermann-avt-zrtp-04 (work in progress),
              July 2007.

   [RFC3388]  Camarillo, G., Eriksson, G., Holler, J., and H.
              Schulzrinne, "Grouping of Media Lines in the Session
              Description Protocol (SDP)", RFC 3388, December 2002.

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

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

   [RFC4346]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.1", RFC 4346, April 2006.

   [RFC4474]  Peterson, J. and C. Jennings, "Enhancements for
              Authenticated Identity Management in the Session
              Initiation Protocol (SIP)", RFC 4474, August 2006.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 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.

   [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.




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   [RFC4771]  Lehtovirta, V., Naslund, M., and K. Norrman, "Integrity
              Transform Carrying Roll-Over Counter for the Secure Real-
              time Transport Protocol (SRTP)", RFC 4771, January 2007.

   [RFC4916]  Elwell, J., "Connected Identity in the Session Initiation
              Protocol (SIP)", RFC 4916, June 2007.


Appendix A.  Overview of Keying Mechanisms

   Based on how the SRTP keys are exchanged, each SRTP key exchange
   mechanism belongs to one general category:



      signaling path:  All the keying is carried in the call signaling
         (SIP or SDP) path.

      media path:  All the keying is carried in the SRTP/SRTCP media
         path, and no signaling whatsoever is carried in the call
         signaling path.

      signaling and media path:  Parts of the keying are carried in the
         SRTP/SRTCP media path, and parts are carried in the call
         signaling (SIP or SDP) path.

   One of the significant benefits of SRTP over other end-to-end
   encryption mechanisms, such as for example IPsec, is that SRTP is
   bandwidth efficient and SRTP retains the header of RTP packets.
   Bandwidth efficiency is vital for VoIP in many scenarios where access
   bandwidth is limited or expensive, and retaining the RTP header is
   important for troubleshooting packet loss, delay, and jitter.

   Related to SRTP's characteristics is a goal that any SRTP keying
   mechanism to also be efficient and not cause additional call setup
   delay.  Contributors to additional call setup delay include network
   or database operations: retrieval of certificates and additional SIP
   or media path messages, and computational overhead of establishing
   keys or validating certificates.

   When examining the choice between keying in the signaling path,
   keying in the media path, or keying in both paths, it is important to
   realize the media path is generally 'faster' than the SIP signaling
   path.  The SIP signaling path has computational elements involved
   which parse and route SIP messages.  The media path, on the other
   hand, does not normally have computational elements involved, and
   even when computational elements such as firewalls are involved, they
   cause very little additional delay.  Thus, the media path can be



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   useful for exchanging several messages to establish SRTP keys.  A
   disadvantage of keying over the media path is that interworking
   different key exchange requires the interworking function be in the
   media path, rather than just in the signaling path; in practice this
   involvement is probably unavoidable anyway.

A.1.  Signaling Path Keying Techniques

A.1.1.  MIKEY-NULL

   MIKEY-NULL [RFC3830] has the offerer indicate the SRTP keys for both
   directions.  The key is sent unencrypted in SDP, which means the SDP
   must be encrypted hop-by-hop (e.g., by using TLS (SIPS)) or end-to-
   end (e.g., by using S/MIME).

   MIKEY-NULL requires one message from offerer to answerer (half a
   round trip), and does not add additional media path messages.

A.1.2.  MIKEY-PSK

   MIKEY-PSK (pre-shared key) [RFC3830] requires that all endpoints
   share one common key.  MIKEY-PSK has the offerer encrypt the SRTP
   keys for both directions using this pre-shared key.

   MIKEY-PSK requires one message from offerer to answerer (half a round
   trip), and does not add additional media path messages.

A.1.3.  MIKEY-RSA

   MIKEY-RSA [RFC3830] has the offerer encrypt the keys for both
   directions using the intended answerer's public key, which is
   obtained from a PKI.

   MIKEY-RSA requires one message from offerer to answerer (half a round
   trip), and does not add additional media path messages.  MIKEY-RSA
   requires the offerer to obtain the intended answerer's certificate.

A.1.4.  MIKEY-RSA-R

   MIKEY-RSA-R An additional mode of key distribution in MIKEY: MIKEY-
   RSA-R [RFC4738] is essentially the same as MIKEY-RSA but reverses the
   role of the offerer and the answerer with regards to providing the
   keys.  That is, the answerer encrypts the keys for both directions
   using the offerer's public key.  Both the offerer and answerer
   validate each other's public keys using a PKI.  MIKEY-RSA-R also
   enables sending certificates in the MIKEY message.

   MIKEY-RSA-R requires one message from offerer to answer, and one



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   message from answerer to offerer (full round trip), and does not add
   additional media path messages.  MIKEY-RSA-R requires the offerer
   validate the answerer's certificate.

A.1.5.  MIKEY-DHSIGN

   In MIKEY-DHSIGN [RFC3830] the offerer and answerer derive the key
   from a Diffie-Hellman exchange.  In order to prevent an active man-
   in-the-middle the DH exchange itself is signed using each endpoint's
   private key and the associated public keys are validated using a PKI.

   MIKEY-DHSIGN requires one message from offerer to answerer, and one
   message from answerer to offerer (full round trip), and does not add
   additional media path messages.  MIKEY-DHSIGN requires the offerer
   and answerer to validate each other's certificates.  MIKEY-DHSIGN
   also enables sending the answerer's certificate in the MIKEY message.

A.1.6.  MIKEY-DHHMAC

   MIKEY-DHHMAC [RFC4650] uses a pre-shared secret to HMAC the Diffie-
   Hellman exchange, essentially combining aspects of MIKEY-PSK with
   MIKEY-DHSIGN, but without MIKEY-DHSIGN's need for a PKI to
   authenticate the Diffie-Hellman exchange.

   MIKEY-DHHMAC requires one message from offerer to answerer, and one
   message from answerer to offerer (full round trip), and does not add
   additional media path messages.

A.1.7.  MIKEY-ECIES and MIKEY-ECMQV (MIKEY-ECC)

   ECC Algorithms For MIKEY [I-D.ietf-msec-mikey-ecc] describes how ECC
   can be used with MIKEY-RSA (using ECDSA signature) and with MIKEY-
   DHSIGN (using a new DH-Group code), and also defines two new ECC-
   based algorithms, Elliptic Curve Integrated Encryption Scheme (ECIES)
   and Elliptic Curve Menezes-Qu-Vanstone (ECMQV) .

   For the purposes of this paper, the ECDSA signature, MIKEY-ECIES, and
   MIKEY-ECMQV function exactly like MIKEY-RSA, and the new DH-Group
   code function exactly like MIKEY-DHSIGN.  Therefore these ECC
   mechanisms aren't discussed separately in this paper.

A.1.8.  Security Descriptions with SIPS

   Security Descriptions [RFC4568] has each side indicate the key it
   will use for transmitting SRTP media, and the keys are sent in the
   clear in SDP.  Security Descriptions relies on hop-by-hop (TLS via
   "SIPS:") encryption to protect the keys exchanged in signaling.




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   Security Descriptions requires one message from offerer to answerer,
   and one message from answerer to offerer (full round trip), and does
   not add additional media path messages.

A.1.9.  Security Descriptions with S/MIME

   This keying mechanism is identical to Appendix A.1.8, except that
   rather than protecting the signaling with TLS, the entire SDP is
   encrypted with S/MIME.

A.1.10.  SDP-DH (expired)

   SDP Diffie-Hellman [I-D.baugher-mmusic-sdp-dh] exchanges Diffie-
   Hellman messages in the signaling path to establish session keys.  To
   protect against active man-in-the-middle attacks, the Diffie-Hellman
   exchange needs to be protected with S/MIME, SIPS, or SIP-Identity
   [RFC4474] and [RFC4474].

   SDP-DH requires one message from offerer to answerer, and one message
   from answerer to offerer (full round trip), and does not add
   additional media path messages.

A.1.11.  MIKEYv2 in SDP (expired)

   MIKEYv2 [I-D.dondeti-msec-rtpsec-mikeyv2] adds mode negotiation to
   MIKEYv1 and removes the time synchronization requirement.  It
   therefore now takes 2 round-trips to complete.  In the first round
   trip, the communicating parties learn each other's identities, agree
   on a MIKEY mode, crypto algorithm, SRTP policy, and exchanges nonces
   for replay protection.  In the second round trip, they negotiate
   unicast and/or group SRTP context for SRTP and/or SRTCP.

   Furthemore, MIKEYv2 also defines an in-band negotiation mode as an
   alternative to SDP (see Appendix A.3.3).

A.2.  Media Path Keying Technique

A.2.1.  ZRTP

   ZRTP [I-D.zimmermann-avt-zrtp] does not exchange information in the
   signaling path (although it's possible for endpoints to indicate
   support for ZRTP with "a=zrtp" in the initial Offer).  In ZRTP the
   keys are exchanged entirely in the media path using a Diffie-Hellman
   exchange.  The advantage to this mechanism is that the signaling
   channel is used only for call setup and the media channel is used to
   establish an encrypted channel -- much like encryption devices on the
   PSTN.  ZRTP uses voice authentication of its Diffie-Hellman exchange
   by having each person read digits to the other person.  Subsequent



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   sessions with the same ZRTP endpoint can be authenticated using the
   stored hash of the previously negotiated key rather than voice
   authentication.

   ZRTP uses 4 media path messages (Hello, Commit, DHPart1, and DHPart2)
   to establish the SRTP key, and 3 media path confirmation messages.
   The first 4 are sent as RTP packets (using RTP header extensions),
   and the last 3 are sent in conjunction with SRTP media packets (again
   as SRTP header extensions).  Note that unencrypted RTP is being
   exchanged until the SRTP keys are established.

A.3.  Signaling and Media Path Keying Techniques

A.3.1.  EKT

   EKT [I-D.mcgrew-srtp-ekt] relies on another SRTP key exchange
   protocol, such as Security Descriptions or MIKEY, for bootstrapping.
   In the initial phase, each member of a conference uses an SRTP key
   exchange protocol to establish a common key encryption key (KEK).
   Each member may use the KEK to securely transport its SRTP master key
   and current SRTP rollover counter (ROC), via RTCP, to the other
   participants in the session.

   EKT requires the offerer to send some parameters (EKT_Cipher, KEK,
   and security parameter index (SPI)) via the bootstrapping protocol
   such as Security Descriptions or MIKEY.  Each answerer sends an SRTCP
   message which contains the answerer's SRTP Master Key, rollover
   counter, and the SRTP sequence number.  Rekeying is done by sending a
   new SRTCP message.  For reliable transport, multiple RTCP messages
   need to be sent.

A.3.2.  DTLS-SRTP

   DTLS-SRTP [I-D.mcgrew-tls-srtp] exchanges public key fingerprints in
   SDP [I-D.fischl-sipping-media-dtls] and then establishes a DTLS
   session over the media channel.  The endpoints use the DTLS handshake
   to agree on crypto suites and establish SRTP session keys.  SRTP
   packets are then exchanged between the endpoints.

   DTLS-SRTP requires one message from offerer to answerer (half round
   trip), and, if the offerer wishes to correlate the SDP answer with
   the endpoint, requires one message from answer to offerer (full round
   trip).  DTLS-SRTP uses 4 media path messages to establish the SRTP
   key.

   This paper assumes DTLS will use TLS_RSA_WITH_3DES_EDE_CBC_SHA as its
   cipher suite, which is the mandatory-to-implement cipher suite in TLS
   [RFC4346].



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A.3.3.  MIKEYv2 Inband (expired)

   As defined in Appendix A.1.11, MIKEYv2 also defines an in-band
   negotiation mode as an alternative to SDP (see Appendix A.3.3).  The
   details are not sorted out in the draft yet on what in-band actually
   means (i.e., UDP, RTP, RTCP, etc.).


Appendix B.  Evaluation Criteria - SIP

   This section considers how each keying mechanism interacts with SIP
   features.

B.1.  Secure Retargeting and Secure Forking

   Retargeting and forking of signaling requests is described within
   Section 3.2.  The following builds upon this description.

   The following list compares the behavior of secure forking, answering
   association, two-time pads, and secure retargeting for each keying
   mechanism.



      MIKEY-NULL  Secure Forking: No, all AORs see offerer's and
         answerer's keys.  Answer is associated with media by the SSRC
         in MIKEY.  Additionally, a two-time pad occurs if two branches
         choose the same 32-bit SSRC and transmit SRTP packets.

         Secure Retargeting: No, all targets see offerer's and
         answerer's keys.  Suffers from retargeting identity problem.

      MIKEY-PSK
         Secure Forking: No, all AORs see offerer's and answerer's keys.
         Answer is associated with media by the SSRC in MIKEY.  Note
         that all AORs must share the same pre-shared key in order for
         forking to work at all with MIKEY-PSK.  Additionally, a two-
         time pad occurs if two branches choose the same 32-bit SSRC and
         transmit SRTP packets.

         Secure Retargeting: Not secure.  For retargeting to work, the
         final target must possess the correct PSK.  As this is likely
         in scenarios were the call is targeted to another device
         belonging to the same user (forking), it is very unlikely that
         other users will possess that PSK and be able to successfully
         answer that call.





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      MIKEY-RSA
         Secure Forking: No, all AORs see offerer's and answerer's keys.
         Answer is associated with media by the SSRC in MIKEY.  Note
         that all AORs must share the same private key in order for
         forking to work at all with MIKEY-RSA.  Additionally, a two-
         time pad occurs if two branches choose the same 32-bit SSRC and
         transmit SRTP packets.

         Secure Retargeting: No.

      MIKEY-RSA-R
         Secure Forking: Yes. Answer is associated with media by the
         SSRC in MIKEY.

         Secure Retargeting: Yes.

      MIKEY-DHSIGN
         Secure Forking: Yes, each forked endpoint negotiates unique
         keys with the offerer for both directions.  Answer is
         associated with media by the SSRC in MIKEY.

         Secure Retargeting: Yes, each target negotiates unique keys
         with the offerer for both directions.

      MIKEYv2 in SDP
         The behavior will depend on which mode is picked.

      MIKEY-DHHMAC
         Secure Forking: Yes, each forked endpoint negotiates unique
         keys with the offerer for both directions.  Answer is
         associated with media by the SSRC in MIKEY.

         Secure Retargeting: Yes, each target negotiates unique keys
         with the offerer for both directions.  Note that for the keys
         to be meaningful, it would require the PSK to be the same for
         all the potential intermediaries, which would only happen
         within a single domain.

      Security Descriptions with SIPS
         Secure Forking: No.  Each forked endpoint sees the offerer's
         key.  Answer is not associated with media.

         Secure Retargeting: No.  Each target sees the offerer's key.

      Security Descriptions with S/MIME
         Secure Forking: No.  Each forked endpoint sees the offerer's
         key.  Answer is not associated with media.




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         Secure Retargeting: No.  Each target sees the offerer's key.
         Suffers from retargeting identity problem.

      SDP-DH
         Secure Forking: Yes. Each forked endpoint calculates a unique
         SRTP key.  Answer is not associated with media.

         Secure Retargeting: Yes. The final target calculates a unique
         SRTP key.

      ZRTP
         Secure Forking: Yes. Each forked endpoint calculates a unique
         SRTP key.  As ZRTP isn't signaled in SDP, there is no
         association of the answer with media.

         Secure Retargeting: Yes. The final target calculates a unique
         SRTP key.

      EKT
         Secure Forking: Inherited from the bootstrapping mechanism (the
         specific MIKEY mode or Security Descriptions).  Answer is
         associated with media by the SPI in the EKT protocol.  Answer
         is associated with media by the SPI in the EKT protocol.

         Secure Retargeting: Inherited from the bootstrapping mechanism
         (the specific MIKEY mode or Security Descriptions).

      DTLS-SRTP
         Secure Forking: Yes. Each forked endpoint calculates a unique
         SRTP key.  Answer is associated with media by the certificate
         fingerprint in signaling and certificate in the media path.

         Secure Retargeting: Yes. The final target calculates a unique
         SRTP key.

      MIKEYv2 Inband
         The behavior will depend on which mode is picked.

B.2.  Clipping Media Before SDP Answer

   Clipping media before receiving the signaling answer is described
   within Section 3.1.  The following builds upon this description.

   Furthermore, the problem of clipping gets compounded when forking is
   used.  For example, if using a Diffie-Hellman keying technique with
   security preconditions that forks to 20 endpoints, the call initiator
   would get 20 provisional responses containing 20 signed Diffie-
   Hellman half keys.  Calculating 20 DH secrets and validating



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   signatures can be a difficult task depending on the device
   capabilities.

   The following list compares the behavior of clipping before SDP
   answer for each keying mechanism.



      MIKEY-NULL
         Not clipped.  The offerer provides the answerer's keys.

      MIKEY-PSK
         Not clipped.  The offerer provides the answerer's keys.

      MIKEY-RSA
         Not clipped.  The offerer provides the answerer's keys.

      MIKEY-RSA-R
         Clipped.  The answer contains the answerer's encryption key.

      MIKEY-DHSIGN
         Clipped.  The answer contains the answerer's Diffie-Hellman
         response.

      MIKEY-DHHMAC
         Clipped.  The answer contains the answerer's Diffie-Hellman
         response.

      MIKEYv2 in SDP
         The behavior will depend on which mode is picked.

      Security Descriptions with SIPS
         Clipped.  The answer contains the answerer's encryption key.

      Security Descriptions with S/MIME
         Clipped.  The answer contains the answerer's encryption key.

      SDP-DH
         Clipped.  The answer contains the answerer's Diffie-Hellman
         response.

      ZRTP
         Not clipped because the session intially uses RTP.  While RTP
         is flowing, both ends negotiate SRTP keys in the media path and
         then switch to using SRTP.






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      EKT
         Not clipped, as long as the first RTCP packet (containing the
         answerer's key) is not lost in transit.  The answerer sends its
         encryption key in RTCP, which arrives at the same time (or
         before) the first SRTP packet encrypted with that key.

            Note: RTCP needs to work, in the answerer-to-offerer
            direction, before the offerer can decrypt SRTP media.

      DTLS-SRTP
         Not clipped.  Keys are exchanged in the media path without
         relying on the signaling path.

      MIKEYv2 Inband
         Not clipped.  Keys are exchanged in the media path without
         relying on the signaling path.

B.3.  Centralized Keying

   Centralized keying is described within Section 3.3.  The following
   builds upon this description.

   The following list describes how each keying mechanism behaves with
   centralized keying (scenario d) and rekeying.



      MIKEY-NULL
         Keying: Yes, if offerer is the mixer.  No, if offerer is the
         participant (end user).

         Rekeying: Yes, via re-Invite

      MIKEY-PSK
         Keying: Yes, if offerer is the mixer.  No, if offerer is the
         participant (end user).

         Rekeying: Yes, with a re-Invite

      MIKEY-RSA
         Keying: Yes, if offerer is the mixer.  No, if offerer is the
         participant (end user).

         Rekeying: Yes, with a re-Invite







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      MIKEY-RSA-R
         Keying: No, if offerer is the mixer.  Yes, if offerer is the
         participant (end user).

         Rekeying: n/a

      MIKEY-DHSIGN
         Keying: No; a group-key Diffie-Hellman protocol is not
         supported.

         Rekeying: n/a

      MIKEY-DHHMAC
         Keying: No; a group-key Diffie-Hellman protocol is not
         supported.

         Rekeying: n/a

      MIKEYv2 in SDP
         The behavior will depend on which mode is picked.

      Security Descriptions with SIPS
         Keying: Yes, if offerer is the mixer.  Yes, if offerer is the
         participant.

         Rekeying: Yes, with a Re-Invite.

      Security Descriptions with S/MIME
         Keying: Yes, if offerer is the mixer.  Yes, if offerer is the
         participant.

         Rekeying: Yes, with a Re-Invite.

      SDP-DH
         Keying: No; a group-key Diffie-Hellman protocol is not
         supported.

         Rekeying: n/a

      ZRTP
         Keying: No; a group-key Diffie-Hellman protocol is not
         supported.

         Rekeying: n/a







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      EKT
         Keying: Yes. After bootstrapping a KEK using Security
         Descriptions or MIKEY, each member originating an SRTP stream
         can send its SRTP master key, sequence number and ROC via RTCP.

         Rekeying: Yes. EKT supports each sender to transmit its SRTP
         master key to the group via RTCP packets.  Thus, EKT supports
         each originator of an SRTP stream to rekey at any time.

      DTLS-SRTP
         Keying: Yes, because with the assumed cipher suite,
         TLS_RSA_WITH_3DES_EDE_CBC_SHA, each end indicates its SRTP key.

         Rekeying: via DTLS in the media path.

      MIKEYv2 Inband
         The behavior will depend on which mode is picked.

B.4.  SSRC and ROC

   In SRTP, a cryptographic context is defined as the SSRC, destination
   network address, and destination transport port number.  Whereas RTP,
   a flow is defined as the destination network address and destination
   transport port number.  This results in a problem -- how to
   communicate the SSRC so that the SSRC can be used for the
   cryptographic context.

   Two approaches have emerged for this communication.  One, used by all
   MIKEY modes, is to communicate the SSRCs to the peer in the MIKEY
   exchange.  Another, used by Security Descriptions, is to use "late
   bindng" -- that is, any new packet containing a previously-unseen
   SSRC (which arrives at the same destination network address and
   destination transport port number) will create a new cryptographic
   context.  Another approach, common amongst techniques with media-path
   SRTP key establishment, is to require a handshake over that media
   path before SRTP packets are sent.  MIKEY's approach changes RTP's
   SSRC collision detection behavior by requiring RTP to pre-establish
   the SSRC values for each session.

   Another related issue is that SRTP introduces a rollover counter
   (ROC), which records how many times the SRTP sequence number has
   rolled over.  As the sequence number is used for SRTP's default
   ciphers, it is important that all endpoints know the value of the
   ROC.  The ROC starts at 0 at the beginning of a session.

   Some keying mechanisms cause a two-time pad to occur if two endpoints
   of a forked call have an SSRC collision.




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   Note: A proposal has been made to send the ROC value on every Nth
   SRTP packet[RFC4771].  This proposal has not yet been incorporated
   into this document.

   The following list examines handling of SSRC and ROC:



      MIKEY-NULL
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.

      MIKEY-PSK
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.

      MIKEY-RSA
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.

      MIKEY-RSA-R
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.

      MIKEY-DHSIGN
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.

      MIKEY-DHHMAC
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.

      MIKEYv2 in SDP
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.

      Security Descriptions with SIPS
         Neither SSRC nor ROC are signaled.  SSRC 'late binding' is
         used.

      Security Descriptions with S/MIME
         Neither SSRC nor ROC are signaled.  SSRC 'late binding' is
         used.

      SDP-DH
         Neither SSRC nor ROC are signaled.  SSRC 'late binding' is
         used.




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      ZRTP
         Neither SSRC nor ROC are signaled.  SSRC 'late binding' is
         used.

      EKT
         The SSRC of the SRTCP packet containing an EKT update
         corresponds to the SRTP master key and other parameters within
         that packet.

      DTLS-SRTP
         Neither SSRC nor ROC are signaled.  SSRC 'late binding' is
         used.

      MIKEYv2 Inband
         Each endpoint indicates a set of SSRCs and the ROC for SRTP
         packets it transmits.


Appendix C.  Evaluation Criteria - Security

   This section evaluates each keying mechanism on the basis of their
   security properties.

C.1.  Public Key Infrastructure

   There are two aspects of PKI requirements -- one aspect is if PKI is
   necessary in order for the mechanism to function at all, the other is
   if PKI is used to authenticate a certificate.  With interactive
   communications it is desirable to avoid fetching certificates that
   delay call setup; rather it is preferable to fetch or validate
   certificates in such a way that call setup isn't delayed.  For
   example, a certificate can be validated while the phone is ringing or
   can be validated while ring-back tones are being played or even while
   the called party is answering the phone and saying "hello".

   SRTP key exchange mechanisms that require a global PKI to operate are
   gated on the deployment of a common PKI available to both endpoints.
   This means that no media security is achievable until such a PKI
   exists.  For SIP, something like sip-certs [I-D.ietf-sip-certs] might
   be used to obtain the certificate of a peer.

      Note: Even if SIP CERTs was deployed, the retargeting problem
      (Appendix B.1) would still prevent successful deployment of keying
      techniques which require the offerer to obtain the actual target's
      public key.

   The following list compares the PKI requirements of each keying
   mechanism, both if a PKI is required for the key exchange itself, and



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   if PKI is only used to authenticate the certificate supplied in
   signaling.



      MIKEY-NULL
         PKI not used.

      MIKEY-PSK
         PKI not used; rather, all endpoints must have some way to
         exchange per-endpoint or per-system pre-shared keys.

      MIKEY-RSA
         The offerer obtains the intended answerer's public key before
         initiating the call.  This public key is used to encrypt the
         SRTP keys.  There is no defined mechanism for the offerer to
         obtain the answerer's public key, although [I-D.ietf-sip-certs]
         might be viable in the future.

      MIKEY-RSA-R
         The offer contains the offerer's public key.  The answerer uses
         that public key to encrypt the SRTP keys that will be used by
         the offerer and the answerer.  A PKI is necessary to validate
         the certificates.

      MIKEY-DHSIGN
         PKI is used to authenticate the public key that is included in
         the MIKEY message, by walking the CA trust chain.

      MIKEY-DHHMAC
         PKI not used; rather, all endpoints must have some way to
         exchange per-endpoint or per-system pre-shared keys.

      MIKEYv2 in SDP
         The behavior will depend on which mode is picked.

      Security Descriptions with SIPS
         PKI not used.

      Security Descriptions with S/MIME
         PKI is needed for S/MIME.  The offerer must obtain the intended
         target's public key and encrypt their SDP with that key.  The
         answerer must obtain the offerer's public key and encrypt their
         SDP with that key.







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      SDP-DH
         PKI not used.

      ZRTP
         PKI not used.

      EKT
         PKI not used by EKT itself, but might be used by the EKT
         bootstrapping keying mechanism (such as certain MIKEY modes).

      DTLS-SRTP
         Remote party's certificate is sent in media path, and a
         fingerprint of the same certificate is sent in the signaling
         path.

      MIKEYv2 Inband
         The behavior will depend on which mode is picked.

C.2.  Perfect Forward Secrecy

   In the context of SRTP, Perfect Forward Secrecy is the property that
   SRTP session keys that protected a previous session are not
   compromised if the static keys belonging to the endpoints are
   compromised.  That is, if someone were to record your encrypted
   session content and later acquires either party's private key, that
   encrypted session content would be safe from decryption if your key
   exchange mechanism had perfect forward secrecy.

   The following list describes how each key exchange mechanism provides
   PFS.



      MIKEY-NULL
         No PFS.

      MIKEY-PSK
         No PFS.

      MIKEY-RSA
         No PFS.

      MIKEY-RSA-R
         No PFS.







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      MIKEY-DHSIGN
         PFS is provided with the Diffie-Hellman exchange.

      MIKEY-DHHMAC
         PFS is provided with the Diffie-Hellman exchange.

      MIKEYv2 in SDP
         The behavior will depend on which mode is picked.

      Security Descriptions with SIPS
         No PFS.

      Security Descriptions with S/MIME
         No PFS.

      SDP-DH
         PFS is provided with the Diffie-Hellman exchange.

      ZRTP
         PFS is provided with the Diffie-Hellman exchange.

      EKT
         No PFS.

      DTLS-SRTP
         PFS is achieved if the negotiated cipher suite includes an
         exponential or discrete-logarithmic key exchange (such as
         Diffie-Hellman or Elliptic Curve Diffie-Hellman [RFC4492]).

      MIKEYv2 Inband
         The behavior will depend on which mode is picked.

C.3.  Best Effort Encryption


      Note: With the ongoing efforts in SDP Capability Negotiation
      [I-D.ietf-mmusic-sdp-capability-negotiation], the conclusions
      reached in this section may no longer be accurate.


   With best effort encryption, SRTP is used with endpoints that support
   SRTP, otherwise RTP is used.

   SIP needs a backwards-compatible best effort encryption in order for
   SRTP to work successfully with SIP retargeting and forking when there
   is a mix of forked or retargeted devices that support SRTP and don't
   support SRTP.




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      Consider the case of Bob, with a phone that only does RTP and a
      voice mail system that supports SRTP and RTP.  If Alice calls Bob
      with an SRTP offer, Bob's RTP-only phone will reject the media
      stream (with an empty "m=" line) because Bob's phone doesn't
      understand SRTP (RTP/SAVP).  Alice's phone will see this rejected
      media stream and may terminate the entire call (BYE) and re-
      initiate the call as RTP-only, or Alice's phone may decide to
      continue with call setup with the SRTP-capable leg (the voice mail
      system).  If Alice's phone decided to re-initiate the call as RTP-
      only, and Bob doesn't answer his phone, Alice will then leave
      voice mail using only RTP, rather than SRTP as expected.

   Currently, several techniques are commonly considered as candidates
   to provide opportunistic encryption:

   multipart/alternative
      [I-D.jennings-sipping-multipart] describes how to form a
      multipart/alternative body part in SIP.  The significant issues
      with this technique are (1) that multipart MIME is incompatible
      with existing SIP proxies, firewalls, Session Border Controllers,
      and endpoints and (2) when forking, the Heterogeneous Error
      Response Forking Problem (HERFP) [I-D.mahy-sipping-herfp-fix]
      causes problems if such non-multipart-capable endpoints were
      involved in the forking.

   SDP Grouping
      A new SDP grouping mechanism (following the idea introduced in
      [RFC3388]) has been discussed which would allow a media line to
      indicate RTP/AVP and another media line to indicate RTP/SAVP,
      allowing non-SRTP-aware endpoints to choose RTP/AVP and SRTP-aware
      endpoints to choose RTP/SAVP.  As of this writing, this SDP
      grouping mechanism has not been published as an Internet Draft.

   session attribute
      With this technique, the endpoints signal their desire to do SRTP
      by signaling RTP (RTP/AVP), and using an attribute ("a=") in the
      SDP.  This technique is entirely backwards compatible with non-
      SRTP-aware endpoints, but doesn't use the RTP/SAVP protocol
      registered by SRTP [RFC3711].

   Probing
      With this technique, the endpoints first establish an RTP session
      using RTP (RTP/AVP).  The endpoints send probe messages, over the
      media path, to determine if the remote endpoint supports their
      keying technique.

   The following list compares the availability of best effort
   encryption for each keying mechanism.



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      MIKEY-NULL
         No best effort encryption.

      MIKEY-PSK
         No best effort encryption.

      MIKEY-RSA
         No best effort encryption.

      MIKEY-RSA-R
         No best effort encryption.

      MIKEY-DHSIGN
         No best effort encryption.

      MIKEY-DHHMAC
         No best effort encryption.

      MIKEYv2 in SDP
         No best effort encryption.

      Security Descriptions with SIPS
         No best effort encryption.

      Security Descriptions with S/MIME
         No best effort encryption.

      SDP-DH
         No best effort encryption.

      ZRTP
         Best effort encryption is done by probing (sending RTP messages
         with header extensions) or by session attribute (see "a=zrtp",
         defined in Section 10 of [I-D.zimmermann-avt-zrtp]).  Current
         implementations of ZRTP use probing.

      EKT
         No best effort encryption.

      DTLS-SRTP
         No best effort encryption.

      MIKEY Inband
         No best effort encryption.





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C.4.  Upgrading Algorithms

   It is necessary to allow upgrading SRTP encryption and hash
   algorithms, as well as upgrading the cryptographic functions used for
   the key exchange mechanism.  With SIP's offer/answer model, this can
   be computionally expensive because the offer needs to contain all
   combinations of the key exchange mechanisms (all MIKEY modes,
   Security Descriptions) and all SRTP cryptographic suites (AES-128,
   AES-256) and all SRTP cryptographic hash functions (SHA-1, SHA-256)
   that the offerer supports.  In order to do this, the offerer has to
   expend CPU resources to build an offer containing all of this
   information which becomes computationally prohibitive.

   Thus, it is important to keep the offerer's CPU impact fixed so that
   offering multiple new SRTP encryption and hash functions incurs no
   additional expense.

   The following list describes the CPU effort involved in using each
   key exchange technique.



      MIKEY-NULL
         No significant computaional expense.

      MIKEY-PSK
         No significant computational expense.

      MIKEY-RSA
         For each offered SRTP crypto suite, the offerer has to perform
         RSA operation to encrypt the TGK

      MIKEY-RSA-R
         For each offered SRTP crypto suite, the offerer has to perform
         public key operation to sign the MIKEY message.

      MIKEY-DHSIGN
         For each offered SRTP crypto suite, the offerer has to perform
         Diffie-Hellman operation, and a public key operation to sign
         the Diffie-Hellman output.

      MIKEY-DHHMAC
         For each offered SRTP crypto suite, the offerer has to perform
         Diffie-Hellman operation.







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      MIKEYv2 in SDP
         The behavior will depend on which mode is picked.

      Security Descriptions with SIPS
         No significant computational expense.

      Security Descriptions with S/MIME
         S/MIME requires the offerer and the answerer to encrypt the SDP
         with the other's public key, and to decrypt the received SDP
         with their own private key.

      SDP-DH
         For each offered SRTP crypto suite, the offerer has to perform
         a Diffie-Hellman operation.

      ZRTP
         The offerer has no additional computational expense at all, as
         the offer contains no information about ZRTP or might contain
         "a=zrtp".

      EKT
         The offerer's Computational expense depends entirely on the EKT
         bootstrapping mechanism selected (one or more MIKEY modes or
         Security Descriptions).

      DTLS-SRTP
         The offerer has no additional computational expense at all, as
         the offer contains only a fingerprint of the certificate that
         will be presented in the DTLS exchange.

      MIKEYv2 Inband
         The behavior will depend on which mode is picked.


Appendix D.  Out-of-Scope

   Discussions concluded that key management for shared-key encryption
   of conferencing is outside the scope of this document.  As the
   priority is point-to-point unicast SRTP session keying, resolving
   shared-key SRTP session keying is deferred to later and left as an
   item for future investigations.










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Authors' Addresses

   Dan Wing
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134
   USA

   Email: dwing@cisco.com


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

   Email: steffen.fries@siemens.com


   Hannes Tschofenig
   Nokia Siemens Networks
   Otto-Hahn-Ring 6
   Munich, Bavaria  81739
   Germany

   Email: Hannes.Tschofenig@nsn.com
   URI:   http://www.tschofenig.com


   Francois Audet
   Nortel
   4655 Great America Parkway
   Santa Clara, CA  95054
   USA

   Email: audet@nortel.com














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