HIPRG                                                      H. Tschofenig
Internet-Draft                                                   Siemens
Expires: January 12, 2006                                       F. Muenz
                                                            M. Shanmugam
                                                           July 11, 2005

                  Using SRTP transport format with HIP

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).


   The Host Identity Protocol is a signaling protocol which adds another
   layer to the Internet model and (optionally) establishes IPsec ESP
   SAs to protect subsequent data traffic.  HIP is an end-to-end
   authentication and key exchange protocol, which supports security and
   mobility in a commendable manner.  This draft explains a Secure Real

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   Time Protocol (SRTP) based mechanism for transmission of user data
   packets, to be used with the Host Identity Protocol (HIP).

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Message Flow . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1   Base Exchange  . . . . . . . . . . . . . . . . . . . . . .  7
     3.2   Rekeying . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.3   Packet Format  . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Key management . . . . . . . . . . . . . . . . . . . . . . . . 13
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     6.1   Normative References . . . . . . . . . . . . . . . . . . . 17
     6.2   Informative References . . . . . . . . . . . . . . . . . . 17
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 18
       Intellectual Property and Copyright Statements . . . . . . . . 19

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1.  Introduction

   Host Identity Protocol (HIP) [I-D.ietf-hip-base] provides a way to
   separate the dual role of IP (end point identifier and locator) by
   adding a new layer between the traditional Network and Transport
   layer .  This separation helps the end host to achieve mobility,
   furthermore, HIP provides better security features (like end-to-end
   authentication, confidentiality for the data traffic etc) than other
   multi6 proposals [I-D.ietf-hip-multi6].

   HIP is based on public key cryptography.  All HIP hosts have a
   public/private key pair.  HIP introduces a new name space called Host
   Identity.  It is nothing but the public key of an asymmetric key
   pair.  It provides a rapid exchange of host identities (public keys)
   between communicating hosts and (optionally) establishes IPsec SAs to
   protect subsequent data traffic.  It is a four-way handshake
   protocol, which supports end-to-end authentication and the data
   traffic may experience IPsec ESP encapsulation.  Since different
   sizes for the public key are possible, it uses the Host Identity Tag
   (HIT), which is the hash of the public key, for operational
   representation.  The HIP header carries HIT (128 bits long), which is
   similar to IPV6 addresses.

   Transport connections and Security Associations between the
   communicating HIP hosts are bound to the HITs only.  IP addresses are
   used for routing purposes only.  Therefore, changes to IP addresses
   do not change the connections or associations.  So, when any of the
   peers move, it uses a readdressing mechanism to update the current
   location of the peer, thereby supporting mobility in a seamless

   Session Initiation Protocol (SIP) is an application layer protocol,
   which is capable of establishing modifying and terminating sessions
   between the hosts.  The SIP architecture uses URIs to uniquely
   identify (maps) the user agents and has various infrastructure
   components like proxy server, redirect server etc., to achieve
   personal mobility.

   SIP, when combined with RTP, can effectively handle multimedia
   applications.  SIP can also invite participants to already existing
   sessions, such as multicast conferences.  Media can be added to (and
   removed from) an existing session.  SIP relies on other security
   protocols like TLS, IPsec, HTTP Digest mechanisms to protect the SIP

   HIP base exchange [I-D.ietf-hip-base] does not describe any transport
   formats for user data.  This document proposes extensions to HIP by
   exporting the relevant parameters to support other key management

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   scheme, like MIKEY.  SRTP proposes MIKEY [RFC3830] as a key
   management protocol.  We propose to use the same  key management
   scheme in HIP.  HIP combined with MIKEY alike scheme can be used for
   SRTP as a key management protocol to exchange Master Keys and create
   a Cryptographic Context (SRTP RFC chapter 3.2).  HIP has to satisfy
   the requirements of SRTP (SRTP RFC chapter 7/8) for a key management
   protocol and has to support the appropriate cryptographic algorithms
   within its transform parameters.

   HIP base exchange provides a mutual authentication of the hosts, but
   does not specify any mechanism for protecting data packets for the
   actual communication.  [I-D.ietf-hip-esp] draft proposes a way to use
   IPsec ESP format with HIP.  In this document, we specify the use of
   SRTP for protecting user data traffic after the HIP base exchange.

   SRTP mandates the use of a external key management protocol (like
   MIKEY) to exchange keys and cryptographic parameters, which are used
   to derive keys (like cipher suites, random number etc.,).  This draft
   proposes a way to exchange the SRTP relevant parameters during the
   HIP base exchange.  Besides this, we inherited the key derivation
   procedure of SRTP to show how the keys will be manipulated and
   maintained for the data traffic.

   This document explains the compatibility of HIP and SIP together with
   the new KEY management scheme.  Section 3 explains the revised base
   exchange, and Section 4 explains the key derivation and future work.

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2.  Terminology

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

   This draft used the terminology defined in  [I-D.ietf-hip-base] and

   The term MKI refers to  Master Key Identifier used in SRTP packets.
   It is similar to SPIs in IPsec.

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3.  Message Flow

   This section explains the integration of SIP and HIP.  The motivation
   to combine HIP and SIP is defined in [I-D.ietf-hip-sip].  SIP uses
   URIs, which bind to an IP address or a host name.  When HIP is used,
   SIP headers will make use of HITs instead of IPs  i.e, SIP URIs will
   be bound to HITs.  A HIT is derived from HI (public key), which
   identify users/hosts and IP addresses.  The resolution of IP from HI/
   HITs can be done via DNS or other mechanisms.  Also, the HI/HITs can
   be  exchanged using SIP/SDP mechanism as desribed in [I-D.ietf-hip-

   Initially the caller sends an INVITE message to its proxy server,
   assuming that the caller is already connected.  Then the caller proxy
   server locates the callee proxy server, possibly by performing a
   particular type of DNS (Domain Name Service) lookup (DNS SRV record).
   DNS will return the HI/HIT of the callee together with one or more IP
   addresses of SIP proxies responsible for the callee.  After
   resolution it forwards the message to a callee proxy server and adds
   a new entry in the SIP header  (for route record routability).  The
   callee proxy receiving the INVITE message consults a location
   database via a location service to resolve the HIT of the callee to
   current IP address.  Finally, it forwards the INVITE to the callee.

   There are several ways how to combine SIP messages with HIP base-
   exchange.  SIP and HIP messages could be combined to reduce
   roundtrips or can be used separately.The latter will be explained in
   detail in this section.

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                    +---------+          +---------+
   +------+         |SIP Proxy|          |SIP Proxy|          +------+
   |Caller|         |Caller   |          |Callee   |          |Callee|
   +------+         +---------+          +---------+          +------+
      +---INVITE-------->|                    |                  |
      |                  +---INVITE---------->|                  |
      |                  |                    +---INVITE-------->|
      |                  |                    |<--OK-------------+
      |                  |<--OK---------------+                  |
      |<--OK-------------+                    |                  |

   +------+                           |HIP RVS  |          +------+
   |Caller|                           |Callee   |          |Callee|
   +------+                           +---------+          +------+
      |                                    |                   |
      |                                    |                   |
      |<==================TCP/UDP Session=====================>|

      Figure 1: SIP and HIP Base Exchange

   Session establishment works in known ways.  First an INVITE is routed
   from the caller to the callee using SIP proxies.  The callee then
   answers with 200 OK and the caller acknowledges with an ACK message
   directly to the callee.  However in this scenario, the SDP of the SIP
   signalling traffic will not include any SRTP parameters (transforms),
   which will be decoupled and delegated to HIP.  SIP only serves as a
   rendezvous protocol for HIP to exchange end-host IP addresses and
   negotiate HIP as the used end-to-end authentication and key exchange

3.1  Base Exchange

   After HIP is chosen there are again two possibilities on how to
   proceed.  Firstly HIP base-exchange may run directly between
   communication partners or secondly the callee might be using HIP
   rendezvous server which is shown in figure 1.

   As explained in the previous sections, HIP allows the use of other
   key management protocols.  Figure 2 explains how the new KEYING
   parameters fit into the HIP base exchange:

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   Initiator                                               Responder

   I1:     ------Trigger exchange--------------------------------->

   R1:    <------ puzzle{HI(R),DH(R)}sig(R)------------------------

   I2:     -{Soln,DH(I), KEYING param.,MKI, enc{HI}keymat }sig(I)->

   R2:    <------ { KEYING param.,MKI, HMAC }sig(R) ---------------

                            Fig 2: Base Exchange

   The Initiator starts the HIP connection by sending the trigger
   message.  This message is nothing but two IPs and HITs of the
   Initiator and the Responder respectively.  The Responder answers with
   the R1 packet, the difference between the actual HIP exchange and the
   proposed mechanism is the removal of the cipher suites, because the
   transforms will be chosen via KEYING parameter.  Since we have to
   avoid the state creation, it sends a precomputed packet.

   Context id = <SSRC, destination HIT/address, destination port> This
   triple SHALL uniquely identiies a cryptographic context (SRTP RFC
   chapter 3.2.3.).  This context id together with MKI will be mapped to
   the master key and cipher suites in KEYING management scheme to find
   the session keys to process the packet.

   Any specific transform parameters needed for the SRTP cryptographic
   context will be exchanged by using SP parameter of KEYING parameter
   in HIP.

      Master Key - derived from Diffie-Hellmann value

      Master Salt - RAND in the KEYING parameter

      MKI         - Master Key Identifier

   Upon receiving it , the Initiator solves the puzzle and sends the I2
   packet with the Diffie Hellmann value and its KEYING parameter.  For
   the explanation of KEYING parameter see above.  The Initiator's HI is
   encrypted by the keying material derived from the master key (Diffie
   Hellmann value), so that the responder can also derive the same key
   using the negotiated cipher suites and Diffie Hellmann value to
   decrypt the HI.  The key management and key derivation is up to the
   KEY scheme.  The whole packet is signed by the Initiator's public

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   The Responder receives the packet verifies the solution, derives the
   key using the Diffie Hellmann value and the KEYING parameter,
   decrypts the HI, using the  keying material ,   and verifies the
   Signature.  The Responder derives all the encryption and
   authentication keys from the Initiator's master (Diffie Hellmann) and
   salt key (KEYING parameter RAND).  The reason for this is that both
   the Initiator and Responder have the same key pairs for providing
   confidentiality for the data traffic.

   Next, the Responder sends its KEYING parameter , the same time stamp,
   random no, the selected cipher suites and HMAC of the whole packet,
   the key for HMAC is derived from the  Master Key. It sends its MKI to
   identify the incoming packet.  The Initiator will  check the HMAC and
   also the Signature to verify the integrity and authenticity of the
   packet.  After this, the HIP association is established and both the
   hosts use their respective master key and it derived keys for
   protecting the traffic.  The Master Key is 128 bit long, which can be
   exchanged using the Diffie Hellmann parameter.

3.2  Rekeying

   Rekeying can be supported using the UPDATE packet of HIP.  The peer
   which wants to rekey should use the UPDATE packet with the
   appropriate parameters.  The mechanism is explained below:

   Initiator                                                  Responder

   Update -Update([seq,REA],DH(R),KEYING param., MKI,HMAC)Sig(I) ----->

   Seq    <-Update seq([ack, REA],DH(I),KEYING param.,MKI,HMAC)Sig(R)-

   Update ---------------Update ACK( ack, HMAC)Sig(I)----------------->
                           Fig 3:Rekeying mechanism

   Figure 3 depicts the rekeying scenario.  Here, assume that the
   Initiator wants to rekey after the Initial exchange.  It can send the
   rekeying parameters in the Update packet.  The same mechanism is
   followed here, the Initiator chooses its Diffie Hellmann value and
   sends it to the Responder.  The key for HMAC has been derived from
   the old Master key.  It also sends a new MKI value to  identify the
   incoming packet.

   The Responder chooses its Diffie Hellmann value, verifies the HMAC
   and Signature.  The other parameters are explained in [I-D.ietf-hip-
   base] draft.  The Responder checks the return routability by sending

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   the Update seq message containing its relevant parameters for the
   rekeying.  After receiving the packet, the Initiator sends the ACK
   thereby both the peers concluding the rekeying procedure and now,
   both of the peers expect to receive the traffic in the new keying

3.3  Packet Format

   This section explains the packet format for the KEYING parameter in
   more detail.

   KEYING parameter contains

      T:    The timestamp, used mainly to prevent replay attacks.

      RAND: Random/pseudo-random byte-string, RAND(nonce) is used as a
      freshness value for the key generation (salts).

      SP: The security policies for the data security protocol. (eg.
      Algorithms and transforms and PRFs supported by the peers).  The
      cipher suites can be negotiated from I2/R2 packet.

      MKI : It is similar to SPI i.e, to identify the Master key and
      also security associations.

      Master Key and its length - obtained from Diffie Hellmann key

      session keys is derived using Master key and SP

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   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |             Type              |             Length            |
   | Encr. transf  | Auth transf   | Encr length                   |
   | Auth Length                   | tag length                    |
   |FEC| Reserved                  | SRTP prefix length            |
   | SRTP prefix length            |                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
   |               SRTP-packets-max-lifetime                       |
   |               Key derivation rate                             |
   |               SRTCP-packets-max-lifetime                      |

              Fig 4:  Key management parameters

   Type:   40000 (experimental identifier range)
   Length: 256 bit
   Value:  Type/Meaning                      | Possible values
           SRTP and SRTCP encr transf        | see below
           SRTP and SRTCP auth transf.       | see below
           tag length                        | 80
           SRTP prefix_length                | variable (default 0)
           Key derivation PRF                | see below
           encr session key length           | 128
           auth session key length           | 160
           key derivation rate               | variable  (default 0)
           SRTP-packets-max-lifetime         | variable
           SRTCP-packets-max-lifetime        | variable
           Forward Error Control             | 2-bits

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   SRTP and SRTCP encr transf. | Value
   NULL                        |  0
   AES_CM                      |  1
   AES_f8                      |  2

   SRTP and SRTCP auth transf. | Value
   NULL                        |  0
   HMAC-SHA1                   |  1

   Key derivation PRF          | Value
   NULL                        |  0
   AES_CM                      |  1

   0                   1                   2                   3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |             Type              |             Length            |
   ~                         MKI (variable)                        ~

                          Fig 5: SRTP MKI parameter

   Type:   40001 (experimental identifier range)
   Length: variable
   Value:  Master Key Identifier (MKI)

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4.  Key management

   This section explains how the key management scheme can be used for
   the data traffic.  After the initial base exchange, both peers have
   the same master key, salt and agreed crypto transforms (including
   pseudo random function).  When the application receives the data
   traffic after the base exchange, an API is invoked and asks the HIP
   daemon for the appropriate key to process the data packet

   The SRTP based key derivation helps to generate the session keys for
   both peers, so that they have the same keys in possession for
   encrypting/decrypting the incoming packets.  It generates three keys
   namely encryption key to provide confidentiality for the data
   packets, authentication key for providing integrity and salt key for
   the AES counter mode.  For that, it uses the master key, salt and
   crypto transforms together with the packet index.

   Figure 6 depicts the example implementation architecture of the
   proposed mechanism:

   -------------+   API         | KEY ENGINE |
    Application |<------------->|            |
                |               |            |
                |               |            |
                |               | HIP daemon |
                |               +------------+
    User space  |
             PF_INET ||          || PF_RAW
                     ||          ||
    Kernel space
                     | TCP|UDP / IP |

          Fig 6: Example Implementation Architecture

   Figure 7 depicts the key derivation, for example, when the peer
   receives a packet it gets the packet index, MKI, which is used for
   identifying the relevant master key and transforms.  Then, the key
   derivation function, which is explained below, will generate the
   required keys.

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             packet index ---+
   +-----------+ master  +--------+ session encr_key
   | ext       | key     |        |---------->
   | key mgmt  |-------->|  key   | session auth_key
   | (optional |         | deriv  |---------->
   | rekey)    |-------->|        | session salt_key
   |           | master  |        |---------->
   +-----------+ salt    +--------+

            Fig 7: SRTP Key Derivation

   For single key derivation (key_derivation_rate = 0), we define x for
   later use in calculating keys using PRF and length of PRF bit string
   output like shown in the following table:

   X        ROC || SEQ     Usage                     PRF output length n
   0x00 000000000000       SRTP encryption               128 bit
   0x01 000000000000       SRTP message auth.            160 bit
   0x02 000000000000       SRTP salting key              112 bit
   0x03 000000000000       SRTCP encryption              128 bit
   0x04 000000000000       SRTCP message auth.           160 bit
   0x05 000000000000       SRTCP salting key             112 bit

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   PRF_n (master_key, x)

   For multiple key derivation (key_derivation_rate = 1,2,...2E24)
   x must be calculated according to the following sequence:

   r = index / key_derivation_rate
   (with "/" defines r = 0 for key_derivation_rate = 0)

   with index is a 48-bit concatenation of the 32 bit Roll Over Counter
   (ROC) and the 16 bit sequence number of the SRTP packet given in the
   SRTP header (ROC||SEQ)

   r must be the same length like index, which results in leading zeros.

   Next concatenate an 8-bit label for selecting the usage with r
   key_id = <label> concatenated with r.

   where <label>  is one of the following>
   0x00 for SRTP encryption
   0x01 for SRTP message authentication
   0x02 for SRTP salting key
   0x03 for SRTCP encryption key
   0x04 for SRTCP authentication key
   0x05 for SRTCP salting key

   Finally, x is calculated by performing key_id XOR master_salt,
   where key_id and master_salt are aligned so that their least
   significant bits agree (right-alignment).

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5.  Security Considerations

   Security is considered throughout this document

   The initial keying material is generated using using Diffie-Hellman
   procedure.  This document extends the usage of UDPATE packet, defined
   in the base specification, for rekeying.  The hosts may rekey for the
   generation of new keying material using Diffie-Hellman procedure.
   This mechanism enjoys the security protection provided by base
   exchange using HMAC and signature verifications.

   In this approach, we have tried to extend the HIP base exchange to
   support SRTP based key management scheme.  We have listed the
   following security mechanisms that are incorporated with this idea:

      DoS: This approach enjoys the merits of HIP like resisting cpu and
      memory exhaustive DoS attacks by forcing the caller to calculate
      the solution for a cryptographic puzzle.  This provides only a
      basic DoS protection for the callee.

      MitM: HIP uses authenticated Diffie-Hellmann key exchange, which
      prevents the man-in-the-middle (MitM) attacks.

      Eavesdropping : Since the data traffic is encrypted, it is
      unreadable for the attackers.

      Authentication: Both peers are authenticated using asymmetric key
      (signature verification) cryptography assuming that public keys
      can be acquired by secure ways.

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6.  References

6.1  Normative References

              Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", draft-ietf-hip-base-02 (work in
              progress), February 2005.

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

6.2  Informative References

              Moskowitz, R., Nikander, P., and P. Jokela, "Host Identity
              Protocol", draft-ietf-hip-esp-00 (work in progress),
              June 2005.

              Tschofenig, H. and A. Nagarajan, "Comparative Analysis of
              Multi6 Proposals using a Locator/Identifier Split",
              October 2004.

              Tschofenig, H., Schulzrinne, H., Henderson, T., Torvinen,
              V., Camarillo, G., and J. ott, "Exchanging Host Identities
              in SIP", October 2004.

   [RFC3261]  Schulzrinne, H., Camarillo, G., Rosenberg, J., Peterson,
              J., Sparks, R., Handley, M., and E. Schooler, "Session
              Initiation Protocol", February 2005.

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

   [RFC3830]  Arrko, J., Carrara, E., Lindholm, F., Naslund, M., and K.
              Norrman, "MIKEY: Multimedia Internet KEYing",
              draft-ietf-hip-base-02 (work in progress), August 2004.

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Authors' Addresses

   Hannes Tschofenig
   Otto-Hahn-Ring 6
   Munich, Bayern  81739

   Email: Hannes.Tschofenig@siemens.com

   Franz Muenz
   University of Applied Sciences
   Lurzenhof 1
   Landshut, Bayern  84036

   Email: franz.muenz@fh-landshut.de

   Murugaraj Shanmugam
   Technical University Hamburg-Harburg
   Schwarzenbergstrasse 95
   Harburg, Hamburg  21075

   Email: murugaraj.shanmugam@tuhh.de

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Tschofenig, et al.      Expires January 12, 2006               [Page 19]