Host Identity Protocol                                      T. Heer, Ed.
Internet-Draft                                                 K. Wehrle
Intended status: Experimental            Distributed Systems Group, RWTH
Expires: September 1, 2009                             Aachen University
                                                                 M. Komu
                                                       February 28, 2009

              End-Host Authentication for HIP Middleboxes

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   The Host Identity Protocol [RFC5201] is a signaling protocol for
   secure communication, mobility, and multihoming that introduces a
   cryptographic namespace.  This document specifies an extension for
   HIP that enables middleboxes to unambiguously verify the identities
   of hosts that communicate across them.  This extension allows
   middleboxes to verify the liveness and freshness of a HIP association
   and, thus, to secure access control in middleboxes.

Requirements Language

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


   [x]        indicates that x is optional.

   {x}        indicates that x is under signature.

   Initiator  is the host that initiates a HIP association
              (cf. HIP base protocol).

   Responder  is the host that responds to the INITIATOR
              (cf. HIP base protocol).

   -->        signifies "Initiator to Responder" communication.

   <--        signifies "Responder to Initiator" communication.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Authentication and Replay Attacks  . . . . . . . . . . . .  5
   2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Signed Middlebox Nonces  . . . . . . . . . . . . . . . . .  6
     2.2.  Identity Verification by Middleboxes . . . . . . . . . . .  8
     2.3.  Failure Signaling  . . . . . . . . . . . . . . . . . . . . 13
     2.4.  Fragmentation  . . . . . . . . . . . . . . . . . . . . . . 13
     2.5.  HIP Parameters . . . . . . . . . . . . . . . . . . . . . . 13
   3.  Security Services for the HIP Control Channel  . . . . . . . . 15
     3.1.  Adversary model and Security Services  . . . . . . . . . . 15
   4.  Security Services for the HIP Payload Channel  . . . . . . . . 16
     4.1.  Access Control . . . . . . . . . . . . . . . . . . . . . . 17
     4.2.  Resource allocation  . . . . . . . . . . . . . . . . . . . 17
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 18
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   8.  Normative References . . . . . . . . . . . . . . . . . . . . . 19
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20

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1.  Introduction

   The Host Identity Protocol (HIP) introduces a new cryptographic
   namespace, based on public keys, in order to secure Internet
   communication.  This namespace allows hosts to securely address and
   authenticate their peers.  HIP was designed to be middlebox-friendly
   and to allow middleboxes to inspect HIP control traffic.  Examples of
   such middleboxes are firewalls and Network Address Translators

   In this context, one can distinguish HIP-aware middleboxes, which are
   designed to process HIP packets, and other middleboxes, which are
   unaware of HIP.  This document addresses only HIP-aware middleboxes
   while the behavior of HIP in combination with HIP-unaware middleboxes
   is specified in [I-D.ietf-hip-nat-traversal].  Moreover, the scope of
   this document is restricted to middleboxes that use HIP in order to
   provide Authentication, Authorization, and Accounting (AAA)-related
   services and, thus, need to authenticate the communicating peers that
   send traffic over the middlebox.  The class of middleboxes this
   document focuses on does not require the end-host to explicitly
   register to the middlebox.  HIP behavior for interacting and
   registering to such middleboxes is specified in [RFC5203].  Thus, we
   focus on middleboxes that build their state based on packets they
   forward (path-coupled signaling).

   An example of such a middlebox is a firewall that only allows traffic
   from certain hosts to traverse.  We assume that access control is
   performed based on Host Identities (HIs).  Such an authenticating
   middlebox needs to observe the HIP Base EXchange (BEX) or a HIP
   mobility update [RFC5206] and check the Host Identifiers (HIs) in the

   Along the lines of [I-D.irtf-hiprg-nat], an authentication solution
   for middleboxes must have some vital properties.  For one, the
   middlebox must be able to unambiguously identify one or both of the
   communicating peers.  Additionally, the solution must not allow for
   new attacks against the middlebox.  This document specifies a HIP
   extension that allows middleboxes to participate in the HIP handshake
   and the HIP update process in order to allow these middleboxes to
   reliably verify the identities of the communicating peers.  To this
   end, this HIP extension defines how middleboxes can interact with
   end-hosts in order to verify their identities.

   Verifying public-key (PK) signatures is costly in terms of CPU
   cycles.  Thus, in addition to authentication capabilities, it is also
   necessary to provide middleboxes with a way of defending against
   resource-exhaustion attacks that target PK signature verification.
   This document defines how middleboxes can utilize the HIP puzzle

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   mechanism defined in [RFC5201] to slow down resource-exhaustion

   The presented authentication extension only targets the HIP control
   channel.  Additional security considerations and possible security
   services for the HIP payload channel are discussed in Section 4.

1.1.  Authentication and Replay Attacks

   Middleboxes may need to verify the HIs in the HIP base exchange
   messages to perform access control based on Host Identities.
   However, passive verification of HIs in the messages is not
   sufficient to ensure the identity of an end-host because of a
   possible replay attack against which the basic HIP protocol as
   specified in [RFC5201] does not provide adequate protection.

   To illustrate the need for additional security measures for HIP-aware
   middleboxes, we briefly outline the replay attack: Assume that the
   legitimate owner of Host Identity Tag (HIT) X establishes a HIP
   association with the legitimate owner of HIT Y at some point in time
   and an attacker A overhears the base exchange and records it.

   Assume that a middlebox M checks HIP HIs in order to restrict traffic
   passing through the box.  At some later point in time, Attacker A
   collaborates with another attacker B. They replay the very same BEX
   over a middlebox M on the communication path.  Note that it is not
   required that the middlebox M was on the communication path between
   the peers when the BEX was recorded.

   The middlebox has no way to distinguish legitimate hosts X and Y from
   the attackers A and B as it can only overhear the BEX passively and
   it cannot can distinguish the replayed BEX from a the genuine
   handshake.  As the attackers overheard the SPI numbers, they can
   taverse the middlebox with "fake" ESP packets with valid SPI numbers,
   and hence, send data across m without proper authentication.  Since
   the middleboxes do not know the integrity and encryption keys for
   ESP, they cannot distinguish valid ESP packets from fake ones.
   Hence, collaborating attackers can use any replayed BEX to falsely
   authenticate to the middlebox and thus impersonate any host.  This is
   problematic in cases in which the middlebox needs to know the
   identity of the peers that communicate across it.  Examples for such
   cases are AAA-related services, such as access control, logging of
   activities, and accounting for traffic volume or connection duration.

   This attack scenario is not addressed by the current HIP
   specifications.  Therefore, this document specifies a HIP extension
   that allows middleboxes to defend against this attack.

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2.  Protocol Overview

   This section gives an overview of the interaction between hosts and
   authenticating middleboxes.  This document describes a framework that
   middleboxes can use to implement authentication of end-hosts and
   leaves its further use to other documents and to middlebox

2.1.  Signed Middlebox Nonces

   The described attack scenario shows the necessity for unambiguous
   end-host identity verification by middleboxes.  Relying on nonces
   generated by the end-hosts is not possible because middleboxes cannot
   verify the freshness of these nonces.  Introducing time-stamps
   restricts the attack to a certain time frame but requires global time
   synchronization and therefore should be avoided.

   The following sections specify how HIP hosts can prove their identity
   by performing a challenge-response protocol between the middlebox and
   the end-hosts.  As the challenge, the middlebox adds information
   (e.g. nonces) to HIP control packets which the end-hosts sign with
   public-key (PK) their signatures and echo back.

   The challenge-response mechanism is similar to the ECHO_REQUEST/
   ECHO_RESPONSE mechanism employed already by HIP end-hosts.  It
   assumes that the end-hosts exchange at least two HIP packets with
   each other.  The middlebox adds a CHALLENGE_REQUEST parameter to the
   first HIP control packet.  Similar to the ECHO_REQUEST parameter in
   the original HIP protocol, this parameter contains an opaque data
   field that must be echoed by its receiver.  The receiver echoes the
   opaque data field in a CHALLENGE_RESPONSE parameter.  The
   CHALLENGE_RESPONSE parameter must be covered by the packet signature,
   thereby proving that the receiver is in possession of the private key
   that corresponds to the HI.

   The middlebox can either verify the identity of the initiator, the
   responder, or both peers, depending on the purpose of the middlebox.
   The choice of which authentication is required left to middlebox


   Middleboxes MAY add CHALLENGE_REQUEST parameters to the R1, I2, and
   to any UPDATE packet.  This parameter contains an opaque data block
   of variable size which the middlebox uses to carry arbitrary data
   (e.g., a nonce).  The HIP packets that carry middlebox challenges may
   contain multiple CHALLENGE_REQUEST parameters, since all middleboxes
   on the path may add these parameters.  Hence, the MBs should restrict

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   the size of the variable data field in the CHALLENG_REQUEST
   parameter.  The total length of the packets SHOULD not exceed 1280
   bytes to avoid IPv6 fragmentation [RFC2460].

   The middleboxes add the CHALLENGE_REQUEST parameter to the
   unprotected part of a HIP message.  Thus it does not corrupt any HMAC
   or public-key signatures that protect the HIP packet.  However, the
   middlebox MUST recompute the IP- and HIP header checksums as defined
   in [RFC5201] and the UDP headers of UDP encapsulated HIP packets as
   defined in [I-D.ietf-hip-nat-traversal].

   An end-host that receives a HIP control packet containing one or
   multiple CHALLENGE_REQUEST parameters must copy the contents of each
   parameter without modification to an CHALLENGE_RESPONSE parameter.
   This end-host MUST send this parameter within the signed part of its
   reply.  Note that middleboxes MAY also add ECHO_REQUEST_UNSIGNED
   parameter as specified in [RFC5201] when the receiver of the
   parameter does not have to sign the contents of the ECHO_REQUEST.

   Middleboxes can delay state creation by utilizing the
   encrypted or otherwise protected information about previous
   authentication steps in the opaque data field.


   When a middlebox injects an opaque blob of data with a
   CHALLENGE_REQUEST parameter, it expects to receive the same data
   without modification as part of a CHALLENGE_RESPONSE parameter in a
   subsequent packet.  The opaque data MUST be copied as it is from the
   corresponding CHALLENGE_REQUEST parameter.  In the case of multiple
   CHALLENGE_REQUEST parameters, their order MUST be preserved by the
   corresponding CHALLENGE_RESPONSE parameters.

   for any purpose, in particular when a middlebox has to carry state
   information in a HIP packet to receive it in the next response
   packet.  The CHALLENGE_RESPONSE MUST be covered by the HIP_SIGNATURE.

   The CHALLENGE_RESPONSE parameter is non-critical.  Depending on its
   local policy, a middlebox can react differently on a missing
   CHALLENGE_RESPONSE parameter.  Possible actions range from degraded
   or restricted service, such as bandwidth limitation, up to refusing
   connections and reporting access violations.

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2.1.3.  Middlebox Puzzles

   As PK operations are costly in terms of CPU cycles, a middlebox has
   to defend itself against resource-exhaustion attacks when verifying
   signatures in HIP packets.  The HIP base protocol [RFC5201] specifies
   a puzzle mechanism to protect the Responder from I2 floods that
   require numerous public-key operations.  However, middleboxes cannot
   utilize this mechanism as there is no defense against a collaborative
   replay attack, which involves a malicious Initiator and a malicious
   Responder.  This section specifies how middleboxes can utilize the
   puzzle mechanism to add their own puzzles to R1, I2, and any UPDATE
   packets.  This allows middleboxes to shelter against Denial of
   Service (DoS) attacks on PK verification.

   The puzzle mechanism for middleboxes utilizes the CHALLENGE_REQUEST
   and CHALLENGE_RESPONSE parameters.  The CHALLENGE_REQUEST parameter
   contains fields for setting the difficulty and the expiration date of
   the puzzle.  In contrast to the PUZZLE parameter in the HIP base
   specifications, there is no dedicated puzzle seed field.  Instead,
   the hash of the opaque data field in the CHALLENGE_REQUEST parameter
   serves as puzzle seed.  The hash is generated by applying the
   responder's hash algorithm (RHASH) to the opaque data field.  The
   destination end-host of the HIP control packet MUST solve the puzzle
   and provide the solution in the CHALLENGE_RESPONSE parameter.  The
   middlebox can set the puzzle difficulty by adjusting the K value in
   the CHALLENGE_REQUEST packet.  The semantics of this field equal the
   semantics of the PUZZLE parameter.  Setting K to 0 signifies that no
   puzzle solution is required.

   As a puzzle increases the delay and computational cost for
   establishing or updating a HIP association, a middlebox SHOULD only
   increase K when it is under attack.  Moreover, middleboxes SHOULD
   distinguish attack directions.  If the majority of the CPU load is
   caused by verifying HIP control messages that arrive from a certain
   interface, middleboxes MAY increase K for HIP control packets that
   leave the interface.  The middlebox chooses the difficultly of the
   puzzle according to its load and local policies.

2.2.  Identity Verification by Middleboxes

   This section describes how middleboxes can influence the BEX and the
   HIP update process in order to verify the identity of the HIP end-

2.2.1.  Identity Verification During BEX

   Middleboxes MAY add CHALLENGE_REQUEST parameters to R1 and I2 packets
   in order to verify the identities of the participating end-hosts.

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   Middleboxes can choose either to authenticate the Initiator, the
   Responder, or both.  Middleboxes MUST NOT add CHALLENGE_REQUEST
   parameters to I1 messages because this would expose the Responder to
   DoS attacks.  Thus, middleboxes MUST let unauthenticated and minimal
   I1 packets traverse.  Minimal means that the I1 packet MUST NOT
   contain more than the minimal set of parameters specified by HIP
   standards or internet drafts.  In particular, the I1 packet MUST NOT
   contain any attached payload.  Figure 1 illustrates the
   authentication process during the BEX.
   Middlebox authentication of a HIP base exchange.

    Main path:

    Initiator               Middlebox                        Responder
     I1                |                 | I1
    -----------------> |                 |---------------------------->
                       |                 |
     R1, + CQ1         | Add CQ          | R1
    <----------------- |                 |<----------------------------
                       |                 |
     I2, {CR1}         | Verify CR1      | I2, {CR1} + CQ2
    -----------------> | Add CQ2         |---------------------------->
                       |                 |
                       |                 |
     R2, {CR2}         | Verify CR2      | R2, {CR2}
    <----------------- |                 |<-----------------------------

    CQ: Middlebox challenge reQuest
    CR: Middlebox challenge Response
    {}: Signature with sender's HI as key

                                 Figure 1

2.2.2.  Identity Verification During Mobility Updates

   HIP rekeying, mobility and multihoming UPDATE mechanisms for non-
   NATted environments are described in [RFC5206].  This section
   describes how middleboxes process UPDATE messages in non-NATted
   environments and leave NATted environments for future revisions of
   the draft.

   The middleboxes can apply middlebox challenges to mobility related
   HIP control messages in the case where both end-hosts are single-
   homed.  The middlebox challenges can be applied both ways as the

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   UPDATE process consists of three packets (U1, U2, U3) which all
   traverse through the same middlebox as shown in Figure 2.

   In cases, in which fewer packets are used for updating an
   association, the following rule applies.


   A HIP host, receiving a CHALLENGE_REQUEST MUST reply with a
   CHALLENGE_RESPONSE in its next UPDATE packet.  If no further UPDATE
   packets are necessary to complete the update procedure, an additional
   UPDATE packet containing the CHALLENGE_RESPONSE MUST be sent.

    Initiator                     Middlebox                   Responder
     U1                            |      |  U1 + CQ1
    -----------------------------> |      | --------------------------->
                                   |      |
     U2, {CR1} + CQ2               |      |  U2, {CR1}
    <----------------------------- |OK    | <---------------------------
                                   |      |
     U3, {CR2}                     |      |  U3, {CR2}
    -----------------------------> |    OK| --------------------------->
    CQ: Middlebox challenge reQuest
    CR: Middlebox challenge Response
    {}: Signature with sender's HI as key

   Middlebox authentication of a HIP mobility update over a single path.

                                 Figure 2

   Middlebox 1 in Figure 2 can verify the identity of the Responder by
   checking its PK signature and the presence of the CHALLENGE_RESPONSE
   in the U2 packet.  If necessary, the middlebox MAY add an
   CHALLENGE_REQUEST for the Initiator of the update.  The middlebox can
   verify the Initiator's identity by verifying its signature and the
   CHALLENGE_RESPONSE in the U3 packet.

2.2.3.  Identity Verification for Multihomed Mobility Updates

   Multihomed hosts may use multiple communication paths during an HIP
   mobility update.  Depending on whether the middlebox is located on
   the communication path between the preferred locators of the hosts or
   not, the middlebox forwards different packets and, thus, needs to
   interact differently with the updates.  Figure 3 I) and II)

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   illustrates an update with Middlebox 1 on the path between the
   Initiator's and the Responder's preferred locators and with Middlebox
   2 on an alternative path.  Middlebox 2 is not located on the path
   between the preferred locators of the HIP end-hosts does not receive
   the U1 message.  Therefore, it will not recognize any
   CHALLENGE_RESPONSE (CR1) in the second UPDATE packet.  Thus, if a
   middlebox encounters non-matching or missing CHALLENGE_RESPONSE
   parameter in an initial update packet, the middlebox SHOULD ignore

   Complying to the RESPONSE RULE stated in Section Section 2.2.2, the
   RESPONDER generates an additional fourth update packet on receiving
   the CHALLENGE_REQUEST.  The update process for a middlebox on the
   preferred communication path (Middlebox 1) and a middlebox off the
   preferred communication path (Middlebox 2) is depicted in Figure 3.

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   I)  Main path:

    Initiator                    Middlebox 1                 Responder
     U1                           |      |  U1 + CQ1
    ----------------------------> |      | --------------------------->
                                  |      |
     U2, {CR1} + CQ2              |      |  U2, {CR1}
    <---------------------------- |OK    | <---------------------------
                                  |      |
     U3, {CR2}                    |      |  U3, {CR2}
    ----------------------------> |    OK| --------------------------->

   II) Alternative path:

    Initiator                    Middlebox 2                 Responder

     U1 (bypasses Middlebox 2)
     U2, {CR1} + CQ3              |      |  U2, {CR1}
    <---------------------------- | wrong| <---------------------------
                                  |      |
     U3', {CR3}                   |      |  U3', {CR3} + CQ4
    ----------------------------> |OK    | ---------------------------->
                                  |      |
     U4, {CR4}                    |      |  U4,  {CR4}
    <---------------------------- |    OK| <---------------------------
    CQ: Middlebox challenge reQuest
    CR: Middlebox challenge Response
    {}: Signature with sender's HI as key

   Middlebox authentication of a HIP mobility update over different

                                 Figure 3

2.2.4.  Identity Signaling During Updates

   As middleboxes have to verify rapidly and forward HIP packets, they
   need to be supplied with all information necessary to do so.  If end-
   hosts hand over communication to a new communication path,
   middleboxes need to be able to learn their Host Identifiers (HIs)
   from the UPDATE packets.  Therefore, all packets that contain a
   CHALLENGE_RESPONSE parameter MUST contain the HOST_ID parameter.

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2.2.5.  Closing of Connections

   At the time being, identity verification during the closing of a HIP
   association is not supported.  Hence, the middlebox MUST preserve the
   state until it expires according to local policies.  An appropriate
   mechanism for middleboxes to verify CLOSE messages by middleboxes
   will be provided in future versions of this document.

2.3.  Failure Signaling

   Middleboxes SHOULD inform the sender of a BEX packet or update packet
   if it does not satisfy the requirements of the middlebox.  Reasons
   for non-satisfactory packets are missing HOST_ID or
   CHALLENGE_RESPONSE parameters.  Other reasons may be middlebox
   policies regarding, for example, insufficient client capabilities or
   or insufficient credentials delivered in a HIP CERT parameter
   [I-D.varjonen-hip-cert].  Options for expressing such shortcomings
   are ICMP packets if no HIP association is established and HIP_NOTIFY
   packets in case of an already established HIP association.  Defining
   this signaling mechanism is future work.

2.4.  Fragmentation

   Analogously to the specification in [RFC5201], HIP aware middleboxes
   SHOULD support IP-level fragmentation and reassembly for IPv6 and
   MUST support IP-level fragmentation and reassembly for IPv4.
   However, when adding CHALLENGE_REQUEST parameters, a middlebox SHOULD
   keep the total packet size below 1280 bytes to avoid packet
   fragmentation in IPv6.

2.5.  HIP Parameters

   This HIP extension specifies four new HIP parameters that allow
   middleboxes to authenticate HIP end-hosts and to protect against DoS


   A middlebox MAY append the CHALLENGE_REQUEST parameter to R1, I2, and
   UPDATE packets.  The structure of the CHALLENGE_REQUEST parameter is
   depicted in the following figure.  The semantics of the K and
   Lifetime fields is identical to the fields defined in the PUZZLE
   parameter in [RFC5201].  The opaque data field serves as nonce and
   puzzle seed value.  To generate the seed corresponding to the 8-byte
   value I in [RFC5201], the receiver of the puzzle applies RHASH to the
   opaque data field and truncates the result to 8-byte length.  Note
   that the opaque data field must provide enough randomness to serve as
   puzzle seed.

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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            |
   | K, 1 byte     |    Lifetime   |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                                                               /
   /                    Opaque, (variable length)                  /
   /                                           +-+-+-+-+-+-+-+-+-+-|
   /                                           |      Padding      |

   Type           65334
   Length         Variable
   K              K is the number of verified bits
   Lifetime       Challenge lifetime 2^(value-32) seconds
   Opaque         Opaque data that serves as nonce and as basis for the
                  puzzle. The puzzle value I is generated by hashing the
                  opaque data field with the hash function RHASH and
                  truncating it to 8-byte length.
   Random #I      Random number


   The CHALLENGE_RESPONSE parameter is the response to the
   CHALLENGE_REQUEST parameter.  The receiver of a CHALLENGE_REQUEST
   parameter SHOULD reply with a CHALLENGE_RESPONSE.  Otherwise, the
   middlebox that added the CHALLENGE_REQUEST parameter MAY decide to
   degrade or deny its service.  The contents of the CHALLENGE_REQUEST
   parameter must be copied to the CHALLENGE_RESPONSE parameter without
   any modification.  If the puzzle difficulty in the CHALLENGE_REQUEST
   parameter is set to any other value except 0, an appropriate puzzle
   solution (adhering to the SOLUTION specifications in [RFC5201]) must
   be provided in the CHALLENGE_RESPONSE parameter.  The
   CHALLENGE_RESPONSE parameter is non-critical and covered by the
   SIGNATURE.  The structure of the CHALLENGE_RESPONSE parameter is
   depicted below:

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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            |
   | K, 1 byte     |    Lifetime   |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                   Puzzle solution #J, 8 bytes                 /
   /                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   /                               |                               /
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               /
   /                   Opaque, (variable length)                   /
   /                                           +-+-+-+-+-+-+-+-+-+-|
   /                                           |      Padding      |

   Type             322
   Length           Variable
   K                K is the number of verified bits
   Opaque           Copied unmodified from the received
                    CHALLENGE_REQUEST parameter
   Puzzle solution  Random number

3.  Security Services for the HIP Control Channel

   In this section, we define the adversary model that the security
   analysis in the later sections will be based on.

3.1.  Adversary model and Security Services

   For discussing the security properties of the proposed HIP extension
   we first define an attacker model.  We assume a Dolev-Yao threat
   model in which an adversary can eavesdrop on all traffic regardless
   of its source and destination.  The adversary can inject arbitrary
   packets with any source and destination addresses.  Consequently, an
   adversary can also replay previously eavesdropped messages.  However,
   the adversary cannot subvert the cryptographic ciphers and hash
   function, nor can it compromise one of the communicating nodes.

   Even in the face of this strong attacker, the proposed HIP extension
   enables middleboxes to verify the identity of the communicating HIP
   peers.  It ensures that both peers are involved in the communication
   and that the HIP BEX or update packets are fresh, i.e. not replayed.

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   It enables the middlebox to verify the source and destination (in
   terms of HIs) of the HIP association and the integrity of RSA and DSA
   signed HIP packets.

4.  Security Services for the HIP Payload Channel

   The presented extension for HIP authentication by middleboxes only
   covers the HIP control channel, i.e., the HIP control messages.
   Depending on the binding between the HIP control and payload channel,
   certain security properties for the payload channel can be derived
   from the strong cryptographic authentication of the end-hosts.
   Assuming that there is a secure binding between packets belonging to
   a payload stream and the control stream, the same security properties
   as in Section 3 apply to the payload stream.

   ESP [RFC5202] is currently the default payload encapsulation format
   for HIP.  A limitation of ESP is that it does not provide a secure
   binding between the HIP control channel and the ESP traffic on a per-
   packet basis.  Hence, the achievable level of security for the
   payload channel is lower compared to the HIP control channel.

   This section discusses security properties of an ESP payload channel
   bound to a HIP control channel.  Depending on the assumed adversary
   model, certain security services are possible.  We briefly describe
   two application scenarios and how they benefit from the resulting
   security services.  For the payload channel, HIP in combination with
   the middlebox authentication scheme offers the following security

   Attribute binding:  Middleboxes can extract certain payload channel
      attributes (e.g. locators and SPIs) from the control channel.
      These attributes can be used to enforce certain restrictions on
      the payload channel, e.g., to exhibit the same attributes as the
      control channel.  The attributes can either be stated explicitly
      in the HIP control packets or can be derived from the IP or UDP
      packets carrying the HIP control messages.

   Host involvement:  Middleboxes can verify whether a certain host is
      involved in the establishment of a HIP association and, thus,
      involved in the establishment of the payload channel.

   Based on these security services we construct two use cases that
   illustrate the use of HIP authentication by middleboxes: access
   control and resource allocation as described in the following

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4.1.  Access Control

   Middleboxes can manage resources based on HIs.  As an example, let us
   assume that a middlebox only forwards HIP payload packets after a
   successful HIP BEX or HIP update.  The middlebox uses the parameters
   in the control channel (specifically IP addresses and SPIs) to filter
   the payload traffic.  The middlebox only forwards traffic from and to
   specific authenticated hosts and drops other traffic.

   The feasibility of subverting the function of the middlebox depends
   on the assumed adversary model.

4.1.1.  Adversary model and Security Services

   If we assume a Dolev-Yao threat model, attribute binding is not
   helpful to aid packet filtering for access control.  An attacker can
   send packets from any IP address and can read packets destined to any
   IP address.  Without per packet verification by the middlebox, such
   an attacker can inject arbitrary forged packets into the HIP payload
   channel and make them traverse the middlebox.  The attacker can also
   read the packets from the HIP payload channel, and hence, communicate
   across the middlebox.  However, the forged packets are disclosed by
   inconsistencies in the ESP sequence numbers, which makes the attack
   visible to the middlebox as well as the HIP end hosts.  Moreover,
   attackers can only inject packets into an already established HIP
   payload channel.  Opening a new payload channel and replaying a
   closing of the channel are not possible.

   An attacker that is not able to send IP packets from an arbitrary
   source address and receive IP packets addressed to any destination,
   cannot use the ESP channel to send fake ESP packets when the
   middleboxes bind HIs and SPI numbers to addresses.  By fixing the set
   of source and destination IP addresses, the opportunity to
   successfully inject packets into the payload channel is limited to
   hosts that can send packets from the same source address as the
   legitimate HIP hosts.  Moreover, an attacker can only receive
   injected packets if it is on the communication path towards the
   legitimate HIP peer.  Attackers cannot open new HIP payload channels
   and thus have no influence on the bound payload stream parameters.
   Finally, attackers cannot close HIP associations of legimitate peers.

4.2.  Resource allocation

   When using HIs to limit the resources (e.g. bandwidth) allocated for
   a certain host, the HIs can be used to authenticate the hosts in a
   similar fashion to the access control illustrated above.  Regarding
   authentication, both use cases share the same strengths and
   weaknesses.  However, the implications for the targeted scenarios

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   differ.  Therefore, we restrict the following discussion to these

4.2.1.  Adversary Model and Security Services

   When assuming an Dolev-Yao threat model, an attacker is able to use
   resources allocated for the payload channel of another host by
   injecting packets into this channel.  Also, the attacker cannot open
   a new payload channel with another host nor can it close an existing

   When binding the IP addresses of the HIP payload channel to the IP
   addresses used in the HIP control channel and assuming an attacker is
   unable to receive IP packets addressed to the IP address of an
   authenticated host, the attacker cannot utilize the resources
   allocated to authenticated host.  However, the attacker can still
   inject packets and waste resources, yet without having any benefit
   other than causing disturbance to the other host.  Specifically, it
   cannot increase the share of resources allocated to itself.  Hence,
   this measure takes incentive from selfish users that try to benefit
   by mounting a DoS attack.  Defense against purely malicious attackers
   that aim at creating disturbance without immediate benefit is
   difficult to achieve and out of scope of this document.

5.  Security Considerations

   This HIP extension specifies how HIP-aware middleboxes interact with
   the handshake and mobility-signaling of the Host Identity Protocol.
   The scope is restricted to the authentication of end-hosts and
   excludes the issue of stronger authentication of ESP traffic at the

   Providing middleboxes with a way of adding puzzles to the HIP control
   packets may cause both HIP peers, including the Responder, to spend
   CPU time on solving these puzzles.  Thus, it is advised that HIP
   implementations for servers employ mechanisms to prevent middlebox
   puzzles from being used as DoS attacks.  Under high CPU load, servers
   can rate limit or assign lower priority to packets containing
   middlebox puzzles.

   If multiple middleboxes add CHALLENGE_REQUEST parameters to a HIP
   control packet, the remaining space in the packet might not be
   sufficient for further CHALLENGE_REQUEST parameters to be added.
   Moreover, as the CHALLENGE_REQUEST must be echoed within a
   CHALLENGE_RESPONSE, the space in the subsequent packet may not be
   sufficient to include all CHALLENGE_RESPONSE parameters.  Thus,
   middleboxes SHOULD keep the size of the nonces small.

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6.  IANA Considerations

   This document specifies two new HIP parameter types.  The preliminary
   parameter type numbers are 322 and 65334.

7.  Acknowledgments

   Thanks to Shaohui Li, and Janne Lindqvist for the fruitful
   discussions on this topic.  Many thanks to Julien Laganier, Stefan
   Goetz, Ari Keranen, Samu Varjonen, Rene Hummen, and Kate Harrison for
   commenting and helping to improve the quality of this document.

8.  Normative References

              Komu, M., Henderson, T., Matthews, P., Tschofenig, H., and
              A. Keraenen, "Basic HIP Extensions for Traversal of
              Network Address Translators",
              draft-ietf-hip-nat-traversal-05 (work in progress),
              October 2008.

              Stiemerling, M., "NAT and Firewall Traversal Issues of
              Host Identity Protocol (HIP)  Communication",
              draft-irtf-hiprg-nat-04 (work in progress), March 2007.

              Heer, T. and S. Varjonen, "HIP Certificates",
              draft-varjonen-hip-cert-01 (work in progress), July 2008.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC5201]  Moskowitz, R., Nikander, P., Jokela, P., and T. Henderson,
              "Host Identity Protocol", RFC 5201, April 2008.

   [RFC5202]  Jokela, P., Moskowitz, R., and P. Nikander, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 5202, April 2008.

   [RFC5203]  Laganier, J., Koponen, T., and L. Eggert, "Host Identity
              Protocol (HIP) Registration Extension", RFC 5203,
              April 2008.

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   [RFC5206]  Nikander, P., Henderson, T., Vogt, C., and J. Arkko, "End-
              Host Mobility and Multihoming with the Host Identity
              Protocol", RFC 5206, April 2008.

Authors' Addresses

   Tobias Heer (editor)
   Distributed Systems Group, RWTH Aachen University
   Ahornstrasse 55
   Aachen  52062

   Phone: +49 241 80 214 36

   Klaus Wehrle
   Distributed Systems Group, RWTH Aachen University
   Ahornstrasse 55
   Aachen  52062

   Phone: +49 241 80 214 30

   Miika Komu
   Helsinki Institute for Information Technology
   Metsanneidonkuja 4

   Phone: +358503841531
   Fax:   +35896949768

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