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Versions: (draft-moskowitz-hip-rg-dex) 00 01 02 03 04 05 draft-ietf-hip-dex

Network Working Group                                  R. Moskowitz, Ed.
Internet-Draft                                            HTT Consulting
Intended status: Standards Track                               R. Hummen
Expires: December 21, 2015                           COMSYS, RWTH Aachen
                                                           June 19, 2015


                        HIP Diet EXchange (DEX)
                       draft-moskowitz-hip-dex-03

Abstract

   This document specifies the Host Identity Protocol Diet EXchange (HIP
   DEX), a variant of the Host Identity Protocol Version 2 (HIPv2).  The
   HIP DEX protocol design aims at reducing the overhead of the employed
   cryptographic primitives by omitting public-key signatures and hash
   functions.  In doing so, the main goal is to still deliver similar
   security properties to HIPv2.

   The HIP DEX protocol is primarily designed for computation or memory-
   constrained sensor/actuator devices.  Like HIPv2, it is expected to
   be used together with a suitable security protocol such as the
   Encapsulated Security Payload (ESP) for the protection of upper layer
   protocol data.  In addition, HIP DEX can also be used as a keying
   mechanism for security primitives at the MAC layer, e.g., for IEEE
   802.15.4 networks.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on December 21, 2015.








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Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  The HIP Diet EXchange (DEX) . . . . . . . . . . . . . . .   4
     1.2.  Memo Structure  . . . . . . . . . . . . . . . . . . . . .   5
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   5
     2.2.  Notation  . . . . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Definitions . . . . . . . . . . . . . . . . . . . . . . .   6
   3.  Host Identity (HI) and its Structure  . . . . . . . . . . . .   7
     3.1.  Host Identity Tag (HIT) . . . . . . . . . . . . . . . . .   8
     3.2.  Generating a HIT from an HI . . . . . . . . . . . . . . .   8
   4.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Creating a HIP Association  . . . . . . . . . . . . . . .   9
       4.1.1.  HIP Puzzle Mechanism  . . . . . . . . . . . . . . . .  10
       4.1.2.  HIP State Machine . . . . . . . . . . . . . . . . . .  11
       4.1.3.  HIP DEX Security Associations . . . . . . . . . . . .  14
       4.1.4.  User Data Considerations  . . . . . . . . . . . . . .  15
   5.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Payload Format  . . . . . . . . . . . . . . . . . . . . .  15
     5.2.  HIP Parameters  . . . . . . . . . . . . . . . . . . . . .  15
       5.2.1.  HIT_SUITE_LIST  . . . . . . . . . . . . . . . . . . .  16
       5.2.2.  DH_GROUP_LIST . . . . . . . . . . . . . . . . . . . .  16
       5.2.3.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .  17
       5.2.4.  HIP_CIPHER  . . . . . . . . . . . . . . . . . . . . .  17
       5.2.5.  ENCRYPTED_KEY . . . . . . . . . . . . . . . . . . . .  17
     5.3.  HIP Packets . . . . . . . . . . . . . . . . . . . . . . .  18
       5.3.1.  I1 - the HIP Initiator Packet . . . . . . . . . . . .  19
       5.3.2.  R1 - the HIP Responder Packet . . . . . . . . . . . .  20
       5.3.3.  I2 - the Second HIP Initiator Packet  . . . . . . . .  22
       5.3.4.  R2 - the Second HIP Responder Packet  . . . . . . . .  23
     5.4.  ICMP Messages . . . . . . . . . . . . . . . . . . . . . .  24
   6.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  24



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     6.1.  Solving the Puzzle  . . . . . . . . . . . . . . . . . . .  24
     6.2.  HIP_MAC Calculation and Verification  . . . . . . . . . .  25
       6.2.1.  CMAC Calculation  . . . . . . . . . . . . . . . . . .  25
     6.3.  HIP DEX KEYMAT Generation . . . . . . . . . . . . . . . .  26
     6.4.  Initiation of a HIP Diet EXchange . . . . . . . . . . . .  29
     6.5.  Processing Incoming I1 Packets  . . . . . . . . . . . . .  29
     6.6.  Processing Incoming R1 Packets  . . . . . . . . . . . . .  29
     6.7.  Processing Incoming I2 Packets  . . . . . . . . . . . . .  29
     6.8.  Processing Incoming R2 Packets  . . . . . . . . . . . . .  32
     6.9.  Processing UPDATE, NOTIFY, CLOSE, and CLOSE_ACK Packets .  33
     6.10. Handling State Loss . . . . . . . . . . . . . . . . . . .  33
   7.  HIP Policies  . . . . . . . . . . . . . . . . . . . . . . . .  33
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  34
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  35
   11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  35
     11.1.  Changes in draft-moskowitz-hip-rg-dex-06 . . . . . . . .  35
     11.2.  Changes in draft-moskowitz-hip-dex-00  . . . . . . . . .  35
     11.3.  Changes in draft-moskowitz-hip-dex-01  . . . . . . . . .  35
     11.4.  Changes in draft-moskowitz-hip-dex-02  . . . . . . . . .  36
     11.5.  Changes in draft-moskowitz-hip-dex-03  . . . . . . . . .  36
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  36
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  36
     12.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Appendix A.  Password-based two-factor authentication    during
                the HIP DEX handshake  . . . . . . . . . . . . . . .  39
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  39

1.  Introduction

   This document specifies the Host Identity Protocol Diet EXchange (HIP
   DEX).  HIP DEX builds on the Base EXchange (BEX) of the Host Identity
   Protocol Version 2 (HIPv2) [RFC7401].  HIP DEX preserves the protocol
   semantics as well as the general packet structure of HIPv2.  Hence,
   it is recommended that [RFC7401] is well-understood before reading
   this document.

   The main differences between HIP BEX and HIP DEX are:

   1.  Minimum collection of cryptographic primitives to reduce the
       protocol overhead.

       *  Static Elliptic Curve Diffie-Hellman key pairs for peer
          authentication and encryption of the session key.

       *  AES-CTR for symmetric encryption and AES-CMAC for MACing
          function.




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       *  A simple fold function for HIT generation.

   2.  Forfeit of Perfect Forward Secrecy with the dropping of ephemeral
       Diffie-Hellman.

   3.  Forfeit of digital signatures with the removal of a hash
       function.  Reliance on ECDH derived key used in HIP_MAC to prove
       ownership of the private key.

   4.  Diffie-Hellman derived key ONLY used to protect the HIP packets.
       A separate secret exchange within the HIP packets creates the
       session key(s).

   5.  Optional retransmission strategy tailored to handle the
       potentially extensive processing time of the employed
       cryptographic operations on computationally constrained devices.

   By eliminating the need for public-key signatures and the ephemeral
   DH key agreement, HIP DEX reduces the computation, energy,
   transmission, and memory requirements for public-key cryptography
   (see [LN08]) in the HIPv2 protocol design.  Moreover, by dropping the
   cryptographic hash function, HIP DEX affords a more efficient
   protocol implementation than HIP BEX with respect to the
   corresponding computation and memory requirements.  This makes HIP
   DEX especially suitable for constrained devices as defined in
   [RFC7228].

   In this document, we focus on the protocol specifications related to
   these differences.  Where differences are not called out explicitly,
   HIP DEX is the same as specified in [RFC7401].

1.1.  The HIP Diet EXchange (DEX)

   The HIP Diet EXchange is a two-party cryptographic protocol used to
   establish a secure communication context between hosts.  The first
   party is called the Initiator and the second party the Responder.
   The four-packet exchange helps to make HIP DoS resilient.  The
   Initiator and the Responder exchange their static Elliptic Curve
   Diffie-Hellman (ECDH) keys in the 2nd and 3rd handshake packet.  The
   parties then authenticate each other in the 3rd and 4th handshake
   packet based on the ECDH-derived keying material.  The Initiator and
   the Responder additionally transmit keying material for the session
   key in these last two handshake packets.  This is to prevent overuse
   of the static ECDH-derived keying material.  Moreover, the Responder
   starts a puzzle exchange in the 2nd packet, with the Initiator
   completing it in the 3rd packet before the Responder performs
   computationally expensive operations or stores any state from the
   exchange.  Hence, HIP DEX operationally is very similar to HIP BEX.



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   The model is also fairly equivalent to 802.11-2007 [IEEE.802-11.2007]
   Master Key and Pair-wise Transient Key, but handled in a single
   exchange.

   HIP DEX does not have the option to encrypt the Host Identity of the
   Initiator in the 3rd packet.  The Responder's Host Identity also is
   not protected.  Thus, contrary to HIPv2, there is no attempt at
   anonymity.

   Data packets start to flow after the 4th packet.  The 3rd and 4th HIP
   packets may carry a data payload in the future.  However, the details
   of this may be defined later.

   An existing HIP association can be updated with the update mechanism
   defined in [RFC7401].  Likewise, the association can be torn down
   with the defined closing mechanism for HIPv2 if it is no longer
   needed.  HIP DEX thereby omits the HIP_SIGNATURE parameters of the
   original HIPv2 specification.

   Finally, HIP DEX is designed as an end-to-end authentication and key
   establishment protocol.  As such, it can be used in combination with
   Encapsulated Security Payload (ESP) [RFC7402] as well as with other
   end-to-end security protocols.  In addition, HIP DEX can also be used
   as a keying mechanism for security primitives at the MAC layer, e.g.,
   for IEEE 802.15.4 networks [IEEE.802-15-4.2011].  It is worth
   mentioning that the HIP DEX base protocol does not cover all the
   fine-grained policy control found in Internet Key Exchange Version 2
   (IKEv2) [RFC5996] that allows IKEv2 to support complex gateway
   policies.  Thus, HIP DEX is not a replacement for IKEv2.

1.2.  Memo Structure

   The rest of this memo is structured as follows.  Section 2 defines
   the central keywords, notation, and terms used throughout the rest of
   the document.  Section 3 defines the structure of the Host Identity
   and its various representations.  Section 4 gives an overview of the
   HIP Diet EXchange protocol.  Sections 5 and 6 define the detailed
   packet formats and rules for packet processing.  Finally, Sections 7,
   8, and 9 discuss policy, security, and IANA considerations,
   respectively.

2.  Terms and Definitions

2.1.  Requirements 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 RFC 2119 [RFC2119].



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2.2.  Notation

   [x]   indicates that x is optional.

   {x}   indicates that x is encrypted.

   X(y)   indicates that y is a parameter of X.

   <x>i   indicates that x exists i times.

   -->   signifies "Initiator to Responder" communication (requests).

   <--   signifies "Responder to Initiator" communication (replies).

   |  signifies concatenation of information-- e.g., X | Y is the
      concatenation of X and Y.

   FOLD (X, K)   denotes the partitioning of X into n K-bit fragments
      and the iterative folding of these fragments via XOR.  The last
      fragment thereby is padded to K bit by appending 0 bits.  Hence, X
      = x_1, x_2, ..., x_n, where x_i is of length K and x_n is padded
      to length K by appending 0 bits.  FOLD then is computed as FOLD(X,
      K) = t_n, where t_i = t_i-1 XOR x_i and t_1 = x_1.

   Ltrunc (M(x), K)   denotes the lowest order K bits of the result of
      the MAC function M on the input x.

2.3.  Definitions

   HIP Diet Exchange (DEX):  The ECDH-based HIP handshake for
      establishing a new HIP association.

   Host Identity (HI):  The static ECDH public key that represents the
      identity of the host.  In HIP DEX, a host proves ownership of the
      private key belonging to its HI by creating a HIP_MAC with the
      derived ECDH key (c.f.  Section 3).

   Host Identity Tag (HIT):  A shorthand for the HI in IPv6 format.  It
      is generated by folding the HI (c.f.  Section 3).

   HIT Suite:  A HIT Suite groups all algorithms that are required to
      generate and use an HI and its HIT.  In particular, these
      algorithms are: 1) ECDH and 2) FOLD.

   HIP association:  The shared state between two peers after completion
      of the DEX.





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   Initiator:  The host that initiates the DEX.  This role is typically
      forgotten once the DEX is completed.

   Responder:  The host that responds to the Initiator in the DEX.  This
      role is typically forgotten once the DEX is completed.

   Responder's HIT Hash Algorithm (RHASH):  In HIP DEX, RHASH is
      redefined as CMAC.  Still, note that CMAC is a message
      authentication code and not a cryptographic hash function.  Thus,
      a mapping from CMAC(x,y) to RHASH(z) must be defined where RHASH
      is used.  Moreover, RHASH has different security properties in HIT
      DEX and is not used for HIT generation.

   Length of the Responder's HIT Hash Algorithm (RHASH_len):  The
      natural output length of RHASH in bits.

   CKDF:  CMAC-based Key Derivation Function.

3.  Host Identity (HI) and its Structure

   In this section, the properties of the Host Identity and Host
   Identity Tag are discussed, and the exact format for them is defined.
   In HIP, the public key of an asymmetric key pair is used as the Host
   Identity (HI).  Correspondingly, the host itself is defined as the
   entity that holds the private key of the key pair.  See the HIP
   architecture specification [I-D.ietf-hip-rfc4423-bis] for more
   details on the difference between an identity and the corresponding
   identifier.

   HIP DEX implementations MUST support the Elliptic Curve Diffie-
   Hellman (ECDH) [RFC6090] key exchange for generating the HI as
   defined in Section 5.2.3.  No additional algorithms are supported at
   this time.

   A compressed encoding of the HI, the Host Identity Tag (HIT), is used
   in the handshake to represent the Host Identity.  The DEX Host
   Identity Tag (HIT) is different from the BEX HIT in two ways:

   o  The HIT suite ID MUST only be a DEX HIT ID (see Section 5.2.1).

   o  The DEX HIT is not generated via a cryptographic hash.  Rather, it
      is a compression of the Host Identity.

   Due to the latter property, an attacker may be able to find a
   collision with a HIT that is in use.  Hence, policy decisions such as
   access control MUST NOT be based solely on the HIT.  Instead, the HI
   of a host SHOULD be considered.




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   Carrying HIs and HITs in the header of user data packets would
   increase the overhead of packets.  Thus, it is not expected that
   these parameters are carried in every packet, but other methods are
   used to map the data packets to the corresponding HIs.  In some
   cases, this allows to use HIP DEX without any additional headers in
   the user data packets.  For example, if ESP is used to protect data
   traffic, the Security Parameter Index (SPI) carried in the ESP header
   can be used to map the encrypted data packet to the correct HIP DEX
   association.

3.1.  Host Identity Tag (HIT)

   With HIP DEX, the Host Identity Tag is a 128-bit value - a
   compression of the HI prepended with a specific prefix.  There are
   two advantages of using a hashed encoding over the actual variable-
   sized Host Identity public key in protocols.  First, the fixed length
   of the HIT keeps packet sizes manageable and eases protocol coding.
   Second, it presents a consistent format for the protocol, independent
   of the underlying identity technology in use.

   The structure of the HIT is based on RFC 7343 [RFC7343], called
   Overlay Routable Cryptographic Hash Identifiers (ORCHIDs), and
   consists of three parts: first, an IANA assigned prefix to
   distinguish it from other IPv6 addresses.  Second, a four-bit
   encoding of the algorithms that were used for generating the HI and
   the compressed representation of HI.  Third, a 96-bit hashed
   representation of the Host Identity.  In contrast to HIPv2, HIP DEX
   employs HITs that are NOT generated by means of a cryptographic hash.
   Instead, the HI is compressed to 96 bits as defined in the following
   section.

3.2.  Generating a HIT from an HI

   The HIT does not follow the exact semantics of an ORCHID as there is
   no hash function in HIP DEX.  Still, its structure is strongly
   aligned with the ORCHID design.  The same IPv6 prefix used in BEX is
   used for DEX.  The DEX HIT suite (see Section 9) is used for the four
   bits of the Orchid Generation Algorithm (OGA) field in the ORCHID.
   The hash representation in an ORCHID is replaced with FOLD(HI,96).

4.  Protocol Overview

   This section gives a simplified overview of the HIP DEX protocol
   operation and does not contain all the details of the packet formats
   or the packet processing steps.  Section 5 and Section 6 describe
   these aspects in more detail and are normative in case of any
   conflicts with this section.  Importantly, the information given in




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   this section focuses on the differences between the HIPv2 and HIP DEX
   protocol specifications.

4.1.  Creating a HIP Association

   By definition, the system initiating a HIP Diet EXchange is the
   Initiator, and the peer is the Responder.  This distinction is
   typically forgotten once the base exchange completes, and either
   party can become the Initiator in future communications.

   The HIP Diet EXchange serves to manage the establishment of state
   between an Initiator and a Responder.  The first packet, I1,
   initiates the exchange, and the last three packets, R1, I2, and R2,
   constitute an authenticated Diffie-Hellman [DH76] key exchange for
   the Master Key SA generation.  This Master Key SA is used by the
   Initiator and the Responder to wrap secret keying material in the I2
   and R2 packets.  Based on the exchanged keying material, the peers
   then derive a Pair-wise Key SA if cryptographic keys are needed,
   e.g., for an ESP-based protection of user data.

   The Initiator first sends a trigger packet, I1, to the Responder.
   The packet contains the HIT of the Initiator and possibly the HIT of
   the Responder, if it is known.  Moreover, the I1 packet initializes
   the negotiation of the Diffie-Hellman group that is used for
   generating the the Master Key SA.  Therefore, the I1 packet contains
   a list of Diffie Hellman Group IDs supported by the Initiator.  Note
   that in some cases it may be possible to replace this trigger packet
   by some other form of a trigger, in which case the protocol starts
   with the Responder sending the R1 packet.  In such cases, another
   mechanism to convey the Initiator's supported DH Groups (e.g., by
   using a default group) must be specified.

   The second packet, R1, starts the actual authenticated Diffie-Hellman
   exchange.  It contains a puzzle -- a cryptographic challenge that the
   Initiator must solve before continuing the exchange.  The level of
   difficulty of the puzzle can be adjusted based on level of trust with
   the Initiator, current load, or other factors.  In addition, the R1
   contains the Responder's Diffie-Hellman parameter and lists of
   cryptographic algorithms supported by the Responder.  Based on these
   lists, the Initiator can continue, abort, or restart the base
   exchange with a different selection of cryptographic algorithms.

   In the I2 packet, the Initiator MUST display the solution to the
   received puzzle.  Without a correct solution, the I2 message is
   discarded.  The I2 also contains a key wrap parameter that carries a
   secret keying material of the Initiator.  This keying material is
   only half the final session key.  The packet is authenticated by the
   sender (Initiator) via a MAC.



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   The R2 packet acknowledges the receipt of the I2 packet and completes
   the handshake.  The R2 contains a key wrap parameter that carries the
   rest of the keying material of the Responder.  The packet is
   authenticated by the sender (Responder) via a MAC.

   The HIP DEX handshake is illustrated below.  The terms "ENC(DH,x)"
   and "ENC(DH,y)" refer to the random values x and y that are wrapped
   based on the Master Key SA (indicated by ENC and DH).  Note that x
   and y each constitute half the final session key material.  The
   packets also contain other parameters that are not shown in this
   figure.

      Initiator                                     Responder

                  I1:
                 --------------------------------->
                                                    remain stateless
                  R1: puzzle, HI
                 <--------------------------------
   solve puzzle
   perform ECDH
   encrypt x
                  I2: solution, HI, ENC(DH,x), mac
                 --------------------------------->
                                                    check puzzle
                                                    perform ECDH
                                                    check mac
                                                    decrypt x
                                                    encrypt y
                  R2: ENC(DH,y), mac
                 <---------------------------------
   check mac
   decrypt y


4.1.1.  HIP Puzzle Mechanism

   The purpose of the HIP puzzle mechanism is to protect the Responder
   from a number of denial-of-service threats.  It allows the Responder
   to delay state creation until receiving the I2 packet.  Furthermore,
   the puzzle allows the Responder to use a fairly cheap calculation to
   check that the Initiator is "sincere" in the sense that it has
   churned enough CPU cycles in solving the puzzle.

   The puzzle mechanism enables a Responder to immediately reject an I2
   packet if it does not contain a valid puzzle solution.  To verify the
   puzzle solution, the Responder only has to compute a single CMAC
   operation.  After a successful puzzle verification, the Responder can



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   securely create session-specific state and perform CPU-intensive
   operations such as a Diffie-Hellman key generation.  By varying the
   difficulty of the puzzle, the Responder can frustrate CPU or memory
   targeted DoS attacks.  Under normal network conditions, the puzzle
   difficulty SHOULD be zero, thus effectively reverting the puzzle
   mechanism to a cookie-based DoS protection mechanism.  Without
   setting the puzzle difficulty to zero under normal network
   conditions, potentially scarce computation resources at the Initiator
   would be churned unnecessarily.

   Conceptually, the puzzle mechanism in HIP DEX is the same as in
   HIPv2.  Hence, this document refers to Sections 4.1.1 and 4.1.2 in
   [RFC7401] for more detailed information about the employed mechanism.
   Notably, the only difference between the puzzle mechanism in HIP DEX
   and HIPv2 is that HIP DEX uses CMAC instead of a hash function for
   solving and verifying a puzzle.  The implications of this change on
   the puzzle implementation are discussed in Section 6.1.

4.1.2.  HIP State Machine

   The HIP DEX state machine has the same states as the BEX state
   machine (see 4.4. in [RFC7401]).  However, HIP DEX features an
   retransmission strategy with an optional packet receipt for the I2.
   The goal of this packet receipt is reducing premature I2
   retransmissions in sensor networks with low computation resources and
   high packet loss [HWZ13].  As a result, there are minor changes to
   the transitioning steps between specific states.  The following
   section documents these differences in the HIP DEX state machine
   compared to HIP BEX.

4.1.2.1.  HIP DEX Retransmission Mechanism

   For the retransmission of I1 and I2 packets, the Initiator adopts the
   retransmission strategy of HIP BEX (see Section 4.4.3. in [RFC7401]).
   This strategy is based on a timeout value that is set to the worst-
   case anticipated round-trip time (RTT).  For each received I1 or I2,
   the Responder sends an R1 or R2, respectively.  This design trait
   enables the Responder to remain stateless until the reception of the
   I2.  The Initiator stops retransmitting I1 or I2 packets after the
   reception of the corresponding R1 or R2.  If the Initiator did not
   receive an R1 after I1_RETRIES_MAX tries, it stops I1
   retransmissions.  Likewise, it stops retransmitting I2 packets after
   I2_RETRIES_MAX unsuccessful tries.

   The Responder SHOULD NOT perform operations related to the Diffie-
   Hellman key exchange or the keying material wrapped in the
   ENCRYPTED_KEY parameters for retransmitted I2 packets.  Instead, it
   SHOULD re-use the previously established state to re-create the R2.



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   The potentially high processing time of an I2 packet at the Responder
   may cause retransmissions if the time required for I2 transmission
   and processing exceeds the RTT-based retransmission timeout.  Thus,
   the Initiator should also take the processing time of I2 packets into
   account.  To this end, the Responder MAY optionally notify the
   Initiator about the anticipated delay if the I2 incurs a considerable
   processing overhead.  The Responder MAY therefore send a NOTIFY
   packet to the Initiator before it commences the ECDH operation.  The
   NOTIFY packet serves as an acknowledgement for the I2 and consists of
   a NOTIFICATION parameter with Notify Message Type I2_ACKNOWLEDGEMENT
   (see Section 5.2.19. in [RFC7401]).  The NOTIFICATION parameter
   contains the anticipated remaining processing time for the I2 packet
   in milliseconds as Notification Data . This processing time can,
   e.g., be estimated by measuring the computation time of the ECDH key
   derivation operation at Responder boot-up.  After the I2 processing
   has finished, the Responder sends the regular R2.

   When the Initiator receives the NOTIFY packet, it resets the I2
   retransmission timer to the processing time indicated by the
   Responder in the NOTIFICATION parameter.  If the indicated processing
   time is shorter than the RTT-based timeout, the Initiator MUST set
   the retransmission timer to the RTT-based timeout.  Additionally, the
   Initiator MUST NOT set a higher retransmission timeout than allowed
   by a local policy.  Hence, I2 retransmissions are never triggered in
   shorter succession than without this optional retransmission
   extension.  Moreover, there is a defined upper bound to which
   unauthenticated NOTIFY messages can delay the handshake in case of
   lost R2 packets.

4.1.2.2.  HIP State Processes

   HIP DEX clarifies or introduces the following new transitions.



















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   System behavior in state I2-SENT, Table 1.

   +---------------------+---------------------------------------------+
   | Trigger             | Action                                      |
   +---------------------+---------------------------------------------+
   | Receive NOTIFY,     | Set I2 retransmission timer according to    |
   | process             | NOTIFY payload and stay at I2-SENT          |
   |                     |                                             |
   | Timeout             | Increment timeout counter                   |
   |                     |                                             |
   |                     | If counter is less than I2_RETRIES_MAX,     |
   |                     | send I2, reset timer to RTT and stay at     |
   |                     | I2-SENT                                     |
   |                     |                                             |
   |                     | If counter is greater than I2_RETRIES_MAX,  |
   |                     | go to E-FAILED                              |
   +---------------------+---------------------------------------------+

        Table 1: I2-SENT - Waiting to finish the HIP Diet EXchange

4.1.2.3.  Simplified HIP State Diagram

   The following diagram shows the major state transitions.  Transitions
   based on received packets implicitly assume that the packets are
   successfully authenticated or processed.

                             +-+        +------------------------------+
        I1 received, send R1 | |        |                              |
                             | v        v                              |
         Datagram to send  +--------------+  I2 received, send R2      |
   Send I1  +--------------| UNASSOCIATED |--------------+             |
        +-+ | +-+          +--------------+              |             |
   send | | | | |                                        |             |
   I1 t | | | | | Alg. not supported, send I1            |             |
   msec v | v | v                                        |             |
       +---------+  I2 received, send R2                 |             |
 +---->| I1-SENT |-------------------------------------+ |             |
 |     +---------+                                     | |             |
 |            |               +----------------------+ | | +-+receive  |
 | send I2+-+ | R1 received,  | I2 received, send R2 | | | | |I2,      |
 | t msec | v v send I2       |                      v v v | v send R2 |
 |     +---------+            |                     +---------+        |
 |  +->| I2-SENT |------------+                     | R2-SENT |<--+    |
 |  |  +---------+                                  +---------+   |    |
 |  |          |                                     |            |    |
 |  |          |                                 data|            |    |
 |  |receive   |                                   or|            |    |
 |  |R1, send  |                           EC timeout| receive I2,|    |



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 |  |I2        |R2 received +--------------+         |     send R2|    |
 |  |          +----------->| ESTABLISHED  |<--------+            |    |
 |  |                       +--------------+                      |    |
 |  |                         |    |     |  receive I2, send R2   |    |
 |  |        recv+------------+    |     +------------------------+    |
 |  |      CLOSE,|                 |                              |    |
 |  |        send|   No packet sent|                              |    |
 |  |   CLOSE_ACK|   /received for |                   timeout    |    |
 |  |            |   UAL min, send |    +---------+<-+ (UAL+MSL)  |    |
 |  |            |           CLOSE +--->| CLOSING |--+ retransmit |    |
 |  |            |                      +---------+    CLOSE      |    |
 +--|------------|----------------------+| |  | |                 |    |
    +------------|-----------------------+ |  | +-----------------+    |
    |            |             +-----------+  +-------------------|----+
    |            +-----------+ | receive CLOSE,   CLOSE_ACK       |    |
    |                        | | send CLOSE_ACK   received or     |    |
    |                        | |                  timeout         |    |
    |                        | |                  (UAL+MSL)       |    |
    |                        v v                                  |    |
    |                       +--------+  receive I2, send R2       |    |
    +-----------------------| CLOSED |----------------------------+    |
                            +--------+       /-------------------------+
                              ^ |   \-------/  timeout (UAL+2MSL),
                              | |              move to UNASSOCIATED
                              +-+
               CLOSE received, send CLOSE_ACK


4.1.3.  HIP DEX Security Associations

   HIP DEX establishes two Security Associations (SA), one for the
   Diffie-Hellman derived key, or Master Key, and one for the session
   key, or Pair-wise Key.

4.1.3.1.  Master Key SA

   The Master Key SA is used to authenticate HIP packets and to encrypt
   selected HIP parameters in HIP DEX packet exchanges.  Since only
   little data is protected by this SA, it can be long-lived with no
   need for rekeying.

   The Master Key SA contains the following elements:

   o  Source HIT

   o  Destination HIT

   o  HIP_Encrypt Key



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   o  HIP_MAC Key

   The HIP_Encrypt and HIP_MAC keys are extracted from the Diffie-
   Hellman derived key as described in Section 6.3.  Their length is
   determined by the HIP_CIPHER.

4.1.3.2.  Pair-wise Key SA

   The Pair-wise Key SA is used to authenticate and to encrypt user
   data.  It is refreshed (or rekeyed) using an UPDATE packet exchange.
   The Pair-wise Key SA elements are defined by the data transform (e.g.
   ESP_TRANSFORM [RFC7402]).

   The keys for the Pair-wise Key SA are derived based on the wrapped
   keying material exchanged in the ENCRYPTED_KEY parameter (see
   Section 5.2.5) of the I2 and R2 packets.  Specifically, the exchanged
   keying material of the two peers is concatenated.  This concatenation
   forms the input to a Key Derivation Function (KDF).  If the data
   transform does not specify its own KDF, the key derivation function
   defined in Section 6.3 is used.  Even though this input is randomly
   distributed, a KDF Extract phase may be needed to get the proper
   length for the input to the KDF Expand phase.

4.1.4.  User Data Considerations

   The User Data Considerations in Section 4.5. of [RFC7401] also apply
   to HIP DEX.  There is only one difference between HIPv2 and HIP DEX.
   Loss of state due to system reboot may be a critical performance
   issue for constrained sensor/actuator devices.  Thus, implementors
   MAY choose to use non-volatile, secure storage for HIP states in
   order for them to survive a system reboot.  This will limit state
   loss during reboots to only those situations with an SA timeout.

5.  Packet Formats

5.1.  Payload Format

   HIP DEX employs the same fixed HIP header and payload structure as
   HIP BEX.  As such, the specifications in Section 5.1 of [RFC7401]
   also apply to HIP DEX.

5.2.  HIP Parameters

   The HIP parameters carry information that is necessary for
   establishing and maintaining a HIP association.  For example, the
   peer's public keys as well as the signaling for negotiating ciphers
   and payload handling are encapsulated in HIP parameters.  Additional
   information, meaningful for end-hosts or middleboxes, may also be



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   included in HIP parameters.  The specification of the HIP parameters
   and their mapping to HIP packets and packet types is flexible to
   allow HIP extensions to define new parameters and new protocol
   behavior.

   In HIP packets, HIP parameters are ordered according to their numeric
   type number and encoded in TLV format.

   HIP DEX reuses the HIP parameters of HIP BEX defined in Section 5.2.
   of [RFC7401] where possible.  Still, HIP DEX further restricts and/or
   extends the following existing parameter types:

   o  HIT_SUITE_LIST is limited to HIT suite ECDH/FOLD.

   o  DH_GROUP_LIST and HOST_ID are restricted to ECC-based suites.

   o  HIP_CIPHER is restricted to NULL-ENCRYPT and AES-128-CTR.

   o  RHASH and RHASH_len are redefined to CMAC for PUZZLE, SOLUTION,
      HIP_MAC (see Section 6.1 and Section 6.2).

   In addition, HIP DEX introduces the following new parameter:

   +------------------+------+----------+------------------------------+
   | TLV              | Type | Length   | Data                         |
   +------------------+------+----------+------------------------------+
   | ENCRYPTED_KEY    | 643  | variable | Encrypted container for key  |
   |                  |      |          | generation exchange          |
   +------------------+------+----------+------------------------------+

5.2.1.  HIT_SUITE_LIST

   The HIT_SUITE_LIST parameter contains a list of the supported HIT
   suite IDs of the Responder.  The HIT suites in DEX are limited to:

        HIT suite              ID
        ECDH/FOLD               8

   Since the HIT of the Initiator is a DEX HIT, the Responder MUST only
   respond with a DEX HIT suite ID.

5.2.2.  DH_GROUP_LIST

   The DH_GROUP_LIST parameter contains the list of supported DH Group
   IDs of a host.  The following ECC curves are supported in HIP DEX:






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        Group                              KDF              Value
        NIST P-256 [RFC5903]               CKDF             7
        NIST P-384 [RFC5903]               CKDF             8
        NIST P-521 [RFC5903]               CKDF             9
        SECP160R1  [SECG]                  CKDF             10

   The ECDH groups 7 - 9 are defined in [RFC5903] and [RFC6090].  ECDH
   group 10 is covered in [SECG].

5.2.3.  HOST_ID

   The HI Algorithms in DEX are limited to:

        Algorithm
        profiles         Values
        ECDH             1

   ECC-based Host Identities are serialized as described in
   Section 5.2.9. of [RFC7401].  The supported curves for the HI in HIP
   DEX are defined in Section 5.2.2.

5.2.4.  HIP_CIPHER

   The HIP ciphers in DEX are limited to:

        Suite ID           Value

        RESERVED           0
        NULL-ENCRYPT       1     ([RFC2410])
        AES-128-CTR        5     ([RFC3686])

   Mandatory implementation: AES-128-CTR.  NULL-ENCRYPTION [RFC2410] is
   included for testing purposes.

5.2.5.  ENCRYPTED_KEY
















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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            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     /                        Encrypted value                        /
     /                                                               /
     /                               +-------------------------------+
     /                               |            Padding            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     Type           643
     Length         length in octets, excluding Type, Length, and
                    Padding
     Encrypted      The value is encrypted using an encryption algorithm
     value          as defined in the HIP_CIPHER parameter.

   The ENCRYPTED_KEY parameter encapsulates a random value that is later
   used in the session key creation process (see Section 6.3).  This
   random value MUST have a length of at least 64 bit.  The puzzle value
   #I and the puzzle solution #J (see [RFC7401]) is used as the
   initialization vector (IV) for the encryption process.  To this end,
   the IV is computed as FOLD(I | J, 128).  The AES-CTR counter is a 16
   bit value that is initialized to zero with the first use.

   Once this encryption process is completed, the "encrypted value" data
   field is ready for inclusion in the Parameter.  If necessary,
   additional Padding for 8-byte alignment is then added according to
   the rules of TLV Format in [RFC7401].

5.3.  HIP Packets

   DEX uses the same eight basic HIP packets as in BEX (see [RFC7401]).
   Four are for the HIP handshake (I1, R1, I2, and R2), one is for
   updating (UPDATE), one is for sending notifications (NOTIFY), and two
   are for closing a HIP association (CLOSE and CLOSE_ACK).  There are
   some differences regarding the included HIP parameters in the
   exchange packets of BEX and DEX.  This section covers these
   differences for the DEX packets.  Packets not discussed here, follow
   the structure defined in [RFC7401].

   An important difference between packets in HIP BEX and HIP DEX is
   that the DIFFIE_HELLMAN and the HIP_SIGNATURE parameters are not
   included in DEX.  Thus, the R1 packet is completely unprotected and
   can be spoofed.  As a result, negotiation parameters contained in the
   R1 packet have to be re-included in later, protected packets in order
   to detect and prevent potential downgrading attacks.  Moreover, the
   I2, R2, UPDATE, NOTIFY, CLOSE, and CLOSE_ACK packets are not covered



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   by a signature and purely rely on the HIP_MAC parameter for packet
   authentication.  The processing of these packets is changed
   accordingly.

   In the future, an OPTIONAL upper-layer payload MAY follow the HIP
   header.  The Next Header field in the header indicates if there is
   additional data following the HIP header.

5.3.1.  I1 - the HIP Initiator Packet

   The HIP header values for the I1 packet:

     Header:
       Packet Type = 1
       SRC HIT = Initiator's HIT
       DST HIT = Responder's HIT, or NULL

     IP ( HIP ( DH_GROUP_LIST ) )

     Minimum size = 48 bytes

   Valid control bits: none

   The I1 packet contains the fixed HIP header and the Initiator's
   DH_GROUP_LIST.  The Initiator's HIT Suite ID MUST be of a HIP DEX
   type as defined in Section 5.2.1.

   Regarding the Responder's HIT, the Initiator may receive this HIT
   either from a DNS lookup of the Responder's FQDN, from some other
   repository, or from a local table.  The Responder's HIT also MUST be
   of a HIP DEX type.  If the Initiator does not know the Responder's
   HIT, it may attempt to use opportunistic mode by using NULL (all
   zeros) as the Responder's HIT.  See also "HIP Opportunistic Mode"
   [RFC7401].

   The Initiator's and the Responder's HITs both determine the DH group
   ID that must be used in order to successfully conclude the triggered
   handshake.  HITs, however, do not include a hint about the DH group
   ID of the ECDH-based Host Identity (HI).  To inform the Responder
   about its employed and its otherwise supported DH Group IDs, the
   Initiator therefore includes a DH_GROUP_LIST parameter in the I1
   packet.  This parameter MUST include the DH group ID that corresponds
   to the currently employed Initiator HIT as the first list element.
   With HIP DEX, the DH_GROUP_LIST parameter MUST only include ECDH
   groups defined in Section 5.2.2.






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   Since this packet is so easy to spoof even if it were protected, no
   attempt is made to add to its generation or processing cost.  As a
   result, the DH_GROUP_LIST in the I1 packet is not protected.

   Implementations MUST be able to handle a storm of received I1
   packets, discarding those with common content that arrive within a
   small time delta.

5.3.2.  R1 - the HIP Responder Packet

   The HIP header values for the R1 packet:

     Header:
       Packet Type = 2
       SRC HIT = Responder's HIT
       DST HIT = Initiator's HIT

     IP ( HIP ( [ R1_COUNTER, ]
                PUZZLE,
                DH_GROUP_LIST,
                HIP_CIPHER,
                HOST_ID,
                HIT_SUITE_LIST,
                TRANSPORT_FORMAT_LIST,
                [ <, ECHO_REQUEST_UNSIGNED >i ])

     Minimum size = 120 bytes

   Valid control bits: A

   If the Responder's HI is an anonymous one, the A control MUST be set.

   The Initiator's HIT MUST match the one received in the I1 packet if
   the R1 is a response to an I1.  If the Responder has multiple HIs,
   the Responder's HIT MUST match the Initiator's request.  If the
   Initiator used opportunistic mode, the Responder may select among its
   HIs as described below.  See also "HIP Opportunistic Mode" [RFC7401].

   The R1 packet generation counter is used to determine the currently
   valid generation of puzzles.  The value is increased periodically,
   and it is RECOMMENDED that it is increased at least as often as
   solutions to old puzzles are no longer accepted.

   The Puzzle contains a Random value #I and the puzzle difficulty K.
   The difficulty K indicates the number of lower-order bits, in the
   puzzle CMAC result, that MUST be zeros (see [RFC7401]).  Responders
   SHOULD set K to zero by default and only increase the puzzle
   difficulty to protect against a DoS attack targeting the DEX



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   handshake.  A puzzle difficulty of zero effectively turns the puzzle
   mechanism into a return-routablility test and is strongly encouraged
   to conserve energy resources as well as to prevent unnecessary
   handshake delay in case of a resource-constrained Initiator during
   normal operation.

   The DH_GROUP_LIST parameter contains the Responder's order of
   preference based on which it chose ECDH key contained in the HOST_ID
   parameter (see below).  This allows the Initiator to determine
   whether its own DH_GROUP_LIST in the I1 packet was manipulated by an
   attacker.  There is a further risk that the Responder's DH_GROUP_LIST
   was manipulated by an attacker, as the R1 packet cannot be
   authenticated in DEX as it can in BEX.  Thus, it is repeated in the
   R2 allowing for a final check at that point.

   The HIP_CIPHER contains the encryption algorithms supported by the
   Responder to protect the key exchange, in the order of preference.
   All implementations MUST support the AES-CTR [RFC3686].

   The HIT_SUITE_LIST parameter is an ordered list of the Responder's
   supported and preferred HIT Suites.  It enables a Responder to notify
   the Initiator about other available HIT suites than the one used in
   the current handshake.  Based on the received HIT_SUITE_LIST, the
   Initiator MAY decide to abort the current handshake and initiate a
   new handshake with a different mutually supported HIT suite.  This
   mechanism can, e.g., be used to move from an initial HIP DEX
   handshake to a HIP BEX handshake for peers supporting both protocol
   variants.

   The HOST_ID parameter depends on the received DH_GROUP_LIST parameter
   and the Responder HIT in the I1 packet.  Specifically, if the I1
   contains a Responder HIT, the Responder verifies that this HIT
   matches the required DH group based on the received DH_GROUP_LIST
   parameter.  In case of a positive result, the Responder then selects
   the corresponding HOST_ID for inclusion in the R1 packet.  Likewise,
   if the Responder HIT in the I1 packet is NULL (i.e., during an
   opportunistic handshake), the Responder chooses its HOST_ID according
   to the Initiator's employed DH group as indicated in the received
   DH_GROUP_LIST parameter and sets the source HIT in the R1 packet
   accordingly.  If the Responder however does not support the DH group
   required by the Initiator or if the Responder HIT in the I1 packet
   does not match the required DH group, the Responder selects the
   mutually preferred and supported DH group based on the DH_GROUP_LIST
   parameter of the I1.  The Responder then includes the corresponding
   ECDH key in the HOST_ID parameter.  This parameter also indicates the
   selected DH group.  Moreover, the Responder sets the source HIT in
   the R2 based on the destination HIT from the I1 packet.  Based on the
   deviating DH group ID in the HOST_ID parameter, the Initiator then



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   SHOULD abort the current handshake and initiate a new handshake with
   the mutually supported DH group as far as local policies (see
   Section 7) permit.

   The TRANSPORT_FORMAT_LIST parameter is an ordered list of the
   Responder's supported and preferred transport format types.  The list
   allows the Initiator and the Responder to agree on a common type for
   payload protection.  Currently, the only transport format defined is
   IPsec ESP [RFC7402].

   The ECHO_REQUEST_UNSIGNED parameters contain data that the sender
   wants to receive unmodified in the corresponding response packet in
   the ECHO_RESPONSE_UNSIGNED parameter.  The R1 packet may contain zero
   or more ECHO_REQUEST_UNSIGNED parameters.

5.3.3.  I2 - the Second HIP Initiator Packet

   The HIP header values for the I2 packet:

     Header:
       Type = 3
       SRC HIT = Initiator's HIT
       DST HIT = Responder's HIT

     IP ( HIP ( [R1_COUNTER,]
                SOLUTION,
                HIP_CIPHER,
                ENCRYPTED_KEY,
                HOST_ID,
                TRANSPORT_FORMAT_LIST,
                HIP_MAC,
                [<, ECHO_RESPONSE_UNSIGNED>i )] )

     Minimum size = 180 bytes

   Valid control bits: A

   The HITs used MUST match the ones used in the R1.

   If the Initiator's HI is an anonymous one, the A control bit MUST be
   set.

   If present in the I1 packet, the Initiator MUST include an unmodified
   copy of the R1_COUNTER parameter received in the corresponding R1
   packet into the I2 packet.

   The Solution contains the Random #I from R1 and the computed #J.  The
   low-order #K bits of the RHASH(I | ... | J) MUST be zero.



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   The HIP_CIPHER contains the single encryption transform selected by
   the Initiator, that it uses to encrypt the ENCRYPTED and
   ENCRYPTED_KEY parameters.  The chosen cipher MUST correspond to one
   of the ciphers offered by the Responder in the R1.  All
   implementations MUST support the AES-CTR transform [RFC3686].

   The HOST_ID parameter contains the Initiator HI corresponding to the
   Initiator HIT.

   The ENCRYPTED_KEY contains an Initiator generated random value that
   MUST be uniformly distributed.  This random value is encrypted with
   the Master Key SA using the HIP_CIPHER encryption algorithm.

   The ECHO_RESPONSE_UNSIGNED contains the unmodified Opaque data copied
   from the corresponding echo request parameter(s).  This parameter can
   also be used for two-factor password authentication as shown in
   Appendix A.

   The TRANSPORT_FORMAT_LIST contains the single transport format type
   selected by the Initiator.  The chosen type MUST correspond to one of
   the types offered by the Responder in the R1.  Currently, the only
   transport format defined is the ESP transport format [RFC7402].

   The MAC is calculated over the whole HIP envelope, excluding any
   parameters after the HIP_MAC, as described in Section 6.2.  The
   Responder MUST validate the HIP_MAC.

5.3.4.  R2 - the Second HIP Responder Packet

   The HIP header values for the R2 packet:

     Header:
       Packet Type = 4
       SRC HIT = Responder's HIT
       DST HIT = Initiator's HIT

     IP ( HIP ( DH_GROUP_LIST,
                HIP_CIPHER,
                ENCRYPTED_KEY,
                HIT_SUITE_LIST,
                TRANSPORT_FORMAT_LIST,
                HIP_MAC)

     Minimum size = 108 bytes

   Valid control bits: none

   The HITs used MUST match the ones used in the I2.



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   The Responder repeats the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST,
   and TRANSPORT_FORMAT_LIST parameters in R2.  These parameters MUST be
   the same as included in R1.  The parameter are re-included here
   because R2 is MACed and thus cannot be altered by an attacker.  For
   verification purposes, the Initiator re-evaluates the selected suites
   and compares the results against the chosen ones.  If the re-
   evaluated suites do not match the chosen ones, the Initiator acts
   based on its local policy.

   The ENCRYPTED_KEY contains an Responder generated random value that
   MUST be uniformly distributed.  This random value is encrypted with
   the Master Key SA using the HIP_CIPHER encryption algorithm.

   The MAC is calculated over the whole HIP envelope, excluding any
   parameters after the HIP_MAC, as described in Section 6.2.  The
   Initiator MUST validate the HIP_MAC.

5.4.  ICMP Messages

   When a HIP implementation detects a problem with an incoming packet,
   and it either cannot determine the identity of the sender of the
   packet or does not have any existing HIP association with the sender
   of the packet, it MAY respond with an ICMP packet.  Any such reply
   MUST be rate-limited as described in [RFC4443].  In most cases, the
   ICMP packet has the Parameter Problem type (12 for ICMPv4, 4 for
   ICMPv6), with the Pointer field pointing to the field that caused the
   ICMP message to be generated.  The problem cases specified in
   Section 5.4. of [RFC7401] also apply to HIP DEX.

6.  Packet Processing

   Due to the adopted protocol semantics and the inherited general
   packet structure, packet processing in HIP DEX only differs from HIP
   BEX in very few places.  Here, we focus on these differences and
   refer to Section 6 in [RFC7401] otherwise.

   The processing of outgoing and incoming application data remains the
   same as in HIP BEX (see Sections 6.1 and 6.2 in [RFC7401]).

6.1.  Solving the Puzzle

   The procedures for solving and verifying a puzzle in HIP DEX are
   strongly based on the corresponding procedures in HIPv2.  The only
   exceptions are that HIP DEX does not use pre-computation of R1
   packets and that RHASH is set to CMAC.  As a result, the pre-
   computation step in in Section 6.3 of [RFC7401] is skipped in HIP
   DEX.




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   Moreover, the Initiator solves a puzzle by computing:
      Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K ) == 0

   Similarly, the Responder verifies a puzzle by computing:
      V := Ltrunc( CMAC( I, HIT-I | HIT-R | J ), K )

   Apart from these modifications, the procedures defined in Section 6.3
   of [RFC7401] also apply for HIP DEX.

6.2.  HIP_MAC Calculation and Verification

   The following subsections define the actions for processing the
   HIP_MAC parameter.

6.2.1.  CMAC Calculation

   The HIP_MAC calculation uses RHASH, i.e., CMAC, as the underlying
   cryptographic function.  The scope of the calculation for HIP_MAC is:

   CMAC: { HIP header | [ Parameters ] }

   where Parameters include all HIP parameters of the packet that is
   being calculated with Type values ranging from 1 to (HIP_MAC's Type
   value - 1) and exclude parameters with Type values greater or equal
   to HIP_MAC's Type value.

   During HIP_MAC calculation, the following applies:

   o  In the HIP header, the Checksum field is set to zero.

   o  In the HIP header, the Header Length field value is calculated to
      the beginning of the HIP_MAC parameter.

   Parameter order is described in [RFC7401].

   The CMAC calculation and verification process is as follows:

   Packet sender:

   1.  Create the HIP packet, without the HIP_MAC or any other parameter
       with greater Type value than the HIP_MAC parameter has.

   2.  Calculate the Header Length field in the HIP header.

   3.  Compute the CMAC using either HIP-gl or HIP-lg integrity key
       retrieved from KEYMAT as defined in Section 6.3.





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   4.  Add the HIP_MAC parameter to the packet and any parameter with
       greater Type value than the HIP_MAC's that may follow.

   5.  Recalculate the Length field in the HIP header.

   Packet receiver:

   1.  Verify the HIP header Length field.

   2.  Remove the HIP_MAC parameter, as well as all other parameters
       that follow it with greater Type value, saving the contents if
       they will be needed later.

   3.  Recalculate the HIP packet length in the HIP header and clear the
       Checksum field (set it to all zeros).

   4.  Compute the CMAC using either HIP-gl or HIP-lg integrity key as
       defined in Section 6.3 and verify it against the received CMAC.

   5.  Set Checksum and Header Length fields in the HIP header to
       original values.

6.3.  HIP DEX KEYMAT Generation

   The HIP DEX KEYMAT process is used to derive the keys for Master Key
   SA as well as for the Pair-wise Key SA.  The keys for the Master Key
   SA are based from the Diffie-Hellman derived key, Kij, produced
   during the HIP Diet EXchange.  The Initiator generates Kij during the
   creation of the I2 packet and the Responder generates Kij once it
   receives the I2 packet.  Hence, I2, R2, UPDATE, CLOSE, and CLOSE_ACK
   packets can contain authenticated and/or encrypted information.

   The keys of the Pair-wise Key SA are not directly used in the HIP DEX
   handshake.  Instead, these keys are made available as payload
   protection keys.  Some payload protection mechanisms have their own
   Key Derivation Function, and if so this mechanism SHOULD be used.
   Otherwise, the HIP DEX KEYMAT process MUST be used to derive the keys
   of the Pair-wise Key SA based on the concatenation of the random
   values that are contained in the exchanged ENCRYPTED_KEY parameters.

   The HIP DEX KEYMAT process consists of two components, CKDF-Extract
   and CKDF-Expand.  The Extract function COMPRESSES a non-uniformly
   distributed key, as is the output of a Diffie-Hellman key derivation,
   to EXTRACT the key entropy into a fixed length output.  The Expand
   function takes either the output of the Extract function or directly
   uses a uniformly distributed key and EXPANDS the length of the key,
   repeatedly distributing the key entropy, to produce the keys needed.




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   The key derivation for the Master Key SA employs both the Extract and
   Expand phases, whereas the Pair-wise Key SA MAY need both the Extract
   and Expand phases if the key is longer than 128 bits.  Otherwise, it
   only requires the Expand phase.

   The CKDF-Extract function is the following operation:

      CKDF-Extract(I, IKM, info) -> PRK

    where

        I          Random #I from the PUZZLE parameter
        IKM        Input input keying material, i.e., either the
                   Diffie-Hellman derived key or the concatenation of
                   the random values of the ENCRYPTED_KEY parameters in
                   the same order as the HITs with sort(HIT-I | HIT-R)
        info       sort(HIT-I | HIT-R) | "CKDF-Extract"
        PRK        a pseudorandom key (of RHASH_len/8 octets)
        |          denotes the concatenation

    The pseudorandom key PRK is calculated as follows:

        PRK     = CMAC(I, IKM | info)

   The CKDF-Expand function is the following operation:


























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      CKDF-Expand(PRK, info, L) -> OKM

        PRK        a pseudorandom key of at least RHASH_len/8 octets
                   (either the output from the extract step or the
                   concatenation of the random values of the
                   ENCRYPTED_KEY parameters in the same order as the
                   HITs with sort(HIT-I | HIT-R))
        info       sort(HIT-I | HIT-R) | "CKDF-Expand"
        L          length of output keying material in octets
                   (<= 255*RHASH_len/8)
        |          denotes the concatenation

    The output keying material OKM is calculated as follows:

    N       =  ceil(L/RHASH_len/8)
    T       =  T(1) | T(2) | T(3) | ... | T(N)
    OKM     =  first L octets of T

    where

        T(0) = empty string (zero length)
        T(1) = CMAC(PRK, T(0) | info | 0x01)
        T(2) = CMAC(PRK, T(1) | info | 0x02)
        T(3) = CMAC(PRK, T(2) | info | 0x03)
        ...

    (where the constant concatenated to the end of each T(n) is a
    single octet.)

   sort(HIT-I | HIT-R) is defined as the network byte order
   concatenation of the two HITs, with the smaller HIT preceding the
   larger HIT, resulting from the numeric comparison of the two HITs
   interpreted as positive (unsigned) 128-bit integers in network byte
   order.

   The initial keys are drawn sequentially in the order that is
   determined by the numeric comparison of the two HITs, with comparison
   method described in the previous paragraph.  HOST_g denotes the host
   with the greater HIT value, and HOST_l the host with the lower HIT
   value.

   The drawing order for initial keys:

   1.  HIP-gl encryption key for HOST_g's outgoing HIP packets

   2.  HIP-gl integrity (CMAC) key for HOST_g's outgoing HIP packets

   3.  HIP-lg encryption key for HOST_l's outgoing HIP packets



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   4.  HIP-lg integrity (CMAC) key for HOST_l's outgoing HIP packets

   The number of bits drawn for a given algorithm is the "natural" size
   of the keys.  For the mandatory algorithms, the following sizes
   apply:

   AES  128 or 256 bits

   If other key sizes are used, they must be treated as different
   encryption algorithms and defined separately.

6.4.  Initiation of a HIP Diet EXchange

   The initiation of a HIP DEX handshake proceeds as described in
   Section 6.6. of [RFC7401].

6.5.  Processing Incoming I1 Packets

   I1 packets in HIP DEX are handled identically to HIP BEX (see
   Section 6.7. in [RFC7401]).  The only differences are that the
   Responder SHOULD select a DEX HIT in the R1 response.  Moreover, as
   R1 packets are neither covered by a signature nor incur the overhead
   of generating an ephemeral Diffie-Hellman key-pair, pre-computation
   of an R1 is only marginally beneficial, but would incur additional
   memory resources.  Hence, the R1 pre-computation is omitted in HIP
   DEX.

6.6.  Processing Incoming R1 Packets

   R1 packets in HIP DEX are handled identically to HIP BEX with the
   following differences (see Section 6.8. in [RFC7401]).  Only step 4
   is omitted in HIP DEX as there is no HIP_SIGNATURE in the R1 packet.

6.7.  Processing Incoming I2 Packets

   Upon receipt of an I2 packet, the system MAY perform initial checks
   to determine whether the I2 packet corresponds to a recent R1 packet
   that has been sent out, if the Responder keeps such state.  For
   example, the sender could check whether the I2 packet is from an
   address or HIT for which the Responder has recently received an I1.
   To this end, the R1 packet may have had Opaque data included that was
   echoed back in the I2 packet.  If the I2 packet is considered to be
   suspect, it MAY be silently discarded by the system.

   Otherwise, the HIP implementation SHOULD process the I2 packet.  This
   includes validation of the puzzle solution, generating the Diffie-
   Hellman key, verifying the MAC, extracting the ENCRYPTED_KEY,
   creating state, and finally sending an R2 packet.



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   The following steps define the conceptual processing rules for
   responding to an I2 packet:

   1.   The system MAY perform checks to verify that the I2 packet
        corresponds to a recently sent R1 packet.  Such checks are
        implementation dependent.  See Appendix A in [RFC7401] for a
        description of an example implementation.

   2.   The system MUST check that the Responder's HIT corresponds to
        one of its own HITs and MUST drop the packet otherwise.

   3.   The system MUST further check that the Initiator's HIT Suite is
        supported.  The Responder SHOULD silently drop I2 packets with
        unsupported Initiator HITs.

   4.   If the system's state machine is in the R2-SENT state, the
        system MUST check if the newly received I2 packet is similar to
        the one that triggered moving to R2-SENT.  If so, it MUST
        retransmit a previously sent R2 packet and the state machine
        stays in R2-SENT.

   5.   If the system's state machine is in the I2-SENT state, the
        system MUST make a comparison between its local and sender's
        HITs (similarly as in Section 6.3).  If the local HIT is smaller
        than the sender's HIT, it should drop the I2 packet, use the
        peer Diffie-Hellman key, ENCRYPTED_KEY keying material and nonce
        #I from the R1 packet received earlier, and get the local
        Diffie-Hellman key, ENCRYPTED_KEY keying material, and nonce #J
        from the I2 packet sent to the peer earlier.  Otherwise, the
        system should process the received I2 packet and drop any
        previously derived Diffie-Hellman keying material Kij and
        ENCRYPTED_KEY keying material it might have generated upon
        sending the I2 packet previously.  The peer Diffie-Hellman key,
        ENCRYPTED_KEY, and the nonce #J are taken from the just arrived
        I2 packet.  The local Diffie-Hellman key, ENCRYPTED_KEY keying
        material, and the nonce I are the ones that were sent earlier in
        the R1 packet.

   6.   If the system's state machine is in the I1-SENT state, and the
        HITs in the I2 packet match those used in the previously sent I1
        packet, the system uses this received I2 packet as the basis for
        the HIP association it was trying to form, and stops
        retransmitting I1 packets (provided that the I2 packet passes
        the additional checks below).

   7.   If the system's state machine is in any other state than R2-
        SENT, the system SHOULD check that the echoed R1 generation
        counter in the I2 packet is within the acceptable range if the



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        counter is included.  Implementations MUST accept puzzles from
        the current generation and MAY accept puzzles from earlier
        generations.  If the generation counter in the newly received I2
        packet is outside the accepted range, the I2 packet is stale
        (and perhaps replayed) and SHOULD be dropped.

   8.   The system MUST validate the solution to the puzzle as described
        in Section 6.

   9.   The I2 packet MUST have a single value in the HIP_CIPHER
        parameter, which MUST match one of the values offered to the
        Initiator in the R1 packet.

   10.  The system must derive Diffie-Hellman keying material Kij based
        on the public value and Group ID in the HOST_ID parameter.  This
        key is used to derive the keys of the Master Key SA as described
        in Section 6.3.  If the Diffie-Hellman Group ID is unsupported,
        the I2 packet is silently dropped.

   11.  The implementation SHOULD also verify that the Initiator's HIT
        in the I2 packet corresponds to the Host Identity sent in the I2
        packet.  (Note: some middleboxes may not able to make this
        verification.)

   12.  The system MUST process the TRANSPORT_FORMAT_LIST parameter.
        Other documents specifying transport formats (e.g.  [RFC7402])
        contain specifications for handling any specific transport
        selected.

   13.  The system MUST verify the HIP_MAC according to the procedures
        in Section 5.2.12.

   14.  If the checks above are valid, then the system proceeds with
        further I2 processing; otherwise, it discards the I2 and its
        state machine remains in the same state.

   15.  The I2 packet may have the A bit set -- in this case, the system
        MAY choose to refuse it by dropping the I2 and the state machine
        returns to state UNASSOCIATED.  If the A bit is set, the
        Initiator's HIT is anonymous and should not be stored
        permanently.

   16.  The system MUST extract the keying material from the
        ENCRYPTED_KEY parameter.  This keying material is a partial
        input to the key derivation process for the Pair-wise Key SA
        (see Section 6.3).





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   17.  The system initializes the remaining variables in the associated
        state, including Update ID counters.

   18.  Upon successful processing of an I2 message when the system's
        state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or R2-
        SENT, an R2 packet is sent and the system's state machine
        transitions to state R2-SENT.

   19.  Upon successful processing of an I2 packet when the system's
        state machine is in state ESTABLISHED, the old HIP association
        is dropped and a new one is installed, an R2 packet is sent, and
        the system's state machine transitions to R2-SENT.

   20.  Upon the system's state machine transitioning to R2-SENT, the
        system starts a timer.  The state machine transitions to
        ESTABLISHED if some data has been received on the incoming HIP
        association, or an UPDATE packet has been received (or some
        other packet that indicates that the peer system's state machine
        has moved to ESTABLISHED).  If the timer expires (allowing for
        maximal amount of retransmissions of I2 packets), the state
        machine transitions to ESTABLISHED.

6.8.  Processing Incoming R2 Packets

   An R2 packet received in states UNASSOCIATED, I1-SENT, or ESTABLISHED
   results in the R2 packet being dropped and the state machine staying
   in the same state.  If an R2 packet is received in state I2-SENT, it
   MUST be processed.

   The following steps define the conceptual processing rules for an
   incoming R2 packet:

   1.  If the system is in any other state than I2-SENT, the R2 packet
       is silently dropped.

   2.  The system MUST verify that the HITs in use correspond to the
       HITs that were received in the R1 packet that caused the
       transition to the I1-SENT state.

   3.  The system MUST verify the HIP_MAC according to the procedures in
       Section 6.2.

   4.  The system MUST re-evaluate the DH_GROUP_LIST, HIP_CIPHER,
       HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in the R2
       and compare the results against the chosen suites.






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   5.  If any of the checks above fail, there is a high probability of
       an ongoing man-in-the-middle or other security attack.  The
       system SHOULD act accordingly, based on its local policy.

   6.  The system MUST extract the keying material from the
       ENCRYPTED_KEY parameter.  This keying material is a partial input
       to the key derivation process for the Pair-wise Key SA (see
       Section 6.3).

   7.  Upon successful processing of the R2 packet, the state machine
       transitions to state ESTABLISHED.

6.9.  Processing UPDATE, NOTIFY, CLOSE, and CLOSE_ACK Packets

   UPDATE, NOTIFY, CLOSE, and CLOSE_ACK packets are handled similarly in
   HIP DEX as in HIP BEX (see Sections 6.11. - 6.15. in [RFC7401]).  The
   only difference is the that the HIP_SIGNATURE is never present and,
   therefore, is not required to be processed by the receiving party.

6.10.  Handling State Loss

   Implementors MAY choose to use non-volatile, secure storage for HIP
   states in order for them to survive a system reboot.  If no secure
   storage capabilities are available, the system SHOULD delete the
   corresponding HIP state, including the keying material.  If the
   implementation does drop the state (as RECOMMENDED), it MUST also
   drop the peer's R1 generation counter value, unless a local policy
   explicitly defines that the value of that particular host is stored.
   An implementation MUST NOT store a peer's R1 generation counters by
   default, but storing R1 generation counter values, if done, MUST be
   configured by explicit HITs.

7.  HIP Policies

   There are a number of variables that will influence the HIP exchanges
   that each host must support.  All HIP DEX implementations SHOULD
   provide for an ACL of Initiator's HI to Responder's HI.  This ACL
   SHOULD also include preferred transform and local lifetimes.
   Wildcards SHOULD also be supported for this ACL.

   The value of the puzzle difficulty #K used in the HIP R1 must be
   chosen with care.  Too high numbers of #K will exclude clients with
   weak CPUs because these devices cannot solve the puzzle within
   reasonable time. #K SHOULD only be raised if a Responder is under
   high load, i.e., it can no longer process all incoming HIP
   handshakes.  Otherwise, the responder SHOULD set #K to 0.





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8.  Security Considerations

   HIP DEX replaces the SIGMA-based authenticated Diffie-Hellman key
   exchange of HIPv2 with an exchange of random keying material that is
   encrypted by a Diffie-Hellman derived key.  Both the Initiator and
   Responder contribute to this keying material.

   o  The strength of the keys for the Pair-wise Key SA is based on the
      quality of the random keying material generated by the Initiator
      and Responder.  Since the Initiator is expected to be a sensor/
      actuator device, there is a natural concern about the quality of
      its random number generator.

   o  DEX lacks Perfect Forward Secrecy (PFS).  If the Initiator's HI is
      compromised, ALL HIP connections protected with that HI are
      compromised.

   o  The puzzle mechanism using CMAC may need further study that it
      does present the desired level of difficulty.

   o  The DEX HIT generation MAY present new attack opportunities;
      further study is needed.

   The R1 packet is unprotected and offers an attacker new resource
   attacks against the Initiator.  This is mitigated by only processing
   a received R1 when the Initiator has previously sent a corresponding
   I1.  Moreover, the Responder repeats the DH_GROUP_LIST, HIP_CIPHER,
   HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST parameters in R2 in order
   to verify that these parameters have not been modified by an attacker
   in the R1 packet.

9.  IANA Considerations

   HIP DEX introduces the following new HIP HIT suite:

   +-------+-------------+----------------------+----------------------+
   | Index | Hash        | Signature algorithm  | Description          |
   |       | function    | family               |                      |
   +-------+-------------+----------------------+----------------------+
   |     5 | FOLD        | ECDH                 | ECDH HI folded to 96 |
   |       |             |                      | bits                 |
   +-------+-------------+----------------------+----------------------+

                            Table 2: HIT Suites

   In addition, this document specified a new HIP Parameter Type defined
   in Section 5.2.5.




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   Moreover, a new HIP Cipher ID is defined in Section 5.2.4.

10.  Acknowledgments

   The drive to put HIP on a cryptographic 'Diet' came out of a number
   of discussions with sensor vendors at IEEE 802.15 meetings.  David
   McGrew was very helpful in crafting this document.

11.  Changelog

   This section summarizes the changes made from draft-moskowitz-hip-rg-
   dex-05, which was the first stable version of the draft.  Note that
   the draft was renamed after draft-moskowitz-hip-rg-dex-06.

11.1.  Changes in draft-moskowitz-hip-rg-dex-06

   o  A major change in the ENCRYPT parameter to use AES-CTR rather than
      AES-CBC.

11.2.  Changes in draft-moskowitz-hip-dex-00

   o  Draft name change.  HIPRG ended in IRTF, HIP DEX is now individual
      submission.

   o  Added the change section.

   o  Added a Definitions section.

   o  Changed I2 and R2 packets to reflect use of AES-CTR for
      ENCRYPTED_KEY parameter.

   o  Cleaned up KEYMAT Generation text.

   o  Added Appendix with C code for the ECDH shared secret generation
      on an 8 bit processor.

11.3.  Changes in draft-moskowitz-hip-dex-01

   o  Numerous editorial changes.

   o  New retransmission strategy.

   o  New HIT generation mechanism.

   o  Modified layout of ENCRYPTED_KEY parameter.

   o  Clarify to use puzzle difficulty of zero under normal network
      conditions.



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   o  Align inclusion directive of R1_COUNTER with HIPv2 (from SHOULD to
      MUST).

   o  Align inclusion of TRANSPORT_FORMAT_LIST with HIPv2 (added to R1
      and I2).

   o  HIP_CIPHER, HIT_SUITE_LIST, and TRANSPORT_FORMAT_LIST must now be
      echoed in R2 packet.

   o  Added new author.

11.4.  Changes in draft-moskowitz-hip-dex-02

   o  Introduced formal definition of FOLD function.

   o  Clarified use of CMAC for puzzle computation in section "Solving
      the Puzzle".

   o  Several editorial changes.

11.5.  Changes in draft-moskowitz-hip-dex-03

   o  Addressed HI crypto agility.

   o  Clarified purpose of secret exchanged via ENCRYPTED_KEY parameter.

   o  Extended the IV in the ENCRYPTED_KEY parameter.

   o  Introduced forward-references to HIP DEX KEYMAT process and
      improved KEYMAT section.

   o  Replaced Appendix A on "C code for ECC point multiplication" with
      short discussion in introduction.

   o  Updated references.

   o  Further editorial changes.

12.  References

12.1.  Normative References

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

   [RFC2410]  Glenn, R. and S. Kent, "The NULL Encryption Algorithm and
              Its Use With IPsec", RFC 2410, November 1998.




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   [RFC3686]  Housley, R., "Using Advanced Encryption Standard (AES)
              Counter Mode With IPsec Encapsulating Security Payload
              (ESP)", RFC 3686, January 2004.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC7343]  Laganier, J. and F. Dupont, "An IPv6 Prefix for Overlay
              Routable Cryptographic Hash Identifiers Version 2
              (ORCHIDv2)", RFC 7343, September 2014.

   [RFC7401]  Moskowitz, R., Heer, T., Jokela, P., and T. Henderson,
              "Host Identity Protocol Version 2 (HIPv2)", RFC 7401,
              April 2015.

   [RFC7402]  Jokela, P., Moskowitz, R., and J. Melen, "Using the
              Encapsulating Security Payload (ESP) Transport Format with
              the Host Identity Protocol (HIP)", RFC 7402, April 2015.

12.2.  Informative References

   [DH76]     Diffie, W. and M. Hellman, "New Directions in
              Cryptography", IEEE Transactions on Information Theory
              vol. IT-22, number 6, pages 644-654, Nov 1976.

   [HWZ13]    Hummen, R., Wirtz, H., Ziegeldorf, J., Hiller, J., and K.
              Wehrle, "Tailoring End-to-End IP Security Protocols to the
              Internet of Things", in Proceedings of IEEE International
              Conference on Network Protocols (ICNP 2013), October 2013.

   [I-D.ietf-hip-rfc4423-bis]
              Moskowitz, R. and M. Komu, "Host Identity Protocol
              Architecture", draft-ietf-hip-rfc4423-bis-11 (work in
              progress), April 2015.

   [IEEE.802-11.2007]
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              11: Wireless LAN Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications", IEEE Standard 802.11, June
              2007, <http://standards.ieee.org/getieee802/
              download/802.11-2007.pdf>.







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Internet-Draft           HIP Diet EXchange (DEX)               June 2015


   [IEEE.802-15-4.2011]
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              15.4: Wireless Medium Access Control (MAC) and Physical
              Layer (PHY) Specifications for Low-Rate Wireless Personal
              Area Networks (WPANs)", IEEE Standard 802.15.4, September
              2011, <http://standards.ieee.org/getieee802/
              download/802.15.4-2011.pdf>.

   [LN08]     Liu, A. and H. Ning, "TinyECC: A Configurable Library for
              Elliptic Curve Cryptography in Wireless Sensor Networks",
              in Proceedings of International Conference on Information
              Processing in Sensor Networks (IPSN 2008), April 2008.

   [RFC5903]  Fu, D. and J. Solinas, "Elliptic Curve Groups modulo a
              Prime (ECP Groups) for IKE and IKEv2", RFC 5903, June
              2010.

   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)", RFC
              5996, September 2010.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228, May 2014.

   [SECG]     SECG, "Recommended Elliptic Curve Domain Parameters", SEC
              2 , 2000, <http://www.secg.org/>.




















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Appendix A.  Password-based two-factor authentication during the HIP DEX
             handshake

   HIP DEX allows to identify authorized connections based on a two-
   factor authentication mechanism.  With two-factor authentication,
   devices that are authorized to communicate with each other are
   required to be pre-provisioned with a shared (group) key.  The
   Initiator uses this pre-provisioned key to encrypt the
   ECHO_RESPONSE_UNSIGNED in the I2 packet.  Upon reception of the I2,
   the Responder verifies that its challenge in the
   ECHO_REQUEST_UNSIGNED parameter in the R1 packet has been encrypted
   with the correct key.  If verified successfully, the Responder
   proceeds with the handshake.  Otherwise, it silently drops the I2
   packet.

   Note that there is no explicit signaling in the HIP DEX handshake for
   this behavior.  Thus, knowledge of two-factor authentication must be
   configured externally prior to the handshake.

Authors' Addresses

   Robert Moskowitz (editor)
   HTT Consulting
   Oak Park, MI
   USA

   EMail: rgm@htt-consult.com


   Rene Hummen
   Chair of Communication and Distributed Systems, RWTH Aachen
   Ahornstrasse 55
   Aachen  52074
   Germany

   EMail: hummen@comsys.rwth-aachen.de
   URI:   http://www.comsys.rwth-aachen.de/team/rene-hummen/














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