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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: July 21, 2016                               COMSYS, RWTH Aachen
                                                        January 20, 2016


                        HIP Diet EXchange (DEX)
                       draft-moskowitz-hip-dex-05

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 July 21, 2016.








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Copyright Notice

   Copyright (c) 2016 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
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   include Simplified BSD License text as described in Section 4.e of
   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 . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Requirements Terminology  . . . . . . . . . . . . . . . .   6
     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 . . . . . . . . . . . . . . . . . . . . . .   9
     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 . . . . . . . . . . . .  15
       4.1.4.  User Data Considerations  . . . . . . . . . . . . . .  16
   5.  Packet Formats  . . . . . . . . . . . . . . . . . . . . . . .  16
     5.1.  Payload Format  . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  HIP Parameters  . . . . . . . . . . . . . . . . . . . . .  16
       5.2.1.  DH_GROUP_LIST . . . . . . . . . . . . . . . . . . . .  17
       5.2.2.  HIP_CIPHER  . . . . . . . . . . . . . . . . . . . . .  17
       5.2.3.  HOST_ID . . . . . . . . . . . . . . . . . . . . . . .  17
       5.2.4.  HIT_SUITE_LIST  . . . . . . . . . . . . . . . . . . .  18
       5.2.5.  ENCRYPTED_KEY . . . . . . . . . . . . . . . . . . . .  18
     5.3.  HIP Packets . . . . . . . . . . . . . . . . . . . . . . .  19
       5.3.1.  I1 - the HIP Initiator Packet . . . . . . . . . . . .  20
       5.3.2.  R1 - the HIP Responder Packet . . . . . . . . . . . .  21
       5.3.3.  I2 - the Second HIP Initiator Packet  . . . . . . . .  23
       5.3.4.  R2 - the Second HIP Responder Packet  . . . . . . . .  24
     5.4.  ICMP Messages . . . . . . . . . . . . . . . . . . . . . .  25
   6.  Packet Processing . . . . . . . . . . . . . . . . . . . . . .  25



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     6.1.  Solving the Puzzle  . . . . . . . . . . . . . . . . . . .  25
     6.2.  HIP_MAC Calculation and Verification  . . . . . . . . . .  26
       6.2.1.  CMAC Calculation  . . . . . . . . . . . . . . . . . .  26
     6.3.  HIP DEX KEYMAT Generation . . . . . . . . . . . . . . . .  27
     6.4.  Initiation of a HIP Diet EXchange . . . . . . . . . . . .  30
     6.5.  Processing Incoming I1 Packets  . . . . . . . . . . . . .  30
     6.6.  Processing Incoming R1 Packets  . . . . . . . . . . . . .  31
     6.7.  Processing Incoming I2 Packets  . . . . . . . . . . . . .  34
     6.8.  Processing Incoming R2 Packets  . . . . . . . . . . . . .  37
     6.9.  Processing Incoming NOTIFY Packets  . . . . . . . . . . .  38
     6.10. Processing UPDATE, CLOSE, and CLOSE_ACK Packets . . . . .  39
     6.11. Handling State Loss . . . . . . . . . . . . . . . . . . .  39
   7.  HIP Policies  . . . . . . . . . . . . . . . . . . . . . . . .  39
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  39
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  40
   10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  41
   11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  41
     11.1.  Changes in draft-moskowitz-hip-rg-dex-06 . . . . . . . .  41
     11.2.  Changes in draft-moskowitz-hip-dex-00  . . . . . . . . .  41
     11.3.  Changes in draft-moskowitz-hip-dex-01  . . . . . . . . .  42
     11.4.  Changes in draft-moskowitz-hip-dex-02  . . . . . . . . .  42
     11.5.  Changes in draft-moskowitz-hip-dex-03  . . . . . . . . .  42
     11.6.  Changes in draft-moskowitz-hip-dex-04  . . . . . . . . .  43
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  43
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  43
     12.2.  Informative References . . . . . . . . . . . . . . . . .  44
   Appendix A.  Password-based two-factor authentication    during
                the HIP DEX handshake  . . . . . . . . . . . . . . .  46
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  46

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.





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

       *  A simple fold function for HIT generation.

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

   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].

   This document focuses on the protocol specifications related to
   differences between HIP BEX and HIP DEX.  Where differences are not
   called out explicitly, the protocol specification of HIP DEX is the
   same as defined 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 DEX 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



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   starts a puzzle exchange in the 2nd packet and the Initiator
   completes this exchange in the 3rd packet before the Responder
   performs computationally expensive operations or stores any state
   from the exchange.  Given this handshake structure, HIP DEX
   operationally is very similar to HIP BEX.  Moreover, the employed
   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 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 this
   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.







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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].

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 segments and
      the iterative folding of these segments via XOR.  I.e., X = x_1,
      x_2, ..., x_n, where x_i is of length K and the last segment 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).





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   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 HIP DEX handshake.

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

   Responder:  The host that responds to the Initiator in the HIP DEX
      handshake.  This role is typically forgotten once the handshake 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 HIP
      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 packets to represent the HI.  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 5.2.4).




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

   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.

   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 HIT 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 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 the HI.  Third, a 96-bit hashed
   representation of the HI.  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 HIPv2
   is used for HIP DEX.  The HIP 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).




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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
   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 handshake 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 ESP-based protection of user data.

   The Initiator first sends a trigger packet, I1, to the Responder.
   This packet contains the HIT of the Initiator and 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
   key 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 handshake
   with a different selection of cryptographic algorithms.



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   In the I2 packet, the Initiator MUST display the solution to the
   received puzzle.  Without a correct solution, the I2 packet 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.

   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



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   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
   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 HIPv2 state
   machine (see 4.4. in [RFC7401]).  However, HIP DEX features a
   retransmission strategy with an optional reception acknowledgement
   for the I2 packet.  The goal of this additional acknowledgement is to
   reduce premature I2 retransmissions in case of devices with low
   computation resources [HWZ13].  As a result, there are minor changes
   regarding the transitions in the HIP DEX state machine.  The
   following section documents these differences compared to HIPv2.

4.1.2.1.  HIP DEX Retransmission Mechanism

   For the retransmission of I1 and I2 packets, the Initiator adopts the
   retransmission strategy of HIPv2 (see Section 4.4.3. in [RFC7401]).
   This strategy is based on a timeout that is set to a value larger
   than the worst-case anticipated round-trip time (RTT).  For each
   received I1 or I2 packet, the Responder sends an R1 or R2 packet,
   respectively.  This design trait enables the Responder to remain
   stateless until the reception and successful processing of the I2
   packet.  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 packet after I1_RETRIES_MAX tries, it stops I1



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   retransmissions.  Likewise, it stops retransmitting the I2 packet
   after I2_RETRIES_MAX unsuccessful tries.

   For repeatedly received I2 packets, the Responder SHOULD NOT perform
   operations related to the Diffie-Hellman key exchange or the keying
   material wrapped in the ENCRYPTED_KEY parameters.  Instead, it SHOULD
   re-use the previously established state to re-create the
   corresponding R2 packet in order to prevent unnecessary computation
   overhead.

   The potentially high processing time of an I2 packet at a (resource-
   constrained) Responder may cause premature 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 the I2 packet at the Responder into account
   for retransmission purposes.  To this end, the Responder MAY notify
   the Initiator about the anticipated delay once the puzzle solution
   was successfully verified and if the remaining I2 packet processing
   incurs a high processing delay.  The Responder MAY therefore send a
   NOTIFY packet (see Section 5.3.6. in [RFC7401]) to the Initiator
   before the Responder commences the ECDH operation.  The NOTIFY packet
   serves as an acknowledgement for the I2 packet 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 two-octet 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 packet.

   When the Initiator receives the NOTIFY packet, it sets the I2
   retransmission timeout to the I2 processing time indicated in the
   NOTIFICATION parameter plus half the RTT-based timeout value.  In
   doing so, the Initiator MUST NOT set the retransmission timeout to a
   higher value than allowed by a local policy.  This is to prevent
   unauthenticated NOTIFY packets from maliciously delaying the
   handshake beyond a well-defined upper bound in case of a lost R2
   packet.  At the same time, this extended retransmission timeout
   enables the Initiator to defer I2 retransmissions until the point in
   time when the Responder should have completed its I2 packet
   processing and the network should have delivered the R2 packet
   according to the employed worst-case estimates.

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 to value in     |
   | process             | I2_ACKNOWLEDGEMENT Notification Data plus   |
   |                     | 1/2 RTT-based timeout value and stay at     |
   |                     | I2-SENT                                     |
   |                     |                                             |
   | Timeout             | Increment trial counter                     |
   |                     |                                             |
   |                     | If counter is less than I2_RETRIES_MAX,     |
   |                     | send I2, reset timer to RTT-based timeout,  |
   |                     | 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.
























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                               +--+       +----------------------------+
              recv I1, send R1 |  |       |                            |
                               |  v       v                            |
                             +--------------+  recv I2, send R2        |
            +----------------| UNASSOCIATED |----------------+         |
   datagram |  +--+          +--------------+                |         |
   to send, |  |  | Alg. not supported,                      |         |
    send I1 |  |  | send I1                                  |         |
     .      v  |  v                                          |         |
     .   +---------+  recv I2, send R2                       |         |
   +---->| I1-SENT |--------------------------------------+  |         |
   |     +---------+            +----------------------+  |  |         |
   |          | recv R1,        | recv I2, send R2     |  |  |         |
   |          v send I2         |                      v  v  v         |
   |       +---------+          |                    +---------+       |
   |  +--->| I2-SENT |----------+     +--------------| R2-SENT |<---+  |
   |  |    +---------+                |              +---------+    |  |
   |  |          |  |recv R2          |        data or|             |  |
   |  |recv R1,  |  |                 |     EC timeout|             |  |
   |  |send I2   +--|-----------------+               |  receive I2,|  |
   |  |          |  |       +-------------+           |      send R2|  |
   |  |          |  +------>| ESTABLISHED |<----------+             |  |
   |  |          |          +-------------+                         |  |
   |  |          |            |  |  |      receive I2, send R2      |  |
   |  |          +------------+  |  +-------------------------------+  |
   |  |          |               +-----------+                      |  |
   |  |          |    no packet sent/received|    +---+             |  |
   |  |          |    for UAL min, send CLOSE|    |   |timeout      |  |
   |  |          |                           v    v   |(UAL+MSL)    |  |
   |  |          |                        +---------+ |retransmit   |  |
   +--|----------|------------------------| CLOSING |-+CLOSE        |  |
      |          |                        +---------+               |  |
      |          |                         | |   | |                |  |
      +----------|-------------------------+ |   | +----------------+  |
      |          |               +-----------+   +------------------|--+
      |          |               |recv CLOSE,      recv CLOSE_ACK   |  |
      |          +-------------+ |send CLOSE_ACK   or timeout       |  |
      |     recv CLOSE,        | |                 (UAL+MSL)        |  |
      |     send CLOSE_ACK     v v                                  |  |
      |                     +--------+  receive I2, send R2         |  |
      +---------------------| CLOSED |------------------------------+  |
                            +--------+                                 |
                             ^ |  |                                    |
   recv CLOSE, send CLOSE_ACK| |  |              timeout (UAL+2MSL)    |
                             +-+  +------------------------------------+






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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 the 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

   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.








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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 resource-constrained 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
   HIPv2.  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
   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 HIPv2 defined in Section 5.2. of
   [RFC7401] where possible.  Still, HIP DEX further restricts and/or
   extends the following existing parameter types:

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

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

   o  HIT_SUITE_LIST is limited to the HIT suite ECDH/FOLD.

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

   In addition, HIP DEX introduces the following new parameter:



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

5.2.1.  DH_GROUP_LIST

   The DH_GROUP_LIST parameter contains the list of supported DH Group
   IDs of a host.  It is defined in Section 5.2.6 of [RFC7401].  With
   HIP DEX, the DH Group IDs are restricted to:

   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] and Appendix D of [RFC7401].  Any ECDH
   used with HIP MUST have a co-factor of 1.

5.2.2.  HIP_CIPHER

   The HIP_CIPHER parameter contains the list of supported cipher
   algorithms to be used for encrypting the contents of the ENCRYPTED
   and ENCRYPTED_KEY parameters.  The HIP_CIPHER parameter is defined in
   Section 5.2.8 of [RFC7401].  With HIP DEX, the Suite IDs are limited
   to:

   Suite ID           Value

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

   Mandatory implementation: AES-128-CTR.  Implementors SHOULD support
   NULL-ENCRYPT ([RFC2410]) for testing/debugging purposes but MUST NOT
   offer or accept this value unless explicitly configured for testing/
   debugging of HIP.

5.2.3.  HOST_ID

   The HOST_ID parameter conveys the Host Identity (HI) along with
   optional information about a host.  It is defined in Section 5.2.9 of
   [RFC7401].



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   HIP DEX uses the public portion of a host's static ECDH key-pair as
   the HI.  Correspondingly, HIP DEX limits the HI algorithms to the
   following profile:

   Algorithm profiles   Value

   ECDH                 11 [RFC6090]   (REQUIRED)

   HIP DEX HIs are serialized equally to the ECC-based HIs in HIPv2 (see
   Section 5.2.9. of [RFC7401]).  The Group ID of the HIP DEX HI is
   encoded in the "ECC curve" field of the HOST_ID parameter.  The
   supported DH Group IDs are defined in Section 5.2.1.

5.2.4.  HIT_SUITE_LIST

   The HIT_SUITE_LIST parameter contains a list of the supported HIT
   suite IDs of the Responder.  Based on the HIT_SUITE_LIST, the
   Initiator can determine which source HIT Suite IDs are supported by
   the Responder.  The HIT_SUITE_LIST parameter is defined in
   Section 5.2.10 of [RFC7401].

   The following HIT Suite IDs are defined for HIP DEX, and the
   relationship between the four-bit ID value used in the OGA ID field
   and the eight-bit encoding within the HIT_SUITE_LIST ID field is
   clarified:

   HIT Suite       Four-bit ID    Eight-bit encoding

   ECDH/FOLD           8             0x80

   Note that the HIP DEX HIT Suite ID allows the peers to distinguish a
   HIP DEX handshake from a HIPv2 handshake.  The Responder MUST respond
   with a HIP DEX HIT suite ID when the HIT of the Initiator is a HIP
   DEX HIT.

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]) are 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

   HIP DEX uses the same eight basic HIP packets as HIPv2 (see
   Section 5.3 of [RFC7401]).  Four of them are for the HIP handshake
   (I1, R1, I2, and R2), one is for updating an association (UPDATE),
   one is for sending notifications (NOTIFY), and two are for closing
   the association (CLOSE and CLOSE_ACK).  There are some differences
   regarding the HIP parameters that are included in the handshake
   packets concerning HIP BEX and HIP 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 HIP 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,



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   the I2, R2, UPDATE, NOTIFY, CLOSE, and CLOSE_ACK packets are not
   covered 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 ) )

   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 5.2.4.

   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 Section 4.1.8 of [RFC7401] for
   detailed information about the "HIP Opportunistic Mode".

   As a compression of the employed HIs, 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, only include the OGA ID identifying a HIP DEX HIT.  They do
   not include information about the specific DH group ID of the
   corresponding HI.  To inform the Responder about its employed and its
   otherwise supported DH Group IDs, the Initiator therefore includes
   the 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 5.2.1.





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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 ])

   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 Section 4.1.8 of [RFC7401] for detailed
   information about the "HIP Opportunistic Mode".

   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 HIP DEX
   handshake.  A puzzle difficulty of zero effectively turns the puzzle



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   mechanism into a return-routablility test and is strongly encouraged
   during normal operation in order to conserve energy resources as well
   as to prevent unnecessary handshake delay in case of a resource-
   constrained Initiator.

   The DH_GROUP_LIST parameter contains the Responder's order of
   preference based on which it chose the 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 HI DEX.  Thus, this parameter is
   repeated in the R2 packet to allow for a final, cryptographically
   secured validation.

   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 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 in the I1 packet.  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 packet based on the destination HIT from the I1
   packet.  Based on the deviating DH group ID in the HOST_ID parameter,



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   the Initiator then 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 )] )

   Valid control bits: A

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

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

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

   The Solution contains the Random #I from the R1 packet and the
   computed #J value.  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 parameter 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 parameter(s) contain 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 parameter 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
   packet.  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 parameter as described in Section 6.2.
   The Responder MUST validate the HIP_MAC parameter.

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)

   Valid control bits: none

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



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   The Responder repeats the DH_GROUP_LIST, HIP_CIPHER, HIT_SUITE_LIST,
   and TRANSPORT_FORMAT_LIST parameters in the R2 packet.  These
   parameters MUST be the same as included in the R1 packet.  The
   parameter are re-included here because the R2 packet 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 parameter 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 parameter.

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, the packet processing in HIP DEX only differs from
   HIPv2 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.

   The parameter order is described in Section 5.2.1 of [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.  Note that the Checksum and Length fields
       contain incorrect values after this step.

6.3.  HIP DEX KEYMAT Generation

   The HIP DEX KEYMAT process is used to derive the keys for the 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 DEX handshake.  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 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

   where

       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 the
   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



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

   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].  The I1 packet contents are specified in
   Section 5.3.1.

6.5.  Processing Incoming I1 Packets

   I1 packets in HIP DEX are handled almost identical to HIPv2 (see
   Section 6.7 of [RFC7401]).  The main differences are that the
   Responder SHOULD select a HIP DEX HIT Suite 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 at the Responder.  Hence, the R1 pre-
   computation SHOULD be omitted in HIP DEX.

   Correspondingly, the modified conceptual processing rules for
   responding to an I1 packet are as follows:

   1.  The Responder MUST check that the Responder's HIT in the received
       I1 packet is either one of its own HITs or NULL.  Otherwise, it
       must drop the packet.

   2.  If the Responder is in ESTABLISHED state, the Responder MAY
       respond to this with an R1 packet, prepare to drop an existing
       HIP security association with the peer, and stay at ESTABLISHED
       state.

   3.  If the Responder is in I1-SENT state, it MUST make a comparison
       between the sender's HIT and its own (i.e., the receiver's) HIT.
       If the sender's HIT is greater than its own HIT, it should drop
       the I1 packet and stay at I1-SENT.  If the sender's HIT is
       smaller than its own HIT, it SHOULD send the R1 packet and stay




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       at I1-SENT.  The HIT comparison is performed as defined in
       Section 6.3.

   4.  If the implementation chooses to respond to the I1 packet with an
       R1 packet, it creates a new R1 according to the format described
       in Section 5.3.2.  It chooses the HI based on the destination HIT
       and the DH_GROUP_LIST in the I1 packet.  If the implementation
       does not support the DH group required by the Initiator or if the
       destination HIT in the I1 packet does not match the required DH
       group, it selects the mutually preferred and supported DH group
       based on the DH_GROUP_LIST parameter in the I1 packet.  The
       implementation includes the corresponding ECDH public key in the
       HOST_ID parameter.  If no suitable DH Group ID was contained in
       the DH_GROUP_LIST in the I1 packet, it sends an R1 packet with
       any suitable ECDH public key.

   5.  If the received Responder's HIT in the I1 packet is not NULL, the
       Responder's in the R1 packet HIT MUST match the destination HIT
       in the I1 packet.  Otherwise, the Responder MUST select a HIT
       with the same HIT Suite as the Initiator's HIT.  If this HIT
       Suite is not supported by the Responder, it SHOULD select a
       REQUIRED HIT Suite from Section 5.2.10 of [RFC7401], which is
       currently RSA/DSA/SHA-256.  Other than that, selecting the HIT is
       a local policy matter.

   6.  The Responder expresses its supported HIP transport formats in
       the TRANSPORT_FORMAT_LIST as described in Section 5.2.11 of
       [RFC7401].  The Responder MUST provide at least one payload
       transport format type.

   7.  The Responder sends the R1 packet to the source IP address of the
       I1 packet.

   Note that only steps 4 and 5 have been changed with regard to the
   processing rules of HIPv2.  The considerations about R1 management
   (except pre-computation) and malformed I1 packets in Sections 6.7.1
   and 6.7.2 of [RFC7401] likewise apply to HIP DEX.

6.6.  Processing Incoming R1 Packets

   R1 packets in HIP DEX are handled identically to HIPv2 (see
   Section 6.8 in [RFC7401]) with the following exceptions: HIP DEX uses
   ECDH public keys as HIs and does not employ signatures.

   The modified conceptual processing rules for responding to an R1
   packet are as follows:





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   1.   A system receiving an R1 MUST first check to see if it has sent
        an I1 packet to the originator of the R1 packet (i.e., it has a
        HIP association that is in state I1-SENT and that is associated
        with the HITs in the R1).  Unless the I1 packet was sent in
        opportunistic mode (see Section 4.1.8 of [RFC7401]), the IP
        addresses in the received R1 packet SHOULD be ignored by the R1
        processing and, when looking up the right HIP association, the
        received R1 packet SHOULD be matched against the associations
        using only the HITs.  If a match exists, the system should
        process the R1 packet as described below.

   2.   Otherwise, if the system is in any state other than I1-SENT or
        I2-SENT with respect to the HITs included in the R1 packet, it
        SHOULD silently drop the R1 packet and remain in the current
        state.

   3.   If the HIP association state is I1-SENT or I2-SENT, the received
        Initiator's HIT MUST correspond to the HIT used in the original
        I1 packet.  Also, the Responder's HIT MUST correspond to the one
        used in the I1 packet, unless this packet contained a NULL HIT.

   4.   If the HIP association state is I1-SENT, and multiple valid R1
        packets are present, the system MUST select from among the R1
        packets with the largest R1 generation counter.

   5.   The system MUST check that the Initiator's HIT Suite is
        contained in the HIT_SUITE_LIST parameter in the R1 packet
        (i.e., the Initiator's HIT Suite is supported by the Responder).
        If the HIT Suite is supported by the Responder, the system
        proceeds normally.  Otherwise, the system MAY stay in state
        I1-SENT and restart the HIP DEX handshake by sending a new I1
        packet with an Initiator HIT that is supported by the Responder
        and hence is contained in the HIT_SUITE_LIST in the R1 packet.
        The system MAY abort the handshake if no suitable source HIT is
        available.  The system SHOULD wait for an acceptable time span
        to allow further R1 packets with higher R1 generation counters
        or different HIT and HIT Suites to arrive before restarting or
        aborting the HIP DEX handshake.

   6.   The system MUST check that the DH Group ID in the HOST_ID
        parameter in the R1 matches the first DH Group ID in the
        Responder's DH_GROUP_LIST in the R1 packet, and also that this
        Group ID corresponds to a value that was included in the
        Initiator's DH_GROUP_LIST in the I1 packet.  If the DH Group ID
        of the HOST_ID parameter does not express the Responder's best
        choice, the Initiator can conclude that the DH_GROUP_LIST in the
        I1 or R1 packet was adversely modified.  In such a case, the
        Initiator MAY send a new I1 packet; however, it SHOULD NOT



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        change its preference in the DH_GROUP_LIST in the new I1 packet.
        Alternatively, the Initiator MAY abort the HIP DEX handshake.
        Moreover, if the DH Group ID indicated in the HOST_ID parameter
        does not match the DH Group ID of the HI employed by the
        Initiator, the system SHOULD wait for an acceptable time span to
        allow further R1 packets with different DH Group IDs to arrive
        before restarting or aborting the HIP DEX handshake.  When
        restarting the handshake, the Initiator MUST consult local
        policies (see Section 7) regarding the use of another, mutually
        supported DH group for the subsequent handshake with the
        Responder.

   7.   If the HIP association state is I2-SENT, the system MAY re-enter
        state I1-SENT and process the received R1 packet if it has a
        larger R1 generation counter than the R1 packet responded to
        previously.

   8.   The R1 packet may have the A-bit set - in this case, the system
        MAY choose to refuse it by dropping the R1 packet and returning
        to state UNASSOCIATED.  The system SHOULD consider dropping the
        R1 packet only if it used a NULL HIT in the I1 packet.  If the
        A-bit is set, the Responder's HIT is anonymous and SHOULD NOT be
        stored permanently.

   9.   The system SHOULD attempt to validate the HIT against the
        received Host Identity by using the received Host Identity to
        construct a HIT and verify that it matches the Sender's HIT.

   10.  The system MUST store the received R1 generation counter for
        future reference.

   11.  The system attempts to solve the puzzle in the R1 packet.  The
        system MUST terminate the search after exceeding the remaining
        lifetime of the puzzle.  If the puzzle is not successfully
        solved, the implementation MAY either resend the I1 packet
        within the retry bounds or abandon the HIP base exchange.

   12.  The system computes standard Diffie-Hellman keying material
        according to the public value and Group ID provided in the
        HOST_ID parameter.  The Diffie-Hellman keying material Kij is
        used for key extraction as specified in Section 6.3.

   13.  The system selects the HIP_CIPHER ID from the choices presented
        in the R1 packet and uses the selected values subsequently when
        generating and using encryption keys, and when sending the I2
        packet.  If the proposed alternatives are not acceptable to the
        system, it may either resend an I1 packet within the retry
        bounds or abandon the HIP base exchange.



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   14.  The system chooses one suitable transport format from the
        TRANSPORT_FORMAT_LIST and includes the respective transport
        format parameter in the subsequent I2 packet.

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

   16.  The system prepares and sends an I2 packet as described in
        Section 5.3.3.

   17.  The system SHOULD start a timer whose timeout value SHOULD be
        larger than the worst-case anticipated RTT, and MUST increment a
        trial counter associated with the I2 packet.  The sender SHOULD
        retransmit the I2 packet upon a timeout and restart the timer,
        up to a maximum of I2_RETRIES_MAX tries.

   18.  If the system is in state I1-SENT, it SHALL transition to state
        I2-SENT.  If the system is in any other state, it remains in the
        current state.

   Note that step 4 from the original processing rules of HIPv2
   (signature verification) has been removed in the above processing
   rules for HIP DEX.  Moreover, step 7 of the HIPv2 processing rules
   has been adapted to account for the fact that HIP DEX uses ECDH
   public keys as HIs.  The considerations about malformed R1 packets in
   Sections 6.8.1 of [RFC7401] also apply to HIP DEX.

6.7.  Processing Incoming I2 Packets

   The processing of I2 packets follows similar rules as HIPv2 (see
   Section 6.9 of [RFC7401]).  The main differences to HIPv2 are that
   HIP DEX introduces a new session key exchange via the ENCRYPTED_KEY
   parameter as well as an I2 reception acknowledgement for
   retransmission purposes.  Moreover, with HIP DEX the Initiator is
   responsible for triggering retransmissions, whereas the Responder
   merely replies to received I2 packets.

   The modified HIP DEX conceptual processing rules for responding to an
   I2 packet are:

   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.




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   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 to see 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 reset the
        R2-SENT timer.  The system SHOULD re-use the previously
        established state to re-create the corresponding R2 packet in
        order to prevent unnecessary computation overhead.

   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 state other than
        R2-SENT, the system SHOULD check that the echoed R1 generation
        counter in the I2 packet is within the acceptable range if the
        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 6.1.




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   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
        keying material 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.  If the
        processing time for the derivation of the Diffie-Hellman keying
        material Kij is likely to cause premature I2 retransmissions by
        the Initiator, the system MAY send a NOTIFY packet before
        starting the key derivation process.  The NOTIFY packet contains
        a NOTIFICATION parameter with Notify Message Type
        I2_ACKNOWLEDGEMENT.  The NOTIFICATION parameter indicates the
        anticipated remaining processing time for the I2 packet in
        milliseconds as two-octet Notification Data.

   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 be 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 6.2.

   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 decrypt 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).

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



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   18.  Upon successful processing of an I2 packet when the system's
        state machine is in state UNASSOCIATED, I1-SENT, I2-SENT, or
        R2-SENT, an R2 packet is sent as described in Section 5.3.4 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 as
        described in Section 5.3.4, 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 a
        maximal amount of retransmissions of I2 packets), the state
        machine transitions to ESTABLISHED.

   Note that steps 11 (encrypted HOST_ID) and 15 (signature
   verification) from the original processing rules of HIPv2 have been
   removed in the above processing rules for HIP DEX.  Moreover, step 10
   of the HIPv2 processing rules has been adapted to account for
   optional extension of the retransmission mechanism.  Step 16 has been
   added to the processing rules.  The considerations about malformed I2
   packets in Sections 6.9.1 of [RFC7401] also apply to HIP DEX.

6.8.  Processing Incoming R2 Packets

   R2 packets in HIP DEX are handled identically to HIPv2 (see
   Section 6.10 of [RFC7401]) with the following exceptions: HIP DEX
   introduces a new session key exchange via the ENCRYPTED_KEY parameter
   and does not employ signatures.

   The modified conceptual processing rules for responding to an R2
   packet are as follows:

   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 I2-SENT state.

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




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

   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 decrypt 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.

   Note that step 4 (signature verification) from the original
   processing rules of HIPv2 has been replaced with a negotiation re-
   evaluation in the above processing rules for HIP DEX.  Moreover, step
   6 has been added to the processing rules.

6.9.  Processing Incoming NOTIFY Packets

   Processing of NOTIFY packets is OPTIONAL.  If processed, any errors
   in a received NOTIFICATION parameter SHOULD be logged.  Received
   errors MUST be considered only as informational, and the receiver
   SHOULD NOT change its HIP state purely based on the received NOTIFY
   packet.

   If a NOTIFY packet is received in state I2-SENT, this packet may be
   an I2 reception acknowledgement of the optional retransmission
   mechanism extension and SHOULD be processed.  The following steps
   define the conceptual processing rules for such incoming NOTIFY
   packets in state I2-SENT:

   1.  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 I2-SENT state.  If this check fails, the NOTIFY
       packet SHOULD be dropped silently.

   2.  If the NOTIFY packet contains a NOTIFICATION parameter with
       Notify Message Type I2_ACKNOWLEDGEMENT, the system SHOULD set the
       I2 retransmission timer to the I2 processing time indicated in
       the NOTIFICATION parameter plus half the RTT-based timeout value.
       The system MUST NOT set the retransmission timeout to a higher
       value than allowed by a local policy.  Moreover, the system
       SHOULD reset the I2 retransmission timer to the RTT-based timeout
       value after the next I2 retransmission.



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6.10.  Processing UPDATE, CLOSE, and CLOSE_ACK Packets

   UPDATE, CLOSE, and CLOSE_ACK packets are handled similarly in HIP DEX
   as in HIP BEX (see Sections 6.11, 6.12, 6.14, and 6.15 of [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.

6.11.  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 #K used in the HIP R1 must be chosen with care.  Values
   of #K that are too high will exclude clients with weak CPUs because
   these devices cannot solve the puzzle within a reasonable amount of
   time. #K should only be raised if a Responder is under high load,
   i.e., it cannot process all incoming HIP handshakes any more.  If a
   Responder is not under high load, #K SHOULD be 0.

8.  Security Considerations

   HIP DEX closely resembles HIPv2.  As such, the security
   considerations discussed in Section 8 of [RFC7401] similarly apply to
   HIP DEX.  HIP DEX, however, 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.
   As a result, the following additional security considerations apply
   to HIP DEX:





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   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 the Responder.  Since the Initiator is expected to be a sensor
      or an actuator device, there is a natural concern about the
      quality of its random number generator.

   o  HIP DEX lacks the Perfect Forward Secrecy (PFS) property of HIPv2.
      Consequently, if an HI is compromised, all HIP connections
      protected with that HI are compromised.

   o  The puzzle mechanism using CMAC may need further study regarding
      the level of difficulty.

   o  The HIP DEX HIT generation may present new attack opportunities.

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

   The optional retransmission extension of HIP DEX is based on a NOTIFY
   packet that the Responder can use to inform the Initiator about the
   reception of an I2 packet.  The Responder, however, cannot protect
   the authenticity of this packet as it did not yet set up the Master
   Key SA.  Hence, an eavesdropping adversary may send spoofed reception
   acknowledgements for an overheard I2 packet and signal an arbitrary
   I2 processing time to the Initiator.  The adversary can, e.g.,
   indicate a lower I2 processing time than actually required by the
   Responder in order to cause premature retransmissions.  To protect
   against this attack, the Initiator SHOULD set the NOTIFY-based
   timeout value to the maximum indicated packet processing time in case
   of conflicting NOTIFY packets.  This allows the legitimate Responder
   to extend the retransmission timeout to the intended length.  The
   adversary, however, can still arbitrarily delay the protocol
   handshake beyond the Responder's actual I2 processing time.  To limit
   the extend of such a maliciously induced handshake delay, this
   specification additionally requires the Initiator not to set the
   NOTIFY-based timeout value higher than allowed by a local policy.

9.  IANA Considerations

   The following changes to the "Host Identity Protocol (HIP)
   Parameters" registries have been made:




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   HIT Suite ID  This document defines the new HIT Suite "ECDH/FOLD"
      (see Section 5.2.4).

   Parameter Type  This document defines the new HIP parameter
      "ENCRYPTED_KEY" with type number 643 (see Section 5.2.5).

   HIP Cipher ID  This document defines the new HIP Cipher ID "AES-
      128-CTR" (see Section 5.2.2).

   HI Algorithm  This document defines the new HI Algorithm "ECDH" (see
      Section 5.2.3).

   ECC Curve Label  This document specifies a new algorithm-specific
      subregistry named "ECDH Curve Label".  The values for this
      subregistry are defined in Section 5.2.1.

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.





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   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.

   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.





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

   o  Updated references.

   o  Further editorial changes.

11.6.  Changes in draft-moskowitz-hip-dex-04

   o  Improved retransmission extension.

   o  Updated and strongly revised packet processing rules.

   o  Updated security considerations.

   o  Updated IANA considerations.

   o  Move the HI Algorithm for ECDH to a value of 11.

   o  Many 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.

   [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.





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   [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-13 (work in
              progress), December 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>.

   [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.




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   [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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