RFC 8928

Internet Engineering Task Force (IETF)                   P. Thubert, Ed.
Request for Comments: 8928                                         Cisco
Updates: 8505                                                B. Sarikaya
Category: Standards Track
ISSN: 2070-1721                                                 M. Sethi
                                                               R. Struik
                                             Struik Security Consultancy
                                                           November 2020

Address-Protected Neighbor Discovery for Low-Power and Lossy Networks


   This document updates the IPv6 over Low-Power Wireless Personal Area
   Network (6LoWPAN) Neighbor Discovery (ND) protocol defined in RFCs
   6775 and 8505.  The new extension is called Address-Protected
   Neighbor Discovery (AP-ND), and it protects the owner of an address
   against address theft and impersonation attacks in a Low-Power and
   Lossy Network (LLN).  Nodes supporting this extension compute a
   cryptographic identifier (Crypto-ID), and use it with one or more of
   their Registered Addresses.  The Crypto-ID identifies the owner of
   the Registered Address and can be used to provide proof of ownership
   of the Registered Addresses.  Once an address is registered with the
   Crypto-ID and a proof of ownership is provided, only the owner of
   that address can modify the registration information, thereby
   enforcing Source Address Validation.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2020 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   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
   2.  Terminology
     2.1.  Requirements Language
     2.2.  Background
     2.3.  Abbreviations
   3.  Updating RFC 8505
   4.  New Fields and Options
     4.1.  New Crypto-ID
     4.2.  Updated EARO
     4.3.  Crypto-ID Parameters Option
     4.4.  NDP Signature Option
     4.5.  Extensions to the Capability Indication Option
   5.  Protocol Scope
   6.  Protocol Flows
     6.1.  First Exchange with a 6LR
     6.2.  NDPSO Generation and Verification
     6.3.  Multi-Hop Operation
   7.  Security Considerations
     7.1.  Brown Field
     7.2.  Threats Identified in RFC 3971
     7.3.  Related to 6LoWPAN ND
     7.4.  Compromised 6LR
     7.5.  ROVR Collisions
     7.6.  Implementation Attacks
     7.7.  Cross-Algorithm and Cross-Protocol Attacks
     7.8.  Public Key Validation
     7.9.  Correlating Registrations
   8.  IANA Considerations
     8.1.  CGA Message Type
     8.2.  Crypto-Type Subregistry
     8.3.  IPv6 ND Option Types
     8.4.  New 6LoWPAN Capability Bit
   9.  References
     9.1.  Normative References
     9.2.  Informative References
   Appendix A.  Requirements Addressed in This Document
   Appendix B.  Representation Conventions
     B.1.  Signature Schemes
     B.2.  Representation of ECDSA Signatures
     B.3.  Representation of Public Keys Used with ECDSA
     B.4.  Alternative Representations of Curve25519

   Authors' Addresses

1.  Introduction

   Neighbor Discovery optimizations for 6LoWPAN networks (aka 6LoWPAN
   ND) [RFC6775] adapts the original IPv6 Neighbor Discovery protocols
   defined in [RFC4861] and [RFC4862] for constrained Low-Power and
   Lossy Networks (LLNs).  In particular, 6LoWPAN ND introduces a
   unicast host Address Registration mechanism that reduces the use of
   multicast compared to the Duplicate Address Detection (DAD) mechanism
   defined in IPv6 ND. 6LoWPAN ND defines a new Address Registration
   Option (ARO) that is carried in the unicast Neighbor Solicitation
   (NS) and Neighbor Advertisement (NA) messages exchanged between a
   6LoWPAN Node (6LN) and a 6LoWPAN Router (6LR).  It also defines the
   Duplicate Address Request (DAR) and Duplicate Address Confirmation
   (DAC) messages between the 6LR and the 6LoWPAN Border Router (6LBR).
   In LLNs, the 6LBR is the central repository of all the Registered
   Addresses in its domain.

   The registration mechanism in "Neighbor Discovery Optimization for
   IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)"
   [RFC6775] prevents the use of an address if that address is already
   registered in the subnet (first come, first served).  In order to
   validate address ownership, "Registration Extensions for IPv6 over
   Low-Power Wireless Personal Area Network (6LoWPAN) Neighbor
   Discovery" [RFC8505] defines a Registration Ownership Verifier (ROVR)
   field.  [RFC8505] enables a 6LR and 6LBR to validate the association
   between the Registered Address of a node and its ROVR.  The ROVR can
   be derived from the link-layer address of the device (using the
   64-bit Extended Unique Identifier (EUI-64) address format specified
   by IEEE).  However, the EUI-64 can be spoofed; therefore, any node
   connected to the subnet and aware of a registered-address-to-ROVR
   mapping could effectively fake the ROVR.  This would allow an
   attacker to steal the address and redirect traffic for that address.
   [RFC8505] defines an Extended Address Registration Option (EARO) that
   transports alternate forms of ROVRs and is a prerequisite for this

   In this specification, a 6LN generates a cryptographic identifier
   (Crypto-ID) and places it in the ROVR field during the registration
   of one (or more) of its addresses with the 6LR(s).  Proof of
   ownership of the Crypto-ID is passed with the first registration
   exchange to a new 6LR and enforced at the 6LR.  The 6LR validates
   ownership of the Crypto-ID before it creates any new registration
   state or changes existing information.

   The protected address registration protocol proposed in this document
   provides the same conceptual benefit as Source Address Validation
   Improvement (SAVI) [RFC7039] in that only the owner of an IPv6
   address may source packets with that address.  As opposed to
   [RFC7039], which relies on snooping protocols, the protection
   provided by this document is based on a state that is installed and
   maintained in the network by the owner of the address.  With this
   specification, a 6LN may use a 6LR for forwarding an IPv6 packet if
   and only if it has registered the address used as the source of the
   packet with that 6LR.

   With the 6LoWPAN adaptation layer in [RFC4944] and [RFC6282], a 6LN
   can obtain better compression for an IPv6 address with an Interface
   ID (IID) that is derived from a Layer 2 (L2) address.  Such
   compression is incompatible with "SEcure Neighbor Discovery (SEND")
   [RFC3971] and "Cryptographically Generated Addresses (CGAs)"
   [RFC3972], since they derive the IID from cryptographic keys.  This
   specification, on the other hand, separates the IID generation from
   cryptographic computations and can enable better compression.

2.  Terminology

2.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

2.2.  Background

   The reader may get additional context for this specification from the
   following references:

   *  "SEcure Neighbor Discovery (SEND)" [RFC3971],

   *  "Cryptographically Generated Addresses (CGA)" [RFC3972],

   *  "Neighbor Discovery for IP version 6 (IPv6)" [RFC4861] ,

   *  "IPv6 Stateless Address Autoconfiguration" [RFC4862], and

   *  "IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
      Overview, Assumptions, Problem Statement, and Goals" [RFC4919].

2.3.  Abbreviations

   This document uses the following abbreviations:

   6BBR:     6LoWPAN Backbone Router
   6LBR:     6LoWPAN Border Router
   6LN:      6LoWPAN Node
   6LR:      6LoWPAN Router
   AP-ND:    Address-Protected Neighbor Discovery
   CGA:      Cryptographically Generated Address
   DAD:      Duplicate Address Detection
   EARO:     Extended Address Registration Option
   ECC:      Elliptic Curve Cryptography
   ECDH:     Elliptic Curve Diffie-Hellman
   ECDSA:    Elliptic Curve Digital Signature Algorithm
   EDAC:     Extended Duplicate Address Confirmation
   EDAR:     Extended Duplicate Address Request
   CIPO:     Crypto-ID Parameters Option
   LLN:      Low-Power and Lossy Network
   NA:       Neighbor Advertisement
   ND:       Neighbor Discovery
   NDP:      Neighbor Discovery Protocol
   NDPSO:    Neighbor Discovery Protocol Signature Option
   NS:       Neighbor Solicitation
   ROVR:     Registration Ownership Verifier
   RA:       Router Advertisement
   RS:       Router Solicitation
   RSAO:     RSA Signature Option
   SHA:      Secure Hash Algorithm
   SLAAC:    Stateless Address Autoconfiguration
   TID:      Transaction ID

3.  Updating RFC 8505

   Section 5.3 of [RFC8505] introduces the ROVR that is used to detect
   and reject duplicate registrations in the DAD process.  The ROVR is a
   generic object that is designed for both backward compatibility and
   the capability to introduce new computation methods in the future.
   Using a Crypto-ID per this specification is the RECOMMENDED method.
   Section 7.5 discusses collisions when heterogeneous methods to
   compute the ROVR field coexist inside a network.

   This specification introduces a new identifier called a Crypto-ID
   that is transported in the ROVR field and used to indirectly prove
   the ownership of an address that is being registered by means of
   [RFC8505].  The Crypto-ID is derived from a cryptographic public key
   and additional parameters.

   The overall mechanism requires the support of Elliptic Curve
   Cryptography (ECC) and a hash function as detailed in Section 6.2.
   To enable the verification of the proof, the Registering Node needs
   to supply certain parameters including a nonce and a signature that
   will demonstrate that the node possesses the private key
   corresponding to the public key used to build the Crypto-ID.

   The elliptic curves and the hash functions listed in Table 1 in
   Section 8.2 can be used with this specification; more may be added in
   the future to the corresponding IANA registry.  The cryptographic
   algorithms used (including the curve and the representation
   conventions) are signaled by the Crypto-Type field in a new IPv6 ND
   Crypto-ID Parameters Option (CIPO) (see Section 4.3) that contains
   the parameters that are necessary for address validation.  A new NDP
   Signature Option (Section 4.4) is also specified in this document to
   carry the resulting signature.  A Nonce Option [RFC3971] is added in
   the NA(EARO) that is used to request the validation, and all three
   options are needed in the NS(EARO) that provides the validation.

4.  New Fields and Options

4.1.  New Crypto-ID

   The Crypto-ID is transported in the ROVR field of the EARO and the
   Extended Duplicate Address Request (EDAR) message and is associated
   with the Registered Address at the 6LR and the 6LBR.  The ownership
   of a Crypto-ID can be demonstrated by cryptographic mechanisms, and
   by association, the ownership of the Registered Address can be

   A node in possession of the necessary cryptographic primitives SHOULD
   use Crypto-ID by default as ROVR in its registrations.  Whether a
   ROVR is a Crypto-ID is indicated by a new "C" flag in the EARO of the
   NS(EARO) message.

   The Crypto-ID is derived from the public key and a modifier as

   1.  The hash function used internally by the signature scheme and
       indicated by the Crypto-Type (see Table 1 in Section 8.2) is
       applied to the CIPO.  Note that all the reserved and padding bits
       MUST be set to zero.

   2.  The leftmost bits of the resulting hash, up to the desired size,
       are used as the Crypto-ID.

   At the time of this writing, a minimal size for the Crypto-ID of 128
   bits is RECOMMENDED unless backward compatibility is needed [RFC8505]
   (in which case it is at least 64 bits).  The size of the Crypto-ID is
   likely to increase in the future.

4.2.  Updated EARO

   This specification updates the EARO to enable the use of the ROVR
   field to transport the Crypto-ID.  The resulting format is as

      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    |    Status     |    Opaque     |
     |Rsvd |C| I |R|T|     TID       |     Registration Lifetime     |
     |                                                               |
    ...            Registration Ownership Verifier (ROVR)           ...
     |                                                               |

               Figure 1: Enhanced Address Registration Option

   Type:  33

   Length:  Defined in [RFC8505] and copied in the "EARO Length" field
      in the associated CIPO.

   Status:  Defined in [RFC8505].

   Opaque:  Defined in [RFC8505].

   Rsvd (Reserved):  3-bit unsigned integer.  It MUST be set to zero by
      the sender and MUST be ignored by the receiver.

   C:  This "C" flag is set to indicate that the ROVR field contains a
      Crypto-ID and that the 6LN MAY be challenged for ownership as
      specified in this document.

   I, R, T:  Defined in [RFC8505].

   TID and Registration Lifetime:  Defined in [RFC8505].

   Registration Ownership Verifier (ROVR):  When the "C" flag is set,
      this field contains a Crypto-ID.

   This specification uses the status codes "Validation Requested" and
   "Validation Failed", which are defined in [RFC8505].

   This specification does not define any new status codes.

4.3.  Crypto-ID Parameters Option

   This specification defines the CIPO.  The CIPO carries the parameters
   used to form a Crypto-ID.

   In order to provide cryptographic agility [BCP201], this
   specification supports different elliptic-curve-based signature
   schemes, indicated by a Crypto-Type field:

   *  The ECDSA256 signature scheme, which uses ECDSA with the NIST
      P-256 curve [FIPS186-4] and the hash function SHA-256 [RFC6234]
      internally, MUST be supported by all implementations.

   *  The Ed25519 signature scheme, which uses the Pure Edwards-Curve
      Digital Signature Algorithm (PureEdDSA) [RFC8032] with the twisted
      Edwards curve Edwards25519 [RFC7748] and the hash function SHA-512
      [RFC6234] internally, MAY be supported as an alternative.

   *  The ECDSA25519 signature scheme, which uses ECDSA [FIPS186-4] with
      the Weierstrass curve Wei25519 (see Appendix B.4) and the hash
      function SHA-256 [RFC6234] internally, MAY also be supported.

   This specification uses signature schemes that target similar
   cryptographic strength but rely on different curves, hash functions,
   signature algorithms, and/or representation conventions.  Future
   specification may extend this to different cryptographic algorithms
   and key sizes, e.g., to provide better security properties or a
   simpler implementation.

       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     |Reserved1|  Public Key Length  |
      |  Crypto-Type  | Modifier      |  EARO Length  |               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+               +
      |                                                               |
      .                                                               .
      .                  Public Key (variable length)                 .
      .                                                               .
      |                                                               |
      |                                                               |
      .                           Padding                             .
      |                                                               |

                   Figure 2: Crypto-ID Parameters Option

   Type:  8-bit unsigned integer.  IANA has assigned value 39; see
      Table 2.

   Length:  8-bit unsigned integer.  The length of the option in units
      of 8 octets.

   Reserved1:  5-bit unsigned integer.  It MUST be set to zero by the
      sender and MUST be ignored by the receiver.

   Public Key Length:  11-bit unsigned integer.  The length of the
      Public Key field in bytes.  The actual length depends on the
      Crypto-Type value and how the public key is represented.  The
      valid values with this document are provided in Table 1.

   Crypto-Type:  8-bit unsigned integer.  The type of cryptographic
      algorithm used in calculation of the Crypto-ID indexed by IANA in
      the "Crypto-Types" subregistry in the "Internet Control Message
      Protocol version 6 (ICMPv6) Parameters" registry (see
      Section 8.2).

   Modifier:  8-bit unsigned integer.  Set to an arbitrary value by the
      creator of the Crypto-ID.  The role of the modifier is to enable
      the formation of multiple Crypto-IDs from the same key pair.  This
      reduces the traceability and, thus, improves the privacy of a
      constrained node without requiring many key pairs.

   EARO Length:  8-bit unsigned integer.  The option length of the EARO
      that contains the Crypto-ID associated with the CIPO.

   Public Key:  A variable-length field; the size is indicated in the
      Public Key Length field.

   Padding:  A variable-length field that completes the Public Key field
      to align to the next 8-byte boundary.  It MUST be set to zero by
      the sender and MUST be ignored by the receiver.

   The implementation of multiple hash functions in a constrained device
   may consume excessive amounts of program memory.  This specification
   enables the use of the same hash function SHA-256 [RFC6234] for two
   of the three supported ECC-based signature schemes.  Some code
   factorization is also possible for the ECC computation itself.

   [CURVE-REPR] provides information on how to represent Montgomery
   curves and (twisted) Edwards curves as curves in short-Weierstrass
   form, and it illustrates how this can be used to implement elliptic
   curve computations using existing implementations that already
   provide, e.g., ECDSA and ECDH using NIST [FIPS186-4] prime curves.
   For more details on representation conventions, refer to Appendix B.

4.4.  NDP Signature Option

   This specification defines the NDP Signature Option (NDPSO).  The
   NDPSO carries the signature that proves the ownership of the Crypto-
   ID and validates the address being registered.  The format of the
   NDPSO is illustrated in Figure 3.

   As opposed to the RSA Signature Option (RSAO) defined in Section 5.2
   of SEND [RFC3971], the NDPSO does not have a key hash field.
   Instead, the leftmost 128 bits of the ROVR field in the EARO are used
   as hash to retrieve the CIPO that contains the key material used for
   signature verification, left-padded if needed.

   Another difference is that the NDPSO signs a fixed set of fields as
   opposed to all options that appear prior to it in the ND message that
   bears the signature.  This allows a CIPO that the 6LR already
   received to be omitted, at the expense of the capability to add
   arbitrary options that would be signed with an RSAO.

   An ND message that carries an NDPSO MUST have one and only one EARO.
   The EARO MUST contain a Crypto-ID in the ROVR field, and the Crypto-
   ID MUST be associated with the key pair used for the digital
   signature in the NDPSO.

   The CIPO may be present in the same message as the NDPSO.  If it is
   not present, it can be found in an abstract table that was created by
   a previous message and indexed by the hash.

       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     |Reserved1|  Signature Length   |
      |                            Reserved2                          |
      |                                                               |
      .                                                               .
      .          Digital Signature  (variable length)                 .
      .                                                               .
      |                                                               |
      |                                                               |
      .                           Padding                             .
      |                                                               |

                       Figure 3: NDP Signature Option

   Type:  IANA has assigned value 40; see Table 2.

   Length:  8-bit unsigned integer.  The length of the option in units
      of 8 octets.

   Reserved1:  5-bit unsigned integer.  It MUST be set to zero by the
      sender and MUST be ignored by the receiver.

   Digital Signature Length:  11-bit unsigned integer.  The length of
      the Digital Signature field in bytes.

   Reserved2:  32-bit unsigned integer.  It MUST be set to zero by the
      sender and MUST be ignored by the receiver.

   Digital Signature:  A variable-length field containing the digital
      signature.  The length and computation of the digital signature
      both depend on the Crypto-Type, which is found in the associated
      CIPO; see Appendix B.  For the values of the Crypto-Type defined
      in this specification, and for future values of the Crypto-Type
      unless specified otherwise, the signature is computed as detailed
      in Section 6.2.

   Padding:  A variable-length field completing the Digital Signature
      field to align to the next 8-byte boundary.  It MUST be set to
      zero by the sender and MUST be ignored by the receiver.

4.5.  Extensions to the Capability Indication Option

   This specification defines one new capability bit in the 6LoWPAN
   Capability Indication Option (6CIO), as defined by [RFC7400], for use
   by the 6LR and 6LBR in IPv6 ND RA messages.

       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 = 1  |   Reserved      |A|D|L|B|P|E|G|
      |                           Reserved                            |

                  Figure 4: New Capability Bit in the 6CIO

   New Option Field:

   A:  1-bit flag.  Set to indicate that AP-ND is globally activated in
      the network.

   The "A" flag is set by the 6LBR that serves the network and is
   propagated by the 6LRs.  It is typically turned on when all 6LRs are
   migrated to this specification.

5.  Protocol Scope

   The scope of the protocol specified here is a 6LoWPAN LLN, typically
   a stub network connected to a larger IP network via a border router
   called a 6LBR per [RFC6775].  A 6LBR has sufficient capability to
   satisfy the needs of DAD.

   The 6LBR maintains registration state for all devices in its attached
   LLN.  Together with the first-hop router (the 6LR), the 6LBR assures
   uniqueness and grants ownership of an IPv6 address before it can be
   used in the LLN.  This is in contrast to a traditional network that
   relies on IPv6 address autoconfiguration [RFC4862], where there is no
   guarantee of ownership from the network, and each IPv6 Neighbor
   Discovery packet must be individually secured [RFC3971].

                 ---+-------- ............
                    |      External Network
                 |     | 6LBR
               o    o   o
        o     o   o     o
           o   o LLN   o    o     o
              o     o
         o       o    o(6LR)
       o      o       | LLN link
            o     o   v

                       Figure 5: Basic Configuration

   In a mesh network, the 6LR is directly connected to the host device.
   This specification mandates that the peer-wise L2 security is
   deployed so that all the packets from a particular host are
   protected.  The 6LR may be multiple hops away from the 6LBR.  Packets
   are routed between the 6LR and the 6LBR via other 6LRs.

   This specification mandates that all the LLN links between the 6LR
   and the 6LBR are protected so that a packet that was validated by the
   first 6LR can be safely routed by other on-path 6LRs to the 6LBR.

6.  Protocol Flows

   The 6LR/6LBR ensures first come, first served by storing the ROVR
   associated to the address being registered upon the first
   registration and rejecting a registration with a different ROVR
   value.  A 6LN can claim any address as long as it is the first to
   make that claim.  After a successful registration, the 6LN becomes
   the owner of the Registered Address, and the address is bound to the
   ROVR value in the 6LR/6LBR registry.

   This specification protects the ownership of the address at the first
   hop (the edge).  Its use in a network is signaled by the "A" flag in
   the 6CIO.  The flag is set by the 6LBR and propagated unchanged by
   the 6LRs.  Once every node in the network is upgraded to support this
   specification, the "A" flag can be set to turn the protection on

   The 6LN places a cryptographic identifier, the Crypto-ID, in the ROVR
   that is associated with the address at the first registration,
   enabling the 6LR to later challenge it to verify that it is the
   original Registering Node.  The challenge may happen at any time at
   the discretion of the 6LR and the 6LBR.  A valid registration in the
   6LR or the 6LBR MUST NOT be altered until the challenge is complete.

   When the "A" flag in a subnet is set, the 6LR MUST challenge the 6LN
   before it creates a Binding with the "C" flag set in the EARO.  The
   6LR MUST also challenge the 6LN when a new registration attempts to
   change a parameter of an already validated Binding for that 6LN, for
   instance, its Source link-layer address.  Such verification protects
   against an attacker that attempts to steal the address of an honest

   The 6LR MUST indicate to the 6LBR that it performed a successful
   validation by setting a status code of 5 ("Validation Requested") in
   the EDAR.  Upon a subsequent EDAR from a new 6LR with a status code
   that is not 5 for a validated Binding, the 6LBR MUST indicate to the
   new 6LR that it needs to challenge the 6LN using a status code of 5
   in the Extended Duplicate Address Confirmation (EDAC).

   The 6LR MUST challenge the 6LN when the 6LBR signals to do so, which
   is done with an EDAC message with a status code of 5.  The EDAC is
   echoed by the 6LR in the NA(EARO) back to the Registering Node.  The
   6LR SHOULD also challenge all its attached 6LNs at the time the 6LBR
   turns the "A" flag on in the 6CIO in orders to detect an issue

   If the 6LR does not support the Crypto-Type, it MUST reply with an
   EARO status code of 10 "Validation Failed" without a challenge.  In
   that case, the 6LN may try another Crypto-Type until it falls back to
   Crypto-Type 0, which MUST be supported by all 6LRs.

   A node may use more than one IPv6 address at the same time.  The
   separation of the address and the cryptographic material avoids the
   need for the constrained device to compute multiple keys for multiple
   addresses.  The 6LN MAY use the same Crypto-ID to prove the ownership
   of multiple IPv6 addresses.  The 6LN MAY also derive multiple Crypto-
   IDs from the same key pair by changing the modifier.

6.1.  First Exchange with a 6LR

   A 6LN registers to a 6LR that is one hop away from it with the "C"
   flag set in the EARO, indicating that the ROVR field contains a
   Crypto-ID.  The Target Address in the NS message indicates the IPv6
   address that the 6LN is trying to register [RFC8505].  The on-link
   (local) protocol interactions are shown in Figure 6.  If the 6LR does
   not have a state with the 6LN that is consistent with the NS(EARO),
   then it replies with a challenge NA(EARO, status=Validation
   Requested) that contains a Nonce Option (shown as NonceLR in
   Figure 6).

       6LN                                                     6LR
        |                                                       |
        |<------------------------- RA -------------------------|
        |                                                       | ^
        |---------------- NS with EARO (Crypto-ID) ------------>| |
        |                                                       | option
        |<- NA with EARO(status=Validation Requested), NonceLR  | |
        |                                                       | v
        |------- NS with EARO, CIPO, NonceLN and NDPSO -------->|
        |                                                       |
        |<------------------- NA with EARO ---------------------|
        |                                                       |
        |                                                       |
        |--------------- NS with EARO (Crypto-ID) ------------->|
        |                                                       |
        |<------------------- NA with EARO ---------------------|
        |                                                       |
        |                                                       |
        |--------------- NS with EARO (Crypto-ID) ------------->|
        |                                                       |
        |<------------------- NA with EARO ---------------------|
        |                                                       |

                    Figure 6: On-Link Protocol Operation

   The Nonce Option contains a nonce value that, to the extent possible
   for the implementation, was never used before.  This specification
   inherits the idea from [RFC3971] that the nonce is a random value.
   Ideally, an implementation uses an unpredictable cryptographically
   random value [BCP106].  But that may be impractical in some LLN
   scenarios with resource-constrained devices.

   Alternatively, the device may use an always-incrementing value saved
   in the same stable storage as the key, so they are lost together, and
   start at a best-effort random value as either the nonce value or a
   component to its computation.

   The 6LN replies to the challenge with an NS(EARO) that includes the
   Nonce Option (shown as NonceLN in Figure 6), the CIPO (Section 4.3),
   and the NDPSO containing the signature.  Both nonces are included in
   the signed material.  This provides a "contributory behavior" that
   results in better security even when the nonces of one party are not
   generated as specified.

   The 6LR MUST store the information associated with a Crypto-ID on the
   first NS exchange where it appears in a fashion that the CIPO
   parameters can be retrieved from the Crypto-ID alone.

   The steps for the registration to the 6LR are as follows:

   Upon the first exchange with a 6LR, a 6LN will be challenged to prove
   ownership of the Crypto-ID and the Target Address being registered in
   the Neighbor Solicitation message.  When a 6LR receives an NS(EARO)
   registration with a new Crypto-ID as a ROVR, and unless the
   registration is rejected for another reason, it MUST challenge by
   responding with an NA(EARO) with a status code of "Validation

   Upon receiving a first NA(EARO) with a status code of "Validation
   Requested" from a 6LR, the Registering Node SHOULD retry its
   registration with a CIPO (Section 4.3) that contains all the
   necessary material for building the Crypto-ID, the NonceLN that it
   generated, and the NDP Signature Option (Section 4.4) that proves its
   ownership of the Crypto-ID and intent of registering the Target
   Address.  In subsequent revalidation with the same 6LR, the 6LN MAY
   try to omit the CIPO to save bandwidth, with the expectation that the
   6LR saved it.  If the validation fails and it gets challenged again,
   then it SHOULD add the CIPO again.

   In order to validate the ownership, the 6LR performs the same steps
   as the 6LN and rebuilds the Crypto-ID based on the parameters in the
   CIPO.  If the rebuilt Crypto-ID matches the ROVR, the 6LN also
   verifies the signature contained in the NDPSO.  At that point, if the
   signature in the NDPSO can be verified, then the validation succeeds.
   Otherwise, the validation fails.

   If the 6LR fails to validate the signed NS(EARO), it responds with a
   status code of "Validation Failed".  After receiving an NA(EARO) with
   a status code of "Validation Failed", the Registering Node SHOULD try
   an alternate Crypto-Type; even if Crypto-Type 0 fails, it may try to
   register a different address in the NS message.

6.2.  NDPSO Generation and Verification

   The signature generated by the 6LN to provide proof of ownership of
   the private key is carried in the NDPSO.  It is generated by the 6LN
   in a fashion that depends on the Crypto-Type (see Table 1 in
   Section 8.2) chosen by the 6LN as follows:

   *  Form the message to be signed, by concatenating the following
      byte-strings in the order listed:

      1.  The 128-bit Message Type tag [RFC3972] (in network byte
          order).  For this specification, the tag is given in
          Section 8.1.  (The tag value has been generated by the editor
          of this specification on <https://www.random.org>.)

      2.  The CIPO.

      3.  The 16-byte Target Address (in network byte order) sent in the
          NS message.  It is the address that the 6LN is registering
          with the 6LR and 6LBR.

      4.  The NonceLR received from the 6LR (in network byte order) in
          the NA message.  The nonce is at least 6 bytes long as defined
          in [RFC3971].

      5.  The NonceLN sent from the 6LN (in network byte order).  The
          nonce is at least 6 bytes long as defined in [RFC3971].

      6.  The 1-byte option length of the EARO containing the Crypto-ID.

   *  Apply the signature algorithm specified by the Crypto-Type using
      the private key.

   Upon receiving the NDPSO and CIPO options, the 6LR first checks that
   the EARO Length in the CIPO matches the length of the EARO.  If so,
   it regenerates the Crypto-ID based on the CIPO to make sure that the
   leftmost bits up to the size of the ROVR match.

   If, and only if, the check is successful, it tries to verify the
   signature in the NDPSO using the following steps:

   *  Form the message to be verified, by concatenating the following
      byte-strings in the order listed:

      1.  The 128-bit Message Type tag given in Section 8.1 (in network
          byte order).

      2.  The CIPO.

      3.  The 16-byte Target Address (in network byte order) received in
          the NS message.  It is the address that the 6LN is registering
          with the 6LR and 6LBR.

      4.  The NonceLR sent in the NA message.  The nonce is at least 6
          bytes long as defined in [RFC3971].

      5.  The NonceLN received from the 6LN (in network byte order) in
          the NS message.  The nonce is at least 6 bytes long as defined
          in [RFC3971].

      6.  The 1-byte EARO Length received in the CIPO.

   *  Verify the signature on this message with the public key in the
      CIPO and the locally computed values using the signature algorithm
      specified by the Crypto-Type.  If the verification succeeds, the
      6LR propagates the information to the 6LBR using an EDAR/EDAC

   *  Due to the first-come, first-served nature of the registration, if
      the address is not registered to the 6LBR, then flow succeeds and
      both the 6LR and 6LBR add the state information about the Crypto-
      ID and Target Address being registered to their respective
      abstract databases.

6.3.  Multi-Hop Operation

   A new 6LN that joins the network autoconfigures an address and
   performs an initial registration to a neighboring 6LR with an NS
   message that carries an EARO [RFC8505].

   In a multi-hop 6LoWPAN, the registration with Crypto-ID is propagated
   to 6LBR as shown in Figure 7, which illustrates the registration flow
   all the way to a 6LoWPAN Backbone Router (6BBR) [RFC8929].

        6LN              6LR             6LBR            6BBR
         |                |               |                |
         |   NS(EARO)     |               |                |
         |--------------->|               |                |
         |                | Extended DAR  |                |
         |                |-------------->|                |
         |                |               | proxy NS(EARO) |
         |                |               |--------------->|
         |                |               |                | NS(DAD)
         |                |               |                | ------>
         |                |               |                |
         |                |               |                | <wait>
         |                |               |                |
         |                |               | proxy NA(EARO) |
         |                |               |<---------------|
         |                | Extended DAC  |                |
         |                |<--------------|                |
         |   NA(EARO)     |               |                |
         |<---------------|               |                |
         |                |               |                |

                      Figure 7: (Re-)Registration Flow

   The 6LR and the 6LBR communicate using ICMPv6 EDAR and EDAC messages
   [RFC8505] as shown in Figure 7.  This specification extends EDAR/EDAC
   messages to carry cryptographically generated ROVR.

   The assumption is that the 6LR and the 6LBR maintain a security
   association to authenticate and protect the integrity of the EDAR and
   EDAC messages, so there is no need to propagate the proof of
   ownership to the 6LBR.  The 6LBR implicitly trusts that the 6LR
   performs the verification when the 6LBR requires it, and if there is
   no further exchange from the 6LR to remove the state, the
   verification succeeded.

7.  Security Considerations

7.1.  Brown Field

   Only 6LRs that are upgraded to this specification are capable of
   challenging a registration and avoiding an attack.  In a brown
   (mixed) network, an attacker may attach to a legacy 6LR and fool the
   6LBR.  So even if the "A" flag could be set at any time to test the
   protocol operation, the security will only be effective when all the
   6LRs are upgraded.

7.2.  Threats Identified in RFC 3971

   Observations regarding the following threats to the local network in
   [RFC3971] also apply to this specification.

   Neighbor Solicitation/Advertisement Spoofing:  Threats in
      Section 9.2.1 of [RFC3971] apply.  AP-ND counters the threats on
      NS(EARO) messages by requiring that the NDPSO and CIPO be present
      in these solicitations.

   Duplicate Address Detection DoS Attack:  Inside the LLN, duplicate
      addresses are sorted out using the ROVR.  A different ROVR for the
      same Registered Address entails a rejection of the second
      registration [RFC8505].  DADs coming from the backbone network are
      not forwarded over the LLN to provide some protection against DoS
      attacks inside the resource-constrained part of the network.
      However, the EARO is present in the NS/NA messages exchanged over
      the backbone network.  This protects against misinterpreting node
      movement as a duplication and enables the Backbone Routers to
      determine which subnet has the most recent registration [RFC8505]
      and is thus the best candidate to validate the registration

   Router Solicitation and Advertisement Attacks:  This specification
      does not change the protection of RS and RA, which can still be
      protected by SEND.

   Replay Attacks:  Nonces should never repeat but they do not need to
      be unpredictable for secure operation.  Using nonces (NonceLR and
      NonceLN) generated by both the 6LR and 6LN ensures a contributory
      behavior that provides an efficient protection against replay
      attacks of the challenge/response flow.  The quality of the
      protection by a random nonce depends on the random number

   Neighbor Discovery DoS Attack:  A rogue node that can access the L2
      network may form many addresses and register them using AP-ND.
      The perimeter of the attack is all the 6LRs in range of the
      attacker.  The 6LR MUST protect itself against overflows and
      reject excessive registration with a status code of 2 "Neighbor
      Cache Full".  This effectively blocks another (honest) 6LN from
      registering to the same 6LR, but the 6LN may register to other
      6LRs that are in its range but not in that of the attacker.

7.3.  Related to 6LoWPAN ND

   The threats and mitigations discussed in 6LoWPAN ND [RFC6775]
   [RFC8505] also apply here, in particular, denial-of-service (DoS)
   attacks against the registry at the 6LR or 6LBR.

   Secure ND [RFC3971] forces the IPv6 address to be cryptographic since
   it integrates the CGA as the IID in the IPv6 address.  In contrast,
   this specification saves about 1 KB in every NS/NA message.  Also,
   this specification separates the cryptographic identifier from the
   registered IPv6 address so that a node can have more than one IPv6
   address protected by the same cryptographic identifier.

   With this specification, the 6LN can freely form its IPv6 address(es)
   in any fashion, thereby enabling either 6LoWPAN compression for IPv6
   addresses that are derived from L2 addresses or temporary addresses
   that cannot be compressed, e.g., formed pseudorandomly and released
   in relatively short cycles for privacy reasons [RFC8064][RFC8065].

   This specification provides added protection for addresses that are
   obtained following due procedure [RFC8505] but does not constrain the
   way the addresses are formed or the number of addresses that are used
   in parallel by a same entity.  An attacker may still perform a DoS
   attack against the registry at the 6LR or 6LBR or attempt to deplete
   the pool of available addresses at L2 or L3.

7.4.  Compromised 6LR

   This specification distributes the challenge and its validation at
   the edge of the network, between the 6LN and its 6LR.  This protects
   against DoS attacks targeted at that central 6LBR.  This also saves
   back-and-forth exchanges across a potentially large and constrained

   The downside is that the 6LBR needs to trust the 6LR to perform the
   checking adequately, and the communication between the 6LR and the
   6LBR must be protected to avoid tampering with the result of the

   If a 6LR is compromised, and provided that it knows the ROVR field
   used by the real owner of the address, the 6LR may pretend that the
   owner has moved, is now attached to it, and has successfully passed
   the Crypto-ID validation.  The 6LR may then attract and inject
   traffic at will on behalf of that address, or let an attacker take
   ownership of the address.

7.5.  ROVR Collisions

   A collision of ROVRs (i.e., the Crypto-ID in this specification) is
   possible, but it is a rare event.  Assuming that the hash used for
   calculating the Crypto-ID is a well-behaved cryptographic hash, and,
   thus, random collisions are the only ones possible, if n = 2^(k) is
   the maximum number of hash values (i.e., a k-bit hash) and p is the
   number of nodes, then (assuming one Crypto-ID per node) the formula 1
   - e^(-p^(2)/(2n)) provides an approximation of the probability that
   there is at least one collision (birthday paradox).

   If the Crypto-ID is 64 bits (the least possible size allowed), the
   chance of a collision is 0.01% for a network of 66 million nodes.
   Moreover, the collision is only relevant when this happens within one
   stub network (6LBR).  In the case of such a collision, an honest node
   might accidentally claim the Registered Address of another legitimate
   node (with the same Crypto-ID).  To prevent such rare events, it is
   RECOMMENDED that nodes do not derive the address being registered
   from the ROVR.

7.6.  Implementation Attacks

   The signature schemes referenced in this specification comply with
   NIST [FIPS186-4] or Crypto Forum Research Group (CFRG) standards
   [RFC8032] and offer strong algorithmic security at roughly a 128-bit
   security level.  These signature schemes use elliptic curves that
   either were specifically designed with exception-free and constant-
   time arithmetic in mind [RFC7748] or have extensive implementation
   experience of resistance to timing attacks [FIPS186-4].

   However, careless implementations of the signing operations could
   nevertheless leak information on private keys.  For example, there
   are micro-architectural side channel attacks that implementors should
   be aware of [breaking-ed25519].  Implementors should be particularly
   aware that a secure implementation of Ed25519 requires a protected
   implementation of the hash function SHA-512, whereas this is not
   required with implementations of the hash function SHA-256 used with
   ECDSA256 and ECDSA25519.

7.7.  Cross-Algorithm and Cross-Protocol Attacks

   The key pair used in this specification can be self-generated, and
   the public key does not need to be exchanged, e.g., through
   certificates, with a third party before it is used.

   New key pairs can be formed for new registrations if the node
   desires.  However, the same private key MUST NOT be reused with more
   than one instantiation of the signature scheme in this specification.
   Also, the same private key MUST NOT be used for anything other than
   computing NDPSO signatures per this specification.

   ECDSA shall be used strictly as specified in [FIPS186-4].  In
   particular, each signing operation of ECDSA MUST use randomly
   generated ephemeral private keys and MUST NOT reuse the ephemeral
   private key k across signing operations.  This precludes the use of
   deterministic ECDSA without a random input for the determination of
   k, which is deemed dangerous for the intended applications this
   document aims to serve.

7.8.  Public Key Validation

   Public keys contained in the CIPO field (which are used for signature
   verification) shall be verified to be correctly formed, by checking
   that this public key is indeed a point of the elliptic curve
   indicated by the Crypto-Type and that this point does have the proper

   For points used with the signature scheme Ed25519, one MUST check
   that this point is not in the small subgroup (see Appendix B.1 of
   [CURVE-REPR]); for points used with the signature scheme ECDSA (i.e.,
   both ECDSA256 and ECDSA25519), one MUST check that the point has the
   same order as the base point of the curve in question.  This is
   commonly called "full public key validation" (again, see Appendix B.1
   of [CURVE-REPR]).

7.9.  Correlating Registrations

   The ROVR field in the EARO introduced in [RFC8505] extends the EUI-64
   field of the ARO defined in [RFC6775].  One of the drawbacks of using
   an EUI-64 as ROVR is that an attacker that is aware of the
   registrations can correlate traffic for the same 6LN across multiple
   addresses.  Section 3 of [RFC8505] indicates that the ROVR and the
   address being registered are decoupled.  A 6LN may use the same ROVR
   for multiple registrations or a different ROVR per registration, and
   the IID must not be derived from the ROVR.  In theory, different 6LNs
   could use the same ROVR as long as they do not attempt to register
   the same address.

   The modifier used in the computation of the Crypto-ID enables a 6LN
   to build different Crypto-IDs for different addresses with the same
   key pair.  Using that facility improves the privacy of the 6LN at the
   expense of storage in the 6LR, which will need to store multiple
   CIPOs that contain the same public key.  Note that if an attacker
   gains access to the 6LR, then the modifier alone does not provide
   protection, and the 6LN would need to generate different key pairs
   and link-layer addresses in an attempt to obfuscate its multiple

8.  IANA Considerations

8.1.  CGA Message Type

   This document defines a new 128-bit CGA Extension Type Tag under the
   "CGA Extension Type Tags" subregistry of the Cryptographically
   Generated Addresses (CGA) Message Type Name Space created by

   Tag: 0x8701 55c8 0cca dd32 6ab7 e415 f148 84d0.

8.2.  Crypto-Type Subregistry

   IANA has created the "Crypto-Types" subregistry in the "Internet
   Control Message Protocol version 6 (ICMPv6) Parameters" registry.
   The registry is indexed by an integer in the interval 0..255 and
   contains an elliptic curve, a hash function, a signature algorithm,
   representation conventions, public key size, and signature size, as
   shown in Table 1, which together specify a signature scheme.
   Detailed explanations are provided in Appendix B.

   The following Crypto-Type values are defined in this document:

   |Crypto-Type     |   0 (ECDSA256)  | 1 (Ed25519)  |  2 (ECDSA25519) |
   |Value           |                 |              |                 |
   |Elliptic Curve  |    NIST P-256   |  Curve25519  |    Curve25519   |
   |                |   [FIPS186-4]   |  [RFC7748]   |    [RFC7748]    |
   |Hash Function   |SHA-256 [RFC6234]|   SHA-512    |SHA-256 [RFC6234]|
   |                |                 |  [RFC6234]   |                 |
   |Signature       |ECDSA [FIPS186-4]|   Ed25519    |ECDSA [FIPS186-4]|
   |Algorithm       |                 |  [RFC8032]   |                 |
   |Representation  |   Weierstrass,  |   Edwards,   |   Weierstrass,  |
   |Conventions     | (un)compressed, | compressed,  | (un)compressed, |
   |                |  MSB/msb-order, |LSB/lsb-order,|  MSB/msb-order, |
   |                |      [SEC1]     |  [RFC8032]   |   [CURVE-REPR]  |
   |Public Key Size |   33/65 bytes   |   32 bytes   |   33/65 bytes   |
   |                |   (compressed/  | (compressed) |   (compressed/  |
   |                |  uncompressed)  |              |  uncompressed)  |
   |Signature Size  |     64 bytes    |   64 bytes   |     64 bytes    |
   |Reference       |     RFC 8928    |   RFC 8928   |     RFC 8928    |

                           Table 1: Crypto-Types

   New Crypto-Type values providing similar or better security may be
   defined in the future.

   Assignment of values for new Crypto-Type MUST be done through IANA
   with either "Specification Required" or "IESG Approval" as defined in
   BCP 26 [RFC8126].

8.3.  IPv6 ND Option Types

   This document registers two new ND option types under the subregistry
   "IPv6 Neighbor Discovery Option Formats":

         | Description                        | Type | Reference |
         | Crypto-ID Parameters Option (CIPO) |  39  | RFC 8928  |
         | NDP Signature Option (NDPSO)       |  40  | RFC 8928  |

                          Table 2: New ND Options

8.4.  New 6LoWPAN Capability Bit

   IANA has made an addition to the subregistry for "6LoWPAN Capability
   Bits" created for [RFC7400] as follows:

                | Bit | Description           | Reference |
                |  9  | AP-ND Enabled (1 bit) | RFC 8928  |

                    Table 3: New 6LoWPAN Capability Bit

9.  References

9.1.  Normative References

              National Institute of Standards and Technology, "Digital
              Signature Standard (DSS)", FIPS 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3971]  Arkko, J., Ed., Kempf, J., Zill, B., and P. Nikander,
              "SEcure Neighbor Discovery (SEND)", RFC 3971,
              DOI 10.17487/RFC3971, March 2005,

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,

   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
              2014, <https://www.rfc-editor.org/info/rfc7400>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <https://www.rfc-editor.org/info/rfc7748>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8505]  Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
              Perkins, "Registration Extensions for IPv6 over Low-Power
              Wireless Personal Area Network (6LoWPAN) Neighbor
              Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,

   [SEC1]     Standards for Efficient Cryptography, "SEC 1: Elliptic
              Curve Cryptography", Version 2, May 2009,

9.2.  Informative References

   [BCP106]   Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,

   [BCP201]   Housley, R., "Guidelines for Cryptographic Algorithm
              Agility and Selecting Mandatory-to-Implement Algorithms",
              BCP 201, RFC 7696, DOI 10.17487/RFC7696, November 2015,

              Samwel, N., Batina, L., Bertoni, G., Daemen, J., and R.
              Susella, "Breaking Ed25519 in WolfSSL", Topics in
              Cryptology - CT-RSA, pp. 1-20, March 2018,

              Struik, R., "Alternative Elliptic Curve Representations",
              Work in Progress, Internet-Draft, draft-ietf-lwig-curve-
              representations-14, 15 November 2020,

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

   [RFC4862]  Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
              Address Autoconfiguration", RFC 4862,
              DOI 10.17487/RFC4862, September 2007,

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, DOI 10.17487/RFC4919, August 2007,

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,

   [RFC6282]  Hui, J., Ed. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,

   [RFC7039]  Wu, J., Bi, J., Bagnulo, M., Baker, F., and C. Vogt, Ed.,
              "Source Address Validation Improvement (SAVI) Framework",
              RFC 7039, DOI 10.17487/RFC7039, October 2013,

   [RFC7217]  Gont, F., "A Method for Generating Semantically Opaque
              Interface Identifiers with IPv6 Stateless Address
              Autoconfiguration (SLAAC)", RFC 7217,
              DOI 10.17487/RFC7217, April 2014,

   [RFC8064]  Gont, F., Cooper, A., Thaler, D., and W. Liu,
              "Recommendation on Stable IPv6 Interface Identifiers",
              RFC 8064, DOI 10.17487/RFC8064, February 2017,

   [RFC8065]  Thaler, D., "Privacy Considerations for IPv6 Adaptation-
              Layer Mechanisms", RFC 8065, DOI 10.17487/RFC8065,
              February 2017, <https://www.rfc-editor.org/info/rfc8065>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,

   [RFC8929]  Thubert, P., Ed., Perkins, C.E., and E. Levy-Abegnoli,
              "IPv6 Backbone Router", RFC 8929, DOI 10.17487/RFC8929,
              November 2020, <https://www.rfc-editor.org/info/rfc8929>.

Appendix A.  Requirements Addressed in This Document

   In this section, the requirements of a secure Neighbor Discovery
   protocol for LLNs are stated.

   *  The protocol MUST be based on the Neighbor Discovery Optimization
      for the LLN protocol defined in [RFC6775].  RFC 6775 utilizes
      optimizations such as host-initiated interactions for sleeping
      resource-constrained hosts and the elimination of multicast
      address resolution.

   *  New options to be added to Neighbor Solicitation messages MUST
      lead to small packet sizes, especially compared with existing
      protocols such as SEND.  Smaller packet sizes facilitate low-power
      transmission by resource-constrained nodes on lossy links.

   *  The registration mechanism SHOULD be extensible to other LLN links
      and not be limited to IEEE 802.15.4 only.  LLN links for which a
      6lo "IPv6 over foo" specification exist, as well as low-power Wi-
      Fi, SHOULD be supported.

   *  As part of this protocol, a mechanism to compute a unique
      identifier should be provided with the capability to form a Link
      Local Address that SHOULD be unique at least within the LLN
      connected to a 6LBR.

   *  The Address Registration Option used in the ND registration SHOULD
      be extended to carry the relevant forms of the unique identifier.

   *  The Neighbor Discovery should specify the formation of a site-
      local address that follows the security recommendations from

Appendix B.  Representation Conventions

B.1.  Signature Schemes

   The signature scheme ECDSA256 corresponding to Crypto-Type 0 is
   ECDSA, as specified in [FIPS186-4], instantiated with the NIST prime
   curve P-256, as specified in Appendix D.1.2 of [FIPS186-4], and the
   hash function SHA-256, as specified in [RFC6234], where points of
   this NIST curve are represented as points of a short-Weierstrass
   curve (see [FIPS186-4]) and are encoded as octet strings in most-
   significant-bit first (msb) and most-significant-byte first (MSB)
   order.  The signature itself consists of two integers (r and s),
   which are each encoded as fixed-size octet strings in MSB and msb
   order.  For further details, see [FIPS186-4] for ECDSA, see
   Appendix B.3 for the encoding of public keys, and see Appendix B.2
   for signature encoding.

   The signature scheme Ed25519 corresponding to Crypto-Type 1 is EdDSA,
   as specified in [RFC8032], instantiated with the Montgomery curve
   Curve25519, as specified in [RFC7748], and the hash function SHA-512,
   as specified in [RFC6234], where points of this Montgomery curve are
   represented as points of the corresponding twisted Edwards curve
   Edwards25519 (see Appendix B.4) and are encoded as octet strings in
   least-significant-bit first (lsb) and least-significant-byte first
   (LSB) order.  The signature itself consists of a bit string that
   encodes a point of this twisted Edwards curve, in compressed format,
   and an integer encoded in LSB and lsb order.  For details on EdDSA
   and the encoding of public keys and signatures, see the specification
   of pure Ed25519 in [RFC8032].

   The signature scheme ECDSA25519 corresponding to Crypto-Type 2 is
   ECDSA, as specified in [FIPS186-4], instantiated with the Montgomery
   curve Curve25519, as specified in [RFC7748], and the hash function
   SHA-256, as specified in [RFC6234], where points of this Montgomery
   curve are represented as points of the corresponding short-
   Weierstrass curve Wei25519 (see Appendix B.4) and are encoded as
   octet strings in MSB and msb order.  The signature itself consists of
   a bit string that encodes two integers (r and s), which are each
   encoded as fixed-size octet strings in MSB and msb order.  For
   further details, see [FIPS186-4] for ECDSA, see Appendix B.3 for the
   encoding of public keys, and see Appendix B.2 for signature encoding.

B.2.  Representation of ECDSA Signatures

   With ECDSA, each signature is an ordered pair (r, s) of integers
   [FIPS186-4], where each integer is represented as a 32-octet string
   according to the FieldElement-to-OctetString conversion rules in
   [SEC1] and where the ordered pair of integers is represented as the
   right concatenation of these representation values (thereby resulting
   in a 64-octet string).  The inverse operation checks that the
   signature is a 64-octet string and represents the left-side and
   right-side halves of this string (each a 32-octet string) as the
   integers r and s, respectively, using the OctetString-to-FieldElement
   conversion rules in [SEC1].  In both cases, the field with these
   conversion rules is the set of integers modulo n, where n is the
   (prime) order of the base point of the curve in question.  (For
   elliptic curve nomenclature, see Appendix B.1 of [CURVE-REPR].)

B.3.  Representation of Public Keys Used with ECDSA

   ECDSA is specified to be used with elliptic curves in short-
   Weierstrass form.  Each point of such a curve is represented as an
   octet string using the Elliptic-Curve-Point-to-Octet-String
   conversion rules in [SEC1], where point compression may be enabled
   (which is indicated by the leftmost octet of this representation).
   The inverse operation converts an octet string to a point of this
   curve using the Octet-String-to-Elliptic-Curve-Point conversion rules
   in [SEC1], whereby the point is rejected if this is the so-called
   point at infinity.  (This is the case if the input to this inverse
   operation is an octet string of length 1.)

B.4.  Alternative Representations of Curve25519

   The elliptic curve Curve25519, as specified in [RFC7748], is a so-
   called Montgomery curve.  Each point of this curve can also be
   represented as a point of a twisted Edwards curve or as a point of an
   elliptic curve in short-Weierstrass form, via a coordinate
   transformation (a so-called isomorphic mapping).  The parameters of
   the Montgomery curve and the corresponding isomorphic curves in
   twisted Edwards curve and short-Weierstrass form are as indicated
   below.  Here, the domain parameters of the Montgomery curve
   Curve25519 and of the twisted Edwards curve Edwards25519 are as
   specified in [RFC7748]; the domain parameters of the elliptic curve
   Wei25519 in short-Weierstrass form comply with Section 6.1.1 of
   [FIPS186-4].  For further details on these curves and on the
   coordinate transformations referenced above, see [CURVE-REPR].

   General parameters (for all curve models):

   p  2^{255}-19
      (=0x7fffffff ffffffff ffffffff ffffffff ffffffff ffffffff ffffffff
   h  8
      (=2^{252} +  0x14def9de a2f79cd6 5812631a 5cf5d3ed)

   Montgomery curve-specific parameters (for Curve25519):

   A  486662
   B  1
   Gu 9 (=0x9)
      (=0x20ae19a1 b8a086b4 e01edd2c 7748d14c 923d4d7e 6d7c61b2 29e9c5a2

   Twisted Edwards curve-specific parameters (for Edwards25519):

   a  -1 (-0x01)
   d  -121665/121666
      (=0x52036cee 2b6ffe73 8cc74079 7779e898 00700a4d 4141d8ab 75eb4dca
      (=0x216936d3 cd6e53fe c0a4e231 fdd6dc5c 692cc760 9525a7b2 c9562d60
   Gy  4/5
      (=0x66666666 66666666 66666666 66666666 66666666 66666666 66666666

   Weierstrass curve-specific parameters (for Wei25519):

      (=0x2aaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaa98
      (=0x7b425ed0 97b425ed 097b425e d097b425 ed097b42 5ed097b4 260b5e9c
      (=0x2aaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa aaaaaaaa
      (=0x20ae19a1 b8a086b4 e01edd2c 7748d14c 923d4d7e 6d7c61b2 29e9c5a2


   Many thanks to Charlie Perkins for his in-depth review and
   constructive suggestions.  The authors are also especially grateful
   to Robert Moskowitz and Benjamin Kaduk for their comments and
   discussions that led to many improvements.  The authors wish to also
   thank Shwetha Bhandari for actively shepherding this document and
   Roman Danyliw, Alissa Cooper, Mirja Kühlewind, Éric Vyncke, Vijay
   Gurbani, Al Morton, and Adam Montville for their constructive reviews
   during the IESG process.  Finally, many thanks to our INT area ADs,
   Suresh Krishnan and Erik Kline, who supported us along the whole

Authors' Addresses

   Pascal Thubert (editor)
   Cisco Systems, Inc.
   Building D
   45 Allee des Ormes - BP1200
   06254 MOUGINS - Sophia Antipolis

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

   Behcet Sarikaya

   Email: sarikaya@ieee.org

   Mohit Sethi
   FI-02420 Jorvas

   Email: mohit@piuha.net

   Rene Struik
   Struik Security Consultancy

   Email: rstruik.ext@gmail.com