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Versions: 00 01 draft-kempf-abk-nd

                                                            James Kempf
   Internet Draft                                          Craig Gentry
   Document: draft-kempf-secure-nd-01.txt                         Alice
                                                             Silverberg
   Expires: December 2002                                     June 2002


     Securing IPv6 Neighbor Discovery Using Address Based Keys (ABKs)



Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.


   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   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."

   The list of current Internet-Drafts can be accessed at
        http://www.ietf.org/ietf/1id-abstracts.txt
   The list of Internet-Draft Shadow Directories can be accessed at
        http://www.ietf.org/shadow.html.


Abstract

   When an IPv6 node receives a Router Advertisement, how does it know
   that the node which sent the advertisement is authorized to announce
   that it routes the prefix? When an IPv6 node receives a Neighbor
   Advertisement message, how does it know that the node sending the
   message is, in fact, authorized to claim the binding? The answer is,
   in the absence of a preconfigured IPsec security association among
   the nodes on the link and the routers, they don't. In this draft, a
   lightweight protocol is described for securing the signaling
   involved in IPv6 Neighbor Discovery. The protocol allows a node
   receiving a Router Advertisement or a Neighbor Advertisement to have
   the confidence that the message was authorized by the legitimate
   owner of the address or prefix being advertised without requiring a
   preconfigured IPsec security association. A certain degree of
   infrastructural support is required, but not any more than is
   currently common for public access IP networks. The protocol is
   based on some results in identity based cryptosystems that allow a
   publicly known identifier to function as a public key.

   Contents

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   1.0  Introduction.................................................2
   2.0  Terminology..................................................3
   3.0  What are Identity Based Cryptosystems?.......................4
   4.0  Digital Signature Calculations...............................5
   5.0  Host and Router Configuration................................5
    5.1 Router Configuration.........................................6
    5.2 Host Configuration...........................................6
   6.0  Securing Router Advertisement................................7
    6.1 Router Advertisement Signature...............................7
    6.2 Verifying a Router Advertisement.............................8
    6.3 Negotiating an Identity based Algorithm......................8
   7.0  Securing Neighbor Discovery..................................9
    7.1 Neighbor Advertisement Signature.............................9
    7.2 Verifying a Neighbor Advertisement...........................9
    7.3 Negotiating an Identity Based Algorithm......................9
   8.0  Option Formats...............................................9
    8.1 Identity Digital Signature Option...........................10
    8.2 Identity Algorithm Option...................................11
   9.0  Identity Based Key Algorithms...............................11
   10.0  Previous Work..............................................12
   11.0  Infrastructure Requirements................................14
   12.0  Security Considerations....................................14
   13.0  References.................................................15
   14.0  Author's Contact Information...............................16
   15.0  Full Copyright Statement...................................17

1.0     Introduction

   The IPv6 Neighbor Discovery protocol described in RFC 2461 [1] plays
   a critical role in last hop network access for IPv6 nodes. The
   protocol allows a IP node joining a link to discover a default
   router, and for nodes on the link, including the routers, to
   discover the link layer address of an IP node on the link to which
   IP traffic must be delivered. Disruption of this protocol can have a
   serious impact on the ability of nodes to send and receive IP
   traffic.

   Yet, security on the protocol is weak. As stated in the Security
   Considerations section of RFC 2461:

      The protocol contains no mechanism to determine which
      neighbors are authorized to send a particular type of
      message...; any neighbor, presumably even in the presence of
      authentication, can send Router Advertisement messages thereby
      being able to cause denial of service. Furthermore, any
      neighbor can send proxy Neighbor Advertisements as well as
      unsolicited Neighbor Advertisements as a potential denial of
      service attack.


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   In [2], a list of threats to IPv6 Neighbor Discovery on multi-access
   links is outlined. The threats don't occur on point to point links
   because the default router and IP address for a host are determined
   by PPP negotiation and so Neighbor Discovery is not required. These
   threats can occur for both wired and wireless public multi-access
   links. They are a particular problem for wireless links, however,
   because even private multi-access links over shared access (as
   opposed to switched) media with link level authentication mechanisms
   such as 802.1x [22] are subject to disruption if an authenticated
   node decides to play the trickster.

   There are two underlying causes of these threats: a router
   advertising a prefix that it is not authorized to route or a node
   claiming an IPv6 address that it is not authorized to claim. These
   threats occur because the messaging involved in Neighbor Discovery
   by default contains no authentication information allowing the
   receiver to authenticate the sender. RFC 2461 recommends using IPsec
   AH authentication [4] if a security association exists, but this is
   a fairly heavyweight solution and is unlikely to be widely
   applicable to public access networks. In particular, a roaming node
   in a foreign public access network is unlikely to have a security
   association with a local access router or with other nodes on the
   same link. Indeed, most of the nodes on the same link may not even
   have the same home ISP as the roaming node. In addition, using IKE
   [28] or any other IPv6 protocol to establish a dynamic security
   association won't work if the protocol requires unsecured Neighbor
   Discovery. Manual keying can be used, but is impractical for public
   access networks.

   In this document, a lightweight protocol that secures IPv6 Neighbor
   Discovery is described. The protocol allows IP nodes to verify that
   a node advertising routing for a local subnet prefix is authorized
   to advertise the prefix, and that information provided in a Neighbor
   Discovery message is authorized by the sending node. A certain
   amount of infrastructure is required, but no more than is currently
   needed for public access IP networks. In particular, no extension of
   the current NAS-based AAA infrastructure [24] nor a global PKI are
   necessary. The protocol depends on some results in identity based
   cryptosystems whereby a publicly known identifier, in this case,
   parts of a node's IP address, can serve as a public key. The
   technique whereby addresses are used to generate public/private key
   pairs is called Address Based Keys (ABKs).

2.0     Terminology

      Address Based Keys (ABKs) - A technique whereby an identity
      based cryptosystem is used to generate a node's public and
      private key from its IPv6 subnet prefix or interface
      identifier.

      Identity based cryptosystem - A cryptographic technique that
      allows a publicly known identifier, such as the IPv6 address,


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      to be used as a public key for authentication, key agreement,
      and encryption.

      Identity based Private Key Generator (IPKG) - An agent that is
      capable of executing an identity based cryptographic algorithm
      to generate a private key when presented with the public
      identifier that will act as the public key. The IPKG is the
      root of trust in identity based crytosystems.

      Public cryptographic parameters - A collection of publicly
      known parameters which the IPKG uses to generate the node's
      private key and which two nodes involved in securing or
      encrypting a message use to perform cryptographic operations.
      The public cryptographic parameters are formed from chosen
      constants and a secret key known only to the IPKG, specific to
      the identity based cryptographic algorithm.

      Network Access Server (NAS) - A server that performs
      Authentication, Authorization, and Accounting (AAA) for nodes
      in a public access network.

   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 [23].

3.0     What are Identity Based Cryptosystems?

   Identity based cryptosystems are a collection of cryptographic
   techniques that allow a publicly known identifier, such as the email
   address or (particularly important in this application) the IP
   address of a node, to function as the public key part of a
   public/private key pair for purposes of digital signature
   calculation, key agreement, and encryption. Section 9.0 provides a
   quick overview of the available algorithms, with an extensive
   reference list. While identity based cryptosystems have been
   investigated for almost 20 years in the cryptographic community,
   they have not been widely discussed in the network security
   community. The reason is unclear, but it might have to do with the
   popularity and algorithmic simplicity of the reigning standard
   Diffie-Hellman technique, or possibly to the fact that, until
   recently, there have been no known identity based cryptographic
   algorithms that can be used to perform encryption. The existing
   algorithms have been restricted to digital signature calculation,
   and therefore have been fairly limited in scope. Hopefully, should
   identity based cryptosystems prove useful to the network security
   community, increased communication between the cryptography and
   network security communities will lead to a refinement of the
   algorithms and applications of identity based algorithms for
   application to securing IPv6 signaling.

   Elliptic curve (EC) algorithms are particularly attractive for
   identity based keys because they work well with small key sizes, are
   computationally efficient on small nodes, such as small wireless

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   devices, and may generate smaller signatures. In addition, while
   non-EC algorithms have been proposed for identity based digital
   signature calculation, at the time of this writing, the most
   efficient way of performing identity based encryption is an EC
   algorithm.

   Identity based cryptosystems work in the following way. A publicly
   known identifier is submitted to an IPKG. In this application, the
   publicly known identifier is either the 64 bit subnet prefix or the
   unique 64 bit interface identifier of an IPv6 address. The IPKG uses
   a particular algorithm to generate the private key and returns it.
   The public and private key can now be used for authentication and
   encryption as is typical in cryptosystems.

4.0     Digital Signature Calculations

   Digital signatures MUST be calculated using the following algorithm:

      sig = SIGN(hash(contents),IPrK,Params)

   where:

      sig      - The digital signature.
      SIGN     - The identity based digital signature algorithm used
                 to calculate the signature.
      hash     - The HMAC-SHA1 one-way hash algorithm.
      IPrK     - The Identity based Private Key.
      Params   - The public cryptographic parameters.
      contents - The message contents to be signed.

   The digital signature MUST be verified using the following
   algorithm:

      IPuK  = IBC-HASH(ID)
      valid = VERIFY(hash(contents),sig,IPuK)

   where:

      IBC-HASH  - A hash function specific to the identity based
                  algorithm that generates the public key from the
                  public identifier.
      ID        - The publicly known identifier used to generate the
                  key.
      IPuK      - The Identity based Public Key.
      sig       - The digital signature.
      VERIFY    - The identity based public key algorithm used to
                  verify the signature.
      Params    - The public cryptographic parameters.
      valid     - 1 if the signature is verified, 0 if not.

5.0     Host and Router Configuration



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   Hosts and last hop routers participating in Neighbor Discovery
   require configuration with the identity based private key and with
   cryptographic parameters before they can secure messaging.

5.1 Router Configuration

   When the ISP or network owner sets up its last hop routers, the
   routers are configured with the 64 bit subnet prefix or prefixes
   that they should advertise. In addition, the ISP uses its IPKG to
   generate a private key per prefix. The router uses this key in
   generating digital signatures on Router Advertisements. The private
   key and the public cryptographic parameters MUST be installed on the
   router through a secure channel. Examples of possible secure
   channels include configuration by a network administrator,
   installation via an NAS-based AAA network capable of secure key
   distribution, installation via a secure message exchange to a server
   with which the router has an IPsec security association, etc.

5.2 Host Configuration

   Hosts require an identity based private key associated with their 64
   bit interface identifier [3] in the IPv6 address, and the public
   cryptographic parameters. There are two possible ways in which the
   host can be configured:

      - Dynamically, when the host is initially authenticated and
        authorized for network access through a secure connection with
        the local network's NAS,

      - Statically, when its home ISP initially assigns the interface
        identifier.

   If the dynamic configuration method is used, the local network must
   keep track of interface identifiers to avoid duplicates. If the
   static configuration method is used, the cryptographic parameters
   for the local network's router must be installed on a roaming host,
   since the router's parameters may not be the same as those for the
   roaming host. Dynamic and static configuration are discussed in the
   next two paragraphs.

   Most public access networks currently require a host to undergo a
   secure authentication and authorization exchange through a NAS prior
   to being able to use the network. Since this exchange is typically
   performed at Layer 2 before any IP signaling, it can be done prior
   to any Neighbor Discovery signaling. The host includes its interface
   identifier in a message to the NAS. The NAS sends the interface
   identifier to the IPKG, where the private key is generated. The
   private key and public cryptographic parameters are then securely
   transferred back to the host where they are installed. The host uses
   this private key for securing IPv6 Neighbor Discovery traffic on the
   foreign network, not for securing any private data, because the key
   belongs to the foreign network. After router discovery, the host
   uses the interface id and subnet prefix from the router to construct

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   the router's IP address using IPv6 Stateless Address
   Autoconfiguration. The hosts on the local link and the last hop
   router then use the public cryptographic parameters and the private
   keys given to them by the network to secure IPv6 Neighbor Discovery
   signaling.

   Some public access networks may not perform secure Layer 2
   authentication and authorization prior to allowing the host to
   perform Neighbor Discovery. In order to accommodate these kinds of
   networks, hosts MUST be configured with public cryptographic
   parameters and a private key by their home ISPs or network
   operators. The messaging for securing Neighbor Discovery includes an
   identifier based on the realm portion of the NAI [25]. The realm
   identifies the host's home ISP. This identifier allows the hosts and
   routers on the local link to authenticate the signaling of guest
   hosts. However, some method is needed to co-ordinate distribution of
   public cryptographic parameters between ISPs.

   ISPs commonly use roaming consortia to provide remote access in
   areas where they do not have POPs. A group of ISPs organize into a
   roaming consortium to facilitate billing settlement and
   authentication. Roaming consortia can be used to support ABKs as
   well. A group of ISPs in a roaming consortium co-ordinate IPKGs so
   that the various ISPs in the consortium can accommodate guest hosts.
   The IPKGs use the same public cryptographic parameters, or are
   organized into an IPKG hierarchy [29]. Any private information (like
   a secret key) would need to be distributed between ISPs by secure
   means, such as a secure AAA connection or by hand.

6.0     Securing Router Advertisement

   In this section, a protocol for securing the IPv6 Router
   Advertisement messages is discussed.

6.1 Router Advertisement Signature

   A Router Advertisement sent by a router configured with a 64 bit
   prefix key contains a digital signature. The signature MUST sign the
   entire message.

   In the signing algorithm described in Section 4.0, the input into
   the HMAC-SHA1 algorithm is the following:

      contents = (chl,fl,rol,rel,rtt,sllao,mtuo,pro,dso,...)


   IPrK in the signing algorithm is the private key having the router's
   64 bit subnet prefix as its public key.

   The digital signature MUST be included in an Identity Digital
   Signature option (see Section 8.1) with the signature, algorithm,
   and realm identifier. An ICMP option is used instead of IPsec AH [4]
   because Neighbor Discovery options that are not recognized by a host

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   are ignored, so a host that can't verify the signature but is
   interested in risking using an unsecured Router Advertisement can
   simply ignore the option as a consequence of normal Neighbor
   Discovery processing, as opposed to having the Router Advertisement
   rejected by IPsec processing.

   The Router Advertisement MUST contain a single Prefix option with
   the prefix for which the key was assigned. If the router also
   announces other prefixes, it MUST advertise them using separate
   Router Advertisements. If the router supports multiple identity
   based algorithms, it MAY include multiple Identity Digital Signature
   options with signatures calculated by the various algorithms, up to
   the path MTU.

6.2 Verifying a Router Advertisement

   An IPv6 host receiving a Router Advertisement with an Identity
   Digital Signature Option verifies that the advertising node is
   authorized to send the advertisement in the following way. If the
   Router Advertisement does not contain a routing prefix option, or if
   it contains more than one routing prefix option, the host SHOULD
   discard the Router Advertisement, unless the host wants to risk
   using an unsecured Router Advertisement. If the host does not
   support one of the algorithms used for signing the message, it
   SHOULD discard the Router Advertisement, unless the host wants to
   risk using an unsecured Router Advertisement.

   The host locates the single routing prefix option and extracts the
   subnet prefix which the sending node claims it is allowed to route.
   The host then uses the verification algorithm in Section 4.0 to
   verify the digital signature using the same value for contents as in
   Section 6.1. In this calculation, ID is the subnet prefix in the
   Prefix option. The identity based algorithm and router public
   cryptographic parameters depend on the algorithm and realm
   identifier in the Identity Digital Signature option.

6.3 Negotiating an Identity based Algorithm

   A lengthy negotiation process for determining which identity based
   algorithm to use is obviously not in the interest of supporting a
   lightweight protocol. However, algorithms do change over time, and
   therefore it is necessary to have some way whereby a host can
   indicate in a Router Solicitation which algorithms it supports. If
   the router cannot provide an authenticator for any of the
   algorithms, it can simply return an unauthenticated Router
   Advertisement and the host can take its chances. For this purpose,
   the host uses an Identity Algorithm option (see Section 8.2).

   For multicast Router Advertisements, the router can include Identity
   Digital Signature options for each algorithm it supports, up to the
   path MTU. Alternatively, the host can be required to solicit the
   Router Advertisement and tell the router what algorithms it supports
   in an Identity Algorithm option.

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7.0     Securing Neighbor Discovery

   A similar procedure is used for securing IPv6 Neighbor Discovery
   messages.

7.1 Neighbor Advertisement Signature

   A Neighbor Advertisement sent message contains a digital signature
   calculated with the private key generated from the 64 bit interface
   identifier and the host public cryptographic parameters. The
   signature MUST be calculated over the entire message.

   The Target Link Layer Address option MUST be included.

   In the signing algorithm described in Section 4.0, the input into
   the hash algorithm is the following:

      contents = (flg,addr,l2addr)

   IPrK is the interface identifier private key.

   The digital signature MUST be included in an Identity Digital
   Signature option (see Section 8.1) with the signature, algorithm,
   and realm identifier. Again, an ICMP option is used instead of IPsec
   AH because Neighbor Discovery options that are not recognized by a
   node are ignored.

7.2 Verifying a Neighbor Advertisement

   An IPv6 node receiving a Neighbor Advertisement with an Identity
   Digital Signature option verifies that the advertising node is
   authorized to send the advertisement in the following way. If the
   receiving node does not support one of the algorithms used for
   encrypting the signature, it SHOULD discard the Neighbor
   Advertisement, unless the node wants to risk using an unsecured
   Neighbor Advertisement.

   The node uses the verification algorithm in Section 4.0 to verify
   the digital signature using the same value for contents as in
   Section 7.1. In this calculation, ID is the sending node's 64 bit
   interface identifier. The identity based algorithm and node public
   cryptographic parameters depend on the algorithm and realm
   identifier in the Identity Digital Signature option.

7.3 Negotiating an Identity Based Algorithm

   A node sending a Neighbor Solicitation message can indicate what
   algorithms it is capable of accepting by including an Identity
   Algorithm option in the message.

8.0     Option Formats


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8.1 Identity Digital Signature Option

   The Identity Digital Signature Option contains a digital signature
   calculated using address based private key. It is always the last
   option in the list. The format of this option, after [1], is:

     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     |       Algorithm Identifier    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                      Realm Identifier                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                                                               |
    +                                                               +
    |               Digital Signature (N  bits)                     |
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

      Type                   8 bit identifier for the option type,
                             assigned by IANA.

      Length                 8 bit unsigned integer giving the
                             option length (including type and
                             length fields) in units of 8 octets.

      Algorithm Identifier   16 bit nonzero algorithm
                             identifier,assigned by IANA, indicating
                             the identity based algorithm used to
                             sign the message.

      Realm Identifier       Either the 64 bit nonzero HMAC-SHA1
                             hash of the realm part of the NAI [25],
                             or zero to indicate that the current
                             network's IPKG and public cryptographic
                             parameters should be used.

      Digital Signature      An N bit field containing the digital
                             signature. The field is zero aligned to
                             the nearest 8 byte boundary. The exact
                             number of bits is depends on the
                             identity based algorithm and use.



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8.2 Identity Algorithm Option

   The Identity Algorithm Option allows a node to indicate which
   identity based keying algorithms it supports for particular realms
   when requesting a Router Advertisement or Neighbor Advertisement.
   The Identity Algorithm Option has the following format:

     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     |       Algorithm Identifier    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Realm Identifier         |         ...                   /
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   where:

      Type                   8 bit identifier for the option type,
                             assigned by IANA.

      Length                 8 bit unsigned integer giving the
                             option length (including type and
                             length fields) in units of 8 octets.

      Algorithm Identifier   16 bit nonzero algorithm
                             identifier,assigned by IANA, indicating
                             the identity based algorithm used to
                             sign the message.

      Realm Identifier       Either the 64 bit nonzero HMAC-SHA1
                             hash of the realm part of the NAI [25],
                             or zero to indicate the current
                             network's algorithm.

   and the option contains as many algorithm identifier-realm
   identifier pairs, in order of preference, as the node supports. The
   option is zero padded to multiples of 8 bytes. The

9.0     Identity Based Key Algorithms

   Shamir [19] introduced the idea of identity based cryptography in
   1984. Practical, provably secure identity based signature schemes
   [12], [11], [13] and Key Agreement Protocols [16] soon followed.
   Practical, provably secure identity based encryption schemes [8],
   [10] have only very recently been found.

   In identity based signature protocols, the node signs a message
   using its private key supplied by its IPKG and the public
   cryptographic parameters. The signature is then verified using the
   node's identity together with the public cryptographic parameters.
   In identity based key agreement protocols, two parties share a
   secret. Each party constructs the secret by using its own private

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   key and the other party's public identity. In identity based
   encryption, the encryptor uses the recipient's public identity to
   encrypt a message, and the recipient uses its private key to decrypt
   the ciphertext.

   As is generally the case with public-key cryptography, the security
   of the systems is based on the difficulty of solving a hard number
   theory problem, such as factoring or a discrete log (or Diffie-
   Hellman) problem.

   Elliptic curves and associated pairings have solved the problem of
   how to do identity based encryption [8], and are used to construct
   identity based signature [18][14][9] and key agreement [18][21]
   protocols.

   There are a number of advantages to using identity based systems
   that are based on elliptic curves and their pairings. One is that
   there are compatible elliptic curve-based signature, key agreement,
   and encryption schemes. This means firstly that the same public
   key/private key pair and public cryptographic parameters can be used
   to do signatures, key agreement, and encryption. Secondly, these
   protocols overlap, so that results of computations and pre-
   computations done for one system can be used in the others. Further,
   there are usually efficiency advantages in using elliptic curves,
   over using other public-key methods. Generally, one obtains shorter
   signatures, shorter ciphertexts, and shorter key lengths for the
   same security as other systems. Efficiency can be further enhanced
   by using abelian varieties in place of elliptic curves [20].

   There are identity based signature schemes [9] using elliptic curves
   and pairings that base their security on the difficulty of solving
   the elliptic curve Diffie-Hellman problem. This is the same
   classical hard problem on which standard Elliptic Curve Cryptography
   (ECC) [17][15] is based. Identity based encryption and key agreement
   schemes using elliptic curves (or abelian varieties) and pairings
   rely on the difficulty of solving the bilinear Diffie-Hellman
   problem.

   Identity based cryptosystems can be constructed with or without key
   escrow. Protocols with key escrow can be performed in fewer passes
   than corresponding systems that do not provide for key escrow.

   Techniques from threshold cryptography allow the master key
   information to be distributed or shared among a number of IPKGs so
   that all of them would have to collude for a node's private key to
   be known to them. Such a scenario would allow for key escrow if
   necessary, by agreement among all the IPKGs, but guards against
   knowledge of the private keys by the IPKGs without their mutual
   agreement.

10.0    Previous Work



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   RFC 3401 [27] describes a protocol for generating randomized
   interface identifiers for the bottom 64 bits of the IPv6 address.
   RFC 3401 is not designed to address any of the security concerns
   raised in RFC 2461; however, it is just designed to provide a
   measure of privacy to users by frustrating attempts to correlate
   particular addresses with particular network activity. Randomized
   interface identifiers can be used if the host is re-keyed every time
   it changes its interface identifier. In practice, this may be
   somewhat impractical in public access networks, unless the ABK is
   being provided by the local network and not the home ISP.


   Cryptographically Generated Addresses (CGAs) [6], also called SUCV
   identifiers [7], are another way to construct a cryptographic
   binding for addresses. In CGAs, the interface identifier is
   generated from the public key, rather than the other way around as
   in ABKS. The primary difference between CGAs and ABKs are the
   following:

      - CGAs use the hash of the public key as the interface id in
        the address suffix, whereas ABKs hash the interface id or
        subnet prefix to form the public key.
      - CGAs allow the node to generate the public key/private key
        pair on its own, whereas ABKs require that the node be
        provided with a private key by the entity that assigns its
        address.
      - ABKs require configuration with the public cryptographic
        parameters because the IPKG uses a master secret to perform
        the private key generation, and the master secret might
        expire or be compromised.

   The consequences of the first point are that CGAs are not
   cryptographically active and therefore a separate mechanism is
   required to distribute the public key. This may be as simple as
   including it as a separate field in the message. In addition,
   CGAs are not "topologically active" and therefore cannot be used
   to sign the subnet prefix in routing.

   The consequences of the second point are that there is less
   computational load on the node for ABKs, since it only has to
   perform signature verification, not public key/private key pair
   generation. However, CGAs can be used in the absence of any
   infrastructure whereas ABKs require the node to be assigned an
   address-based private key.

   The consequences of the third point are nodes must be
   preconfigured with the private key and public cryptographic
   parameters for the operation. In principle, this is no different
   than key distribution in Diffie-Hellman. In this case, either
   dynamic or static configuration of the private key and public
   cryptographic parameters is performed, but in a way that doesn't
   require Neighbor Discovery.


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11.0    Infrastructure Requirements

   As mentioned previously, ABKs require a certain amount of
   infrastructure to generate the private keys from the subnet
   prefix and interface ids. This requirement, in and of itself, is
   a hindrance for ad hoc networking designs that call for nodes to
   simply autoconfigure their addresses without requiring an ISP or
   network operator to be involved. For networks that are run by
   ISPs or enterprises, this requirement is not likely to be a
   problem, however.

   ABKs place certain constraints on address provisioning. In
   particular, an address used for ABK cannot be assigned using DHCP
   [30]. To the extent DHCP requires Neighbor Discovery, there is a
   bootstrapping problem in using a DHCP address for ABK. An address
   used for ABK can be constructed using IPv6 Stateless Address
   Autoconfiguration [26] as long as the node performing the
   Stateless Address Autoconfiguration has an ABK interface id and
   private key for the suffix 64 bits of the address and no
   duplicate is detected. Indeed, the same mechanism described here
   to secure Neighbor Discovery could also be used to secure
   Stateless Address Autoconfiguration.

   With some identity based algorithms, the IPKG maintains a copy of
   the private key, the so-called "key escrow" property.
   Consequently, the address assignor's IPKG knows the private keys
   for every address, and can potentially snoop authenticated or
   encrypted traffic. However, the ABK is only being used to secure
   IPv6 signaling traffic and not sensitive private data. Both the
   network operator and the legitimate client/user have an interest
   in seeing efficient operation of the network. Most users today
   are happy to trust their ISPs with their credit card number,
   trusting their ISP to guard their ABK is probably of equal or
   lesser extent.

   If a group of ISPs in a roaming consortium choose to support
   ABKs, they have to co-ordinate in order to share a master key.
   There are techniques that allow secure generation of ABKs in such
   circumstances, but in principle ISPs in a roaming consortium must
   trust each other for billing and settlement, so business
   procedures and computational mechanisms for guarding privileged
   information are likely to be in place. A collection of ISPs that
   share a contract for IPKGs will allow their customers to securely
   use their networks, others will either get insecure or no
   service, just as is the case currently with roaming. The
   practical considerations involving co-ordinating the IPKGs
   between ISPs can be considerably reduced by using a hierarchical
   key generation system, such as is described in [29].

12.0    Security Considerations

   The computation involved in verifying Neighbor Discovery messages
   could be utilized by an attacker to mount a "computational DoS

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   attack." The attacker bombards the victim with bogus Neighbor
   Discovery messages, which the victim is forced to verify. This ties
   the victim up in performing cryptography on the messages.

13.0    References

    [1] Narten, T., Nordmark, E., and Simpson, W., "Neighbor Discovery
        for IP Version 6 (IPv6)", RFC 2461, December, 1998.
    [2] Kempf, J., and Nordmark, E., "Threat Analysis for IPv6 Public
        Multi-Access Links," draft-kempf-netaccess-threats-00.txt, a
        work in progress.
    [3] Hinden, R., and Deering,S., " IP Version 6 Addressing
        Architecture", RFC 2373, July 1998.
    [4] Kent, S., and Atkinson, R., " IP Authentication Header," RFC
        2402, November 1998.
    [5] Droms, R. (ed), " Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", draft-ietf-dhc-dhcpv6-23.txt, a work in progress.
    [6] O'Shea, G., and Roe, M., "Child-proof Authentication for MIPv6
        (CAM)", ACM Computer Communications Review, April, 2001.
    [7] Montenegro, G., and Castellucia, C., "SUCV Identifiers and
        Addresses," draft-montenegro-sucv-02.txt, a work in progress.
    [8] D. Boneh and M. Franklin, "Identity based encryption from the
        Weil pairing", Advances in Cryptology --- Crypto 2001, Lecture
        Notes in Computer Science 2139, (2001), Springer,  213-229,
        http://www.cs.stanford.edu/~dabo/papers/ibe.pdf
    [9] J. C. Cha and J. H. Cheon, "An Identity-Based Signature from
        Gap Diffie-Hellman Problem", Cryptology ePrint Archive: Report
        2002/018, http://eprint.iacr.org/2002/018/
    [10] C. Cocks, "An identity based encryption scheme based on
        quadratic residues", http://www.cesg.gov.uk/technology/id-
        pkc/media/ciren.
    [11] U. Feige, A. Fiat, and A. Shamir, "Zero-knowledge Proofs of
        Identity", Journal of Cryptology 1, (1988), 77-94.
    [12] A. Fiat and A. Shamir, "How to prove yourself: Practical
        solutions to identification and signature problems", Advances
        in Cryptology --- Crypto '86, Lecture Notes in Computer Science
        263, 1986), Springer,  186-194.
    [13] L. C. Guillou and J.-J. Quisquater, "A practical zero-knowledge
        protocol fitted to security microprocessors minimizing both
        transmission and memory", Advances in Cryptology --- EUROCRYPT
        '88, Lecture Notes in Computer Science 330, (1988), Springer,
        123-128.
    [14] F. Hess, "Exponent Group Signature Schemes and Efficient
        Identity Based Signature Schemes Based on Pairings", Cryptology
        ePrint Archive: Report 2002/012,
        http://eprint.iacr.org/2002/012/
    [15] N. Koblitz, "Elliptic curve cryptosystems", Mathematics of
        Computation 48 (1987), 203-209.
    [16] U. Maurer and Y. Yacobi, "Non-interactive public-key
        cryptography," Advances in Cryptology --- Eurocrypt '92,
        Lecture Notes in Computer Science 658,(1993), Springer, 458-
        460.


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    [17] V. S. Miller, "Uses of elliptic curves in cryptography",
        Advances in Cryptology --- Crypto'85, Lecture Notes in Computer
        Science 218, (1986), Springer, 417-426.
    [18] R. Sakai, K. Ohgishi, and M. Kasahara, "Cryptosystems based on
        pairing", SCIC 2000-C20, Okinawa, Japan, January 2000
    [19] A. Shamir, "Identity-Based Cryptosystems and Signature
        Schemes", Advances in Cryptology --- Crypto '84, Lecture Notes
        in Computer Science 196, (1984), Springer, 47-53.
    [20] A. Silverberg and K. Rubin, "The best and worst of
        supersingular abelian varieties in cryptology", Cryptology e-
        Print Archive: Report 2002/006,
        http://eprint.iacr.org/2002/006/
    [21] N. P. Smart, "An identity Based authenticated key agreement
        protocol based on the Weil pairing", Cryptology ePrint Archive:
        Report 2001/111, http://eprint.iacr.org/2001/111/
    [22] "802.1x - Port Based Access Control", IEEE Standard for Local
        and Metropolitan Area Networks, 2001.
    [23] Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", RFC 2119, March 1997.
    [24] Mitton, D., and Beadles, M., "Network Access Server
        Requirements Next Generation (NASREQNG) NAS  Model", RFC 2881,
        July 2000.
    [25] Aboba, B., and Beadles, M., "The Network Access Identifier",
        RFC 2486, January, 1999.
    [26] Thomas, S., and Narten, T., "IPv6 Address Stateless
        Autoconfiguration", RFC 2462, December, 1998.
    [27] Narten, T., and Draves, R., "Privacy Extensions for Stateless
        Address Autoconfiguration in IPv6", RFC 3041, January, 2001.
    [28] Harkins, D., and Carrel, D., "The Internet Key Exchange (IKE)",
        RFC 2409, November, 1998.
    [29] Gentry, C., and Silverberg. A., "Hierarchical ID-based
        Cryptography," http://eprint.iacr.org/2002/056/.
    [30] Droms, R., et. al., "Dynamic Host Configuration Protocol for
        IPv6," draft-ietf-dhc-dhcpv6-26.txt, a work in progress.

14.0    Author's Contact Information

   James Kempf                          Phone: +1 408 451 4711
   DoCoMo Labs USA                      Email: kempf@docomolabs-usa.com
   180 Metro Drive, Suite 300
   San Jose, CA 95430
   USA

   Craig Gentry                 Phone: +1 408 451 4723
   DoCoMo Labs USA              Email: cgentry@docomolabs-usa.com
   180 Metro Drive, Suite 300
   San Jose, CA 95430
   USA

   Alice Silverberg             Phone: +1 614 292 4975
   Department of Mathematics    Email: silver@math.ohio-state.edu
   Ohio State University
   Columbus, OH 43210

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   USA

15.0    Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.
   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph
   are included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.  The limited permissions granted above are perpetual and
   will not be revoked by the Internet Society or its successors or
   assigns.  This document and the information contained herein is
   provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE
   INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
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