Internet Engineering Task Force                            G. Montenegro
INTERNET DRAFT                                           C. Castelluccia
                                                             April, 2001
   Statistically Unique and Cryptographically Verifiable Identifiers
and Addresses

Status of This Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC 2026.

   Comments should be submitted to the HIP and Mobile IP mailing lists
   at and,

   Distribution of this memo is unlimited.

   This document is an Internet-Draft.  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
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   at any time.  It is inappropriate to use Internet-Drafts as
   reference material or to cite them other than as ``work in

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at


   This document addresses the identifier ownership problem.  It
   does so by using characteristics of Statistic Uniqueness and
   Cryptographic Verifiability (SUCV) of certain entities which
   this document calls SUCV Identifiers (SUCV ID's).  This note
   also proposes using these SUCV characteristics in related
   entities called SUCV Addresses in order to severely limit
   certain classes of denial of service attacks and hijacking
   attacks. SUCV addresses are particularly applicable to solve the
   'address ownership' problem that severely undermines confidence
   in mechanisms like Binding Updates in Mobile IP for IPv6.

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Table of Contents

1.0 Introduction ..................................................    3
2.0 Related Issues ................................................    3
   2.1 Address Ownership ..........................................    3
   2.2 Purpose Built Keys [PBK] ...................................    4
3.0 Overview of the Proposal ......................................    4
4.0 Statistical Uniqueness and Cryptographic Verifiability
(SUCV) ............................................................    5
   4.1 SUCV ID's ..................................................    6
   4.2 SUCV ID's versus Addresses .................................    7
   4.3 SUCV Addresses .............................................    8
   4.4 Hash ID Size Considerations ................................    8
5.0 HIP IPv6 Accomodation Mode ....................................    9
6.0 Use of SUCV Addresses for Mobile IPv6 .........................   11
   6.1 Protocol Overview ..........................................   12
7.0 Security Considerations .......................................   14
8.0 Conclusions ...................................................   14
References ........................................................   14
Authors' addresses ................................................   15

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1.0 Introduction

   This document addresses the identifier ownership problem
   [ADDROWN] by using characteristics of Statistic Uniqueness and
   Cryptographic Verifiability (SUCV) of certain entities which
   this document calls SUCV Identifiers (SUCV ID's).  This note
   also proposes using these SUCV characteristics in related
   entities called SUCV Addresses in order to severely limit
   certain classes of denial of service attacks and hijacking
   attacks. SUCV addresses can solve the 'address ownership'
   problem that severely undermines confidence in mechanisms like
   Binding Updates in Mobile IP for IPv6.

2.0 Related Issues

2.1 Address Ownership

   [ADDROWN] argues that there is a fundamental problem in handling
   operations like Binding Updates (BU's) in Mobile IP for IPv6
   [MIPv6], source routing, etc) that allows hosts to modify how
   other hosts route packets to a certain destination.  The problem
   is that these operations can be misused by rogue nodes to
   redirect traffic away from its legitimate destination.
   Authentication does not solve this problem. Even if a node
   unequivocally identifies itself, this has no bearing on its
   rights to modify how packets to any given address are routed.
   This is true even if its packets currently seem to emanate from
   the address in question. This last point is obvious if one
   considers DHCP leased addresses. It is imperative not to allow
   any node to redirect traffic for a DHCP address for which it
   held a valid lease previously. This would allow it to hijack
   traffic meant for the current valid user of the address in
   question. Hence, protection against hijacking of valid addresses
   requires cryptographic authorization for operations that modify
   routing (BU's, source routing, etc). One way to show
   authorization is by showing that the requesting node owns the
   address for which routing information is being altered. Quoting

      Currently there exists no specified mechanism for proving
      address ownership in Internet-wide scale.

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2.2 Purpose Built Keys [PBK]

   Purpose built keys [PBK] have been proposed as a foundation for
   solving scaling and cost concerns associated with some uses of
   BU's [MIPv6]. It has also been suggested that such keys [PBK]
   can solve the authorization problem that is at the heart of the
   address ownership issue.

   The proposal is succintly to:

   1. Generate a temporary public/private pair

   2. Generate an EID = hash(public component)

       a. the initiator sends EID to responder along with initial
          protocol exchanges.

       b. the initiator sends public component to responder along
          with subsequent exchanges.

   3. The initiator sends the BU and its EID signed with its
      private key to the responder.

   Notice that the exchange at step 2 must be secure in order to
   avoid intruder-in-the-middle attacks, but it is an improvement
   over cookies.

   Several problems linger:

   1. This does NOT really address the address ownership problem of
      any publicly routable addresses

   2. It is not specified how the EID and public component of the
      PK are sent by the initiator to the responder

   3. Preventing or limiting hijacking and intruder-in-the-middle
      attacks depend on this sequence if not clearly specified.

   By the time the issues are worked out, [PBK] will look very
   similar to an existing proposal [HIPARCH].  Because of this, it
   may be simpler to base a solution on HIP.

3.0 Overview of the Proposal

   We assume that we have a network in which the nodes inherently
   distrust each other, and in which a global or centralized PKI
   (Public Key Infrastructure) or KDC (Key Distribution Center) is

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   not available.

   The goal is to arrive at some fundamental assumptions about
   trust on top of which one can build some useful peer-to-peer
   communication using opportunistic security.

   But in such a network, is there a default rule we can follow
   safely?  We posit this is it:

      Default Trust Rule:

         Redirect operations are allowed only with addresses which
         are bound (via a cryptographically sound binding) to the
         requesting entity.

   The above rule (to be refined later) constitutes the only rule
   that operates by default, allowing any other more dangerous
   operation only if authorized by strong cryptographic

   Furthermore, we suggest it be based on HIP for the following

   - HIP provides the types of identifiers required by the above

   - The HIP protocol handshake [HIPPROT] is a foundation for a
     very solid sequence resistant to denial-of-service attacks.

   Nevertheless, this document proposes a specific use or 'profile'
   of HIP as applied to environments without DNS (particularly
   secure DNS), a centralized PKI, or a Key Distribution Center.
   Rather, we favor the opportunistic mode in HIP [HIPIMPL], and
   adapt and apply it to Mobile IPv6 (as detailed below). In order
   not to hinder deployment, a primary consideration has been to
   minimize the modifications of existing protocols and network

   Furthermore, at the end of this document we note that there are
   other areas that can benefit from similar adaptations of HIP's
   opportunistic mode. Their details, however, are left for further

4.0 Statistical Uniqueness and Cryptographic Verifiability (SUCV)

   In the absence of a third party, how does a principal prove
   ownership of its identity to a peer?

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   Notice that usual owner verification relies on a third party to
   provide this function.

   In this proposal, the principal self-generates a private/public
   key pair. It uses the public key as its identity and proves its
   ownership by signing it with its private key. The recipient
   verifies the signature, and, consequently, the ownership of the

4.1 SUCV ID's

   It is much more practical for protocols to use fixed length
   identifiers (representations of identities). Because of this, we
   do not use the public key itself as the identifier, but its hash
   (as in HIP).

   These identifiers have a strong cryptographic binding with their
   public components (of their private-public keys). This is
   exactly the purpose that certificates have.  Let's call them
   Statistically Unique Cryptographically Verifiable ID's, or SUCV

   Because of this, once a CN obtains information about one of
   these identifiers, it has a strong cryptographic assurance about
   which entity created it. Not only that, it knows that this
   identifier is owned and used exclusively by only one node in the
   universe: its peer in the current exchange. Thus, it is safe to
   allow that peer to effect changes (via BU's, for example) on how
   this address or identifier is routed to. Notice that with
   publically routable addresses, this assurance is much harder to
   arrive at, given that the address may be 'loaned' to (not owned
   by) the peer in question, perhaps thanks to the good service of
   a DHCP server.

   Despite the fact that currently there is no way to prove address
   ownership in the Internet, these considerations lead to the
   following fundamental assumption:

      Default Trust Rule

         Redirect operations are only allowed to affect routing for
         entities which have the SUCV property.

   The above constitutes perhaps the only rule that operates by
   default, allowing any other more dangerous operation only if
   authorized by strong cryptographic mechanisms

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4.2 SUCV ID's versus Addresses

   What should one use: pure identifiers with no routing
   significance or addresses?

   For example, in the Mobile IPv6 case, a node starts using its
   home address, and issues binding updates as it moves.

   With a home address that is not an SUCV ID it is never evident
   to the CN that whoever was sitting on this address actually owns
   it. At the very most, the mobile node can prove that at some
   point it was sitting on a certain address, and later it can
   prove it is still the same node, but now sitting on another

   But it cannot prove ownership.  Ignoring this subtle distinction
   leads to DOS and hijacking attacks.

   Problems may also arise because of honest mistakes in
   configuration. For example, say node A was originally sitting on
   CoA, and then moved on to CoA'. Suppose it then asks its CN's
   to redirect traffic to CoA'. In the meanwhile, the original
   CoA may have been assigned to another node B, perhaps by the
   DHCP server that rightfully 'owns' that address. The result is
   that now traffic meant for B has been redirected to A at its new

   Relying on ingress filtering may limit the risk, but
   essentially, the only way for a node to prove ownership of an
   identifier (in the absence of any other centralized or global
   mechanism), is for it to prove that it created this
   statistically unique series of bits.

   The intent is to use an identifier instead of an address.  Using
   identifiers that satisfy the SUCV conditions outlined above, it
   is possible to gain the tremendous advantage that other nodes
   can safely believe the node when it claims ownership of that
   identifier.  Hence they can safely heed its redirects when it
   says that it is now available at some different CoA (and later
   at another).  Furthermore, you do not rely on ingress filtering
   to limit exposure.

   A major advantage to using an address is that the data traffic
   need not carry extra information in the packet to guarantee
   proper delivery by routing. Because of this it is advantageous
   to create addresses that are both routable and satisfy the SUCV
   property: SUCV addresses.

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   Another advantage to using an SUCV address is that this address
   can be registered in the DNS and the host does not have to worry
   about communicating securely this identifier to its correspondent
   node. The correspondent node will just get it from the DNS.

4.3 SUCV Addresses

   In IPv6, addresses that satisfy the SUCV property may be
   obtained as follows:

   - use the top 64 bits from your routing prefix (as in rfc3041)

   - define the bottom 64 bits as an SUCV ID (called the HID or
     'hash' ID). Use these 64 bits instead of the 'interface
     identifier' in IPv6 [IPV6ADDR].

   The resultant 128 bit field is an identifier that is also
   routable, avoiding the need for taking extra space in the packet
   by sending routing options. Notice that even after moving, it is
   possible to reuse the 'HID' portion of the address with the new
   network prefix at the new location. Thus it is possible to reuse
   the HID with different CoA's.

   Nevertheless, by snooping on binding updates, it is possible for
   an attacker to learn the original network prefix used by the
   home address. This tells an eavesdropper where this home address
   began to be used, and to which network it belongs, potentially
   important information.

   On the other hand, if you use a 'pure' SUCV ID (without any
   routing significance), then your packets will always need extra
   information somewhere to assure they are routed properly.
   Eavesdroppers may still know where that identity is at any
   particular point in time. But at least they will not know where
   (under what prefix) that identity began to be used.

   For further details on SUCV address please refer to section

4.4 Hash ID Size Considerations

   In SUCV addresses, one of the lower 64 bits is reserved as the
   local/universal bit (the 'u' bit), so only 63 bits are actually
   usable as a hash.

   Suppose the hash function produces an n-bit long output. If we

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   are trying to find some input which will produce some target
   output value y, then since each output is equally likely we
   expect to have to try 2^(n-1) possible input values on average.

   If we are trying to find a collision, then by the birthday
   paradox we would expect that after trying 1.2*2^n/2 possible
   input values we would a 50% probability of collision [BIRTH].

   So if n=63, you need 1.2*2^31.5 i.e. 3.64*10^9 tries on average
   before having a collision.  This is acceptable especially if you
   consider that this collision is actually harmfull only if the 2
   hosts (that collide) are in the same site (i.e.  they have the
   same network prefix), and have the same correspondent nodes.
   This is very unlikely.  Additionally, if the collision is not
   deliberate the duplicate address detection (DAD) will detect

   If an attacker wishes to impersonate a given SUCV address, it
   must attempt 2^62 (i.e. approximately 4.8*10^18) tries to find a
   public key that hashes to this SUCV address.  If the attacker
   can do 1 million hashes per second it will need 142,235 years.
   If the attacker can hash 1 billion hashes per second it will
   still need 142 years

   If we consider that the SUCV Addresses are renewed every 24
   hours (as suggested in RFC3041), an attacker would then be able
   to hash 5.3*.10^13 hashes/second in order to be able to find a
   public key that hashes to the SUCV HID of a given host...

   Note that the previous analysis only considers the cost of
   computing the hash of the public key. Prior to this step, an
   attacker must also generate a valid (public, private) key pair.
   This is also a very computionally expensive operation.

   As a conclusion SUCV addresses as used in this document provide
   more than enough security.

5.0 HIP IPv6 Accomodation Mode

   Using these ID's or addresses depends on also communicating
   safely the SUCV portion, and this, in turn is dependent on the
   packet sequencing, etc.  These concerns are not addresses at all
   in the PBK draft. On the other hand, HIP includes mechanisms and
   detailed considerations in this regard (protection against
   replay, DOS and MITM attacks). This is why this note proposes
   that a solution be drafted based heavily on HIP [HIPARCH,

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   To obtain an IPv6 SUCV address, we define a HIT-64 format and
   use its lower 64 bits to form the relevant IPv6 addresses (this
   constitutes HIP's IPv6 accomodation mode). So, for example, the
   HIT-64 is used to form global aggregatable addresses which start
   thus [IPV6ADDR]:

      Aggregatable Global Unicast Addresses 001

   As well as to derive link-local and site-local addresses which
   start thus:

      Link-Local Unicast Addresses          1111 1110 10
      Site-Local Unicast Addresses          1111 1110 11

   The HIT-64 format (section 5.2 of [HIPARCH]) is defined as

   - first 64 bits:
      - Bit 0 is one
      - HAA field (next 63 bits) formed as follows:
         - RAA: 23 bits of registered assigning authority assigned
           by ICANN (suggested value: 0)
         - RI: 40 bits of  registered identity assigned
           sequentially by the RAA. (suggested value: 0)
   - last 64 bits:
        - derived from a hash of the host identity
        - used as the interface identifier in IPv6 addresses

   The IPv6 accomodation mode consists in using a HIT-64 to form an
   IPv6 address.

   A HIT-64 derived IPv6 Aggregatable Global Unicast Address
   (RFC2374) is formed as follows:

   - top 64 bits: as per the valid prefix at the link the device is on

   - bottom 64 bits:  interface identifier obtained by taking the
     last 64 bits of the above HIT-64, and setting bit 6 (the
     left-most bit is numbered 0) to one.  This creates an
     interface identifier with universal significance.

   From this IPv6 address, other non-global scope addresses are
   derived. For example,  a node uses the bottom 64 bits to form
   the site-local and link-local addresses on the same prefix
   (link) as the aggregatable global unicast address
   (alternatively, if a non-global address is formed first, it is
   used to form the others). Furthermor, if this address is used in
   conjuction with Mobile IP for IPv6 [MIPV6], the Home Agent uses

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   the Prefix Length information within a "home registration"
   binding update to form the corresponding link-local and
   site-local addresses for the Mobile Node, and defend them for
   purposes of Duplicate Address Detection.

   Any of the addresses formed as described above constitute SUCV

6.0 Use of SUCV Addresses for Mobile IPv6

   In Mobile IPv6, a mobile host obtains a new address, a CoA, each
   time it moves. It then registers the binding between its home
   address and its new CoA with its home agent and correspond
   nodes.  Correspondent nodes in posession of such a binding can
   send packets to the mobile node directly at its current CoA.
   Instead, sending packets to the mobile node's home address
   implies sub-optimal routing as they first proceed to the mobile
   node's home link, where they are intercepted by the home agent
   and forwarded to the mobile node's CoA.

   However, the correspond nodes MUST get the assurance the home
   address actually belongs to the mobile node. Otherwise, an
   attacker could send a binding update with a victim's home
   address, thus redirecting all the victim's packets.

   Additionally, the correspond nodes SHOULD get the assurance that
   the CoA actually belongs to the mobile node.  Otherwise, any
   host could use the address of another victim as its CoA. The
   packets that were initially addressed to the first victim will
   then be sent to the victim. Depending on how much traffic this
   implies, this could be used as a denial-of-service attack.

   These attacks can be avoided if, in order to accept and process
   a binding update, a correspond host requires a mobile node to
   prove ownership of its home address and its CoA. If ownership is
   proven, the correspond node has the assurance that the mobile
   node is not hijacking some other node's address, and that it is
   not directing packets at some other node's one's address.

   The solution that we propose is that a mobile node uses the
   method describes previously to configure its Home Address and
   its CoA (the same HID and public/private key pair is used for
   the home address and the CoA).

   By verifying the signature and the HID, the correspond host has
   the assurance that the mobile host is not using some other
   node's home address and CoA.

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   As described in [MIPPRIV], the use of SUCV Identifier for Mobile
   IPv6 is useful when a mobile node wishes to hide its home
   address. Indeed the home address can reveal a lot of information
   about a mobile node. [MIPPRIV] proposes to use a random
   Temporary Mobile Identifier (TMI) in place of the home address.
   By using a SUCV ID as a TMI, a mobile node will be able to prove
   ownership of the TMI and avoid hijacking attacks.

6.1 Protocol Overview

   The following protocol sequence is based on [HIPPROT]. However,
   it has been modified for Mobile IPv6 based on the following

   - the goal is to secure binding updates sent by a mobile node to
     an arbitrary correspondent node

   - the protocol should not rely on a third party (i.e. a home
     agent, mobility anchor point, global PKI, central key
     distribution center, etc)

   - HIP is used for 'opportunistic security' so there is no
     reliance on DNS

   - not all nodes need to use SUCV addresses, only those that wish
     their binding updates to be heeded (mobile nodes)

   - not all nodes need to verify the validity of SUCV addresses,
     only those that wish to safely heed binding updates in order
     to populate its binding cache

   The proposed protocol that a mobile host uses to send a BU to its CN
   is the following:

      msg1- The MN sends a BU HELLO message (just to initiate the
            exchange) to its correspond node.  This message
            contains a Nonce, N1.

      msg2- The CN replies with a message that contains the
            following:  N1, HIP Cookie request, SPI, Diffie-Hellman
            value, ESP transform (list of supported ESP modes).

            In order to defend against msg1 storms, a host might
            use the same DH value for a period of time.  A HIP
            cookie request contains a random number I, the hash of
            I concatenated with a random value J, and K, the number
            of bits that must match as per [HIPPROT].

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            When the MN receives msg2, it verifies that the nonce
            N1 is the same than the one that was sent in the msg1.
            It then computes the HIP Cookie reply by finding J and
            replies with msg3.

      msg3- The MN replies with a message that contains the
            following:  HIP Cookie reply, Public key,
            Diffie-Hellman value, ESP transform (the selected ESP
            modes) and BU.  This message MUST be signed by the MN
            with its private key.

            Note that the home address contained in the BU is
            either a SUCV Address or a SUCV Identifier. The CoA is
            either a SUCV Address or a regular address. By using a
            CoA SUCV address, a CN has the assurance the the CoA
            belongs to the MN and has not been stolen.

   When the CN receives the msg3, it can verify the ownership of
   the Home and CoA addresses and authenticate the BU because it is

   The MN and CN can then derive a session key (using the ephemeral
   D-H value), and use it in conjunction with IPSec to authenticate
   other subsequent BU (if any) as it is done in current MIPv6.

   As long as the MN uses the same HID interface identifier for its
   CoA, it does not have to prove the CoA ownership and IPSec
   authentication mechanism is fine.  If for any reason the MN
   configures its CoA with a new interface identifier, it MUST
   restart the whole protocol (i.e. msg1, msg2, msg3).

   This proposal does not require any prerequisite between the MN
   and the CN. By using a Home Address SUCV, that is generated by
   hashing a public key, and signing message 2 with the
   corresponding private key a MN can prove the ownership of its
   Home Address.

   Because our proposal is heavily based on HIP, it is resistant to
   denial-of service attacks.  Because our proposal is based on
   SUCV Home Address, it is resistant to Man-in the Middle
   attacks.  An attacker won't be able to redirect the traffic
   destined to a particular SUCV Home Address unless it can find a
   (public, private) key pair such that the hash of the public
   component is equal to the least significant 64 bits in the SUCV
   Home Address.  This is computationally infeasable (see section

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7.0 Security Considerations

   The technique introduced in this document is meant to increase
   the level of security in the Internet.

   This document explains the concept of statistical uniqueness and
   cryptographic verifiability (SUCV), specially as it applies to
   IPv6 addresses in the form of SUCV addresses. The SUCV
   characteristics are used to prove address ownership, thus
   preventing a class of attacks which exploit this fault in many
   types of commands.  In particular, commands which alter how an
   address is treated by peers or by the routing infrastructure can
   be used to launch denial of service attacks or hijacking
   attacks. Proving address ownership eliminates these attacks.
   However, given that this technique is meant to be used primarily
   in the absence of global infrastructures, the possibility of man
   in the middle attacks does remain. Nevertheless, since the
   protocol used here is based on HIP, these attacks are limited by
   the use of cookies and client puzzles.

8.0 Conclusions

   The present document focuses on the use of the SUCV property to
   enhance the security of exchanges between an arbitrary pair of
   peers in the absence of any third party.  In particular, we
   propose that SUCV addresses be used to solve the issue of
   securing binding updates in Mobile IPv6.

   Recent micro-mobility management protocols (such as HAWAII or
   Cellular IP) propose to use specialized path setup schemes which
   install host-based forwarding entries in specific routers to
   support intra-domain micro-mobility. In order to avoid trafic
   redirection, routers need to verify the ownership of an address
   used by a mobile host before adding an entry for that particular
   mobile host in its routing table. SUCV addresses or identifiers
   also can be very useful for that purpose.


[ADDROWN] Pekka Nikander, "An Address Ownership Problem in IPv6",
   draft-nikander-ipng-address-ownership-00.txt, February 2001.


[HIPARCH] Bob Moskowitz, "HIP Architecture,"

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[HIPIMPL] Bob Moskowitz, "HIP Implementation,"

[HIPPROT] Bob Moskowitz, "Host Identity Payload and Protocol,"

[IPV6ADDR] Hinden, Deering, "IPv6 Addressing Architecture,"

[MIPPRIV] Castelluccia, Dupont, "A Simple Privacy Extension for
   Mobile IPv6," draft-castelluccia-mobileip-privacy-00.txt,
   February 2001.

[MIPV6] C. Perkins, "Mobile IP for IPv6,",

[PBK] Bradner, Mankin, Schiller, "Purpose Built Keys,"

[RFC3041] T. Narten, R. Draves "Privacy Extensions for Stateless
   Address Autoconfiguration in IPv6," RFC 3041.

[WeakMD5] H. Dobbertin, "Cryptanalysis of MD5 Compress,"

Authors' addresses

   Questions about this document may be directed to:

          Gabriel Montenegro
          Sun Microsystems Laboratories, Europe
          29, chemin du Vieux Chene
          38240 Meylan, FRANCE

          Voice:  +33 476 18 80 45

          Claude Castelluccia
          INRIA Rhone-Alpes
          655 avenue de l'Europe
          38330 Montbonnot Saint-Martin
          phone: +33 4 76 61 52 15
          fax:   +33 4 76 61 52 52

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