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Versions: (draft-arkko-send-cga) 00 01 02 03 04 05 06 RFC 3972

Securing Neighbor Discovery                                      T. Aura
Internet-Draft                                        Microsoft Research
Expires: May 28, 2004                                  November 28, 2003


              Cryptographically Generated Addresses (CGA)
                         draft-ietf-send-cga-03

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.

   This Internet-Draft will expire on May 28, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2003). All Rights Reserved.

Abstract

   This document describes a method for binding a public signature key
   to an IPv6 address in the Secure Neighbor Discovery (SEND) protocol.
   Cryptographically Generated Addresses (CGA) are IPv6 addresses where
   the interface identifier is generated by computing a cryptographic
   one-way hash function from a public key and auxiliary parameters. The
   binding between the public key and the address can be verified by
   re-computing the hash value and by comparing the hash with the
   interface identifier. Messages sent from an IPv6 address can be
   protected by attaching the public key and auxiliary parameters and by
   signing the message with the corresponding private key. The
   protection works without a certification authority or other security
   infrastructure.




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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  CGA Address Format . . . . . . . . . . . . . . . . . . . . . .  4
   3.  CGA Parameters and Hash Values . . . . . . . . . . . . . . . .  6
   4.  CGA Generation . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  CGA Verification . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  CGA Signatures . . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   7.1 Security Goals and Limitations . . . . . . . . . . . . . . . . 13
   7.2 Hash extension . . . . . . . . . . . . . . . . . . . . . . . . 13
   7.3 Privacy Considerations . . . . . . . . . . . . . . . . . . . . 15
   7.4 Related protocols  . . . . . . . . . . . . . . . . . . . . . . 16
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
       Normative References . . . . . . . . . . . . . . . . . . . . . 18
       Informative References . . . . . . . . . . . . . . . . . . . . 19
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 19
   A.  Example of CGA Generation  . . . . . . . . . . . . . . . . . . 21
   B.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 23
       Intellectual Property and Copyright Statements . . . . . . . . 24































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1. Introduction

   This document specifies a method for securely associating a
   cryptographic public key with an IPv6 address in the Secure Neighbor
   Discovery (SEND) protocol [I-D.ietf-send-ndopt]. The basic idea is to
   generate the interface identifier (i.e., the rightmost 64 bits) of
   the IPv6 address by computing a cryptographic hash of the public key.
   The resulting IPv6 addresses are called cryptographically generated
   addresses (CGA). The corresponding private key can then be used to
   sign messages sent from the address.

   This document specifies:

   o  how to create CGA addresses from the cryptographic hash of a
      public key and auxiliary parameters,

   o  how to verify the association between the public key and the CGA
      address, and

   o  how to generate and verify a CGA signature.

   In order to verify the association between the address and the public
   key, the verifier needs to know the address itself, the public key,
   and the values of the auxiliary parameters. No additional security
   infrastructure, such as a public key infrastructure (PKI),
   certification authorities, or other trusted servers, is needed.

   The address format and the CGA parameter format are defined in
   Sections 2 and 3. Detailed algorithms for generating addresses and
   for verifying them are given in Sections 4 and 5, respectively.
   Section 6 defines the procedures for generating and verifying CGA
   signatures. The security considerations in Section 7 include
   limitations of CGA-based authentication, the reasoning behind the
   hash extension technique that enables effective hash lengths above
   the 64-bit limit of the interface identifier, the implications of CGA
   addresses on privacy, and protection against related-protocol
   attacks.

   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 [RFC2119].










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2. CGA Address Format

   When talking about addresses, this document refers to IPv6 addresses
   where the leftmost 64 bits of a 128-bit address form the subnet
   prefix and the rightmost 64 bits of the address form the interface
   identifier. [RFC3513] We number the bits of the interface identifier
   starting from bit 0 on the left.

   A cryptographically generated address (CGA) has a security parameter
   (Sec), which determines its strength against brute-force attacks. The
   security parameter is a 3-bit unsigned integer and it is encoded in
   the three leftmost bits (i.e., bits 0-2) of the interface identifier.
   This can be written as:

       Sec = (interface identifier & 0xe000000000000000) >> 61

   The CGA address is associated with a set of parameters, which consist
   of a public key and auxiliary parameters. Two hash values Hash1 (64
   bits) and Hash2 (112 bits) are computed from the parameters. The
   formats of the public key and auxiliary parameters and the way to
   compute the hash values are defined in Section 3.

   A cryptographically generated address (CGA) is defined as an IPv6
   address that satisfies the following two conditions:

   o  The 16*Sec leftmost bits of the second hash value Hash2 are zero.

   o  The rightmost 64 bits of the first hash value Hash1 equal the
      interface identifier of the address. Bits 0, 1, 2, 6 and 7 (i.e.,
      the bits that encode the security parameter Sec and the "u" and
      "g" bits) are ignored in the comparison.

   The above definition can be stated in terms of the following two bit
   masks:

     Mask1 (112 bits) = 0x0000000000000000000000000000  if Sec=0,
                        0xffff000000000000000000000000  if Sec=1,
                        0xffffffff00000000000000000000  if Sec=2,
                        0xffffffffffff0000000000000000  if Sec=3,
                        0xffffffffffffffff000000000000  if Sec=4,
                        0xffffffffffffffffffff00000000  if Sec=5,
                        0xffffffffffffffffffffffff0000  if Sec=6, and
                        0xffffffffffffffffffffffffffff  if Sec=7

     Mask2 (64 bits)  = 0x1cffffffffffffff

   A cryptographically generated address is an IPv6 address for which
   the following two equations hold:



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       Hash1 & Mask2  ==  interface identifier & Mask2
       Hash2 & Mask1  ==  0x0000000000000000000000000000

















































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3. CGA Parameters and Hash Values

   Each CGA address is associated with a public key and auxiliary
   parameters. The public key is formatted as a DER-encoded
   [ITU.X690.2002] ASN.1 structure of the type SubjectPublicKeyInfo
   defined in the Internet X.509 certificate profile [RFC3280]. The
   public key SHOULD be an RSA encryption key with the object identifier
   rsaEncryption (i.e., "1.2.840.113549.1.1.1") and the subject public
   key field SHOULD be formatted as an ASN.1 data structure of the type
   RSAEncryptionKey defined in [PKCS.1.2002]. The RSA key length SHOULD
   be at least 384 bits. Using any other public key type or format is
   strongly discouraged as it will result in incompatible CGA
   implementations.

   The auxiliary parameters are the following three unsigned integers:

   o  a 128-bit modifier, which can get any value,

   o  a 64-bit subnet prefix, which is equal to the subnet prefix of the
      CGA address, and

   o  an 8-bit collision count, which can get values 0, 1 and 2.

   We use the name CGA Parameters for the data structure that is the
   concatenation of the 16-octet modifier, the 8-octet subnet prefix,
   the 1-octet collision count, and the variable-length encoded public
   key (i.e., the SubjectPublicKeyInfo structure).

   The two hash values are computed with the SHA-1 hash algorithm
   [FIPS.180-1.1995] from the public key and auxiliary parameters. When
   computing Hash1, the input to the SHA-1 algorithm is the CGA
   Parameters data structure. The 64-bit Hash1 is obtained by taking the
   leftmost 64 bits of the 160-bit SHA-1 hash value.

   When computing Hash2, the input is the same CGA Parameters data
   structure except that the subnet prefix and collision count are set
   to zero. The 112-bit Hash2 is obtained by taking the leftmost 112
   bits of the 160-bit SHA-1 hash value.













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4. CGA Generation

   The process of generating a new CGA address takes three input values:
   a 64-bit subnet prefix, the public key of the address owner as a
   DER-encoded ASN.1 structure of the type SubjectPublicKeyInfo, and the
   security parameter Sec, which is an unsigned 3-bit integer. The cost
   of generating a new CGA address depends on the security parameter
   Sec, which gets values from 0 to 7.

   A CGA address and associated parameters SHOULD be generated as
   follows:

   1.  Set the modifier to a random or pseudorandom 128-bit value.

   2.  Concatenate the modifier, 9 zero octets, and the encoded public
       key. Execute the SHA-1 algorithm on the concatenation. Take the
       112 leftmost bits of the SHA-1 hash value. The result is Hash2.

   3.  Compare the 16*Sec leftmost bits of Hash2 with zero. If they are
       all zero (or if Sec=0), continue with step (4). Otherwise,
       increment the modifier and go back to step (2).

   4.  Set the 8-bit collision count to zero.

   5.  Concatenate the final modifier value, the subnet prefix, the
       collision count and the encoded public key. Execute the SHA-1
       algorithm on the concatenation. Take the 64 leftmost bits of the
       SHA-1 hash value. The result is Hash1.

   6.  Form an interface identifier from Hash1 by writing the value of
       Sec into the three leftmost bits and by setting bits 6 and 7
       (i.e., the "u" and "g" bits) both to zero.

   7.  Concatenate the 64-bit subnet prefix and the 64-bit interface
       identifier to form a 128-bit IPv6 address.

   8.  If an address collision is detected, increment the collision
       count and go back to step (5). However, after three collisions,
       stop and report the error.

   9.  Form the CGA Parameters data structure by concatenating the final
       modifier value, the subnet prefix, the final collision count
       value, and the encoded public key.

   The output of the address generation algorithm is a new CGA address
   and a CGA Parameters data structure.

   The initial value of the modifier in step (1) is chosen randomly in



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   order to make addresses generated from the same public key unlinkable
   to enhance privacy (see Section 7.3). The quality of the random
   number generator does not affect the strength of the binding between
   the address and the public key.

   For Sec=0, the above algorithm is deterministic and relatively fast.
   Nodes that implement CGA generation MAY always use the security
   parameter value Sec=0. If Sec=0, steps (2)-(3) of the generation
   algorithm can be skipped.

   For Sec values greater than 0, the above algorithm is not guaranteed
   to terminate after a certain number of iterations. The brute-force
   search in steps (2)-(3) takes O(2^(16*Sec)) iterations to complete.
   It is intentional that generating CGA addresses with high Sec values
   is infeasible with current technology.

   If the subnet prefix of the address changes but the address owner's
   public key does not, the old modifier value MAY be reused. If it is
   reused, the algorithm SHOULD be started from step (4). This avoids
   repeating the expensive search for an acceptable modifier value.

   Note that this document does not specify whether duplicate address
   detection should be performed and how the detection is done. Step (8)
   only defines what to do if some form of duplicate address detection
   is performed and an address collision is detected.


























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5. CGA Verification

   CGA verification takes as input an IPv6 address and a CGA Parameters
   data structure. The CGA Parameters consist of the concatenated
   modifier, subnet prefix, collision count and public key. The
   verification either succeeds or fails.

   The CGA address MUST be verified with the following steps:

   1.  Check that the collision count in the CGA Parameters data
       structure is 0, 1 or 2. The CGA verification fails if the
       collision count is out of the valid range.

   2.  Check that the subnet prefix in the CGA Parameters data structure
       is equal to the subnet prefix (i.e., the leftmost 64 bits) of the
       address. The CGA verification fails if the prefix values differ.

   3.  Execute the SHA-1 algorithm on the CGA Parameters data structure.
       Take the 64 leftmost bits of the SHA-1 hash value. The result is
       Hash1.

   4.  Compare Hash1 with the interface identifier (i.e., the rightmost
       64 bits) of the address. Differences in the three leftmost bits
       and in bits 6 and 7 (i.e., the "u" and "g" bits) are ignored. If
       the 64-bit values differ (other than in the five ignored bits),
       the CGA verification fails.

   5.  Read the security parameter Sec from the three leftmost bits of
       the 64-bit interface identifier of the address. (Sec is an
       unsigned 3-bit integer.)

   6.  Concatenate the modifier, 9 zero octets, and the public key.
       Execute the SHA-1 algorithm on the concatenation. Take the 112
       leftmost bits of the SHA-1 hash value. The result is Hash2.

   7.  Compare the 16*Sec leftmost bits of Hash2 with zero. If any one
       of them is non-zero, the CGA verification fails. Otherwise, the
       verification succeeds. (If Sec=0, the CGA verification never
       fails at this step.)

   If the verification succeeds, the verifier knows that the public key
   in the CGA Parameters is the authentic public key of the address
   owner. The verifier can extract the public key by removing 25 bytes
   from the beginning of the CGA Parameters and by decoding the
   following SubjectPublicKeyInfo data structure. Future versions of
   this specification may add new fields to the end of the CGA
   Parameters and the verifier SHOULD ignore any unrecognized data that
   follows the encoded public key.



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   Note that the values of bits 6 and 7 (the "u" and "g" bits) of the
   interface identifier are ignored during CGA verification. After the
   verification succeeds, the verifier SHOULD process all CGA addresses
   the in the same way regardless of the Sec, modifier and collision
   count values. In particular, the verifier SHOULD NOT have any
   security policy that differentiates between addresses based on the
   value of Sec. That way, the address generator is free choose any
   value of Sec.

   All nodes that implement CGA verification MUST be able to process all
   security parameter values Sec = 0, 1, 2, 3, 4, 5, 6, 7. The
   verification procedure is relatively fast and always requires a
   constant amount of computation. If Sec=0, the verification never
   fails in steps (6)-(7) and these steps can be skipped.

   Nodes that implement CGA verification SHOULD be able to process RSA
   public keys that have the OID rsaEncryption and key length between
   384 and 2048 bits. Implementations MAY support longer keys. Future
   versions of this specification may recommend support for longer keys.
































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6. CGA Signatures

   This section defines the procedures for generating and verifying CGA
   signatures. In order to sign a message, a node needs the CGA address,
   the associated CGA Parameters data structure, the message, and the
   private cryptographic key that corresponds to the public key in the
   CGA Parameters. The node also needs to have a 128-bit type tag for
   the message from CGA Message Type name space.

   To sign a message, a node SHOULD do the following:

   o  Concatenate the 128-bit type tag and the message. The
      concatenation is the message to be signed in the next step.

   o  Generate the RSA signature using the RSASSA-PKCS1-v1_5
      [PKCS.1.2002] signature algorithm with the SHA-1 hash algorithm.
      The inputs to the generation operation are the private key and the
      concatenation created above.

   The SEND protocol specification [I-D.ietf-send-ndopt] defines several
   messages that contain a signature in the Signature Option. The SEND
   protocol specification also defines a type tag from the CGA Message
   Type name space. The same type tag is used for all the SEND messages
   that have the Signature Option. This type tag is an IANA-allocated
   128 bit integer that has been chosen in random to prevent accidental
   type collision with messages of other protocols that use the same
   public key but may or may not use IANA-allocated type tags.

   The CGA address, the CGA Parameters data structure, the message, and
   the signature are sent to the verifier. The SEND protocol
   specification defines how this data is sent in SEND protocol
   messages. Note that the 128-bit type tag is not included in the SEND
   protocol messages because the verifier knows its value implicitly
   from the ICMP message type field in the SEND message.

   In order to verify a signature, the verifier needs the CGA address,
   the associated CGA Parameters data structure, the message, and the
   signature. The verifier also needs to have the 128-bit type tag for
   the message.

   To verify the signature, a node SHOULD do the following:

   o  Verify the CGA address as defined in Section 5. The inputs to the
      CGA verification are the CGA address and the CGA Parameters data
      structure.

   o  Concatenate the 128-bit type tag and the message. The
      concatenation is the message whose signature is to be verified in



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      the next step.

   o  Verify the RSA signature using the RSASSA-PKCS1-v1_5 [PKCS.1.2002]
      algorithm with the SHA-1 hash algorithm. The inputs to the
      verification operation are the public key (i.e., the
      RSAEncryptionKey structure from the SubjectPublicKeyInfo structure
      that is a part of the CGA Parameters data structure), the
      concatenation created above, and the signature.

   The verifier accepts the signature as authentic only if both the CGA
   verification and the signature verification succeed.








































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7. Security Considerations

7.1 Security Goals and Limitations

   The purpose of CGA addresses is to prevent stealing and spoofing of
   existing IPv6 addresses. The public key of the address owner is bound
   cryptographically to the address. The address owner can use the
   corresponding private key to assert its ownership of the address and
   to sign SEND messages sent from the address.

   It is important to understand that an attacker can create a new
   address from an arbitrary subnet prefix and its own public key.  What
   the attacker cannot do is to impersonate somebody else's address.
   This is because the attacker would have to find a collision of the
   cryptographic hash value Hash1. (The property of the hash function
   needed here is called second pre-image resistance or weak collision
   resistance.)

   For each valid CGA Parameters data structure, there are 4*(Sec+1)
   different CGA addresses that match the value. This is because
   decrementing the Sec value in the three leftmost bits of the
   interface identifier does not invalidate the address, and the "u" and
   "g" bits can be chosen freely. In SEND, this fact does not have any
   security or implementation implications.

   Another limitation of CGA addresses is that there is no mechanism for
   proving that an address is not a CGA address. Thus, an attacker could
   take someone else's CGA address and present it as a non-CGA address
   (e.g., as an RFC-3041 address). An attacker does not benefit from
   this because although SEND nodes accept both signed and unsigned
   messages from every address, they give priority to the information in
   the signed messages.

7.2 Hash extension

   As computers become faster, the 64 bits of the interface identifier
   will not be sufficient to prevent attackers from searching for hash
   collisions. It helps somewhat that we include the subnet prefix of
   the address in the hash input. This prevents the attacker from using
   a single pre-computed database to attack addresses with different
   subnet prefixes. The attacker needs to create a separate database for
   each subnet prefix. Link-local addresses are, however, left
   vulnerable because the same prefix is used by all IPv6 nodes.

   In the long term, some kind of hash extension technique must be used
   to counter the effect of faster computers. Otherwise, the CGA
   technology could become outdated after 5-20 years. The idea in this
   document is to increase the cost of both address generation and



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   brute-force attacks by the same parameterized factor while keeping
   the cost of address use and verification constant. This provides
   protection also for link-local addresses. Introduction of the hash
   extension is the main difference between this document and earlier
   CGA proposals [OR01][Nik01][MC02].

   To achieve the effective extension of the hash length, the input to
   the second hash function Hash2 is modified (by changing the modifier
   value) until the leftmost 16*Sec bits of the hash value are zero.
   This increases the cost of address generation approximately by a
   factor of 2^(16*Sec). It also increases the cost of brute-force
   attacks by the same factor. That is, the cost of creating a CGA
   Parameters data structure that binds the attacker's public key with
   somebody else's address is increased from O(2^59) to
   O(2^(59+16*Sec)). The address generator may choose the security
   parameter Sec depending on its own computational capacity, perceived
   risk of attacks, and the expected lifetime of the address. Currently,
   Sec values between 0 and 2 are sufficient for most IPv6 nodes. As
   computers become faster, higher Sec values will slowly become useful.

   Theoretically, if no hash extension is used (i.e., Sec=0) and a
   typical attacker is able to tap into N local networks at the same
   time, an attack against link-local addresses is N times as efficient
   as an attack against addresses of a specific network. The effect
   could be countered by using a slightly higher Sec value for
   link-local addresses. When higher Sec values (such that 2^(16*Sec) >
   N) are used for all addresses, the relative advantage of attacking
   link-local addresses becomes insignificant.

   The effectiveness of the hash extension depends on the assumption
   that the computational capacities of the attacker and the address
   generator will grow at the same (potentially exponential) rate. This
   is not necessarily true if the addresses are generated on low-end
   mobile devices where the main design goals are lower cost and smaller
   size rather than increased computing power. But there is no reason
   for doing so. The expensive part of the address generation (steps
   (1)-(3) of the generation algorithm) may be delegated to a more
   powerful computer. Moreover, this work can be done in advance or
   offline, rather than in real time when a new address is needed.

   In order to make it possible for mobile nodes whose subnet prefix
   changes frequently to use Sec values greater than 0, we have decided
   not to include the subnet prefix in the input of Hash2. The result is
   weaker than if the subnet prefix were included in the input of both
   hashes. On the other hand, our scheme is at least as strong as using
   the hash extension technique without including the subnet prefix in
   either hash. It is also at least as strong as not using the hash
   extension but including the subnet prefix. This trade-off was made



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   because mobile nodes frequently move to insecure networks where they
   are at the risk of denial-of-service (DoS) attacks, for example,
   during the duplicate address detection procedure.

   In most networks, the goal of Secure Neighbor Discovery and CGA-based
   authentication is to prevent denial-of-service attacks. Therefore, it
   is usually sensible to start by using a low Sec value and to replace
   addresses with stronger ones only when denial-of-service attacks
   based on brute-force search become a significant problem. (If CGA
   addresses were used as a part of a strong authentication or secrecy
   mechanisms, it would be necessary to start with higher Sec values.)

   The collision count value is used to modify the input to Hash1 if
   there is an address collision. It is important not to allow collision
   count values higher than 2. First, it is extremely unlikely that
   three collisions would occur and the reason is certain to be either a
   configuration or implementation error or a denial-of-service attack.
   (When the SEND protocol is used, the deliberate collisions caused by
   a DoS attacker are detected and ignored.) Second, an attacker who is
   doing a brute-force search to match a given CGA address can try all
   different values of collision count without repeating the brute-force
   search for the modifier value. Thus, the more different values are
   allowed for the collision count, the less effective the
   hash-extension technique is in preventing brute-force attacks.

7.3 Privacy Considerations

   CGA addresses can give the same level pseudonymity as the IPv6
   address privacy extensions defined in RFC 3041 [RFC3041]. An IP host
   can generate multiple pseudorandom CGA addresses by executing the CGA
   generation algorithm of Section 4 multiple times and by using every
   time a different random or pseudorandom initial value for the
   modifier. The host should change its address periodically as in
   [RFC3041]. When privacy protection is needed, the (pseudo)random
   number generator used in address generation SHOULD be strong enough
   to produce unpredictable and unlinkable values.

   There are two apparent limitations to this privacy protection.
   However, as we will explain below, neither limitation is very
   serious.

   First, the high cost of address generation may prevent hosts that use
   a high Sec value from changing their address frequently. This problem
   is mitigated by the fact that the expensive part of the address
   generation may be done in advance or offline, as explained in the
   previous section. It should also be noted that the nodes that benefit
   most from high Sec values (e.g., DNS servers, routers, and data
   servers) usually do not require pseudonymity, while the nodes that



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   have high privacy requirements (e.g., client PCs and mobile hosts)
   are unlikely targets for expensive brute-force attacks and can do
   with lower Sec values.

   Second, the public key of the address owner is revealed in the
   authenticated SEND messages. This means that if the address owner
   wants to be pseudonymous towards the nodes in the local links that it
   accesses, it should not only generate a new address but also a new
   public key. With typical local-link technologies, however, a node's
   link-layer address is a unique identifier for the node. As long as
   the node keeps using the same link-layer address, it makes little
   sense to ever change the public key for privacy reasons.

7.4 Related protocols

   While this document defines CGA addresses only for the purposes of
   Secure Neighbor Discovery, other protocols could be defined elsewhere
   that use the same addresses and public keys. This raises the
   possibility of related-key attacks where a signed message from one
   protocol is replayed in another protocol. This means that other
   protocols (perhaps designed without an intimate knowledge of SEND)
   could endanger the security of SEND.

   To prevent the related-protocol attacks, a type tag is prepended to
   every message before signing it. The type-tags are 128-bit randomly
   chosen values, which prevents accidental type collisions with even
   poorly designed protocols that do not use any type tags.

   Finally, some cautionary notes should be made about using CGA-based
   authentication for other purposes than SEND. First, the other
   protocols should use type tags in all signed messages in the same way
   as SEND does. Because of the possibility of related-protocol attacks,
   it is advisable to use the public key only for signing and not for
   encryption. Second, CGA-based authentication is particularly suitable
   for securing neighbor discovery [RFC2461] and duplicate address
   detection [RFC2462] because these are network-layer signaling
   protocols where IPv6 addresses are natural endpoint identifiers. In
   any protocol that aims to protect higher-layer data, CGA-based
   authentication alone is not sufficient and there must also be a
   secure mechanism for mapping higher-layer identifiers, such as DNS
   names, to IP addresses.










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8. IANA Considerations

   This document defines a new CGA Message Type name space for use as
   type tags in messages may be signed using CGA signatures. The values
   in this name space are 128-bit integers. Values in this name space
   are allocated as First Come First Served [RFC2434]. IANA assigns new
   128-bit values directly without a review.

   The new values SHOULD be generated with a strong random-number
   generator by the requester. Continuous ranges of at most 256 values
   can be allocated provided that the 120 most significant bits of the
   values have been generated with a strong random-number generator. It
   is not necessary for IANA to verify the randomness of the requested
   values. The name space is essentially unlimited and any number of
   individual values or ranges of at most 256 values can be allocated.

   CGA Message Type values for private use MAY be generated with a
   strong random-number generator without IANA allocation.

   This document does not define any new values in any name space.































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Normative References

   [I-D.ietf-send-ndopt]
              Arkko, J., Kempf, J., Sommerfeld, B., Zill, B. and P.
              Nikander, "SEcure Neighbor Discovery (SEND)",
              draft-ietf-send-ndopt-00 (work in progress), October 2003.

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

   [RFC3513]  Hinden, R. and S. Deering, "Internet Protocol Version 6
              (IPv6) Addressing Architecture", RFC 3513, April 2003.

   [RFC3280]  Housley, R., Polk, W., Ford, W. and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

   [ITU.X690.2002]
              International Telecommunications Union, "Information
              Technology - ASN.1 encoding rules: Specification of Basic
              Encoding Rules (BER), Canonical Encoding Rules (CER) and
              Distinguished Encoding Rules (DER)", ITU-T Recommendation
              X.690, July 2002.

   [RFC2434]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 2434,
              October 1998.

   [PKCS.1.2002]
              RSA Laboratories, "RSA Encryption Standard, Version 2.1",
              Public-Key Cryptography Standard PKCS 1, June 2002.

   [FIPS.180-1.1995]
              National Institute of Standards and Technology, "Secure
              Hash Standard", Federal Information Processing Standards
              Publication FIPS PUB 180-1, April 1995, <http://
              www.itl.nist.gov/fipspubs/fip180-1.htm>.













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Informative References

   [AAKMNR02]
              Arkko, J., Aura, T., Kempf, J., Mantyla, V., Nikander, P.
              and M. Roe, "Securing IPv6 neighbor discovery and router
              discovery", ACM Workshop on Wireless Security (WiSe 2002),
              Atlanta, GA USA , September 2002.

   [Aura03]   Aura, T., "Cryptographically Generated Addresses (CGA)",
              6th Information Security Conference (ISC'03), Bristol, UK
              , October 2003.

   [MC02]     Montenegro, G. and C. Castelluccia, "Statistically unique
              and cryptographically verifiable identifiers and
              addresses", ISOC Symposium on Network and Distributed
              System Security (NDSS 2002), San Diego, CA USA , February
              2002.

   [RFC3041]  Narten, T. and R. Draves, "Privacy Extensions for
              Stateless Address Autoconfiguration in IPv6", RFC 3041,
              January 2001.

   [RFC2461]  Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December
              1998.

   [Nik01]    Nikander, P., "A scaleable architecture for IPv6 address
              ownership", draft-nikander-addr-ownership-00 (work in
              progress), March 2001.

   [OR01]     O'Shea, G. and M. Roe, "Child-proof authentication for
              MIPv6 (CAM)", ACM Computer Communications Review 31(2),
              April 2001.

   [RFC2462]  Thomson, S. and T. Narten, "IPv6 Stateless Address
              Autoconfiguration", RFC 2462, December 1998.















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Author's Address

   Tuomas Aura
   Microsoft Research
   Roger Needham Building
   7 JJ Thomson Avenue
   Cambridge  CB3 0FB
   United Kingdom

   Phone: +44 1223 479708
   EMail: tuomaura@microsoft.com








































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Appendix A. Example of CGA Generation

   We generate a CGA address with Sec=1 from the subnet prefix fe80::
   and the following public key:

   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The modifier is initialized to a random value 89a8 a8b2 e858 d8b8
   f263 3f44 d2d4 ce9a. The input to Hash2 is:

   89a8 a8b2 e858 d8b8 f263 3f44 d2d4 ce9a 0000 0000 0000 0000 00
   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The 112 first bits of the SHA-1 hash value computed from the above
   input is Hash2=436b 9a70 dbfd dbf1 926e 6e66 29c0. This does not
   begin with 16*Sec=16 zero bits. Thus, we must increment the modifier
   and recompute the hash. The new input to Hash2 is:

   89a8 a8b2 e858 d8b8 f263 3f44 d2d4 ce9b 0000 0000 0000 0000 00
   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The new hash value is Hash2=0000 01ca 680b 8388 8d09 12df fcce. The
   16 leftmost bits of Hash2 are all zero. Thus, we found a suitable
   modifier. (We were very lucky to find it so soon.)

   The input to Hash1 is:

   89a8 a8b2 e858 d8b8 f263 3f44 d2d4 ce9b fe80 0000 0000 0000 00
   305c 300d 0609 2a86 4886 f70d 0101 0105 0003 4b00 3048 0241
   00c2 c2f1 3730 5454 f10b d9ce a368 44b5 30e9 211a 4b26 2b16
   467c b7df ba1f 595c 0194 f275 be5a 4d38 6f2c 3c23 8250 8773
   c786 7f9b 3b9e 63a0 9c7b c48f 7a54 ebef af02 0301 0001

   The 64 first bits of the SHA-1 hash value of the above input are
   Hash1=fd4a 5bf6 ffb4 ca6c. We form an interface identifier from this
   by writing Sec=1 into the three leftmost bits and by setting bits 6
   and 7 (the "u" and "g" bits) to zero. The new interface identifier is
   3c4a:5bf6:ffb4:ca6c.




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   Finally, we form the IPv6 address fe80::3c4a:5bf6:ffb4:ca6c. This is
   the new CGA address. No address collisions were detected this time.
   (Collisions are very rare.) The CGA Parameters data structure
   associated with the address is the same as the input to Hash1 above.















































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Appendix B. Acknowledgments

   The author gratefully acknowledges the contributions of Jari Arkko,
   Francis DuPont, Pasi Eronen, Christian Huitema, Pekka Nikander,
   Michael Roe, Dave Thaler, and several other participants of the IETF
   working group.













































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   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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