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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: October 15, 2004                                 April 16, 2004


              Cryptographically Generated Addresses (CGA)
                         draft-ietf-send-cga-06

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 October 15, 2004.

Copyright Notice

   Copyright (C) The Internet Society (2004). 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 Format . . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  CGA Parameters and Hash Values . . . . . . . . . . . . . . . .  5
   4.  CGA Generation . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  CGA Verification . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  CGA Signatures . . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
     7.1   Security Goals and Limitations . . . . . . . . . . . . . . 12
     7.2   Hash extension . . . . . . . . . . . . . . . . . . . . . . 13
     7.3   Privacy Considerations . . . . . . . . . . . . . . . . . . 15
     7.4   Related protocols  . . . . . . . . . . . . . . . . . . . . 15
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
   9.1   Normative References . . . . . . . . . . . . . . . . . . . . 17
   9.2   Informative References . . . . . . . . . . . . . . . . . . . 18
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 19
   A.  Example of CGA Generation  . . . . . . . . . . . . . . . . . . 19
   B.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
       Intellectual Property and Copyright Statements . . . . . . . . 21






























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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 address is called a cryptographically generated
   address (CGA). The corresponding private key can then be used to sign
   messages sent from the address.

   This document specifies:

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

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

   o  how to sign a message that is sent from the CGA, and how to verify
      the 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. The verifier can then go
   on to verify messages signed by the owner of the public key (i.e.,
   the address owner). No additional security infrastructure, such as a
   public key infrastructure (PKI), certification authorities, or other
   trusted servers, is needed.

   It is important to note that because CGAs themselves are not
   certified, an attacker can create a new CGA from any subnet prefix
   and its own (or anyone else's) public key. What the attacker cannot
   do is to take a CGA created by someone else and send signed messages
   that appear to come from the owner of that address.

   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 security, the reasoning behind the hash
   extension technique that enables effective hash lengths above the
   64-bit limit of the interface identifier, the implications of CGAs 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



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   be interpreted as described in [RFC2119].

2.  CGA 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 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 is defined as an IPv6 address
   that satisfies the following two conditions:

   o  The first hash value, Hash1, equals 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 from the standard
      IPv6 address architecture format of interface identifiers
      [RFC3513]) are ignored in the comparison.

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

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

     Mask1 (64 bits)  = 0x1cffffffffffffff

     Mask2 (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



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                        0xffffffffffffffffffffffffffff  if Sec=7

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

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


3.  CGA Parameters and Hash Values

   Each CGA is associated with a CGA Parameters data structure, which
   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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                      Modifier (16 octets)                     +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                    Subnet Prefix (8 octets)                   +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |Collision Count|                                               |
       +-+-+-+-+-+-+-+-+                                               |
       |                                                               |
       ~                  Public Key (variable length)                 ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       ~           Extension Fields (optional, variable length)        ~
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Modifier

      This field contains a 128-bit unsigned integer, which can be any
      value. The modifier is used during CGA generation to implement the
      hash extension and to enhance privacy by adding randomness to the
      address.





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   Subnet Prefix

      This field contains the 64-bit subnet prefix of the CGA.

   Collision Count

      This is an 8-bit unsigned integer, which MUST be 0, 1 or 2. The
      collision count is incremented during CGA generation to recover
      from an address collision detected by duplicate address detection.

   Public Key

      This is a variable length field containing the public key of the
      address owner. The public key MUST be formatted as a DER-encoded
      [ITU.X690.2002] ASN.1 structure of the type SubjectPublicKeyInfo
      defined in the Internet X.509 certificate profile [RFC3280]. SEND
      SHOULD use an RSA public/private key pair. When RSA is used, the
      algorithm identifier MUST be rsaEncryption, which is
      1.2.840.113549.1.1.1, and the RSA public key MUST be formatted
      using the RSAPublicKey type as specified in Section 2.3.1 of RFC
      3279 [RFC3279]. The RSA key length SHOULD be at least 384 bits.
      Other public key types are undesirable in SEND since they may
      result in incompatibilities between implementations. The length of
      this field is determined by the ASN.1 encoding.

   Extension Fields

      This is an optional variable-length field, which is not used in
      the current specification. Future versions of this specification
      may use this field for additional data items that need to be
      included in the CGA Parameters data structure. IETF standards
      action is required to specify the use of the extension fields.
      Implementations MUST ignore the value of any unrecognized
      extension fields.

   The two hash values MUST be computed as follows. The SHA-1 hash
   algorithm [FIPS.180-1.1995] is applied to the CGA 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. Note that the hash values are computed over
   the entire CGA Parameters data structure, including any unrecognized
   extension fields.





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4.  CGA Generation

   The process of generating a new CGA 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 depends exponentially on the security
   parameter Sec, which can have values from 0 to 7.

   A CGA and associated parameters SHOULD be generated as follows:

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

   2.  Concatenate from left to right 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 by one and go back to step (2).

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

   5.  Concatenate from left to right 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 with the subnet prefix
       to the left and interface identifier to the right as in a
       standard IPv6 address [RFC3513].

   8.  Perform duplicate address detection if required, as per
       [I-D.ietf-send-ndopt]. If an address collision is detected,
       increment the collision count by one and go back to step (5).
       However, after three collisions, stop and report the error.

   9.  Form the CGA Parameters data structure by concatenating from left
       to right 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 and a CGA



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   Parameters data structure.

   The initial value of the modifier in step (1) SHOULD be chosen
   randomly in order to make addresses generated from the same public
   key unlinkable, which enhances 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. Implementations that
   have no strong random numbers available MAY use a non-cryptographic
   pseudo-random number generator that is initialized with the current
   time of day.

   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 CGAs with high Sec values is
   infeasible with current technology.

   Implementations MAY use optimized or otherwise modified versions of
   the above algorithm for CGA generation. However, the output of any
   such modified versions of the algorithm MUST fulfill the following
   two requirements. First, the resulting CGA and CGA Parameters data
   structure MUST be formatted as specified in Sections 2-3. Second, the
   CGA verification procedure defined in Section 5 MUST succeed when
   invoked on the output of the CGA generation algorithm. It is
   important to note that some optimizations involve trade-offs between
   privacy and the cost of address generation.

   One optimization is particularly important. 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 optimization avoids repeating
   the expensive search for an acceptable modifier value but may, in
   some situations, make it easier for an observer to link two addresses
   to each other.

   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.

   Future versions of this specification may specify additional inputs
   to the CGA generation algorithm, which are concatenated as extension
   fields to the end of the CGA Parameters data structure. This document



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   does not specify how such data fields are handled during CGA
   generation.

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, public key, and optional
   extension fields. The verification either succeeds or fails.

   The CGA 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 from left to right the modifier, 9 zero octets, and
       the public key, and any extension fields that follow the public
       key in the CGA Parameters data structure. 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 fails at any step, the execution of the algorithm
   MUST be stopped immediately. On the other hand, if the verification
   succeeds, the verifier knows that the public key in the CGA



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   Parameters is the authentic public key of the address owner. The
   verifier can extract the public key by removing 25 octets from the
   beginning of the CGA Parameters and by decoding the following
   SubjectPublicKeyInfo data structure.

   Note that the values of bits 6 and 7 (the "u" and "g" bits) of the
   interface identifier are ignored during CGA verification. In the SEND
   protocol, after the verification succeeds, the verifier SHOULD
   process all CGAs in the same way regardless of the Sec, modifier and
   collision count values. In particular, the verifier in the SEND
   protocol 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 at most
   two computations of the SHA-1 hash function. If Sec=0, the
   verification never fails in steps (6)-(7) and these steps can be
   skipped.

   Nodes that implement CGA verification for SEND SHOULD be able to
   process RSA public keys that have the algorithm identifier
   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.

   Implementations of CGA verification MUST ignore the value of any
   unrecognized extension fields that follow the public key in the CGA
   Parameters data structure. However, implementations MUST include any
   such unrecognized data in the hash input when computing Hash1 in step
   (3) and Hash2 in step (6) of the CGA verification algorithm. This is
   important to ensure upward compatibility with future extensions.

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, 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 the CGA Message Type name space.

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

   o  Concatenate the 128-bit type tag (in network byte order) and the
      message with the type tag to the left and the message to the
      right. The concatenation is the message to be signed in the next



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      step.

   o  Generate the RSA signature using the RSASSA-PKCS1-v1_5 [RFC3447]
      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 at 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, the CGA Parameters data structure, the message, and the
   signature are sent to the verifier. The SEND protocol specification
   defines how these data items are 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. See the SEND
   specification [I-D.ietf-send-ndopt] for precise information about how
   SEND handles the type tag.

   In order to verify a signature, the verifier needs the CGA, 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 as defined in Section 5. The inputs to the CGA
      verification are the CGA and the CGA Parameters data structure.

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

   o  Verify the RSA signature using the RSASSA-PKCS1-v1_5 [RFC3447]
      algorithm with the SHA-1 hash algorithm. The inputs to the
      verification operation are the public key (i.e., the RSAPublicKey
      structure from the SubjectPublicKeyInfo structure that is a part
      of the CGA Parameters data structure), the concatenation created
      above, and the signature.

   The verifier MUST accept 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 CGAs 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 (or someone
   else's) public key because CGAs are not certified. 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 [MOV97].)

   For each valid CGA Parameters data structure, there are 4*(Sec+1)
   different CGAs 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 verifier ignores the values of
   the "u" and "g" bits. In SEND, this fact does not have any security
   or implementation implications.

   Another limitation of CGAs is that there is no mechanism for proving
   that an address is not a CGA. Thus, an attacker could take someone
   else's CGA and present it as a non-cryptographically-generated
   address (e.g., as an RFC-3041 address [RFC3041]). 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.

   The minimum RSA key length required for SEND is only 384 bits. So
   short keys are vulnerable to integer-factoring attacks and cannot be
   used for strong authentication or secrecy. On the other hand, the
   cost of factoring 384-bit keys is currently high enough to prevent
   most denial-of-service attacks. Implementations that initially use
   short RSA keys SHOULD be prepared switch to longer keys when
   denial-of-service attacks arising from integer factoring become a
   problem.

   The impact of a key compromise on CGAs depends on the application for
   which they are used. In SEND, it is not a major concern. If the
   private signature key is compromised because the SEND node itself has
   been compromised, the attacker does not need to spoof SEND messages
   from the node. When it is discovered that a node has been
   compromised, a new signature key and a new CGA SHOULD be generated.



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   On the other hand, if the RSA key is compromised because
   integer-factoring attacks for the chosen key length have become
   practical, the key needs to be replaced with a longer one, as
   explained above. In either case, the address change effectively
   revokes the old public key. It is not necessary to have any
   additional key revocation mechanism or to limit the lifetimes of the
   signature keys.

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 order to prevent the CGA technology from becoming outdated as
   computers become faster, the hash technique used to generate CGAs
   must be extended somehow. The chosen extension technique is to
   increase the cost of both address generation and 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 using a slightly higher Sec value for link-local



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   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
   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
   signatures 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 CGAs
   were used as a part of a strong authentication or secrecy mechanism,
   it might 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, 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 can try all different
   values of collision count without repeating the brute-force search
   for the modifier value. Thus, if higher values are allowed for the
   collision count, the hash extension technique becomes less effective
   in preventing brute force attacks.




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7.3  Privacy Considerations

   CGAs can give the same level pseudonymity as the IPv6 address privacy
   extensions defined in RFC 3041 [RFC3041]. An IP host can generate
   multiple pseudorandom CGAs by executing the CGA generation algorithm
   of Section 4 multiple times and using a different random or
   pseudorandom initial value for the modifier every time. 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. Advice on random number generation can be
   found in [RFC1750].

   There are two apparent limitations to this privacy protection.
   However, as will be explained 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
   have high privacy requirements (e.g., client PCs and mobile hosts)
   are unlikely targets for expensive brute-force DoS attacks and can do
   with lower Sec values.

   Second, the public key of the address owner is revealed in the signed
   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 CGAs 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-protocol 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. What makes this threat even more
   significant is that the attacker could take someone else's public key
   and create a CGA from it, and then replay signed messages from a



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   protocol that has nothing to do with CGAs or IP addresses.

   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. Moreover,
   the SEND protocol includes the sender's CGA address in all signed
   messages. This makes it even more difficult for an attacker to take
   signed messages from some other context and to replay them as SEND
   messages.

   Finally, a strong cautionary note needs to be made about using CGA
   signatures for other purposes than SEND. First, the other protocols
   MUST include a type tag and the sender address in all signed messages
   in the same way as SEND does. Each protocol MUST define its own type
   tag values as explained in Section 8. Moreover, because of the
   possibility of related-protocol attacks, the public key MUST be used
   only for signing and it MUST NOT be used for encryption. Second, the
   minimum RSA key length of 384 bits may be too short for many
   applications and the impact of key compromise on the particular
   protocol needs to be evaluated. Third, CGA-based authorization 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 uses other identifiers, such as DNS
   names, CGA signatures alone are not a sufficient security mechanism.
   There must also be a secure way of mapping the other identifiers to
   IPv6 addresses. If the goal is not to verify claims about IPv6
   addresses, CGA signatures are probably not the right solution.

8.  IANA Considerations

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

   The requester SHOULD generate the new values with a strong
   random-number generator. Continuous ranges of at most 256 values can
   be requested provided that the 120 most significant bits of the
   values have been generated with a strong random-number generator.

   IANA does not generate random values for the requester. IANA
   allocates requested values without verifying the way in which they
   have been generated. The name space is essentially unlimited and any
   number of individual values and ranges of at most 256 values can be
   allocated.



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   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.

9.  References

9.1  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-03 (work in progress), January 2003.

   [RFC3279]  Bassham, L., Polk, W. and R. Housley, "Algorithms and
              Identifiers for the Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 3279, April 2002.

   [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.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

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

   [FIPS.180-1.1995]
              National Institute of Standards and Technology, "Secure
              Hash Standard", Federal Information Processing Standards



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              Publication FIPS PUB 180-1, April 1995, <http://
              www.itl.nist.gov/fipspubs/fip180-1.htm>.

9.2  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.

   [RFC1750]  Eastlake, D., Crocker, S. and J. Schiller, "Randomness
              Recommendations for Security", RFC 1750, December 1994.

   [MOV97]    Menezes, A., van Oorschot, P. and S. Vanstone, "Handbook
              of Applied Cryptography", CRC Press , 1997.

   [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

Appendix A.  Example of CGA Generation

   We generate a CGA 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 are Hash2=436b 9a70 dbfd dbf1 926e 6e66 29c0. This does not
   begin with 16*Sec=16 zero bits. Thus, we must increment the modifier
   by one 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:




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   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.

   Finally, we form the IPv6 address fe80::3c4a:5bf6:ffb4:ca6c. This is
   the new CGA. 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.

Appendix B.  Acknowledgements

   The author gratefully acknowledges the contributions of Jari Arkko,
   Francis Dupont, Pasi Eronen, Christian Huitema, James Kempf, Pekka
   Nikander, Michael Roe, Dave Thaler, and other participants of the
   SEND working group.




























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