Network Working Group                                           A. Kukec
Internet-Draft                                      University of Zagreb
Intended status: Standards Track                             S. Krishnan
Expires: August 16, September 7, 2010                                      Ericsson
                                                                S. Jiang
                                            Huawei Technologies Co., Ltd
                                                       February 12,
                                                           March 6, 2010

                       SEND Hash Threat Analysis


   This document analysis the use of hashes in SEND, possible threats
   and the impact of recent attacks on hash functions used by SEND.
   Current SEND specification [rfc3971] uses the SHA-1 [sha-1] hash
   algorithm and X.509 certificates [rfc5280] and does not provide
   support for the hash algorithm agility.  The purpose of the document
   is to provide analysis of possible hash threats and to decide how to
   encode the hash agility support in SEND.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5  4
   3.  Impact of collision attacks on SEND  . . . . . . . . . . . . .  6  5
     3.1.  Attacks against CGAs in stateless autoconfiguration  . . .  6  5
     3.2.  Attacks against X.509 certificates in ADD process  . . . .  7  6
     3.3.  Attacks against the Digital Signature in the RSA
           Signature option . . . . . . . . . . . . . . . . . . . . .  8  7
     3.4.  Attacks against the Key Hash field in the RSA
           Signature option . . . . . . . . . . . . . . . . . . . . .  8  7
   4.  Support for the hash agility in SEND . . . . . . . . . . . . .  9  8
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11 10
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 12
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14

1.  Introduction

   SEND [rfc3971] uses the SHA-1 hash algorithm to generate
   Cryptographically Generated Addresses (CGA) [rfc3972], the contents
   of the Key Hash field and the Digital Signature field of the RSA
   Signature option.  It also uses a hash algorithm (SHA-1, MD5, etc.)
   within the digital signature in X.509 certificates [rfc5280] for the
   router authorization in the Authorizaton Delegation Discovery (ADD)

   There is a great variaty of hash functions, but only MD5 and SHA-1
   are in the wide use, which is also the case for SEND.  They both
   derive from MD4, which has been well known for its weaknesses.  First
   hash attacks affected the compression function of MD5, while the
   latest hash attacks against SHA variants delivered colliding hashes
   in significantlly smaller number of rounds compared to the brute
   force attack number of rounds [sha1-coll].  Apart from the
   aforementioned hash attacks, researchers also demonstrated attacks
   against X.509 certificates.  They demonstrated colliding X.509
   certificates with MD5 hash, both with the same and different
   distinguished names [new-hashes] [x509-coll].

   Depending on the way how the Internet protocol uses the hash
   algorithm, Internet protocol can be affected by the weakness of the
   underlaying hash function.  This document analyzes uses of hash
   algorithms in SEND, possible vulnerabilities that hash attacks could
   introduce to SEND, and offers suggestions on how to make SEND
   resistant to such attacks.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [rfc2119].

3.  Impact of collision attacks on SEND

   Due to the hash attacks demonstrated on the aforesaid hash algorithms
   a study was performed to assess the threat of these attacks on the
   cryptographic hash usage in Internet protocols.  This document
   analyzes the hash usage in SEND following the recommended approach
   [rfc4270] [new-hashes].

   Basic cryptographic properties of a hash function are that it is both
   one-way and collision free.  There are two attacks against the one-
   way property, the first-preimage attack and the second-preimage
   attack.  In the first-preimage attack, given a knowledge of a
   particular hash value h, an attacker finds an input message m such
   that hash(m) = h.  The second-preimage attack deals with fixed
   messages.  Given a knowledge of a fixed value m used as the input
   message to the hash function, an attacker finds a different value m'
   that yields hash(m)=hash(m').  Supposing that the hash function
   produces an n-bit long output, since each output is equally likely,
   an attack takes an order of 2^n operations to be successful.  Due to
   the birthday attack, if the hash function is supplied with a random
   input, it returns one of the k equally-likely values, and the number
   of operations can be reduced to the number of 1.2*2^(n/2) operations.
   Attack against the collision-free property deals with two fixed
   messages, both produced by an attacker.  What happens is that the
   attacker produces two different messages, m and m', such that
   hash(m)=hash(m').  Up to date, all demonstrated attacks are attacks
   against a collision-free property.  Attacks against the one-way
   property are not yet feasible [rfc4270].

   The strength of Internet protocol does not have to be necessarily
   affected by the weakness of the underlaying hash function.  The
   appropriate way of use of the hash algorithm will keep the protocol
   immune, no matter of the hash algorithm weaknesses.  Out of many
   possible hash algorithm uses, such as non-repudiable digital
   signatures, certificate digital signatures, message authentication
   with shared secrets, fingerprints, only the first two can introduce
   weaknesses to the Internet protocol [rfc4270].  The rest of the
   section analyzes the impact of hash attacks, mainly collision
   attacks, on SEND by the cases of use.  Through our analysis, we also
   discuss whether we should support the hash agility in SEND.

3.1.  Attacks against CGAs in stateless autoconfiguration

   Hash functions are used in the stateless autoconfiguration process
   which is based on CGAs.  Impacts of collision attacks on current uses
   of CGAs and the CGA hash agility are analyzed in the update of the
   CGA specification [rfc4982].  CGAs provide the proof-of-ownership of
   the sender's private key corresponding to the public key used to
   generate the CGA.  Simply stated, their main purpose is to assure
   that the sender of the message is the same as the sender of the
   previous message.  As such, CGAs do not deal with the non-repudiation
   feature.  The collision attack against the CGA assumes that the
   attacker generates two different, colliding sets of CGA Parameters
   that result in the same hash value.  Since CGAs do not deal with the
   non-repudiation feature, and both CGA Parameters sets are chosen by
   the attacker itself, this attack does not introduce any
   vulnerabilities to SEND.  If pre-image attacks were feasible, an
   attacker would find colliding CGA Parameters for the victim's CGA,
   and produce the Key Hash field and the Digital Signature field
   afterwards using the new public key.  Since the strength of all
   hashes in SEND depends on the strength of the CGA, the pre-image
   attack is potentially dangerous, but it is not yet feasible.

3.2.  Attacks against X.509 certificates in ADD process

   Another use of hash functions is for the router authorization in the
   ADD process.  Router sends to a host a certification path, which is a
   path between a router and the host's trust anchor, consisting of
   X.509 certificates.  Researchers demonstrated attacks against X.509
   certificates with MD5 signature in 2005 [new-hashes] and in 2007
   [x509-coll].  In 2005 researchers constructed colliding certificates
   with the same distinguished name, different public keys, and
   identical signatures.  Potential problem for the attacker here is
   that two certificates with the same identity can be easily revealed
   by the appropriately configured Certification Authority that does not
   allowe to provide two certificates with the same identities.  Human-
   readable fields significantly complicate the attack.  In case of the
   identity field an attacker is faced with the problem of the
   prediction and the generation of the two different, false but
   meaningful identities, which at the end might be revealed by the
   Certification Authority.  Thus, although theoretically possible,
   real-world circumstances such as the context of the human-readable
   fields, make these attacks with colliding certificates with the same
   identities impossible.  In 2007 researchers demonstrated colliding
   certificates which differ in the identity data and in the public key,
   but still result in the same signature value.  Even in this case, the
   real-world scenarios prevent the hash algorithm weaknesses to
   introduce vulnerabilities to X.509 certificates or to SEND.  Even if
   an attacker produced such two colliding certificates in order to
   claim that he was someone else, he still needs to predict the content
   of all fields (some of them are human-readable fields) appearing
   before the public key, e.g. the serial number and validity periods.
   Although a relying party cannot verify the content of these fields
   (each certificate by itself is unsuspicious), the Certification
   Authority keeps track of those fields and it can reveal the false
   certificate during the fraud analysis.  Even though real-world
   scenarios make SEND immune to recent hash attacks introduced through
   X.509 certificates, theoretically they are possible.  Regarding X.509
   certificates in SEND, biggest concer are potential attacks against
   the RFC3779 IP address extension which would enable the bogus router
   to advertize the changed IP prefix range (if the IP prefix range
   used), although, not broader than the prefix range of the parent
   certificate in the ADD chain.  Adding some form of randomness to the
   such human-readble data such would prevent attacks, which can be
   considered once when the collision attack improve.

3.3.  Attacks against the Digital Signature in the RSA Signature option

   The computation of the Digital Signature field is described
   [rfc3971].  It is produced as the SHA-1 hash over the IPv6 addresses,
   the ICMPv6 header, the ND message and other fields, e.g. the Message
   Type Tag and ND options, and signed with the sender's private key.
   Private key corresponds to the public key in the CGA parameters
   structure.  It is usually authorized through CGAs.  The Digital
   Signature field the example of the non-repudiation digital singature,
   and it is vulnerable to recent collision attacks.  Possible attacks
   on such explicit digital signature is a typical non-repudiation
   attack in which the attacker produces two different messages, m and
   m', where hash(m) = hash(m').  He underlays one of the messages to be
   signed with the key authorized through CGAs, but uses another message
   afterwards.  However, the structure of at least one of two messages
   in a collision attack is strictly predefined.  The previous
   requirement makes this collision attack to be much more then the
   simple collision attack.  It requires the attacker to know or predict
   the communication context.  Theoretically this attack could harm
   SEND, but in real-world situation is to achieve it.

3.4.  Attacks against the Key Hash field in the RSA Signature option

   The Key Hash field in the RSA Signature option is a SHA-1 hash of the
   public key from the CGA Parameters structure in the CGA option.  It
   is a fingerprint that provides the integrity protection.
   Fingerprints are generally not affected by the collision attacks
   because they involve random data as one of the inputs, which prevents
   recent collision attacks.  In addition, context of the SEND message
   and the protocol makes this attack unable to introduce new
   vulnerabilities to SEND.  An attacker has to produce both keys, k and
   k', such that hash(k) = hash(k').  Since the key is authorized
   through CGA, and possibily through the certification in the ADD
   process, this attack is of no use for the attacker.  The pre-image
   attack against the Key Hash field, if it was possible, would affect
   SEND since the Key Hash field contains a non human-readable data.

4.  Support for the hash agility in SEND

   Previous section showed that recent hash attacks against CGAs and
   fingerprints (Key Hash field of the Send message) do not introduce
   new vulnerabilities to SEND.  Digital signatures in the Digital
   Signature field of the SEND message and in the X.509 certificate
   theoretically could introduce new vulnerabilities to SEND, but only
   in limited circumstances.  SEND context prevents those attacks of
   almost any use in the real-world scenarios.

   However, recent attacks indicate the possibility for the future
   improved real-world attacks.  Researchers advise to migrate away from
   currently used hash algorithms.  In November 2007, NIST announced an
   opened competition for a new SHA-3 function.  The selection of a
   winning function will be in 2012.  In order to increase the future
   security of SEND, we suggest the support for the hash and algorithm
   agility in SEND.

   o  The most effective and secure would be to bind the hash function
      option with something that can not be changed at all, like
      [rfc4982] does for CGA.  It encodes the hash function information
      into addresses.  We could decide to use by default the same hash
      function in SEND as in CGA.  The security of all hashes in SEND
      depends on CGA, i.e. if an attacker breaks CGA, all other hashes
      are automatically broken.  The use of the hash algorithm embedded
      in CGA protects from the bidding down attacks.  From the security
      point of view, at the moment, this solution is more reasonable
      then defining different hash algorithm for each hash.  The
      disadvantage of this solution is that it introduces the limitation
      for SEND to be used exclusively with CGAs.

   o  Another solution is to incorporate the Hash algorithm option into
      the SEND message.  This solution is vulnerable to the bidding down

   o  The third possible solution is to encode the algorithm in the CGA.
      This would reduce the strength of the CGA and make it vulnerable
      to brute force attacks.

   o  Possible solution is also the hybrid solution which would require
      the hash algorithm to be the same as CGA, if CGA option is
      present, and to use the Hash agility option if the CGA option is
      not present.  In such way, SEND is not bound exclusively to CGA.

   o  None of the previous solutions supports the negotiation of the
      hash function.  One of possible solutions is the negotiation
      approach for the SEND hash agility based on the Supported
      Signature Algorithm option described in [sig-agility].  Based on
      the processing rules described in [sig-agility] nodes find the
      intersection between the sender's and the receiver's supported
      signature algorithms set.

5.  Security Considerations

   This document analyzes the impact of hash attacks in SEND and offeres
   a higher security level for SEND by providing solution for the hash
   agility support.

   The negotiation approach for the hash agility in SEND based on the
   Supported Signature Algorithms option is vulnerable to bidding-down
   attacks, which is usual in the case of any negotiation approach.
   This issue can be mitigated with the appropriate local policies.

6.  Security Considerations

   There are no IANA actions resulting from this document.

7.  References


7.1.  Normative References

              Bellovin, S. and E. Rescorla, "Deploying a New Hash
              Algorithm", November 2005.

              Cheneau, T., Maknavicius, M., Sean, S., and M. Vanderveen,
              "Support for Multiple Signature Algorithms in
              Cryptographically generated Addresses (CGAs)",
              draft-cheneau-cga-pk-agility-00 (work in progress),
              February 2009.

   [rfc3971]  Arkko, J., Kempf, J., Zill, B., and P. Nikander, "SEcure
              Neighbor Discovery (SEND)", RFC 3971, March 2005.

   [rfc3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, March 2005.

   [rfc4270]  Hoffman, P. and B. Schneier, "Attacks on Cryptographic
              Hashed in Internet Protocols", RFC 4270, November 2005.

   [rfc4982]  Bagnulo, M. and J. Arrko, "Support for Multiple Hash
              Algorithms in Cryptographically Generated Addresses
              (CGAs)", RFC 4982, July 2007.

              Cheneau, T. and M. Maknavicius, "Signature Algorithm
              Agility in the Secure Neighbor Discovery (SEND) Protocol",
              draft-cheneau-send-sig-agility-01 (work in progress),
              May 2010.


7.2.  Informative References

   [rfc2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", RFC 2119, March 1997.

   [rfc5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC rfc5280, May 2008.

   [sha-1]    NIST, FIBS PUB 180-1, "Secure Hash Standard", April 1995.

              Wang, X., Yin, L., and H. Yu, "Finding Collisions in the
              Full SHA-1. CRYPTO 2005: 17-36", 2005.

              Stevens, M., Lenstra, A., and B. Weger, "Chosen-Prefix
              Collisions for MD5 and Colliding X.509 Certificates for
              Different Identitites. EUROCRYPT 2007: 1-22", 2005.

Authors' Addresses

   Ana Kukec
   University of Zagreb
   Unska 3


   Suresh Krishnan
   8400 Decarie Blvd.
   Town of Mount Royal, QC


   Sheng Jiang
   Huawei Technologies Co., Ltd
   KuiKe Building, No.9 Xinxi Rd.,
   Shang-Di Information Industry Base, Hai-Dian District, Beijing
   P.R. China