Network Working Group                                           A. Kukec
Internet-Draft                                      University of Zagreb
Intended status: Informational                               S. Krishnan
Expires: April 19, May 13, 2010                                           Ericsson
                                                                S. Jiang
                                            Huawei Technologies Co., Ltd
                                                        October 16,
                                                        November 9, 2009

                       SeND Hash Threat Analysis
                     draft-ietf-csi-hash-threat-04
                     draft-ietf-csi-hash-threat-05

Abstract

   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 PKIX 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.

Status of this Memo

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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Abstract

   This  Code Components extracted from this document analysis the use of hashes must
   include Simplified BSD License text as described in SeND, possible threats
   and the impact Section 4.e of recent attacks on hash functions used by SeND.
   Current SeND specification [rfc3971] uses the SHA-1 [sha-1] hash
   algorithm and PKIX 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 Trust Legal Provisions and to decide how to
   encode the hash agility support are provided without warranty as
   described in SeND. the BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Impact of collision attacks on SeND  . . . . . . . . . . . . .  5
     3.1.  4
     2.1.  Attacks against CGAs in stateless autoconfiguration  . . .  5
     3.2.  4
     2.2.  Attacks against PKIX certificates in ADD process . . . . .  6
     3.3.  5
     2.3.  Attacks against the Digital Signature in the RSA
           Signature option . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  6
     2.4.  Attacks against the Key Hash field in the RSA
           Signature option . . . . . . . . . . . . . . . . . . . . .  7
   4.  6
   3.  Support for the hash agility in SeND . . . . . . . . . . . . .  8
     4.1.  7
     3.1.  The negotiation approach for the SEND hash agility . . . .  8
   5.  7
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   6.  9
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   7. 10
   6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     7.1. 11
     6.1.  Normative References . . . . . . . . . . . . . . . . . . . 12
     7.2. 11
     6.2.  Informative References . . . . . . . . . . . . . . . . . . 12 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13 12

1.  Introduction

   SEND [rfc3971] uses the SHA-1 hash algorithm to generate 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.)
   in the PKIX certificates [rfc5280] used for the router authorization
   in the ADD process.  Recently there have been demonstrated attacks
   against the collision free property of such hash functions
   [sha1-coll], and attacks against the PKIX X.509 certificates that use
   the MD5 hash algorithm [x509-coll] This document analyzes the effects
   of those attacks and other possible hash attacks on the SEND
   protocol.  The document proposes changes to make it resistant to such
   attacks.

2.  Terminology

   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 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 [rfc4270].  This
   document analyzes the hash usage in SEND following the approach
   recommended by [rfc4270] and [new-hashes].

   The 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 value hash(m), an attacker finds an input message m' such
   that hash(m') yields hash(m).  The second-preimage attack deals with
   the 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.  However, attacks against the one-way property are not
   feasible yet [rfc4270].  Up to date, all demonstrated attacks are
   attacks against a collision-free property, in which an attacker
   produces two different messages m and m' such that hash(m)=hash(m').
   The rest of the document deals with the collision attacks.

   We will analyze the impact of hash attacks on SeND case by case in
   this section.  Through our analysis, we also discuss whether we
   should support the hash agility in SeND.

3.1.

2.1.  Attacks against CGAs in stateless autoconfiguration

   Hash functions are used in the stateless autoconfiguration process
   that is based on CGAs.  Impacts of collision attacks on current uses
   of CGAs are analyzed in the update of the CGA specification
   [rfc4982], which also enables CGAs to support the hash agility.  CGAs
   provide the proof-of-ownership of the private key corresponding to
   the public key used to generate the CGA.  CGAs do not deal with the
   non-repudiation feature, while collision attacks are mainly about
   affecting the non-repudiation feature, i.e. in the collision attack
   against the CGA both of the CGA Parameters sets are choosen by an
   attacker, which is not useful in the real-world scenarios.
   Therefore, as [rfc4982] points out CGA based protocols, including
   SeND, are not affected by the recent collision attacks.  Regarding
   the pre-image attacks, if pre-image attacks were feasible, an
   attacker would manage to find the new CGA Parameters based on the
   associated, 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.

2.2.  Attacks against PKIX certificates in ADD process

   The second 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 hosts's trust anchor, consisting
   of PKIX certificates.  Researchers demonstrated attacks against PKIX
   certificates with MD5 signature, in 2005 [new-hashes] and in 2007
   [x509-coll].  In 2005 were constructed the original and the false
   certificate that had the same identity data and the same digital
   signature, but different public keys [new-hashes].  The problem for
   the attacker is that two certificates with the same identity are not
   actually useful in real-world scenarios, because the Certification
   Authority is not allowed to provide such two certificates.  In
   addition, attacks against the human-readable fields demand much more
   effort than the attacks against non human-readable fields, such as a
   public key field.  In case of the identity field, an attacker is
   faced with the problem of the prediction and the generation of the
   false but meaningful identity data, which at the end might be
   revealed by the Certification Authority.  Thus, in practice,
   collision attacks do not affect non human-readable parts of the
   certificate.  In 2007 were demonstrated certificates which differ in
   the identity data and in the public key, but still result in the same
   signature value.  In such attack, even if an attacker produced such
   two certificates in order to claim that he was someone else, he still
   needs to predict the content of all 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.  Regarding certificates in SeND,
   potentially dangerous are attacks against the X.509 certificate
   extensions.  For example, an attack against the IP address extension
   would enable the router to advertize the changed IP prefix range,
   although, not broader than the prefix range of the parent certificate
   in the ADD chain.

   The public-private key pair associated to the Router Authorization
   Certificate in the ADD process is used both for the CGA generation
   and for the message signing.  Since in the future CGAs might be used
   only with certificates, attacks against certificates are even more
   dangerous.  Generally, the most dangerous are attacks against middle-
   certificates in the certification path, where for the cost of the one
   false certificate, an attacker launches an attack on multiple
   routers.  Regarding the attacks against certificates in SEND, the
   only attack that SEND is not suspectable to, is an attack against the
   Trust Anchor's certificate because it is not exchanged in SeND
   messages, i.e. it is not the part of the certification path in the
   ADD process.

3.3.

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

   The computation of the Digital Signature field is described in
   [rfc3971].  The digital signature in the RSA Signature option 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 [rfc3971], that is signed with the sender's private
   key.  The sender's private key corresponds to the public key in the
   CGA parameters structure.  It is usually authorized through CGAs.
   Possible attack on such explicit digital signature is a typical non-
   repudiation attack, i.e. the Digital Signature field is vulnerable to
   the collision attack.  An 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.  If a SEND attacker would manage to find the
   collision in the message made of the ICMPv6 and ND fields mentioned
   in the beginning of the paragraph, he could underlay the false
   message.  The fields that make sense to be changed are ND fields, as
   opposite to the ICMPv6 fields which would be easy to reveal.
   However, as denoted in [rfc4270], the structure of at least one of
   two messages (m, m') in a collision attack is strictly predefined.
   The previous requirement complicates the collision attack, but we
   have to take into account that a variant of SHA-1 was already
   affected by recent collision attacks and we have to prepare for
   future improved attacks.

3.4.

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

   The Key Hash field is a SHA-1 hash of the public key from the CGA
   Parameters structure in the CGA option.  The pre-image attack against
   the Key Hash field is potentially dangerous, as in the case of all
   other hashes in SEND, because the Key Hash field contains a non
   human-readable data.  However the Key Hash field is not suspectable
   to the collision attack, since in the collision attack an attacker
   itself chooses both keys, k and k', where hash(k) = hash(k').  The
   reason for the former is that the associated public key is already
   authorized through the use of CGAs, and possibly the certification
   path in the ADD process.

4.

3.  Support for the hash agility in SeND

   While all of analyzed hash functions in SeND are theoretically
   affected by hash attacks, these attacks indicate the possibility of
   future real-world attacks.  In order to increase the future security
   of SeND, we suggest the support for the hash and algorithm agility in
   SeND. following possible approaches can be used.

   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 - encoding the hash function information
      into addresses.  But, there is no possibilty to do that in SeND.
      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, ie.
      if an attacker could break CGA, all other hashes are automatically
      broken.  From the security point of view, at the moment, this
      solution is more reasonable then defining different hash algorithm
      for each hash.  Additionally, if using the hash algorithm from the
      CGA, no bidding down attacks are possible.  On the other hand,
      this solution introduces the limition for SEND to be used
      exclusively with CGAs.

   o  Another solution is to incorporate the Hash algorithm option into
      the SeND message.  However, if the algorithm is defined in the
      Hash algorithm option in the SeND message, it is vulnerable to the
      bidding down attack.

   o  The third possible solution is to encode the algorithm in the CGA.
      However, this will reduce the strength of the CGAs and make them
      vulnerable to brute force attacks.

   o  One of the possible solutions is also the hybrid solution, i.e. to
      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.

4.1.

3.1.  The negotiation approach for the SEND hash agility

   None of the previous solutions supports the negotiation of the hash
   function.  We suggest the  Another possibility is to use a negotiation approach for
   the SEND hash agility such as the one 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, as well as the preferred signature
   algorithm.  When producing and validating hashes in SEND, nodes MUST must
   observe the following rules:

   o  In the ADD process, if any of the certificates in the
      certification path uses the signature algorithm which is not one
      of the signature algorithms negotiated in the signature agility
      process through the use of the Supported Signature Algorithms
      option, nodes MUST must reject the Router Authorization certificate.

   o  In order to produce the Digital Signature field, nodes MUST must use
      the signature algorithm negotiated in the signature agility
      process through the use of the Supported Signature Algorithms
      option.

   o  In order to produce the Key Hash field, nodes MUST must use the hash
      algorithm defined associated to the signature algorithm negotiated
      in the signature agility process through the use of the Supported
      Signature Algorithms option.

5.

4.  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.

5.  IANA Considerations

   There have been no IANA considerations so far in this document.

7.

6.  References

7.1.

6.1.  Normative References

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

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

   [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.

7.2.

6.2.  Informative References

   [new-hashes]
              Bellovin, S. and E. Rescorla, "Deploying a New Hash
              Algorithm", November 2005.

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

   [sha1-coll]
              Wang, X., Yin, L., and H. Yu, "Finding Collisions in the
              Full SHA-1. CRYPTO 2005: 17-36", 2005.

   [sig-agility]
              Cheneau, T. T., Maknavicius, M., Shen, S., and M. Maknavicius, Vanderveen,
              "Signature Algorithm Agility in the Secure Neighbor
              Discovery (SEND) Protocol",
              draft-cheneau-send-sig-agility-00
              draft-cheneau-csi-send-sig-agility-00 (work in progress),
              February
              October 2009.

   [x509-coll]
              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
   Zagreb
   Croatia

   Email: ana.kukec@fer.hr

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

   Email: suresh.krishnan@ericsson.com

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

   Email: shengjiang@huawei.com