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IETF                                                      Jitendra Kumar
Internet-Draft                                          Balaji Rajendran
Intended status: Best Current Practice                  Bindhumadhava BS
Expires: August 8, 2019                                  C-DAC Bangalore
                                                       February 04, 2019


 Enhanced XML Digital Signature Algorithm to Mitigate Wrapping Attacks
           draft-enhanced-xml-digital-signature-algorithm-01

Abstract

   XML signature standard [RFC3275]identifies signed elements by their
   unique identities in the XML document.  However this allows shifting
   of XML elements from one location to another within the same XML
   document, without affecting the ability to verify the signature.
   This flexibility paves the way for an attacker to tweak the original
   XML message without getting noticed by the receiver, leading to XML
   Signature wrapping or rewriting attacks.  This document proposes to
   use absolute XPath as a "Positional Token" and modifies the existing
   XML Digital Signature algorithm to overcome this attack.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   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."

   This Internet-Draft will expire on August 8, 2019.

Copyright Notice

   Copyright (c) 2019 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  XML Digital Signature structure . . . . . . . . . . . . . . .   3
   3.  Suggested Modified Algorithm  . . . . . . . . . . . . . . . .   3
     3.1.  Algorithm for XML Signature . . . . . . . . . . . . . . .   4
     3.2.  Algorithm for verification of Signature . . . . . . . . .   4
       3.2.1.  Verifying SignedInfo Element Digest with Decrypted
               Digest from SignatureValue element  . . . . . . . . .   5
   4.  Simple Example  . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Algorithm Validation  . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Mitigation of XML Signature wrapping attacks  . . . . . .   9
     5.2.  Mitigation of XML elements jumbling type of wrapping
           attacks . . . . . . . . . . . . . . . . . . . . . . . . .   9
   6.  Conclusion  . . . . . . . . . . . . . . . . . . . . . . . . .   9
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   McIntosh and Austel have illustrated that a SOAP message with XML
   Digital Signature (described in wrapping_attack [wrapping_attack])
   can be forged without invalidating the signature and they have
   further illustrated that a SOAP message content, protected by an XML
   Digital Signature, as specified in WS-Security(refer, WS-Security
   [WS-Security]) can be forged without invalidating the signature.
   This attack is possible because the XML Digital Signature refers to a
   signed element in XML document in a way without giving significance
   to the position within the XML document.  An attacker may inject
   additional nodes replacing the signed nodes while still preserving
   the signed nodes inside the document at different levels in the
   hierarchy of the XML tree, such that it results in successful
   signature verification thereby resulting in XML Re-Writing or
   Wrapping attack.






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1.1.  Requirements Language

   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].

2.  XML Digital Signature structure

   XML Signatures (described in RFC3275 [RFC3275]) are applied to
   arbitrary digital content (data objects).Data objects are digested,
   the resulting value is placed in an element (with other information)
   and that element is then digested and cryptographically signed.XML
   digital signatures are represented by the Signature element which has
   the following structure (where "?" denotes zero or one occurrence;
   "+" denotes one or more occurrences; and "*" denotes zero or more
   occurrences):

           <Signature ID?>
            <SignedInfo>
              <CanonicalizationMethod/>
              <SignatureMethod/>
              (<Reference URI? >
                (<Transforms>)?
                <DigestMethod>
                <DigestValue>
              </Reference>)+
            </SignedInfo>
            <SignatureValue>
           (<KeyInfo>)?
           (<Object ID?>)*
          </Signature>


   Signatures are related to data objects via URIs [URI].  Within an XML
   document, signatures are related to local data objects via fragment
   identifiers.

3.  Suggested Modified Algorithm

   As XML requests are prone to XML Signature wrapping attacks and these
   vulnerabilities stems from the usage of ID (Identity) to identify the
   signed XML subtree.  There are many solutions proposed to mitigate
   such attacks but still,such attacks can't be fully eliminated.  In
   this document, we have proposed the addition of XPath as a doping to
   the XML element being signed to mitigate XML Signature wrapping
   attacks.  We propose to use "Absolute XPath" instead of ID in
   <Reference> node's "URI" attribute to refer to the signed element.
   Absolute XPath can be used as "Positional Token", as this token



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   exactly points to the position of the element being signed.  During
   the signing process, this "Positional Token" gets added as an
   attribute e.g.  PosToken= "Absolute XPath") to the element that is
   subjected to be signed.This absolute XPath as a "Positional Token"
   would identify the signed element in XML Signature and addition of
   this "Positional Token" as an attribute to the element being signed
   would eliminate the chances of XML Signature Wrapping attacks wherein
   the calculated digest of the signed element in forged XML document
   will not match with the respective digest value in <DigestValue> node
   during signature validation process.  We propose a modified XML
   signature algorithm which suggests usage of absolute XPath as a
   "Positional Token" and it will be used during signing as well as
   during signature validation process.  The algorithms proposed are as
   follows:

3.1.  Algorithm for XML Signature


                1.      KS=Load(Keystore.JKS) //Load certificates and keys
                2.      For each element subjected to be signed(represented
                        by its "id" attribute value) {
                3.      ABSXpath= "Absolute XPath" of element to be signed
                        as identified with its "Id" attribute value
                4.      ProtectTree=Node as identified by ABSXpath
                5.      MixedElement=AppendSyntacticToken(ProtectTree,  ABSXpath)
                        /*Append a Positional Token as an attribute,
                        "PosToken= ABSXpath" to the ProtectTree */
                6.      H=Hash(MixedElement)
                7.      Add ABSXpath  to <Reference> node's "URI" attribute value
                8.      Enclose H to <DigestValue> node inside the <Reference> node,
                        as defined in XML Signature standard.
                9.      }
                10.     SignedInfoHash=calculate hash of <SignedInfo> element
                        /* Calculate the digest of the <SignedInfo> element */
                11.     SignedXML=Encrypt(SignedInfoHash , KS.PrivateKey)
                        /*Signing that digest and enclosing the signature value
                        in a <SignatureValue> element */



3.2.  Algorithm for verification of Signature










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                1.      SignInfoDigest=Calculate digest of the <SignedInfo> element
                2.      SignatureValueContent= content inside <SignatureValue> node
                3.      Flag=VerifySignature(Public Key, SignatureValueContent, SignInInfoDigest)
                4.      If(Flag){
                5.      Ids=All  URI's in <Reference> nodes inside the <SignedInfo> node
                6.      For each  Id from Ids){
                7.      ABSXpath=Get the content of Id
                8.      Subtree=Get the sub tree identified by ABSXpath
                9.      MixedElement =AppendSyntacticTokenSubTree(Subtree, ABSXpath)
                        /* Append a Positional Token as an attribute,
                                "PosToken= ABSXpath" to the Subtree  */
                10.     H=Hash (MixedElement)/* generate hash value of signed elements. */
                11.     Digest=Get digest value under the  <Reference>
                        node and inside <DigestValue> node, whose "URI" is equal to Id
                12.     If(H!=Digest){
                13.     return "Signature Validation Failed"
                14.     }else{
                15.     return "Signature Validation Successful"
                16.     }
                17.     } //For loop
                18.     else
                19.     return "Signature Validation Failed"
                20.     }



3.2.1.  Verifying SignedInfo Element Digest with Decrypted Digest from
        SignatureValue element


                        1.      VerifySignature(PublicKey, SignatureValueContent, SignInInfoDigest){
                        2.      DecryptedDigest=Decrypt SignatureValueContent with PublicKey
                        3.      If(DecryptedDigest!=SignInInfoDigest){
                        4.      return False
                        5.      }
                        6.      else{
                        7.      return True
                        8.      }
                        9.      }



4.  Simple Example








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   The <Signature> Lets consider an XML document as an example:

   <?xml version="1.0"?>
   <PatientRecord>
           <Visit date="10pm March 2018">
                   <Account id="id1">1234</Account>
                   <Name>ABC</Name>
                   <Diagnosis>Kidney Function Test</Diagnosis>
           </Visit>
           <Visit date="12pm May 2018">
                   <Account id="id2">1235</Account>
                   <Name>DEF</Name>
                   <Diagnosis>Liver Function Test</Diagnosis>
           </Visit>
   </PatientRecord>


                                 Figure 1

































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   Existing XML Signature algorithm would produce a <Signature> element
   for the XML document mentioned in Figure 1, as follows:

<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
        <SignedInfo>
                <CanonicalizationMethod
                        Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#WithComments" />
                <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
                <Reference URI="#id1">
                        <Transforms>
                                <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                                <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                        </Transforms>
                        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
                        <DigestValue>.................</DigestValue>
                </Reference>
                <Reference URI="#id2">
                        <Transforms>
                                <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                                <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                        </Transforms>
                        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
                        <DigestValue>................</DigestValue>
                </Reference>
        </SignedInfo>
        <SignatureValue>
                ..........
        </SignatureValue>
        <KeyInfo>
                <X509Data>
                        <X509Certificate>
                        .............................
                        </X509Certificate>
                </X509Data>
        </KeyInfo>
</Signature>















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   The proposed XML Signature algorithm would produce a <Signature>
   element for the XML document mentioned in Figure 1, which is
   described in Figure 2.  The "Positional Token" as an attribute
   e.g.(PosToken= "Absolute XPath") is used according to the proposed
   algorithm Section 3.1.  Now, <DigestValue> elements inside
   <Signature> element will also contain the trace of "Positional
   Token", hence the relative position of signed elements in the given
   XML document:

<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
        <SignedInfo>
                <CanonicalizationMethod
                        Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#WithComments" />
                <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#rsa-sha1" />
                <Reference URI="/PatientRecord/Visit[1]/Account[@id='id1']">
                        <Transforms>
                                <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                                <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                        </Transforms>
                        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
                        <DigestValue>.................</DigestValue>
                </Reference>
                <Reference URI="/PatientRecord/Visit[2]/Account[@id='id2']">
                        <Transforms>
                                <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
                                <Transform Algorithm="http://www.w3.org/2001/10/xml-exc-c14n#" />
                        </Transforms>
                        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
                        <DigestValue>................</DigestValue>
                </Reference>
        </SignedInfo>
        <SignatureValue>
                ............
        </SignatureValue>
        <KeyInfo>
                <X509Data>
                        <X509Certificate>
                        .............................
                        </X509Certificate>
                </X509Data>
        </KeyInfo>
</Signature>


                                 Figure 2






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5.  Algorithm Validation

   In this section, we evaluate how the suggested algorithm can mitigate
   the various scenarios of XML wrapping attacks.

5.1.  Mitigation of XML Signature wrapping attacks

   XML Signature Wrapping attacks are possible because of the inherent
   flaw in the signature verification algorithm that identifies the
   position of signed element using ID.  This makes it possible to move
   the signed element anywhere easily within the document and still, the
   document would retains its ability to verify its signature.So, in our
   proposed algorithm, we have suggested the use of absolute XPath in
   place of ID for identifying the position of signed elements.Absolute
   XPath has two-fold advantages as it can easily identify the position
   of the signed element within the XML document and it fixes both the
   vertical and horizontal axis of the signed element exactly.  The
   absolute XPath expression to identify the signed element will not be
   same in a forged document.  The signature validation will fail at
   step-8, of algorithm in Section 3.2, as there is no such node,
   Further, if the attacker modifies the URI attribute and tries to
   perform XML Signature wrapping attack, the digest of <SignedInfo>
   will not match and signature validation will fail at step-4 of the
   algorithm in Section 3.2.

5.2.  Mitigation of XML elements jumbling type of wrapping attacks

   This type of XML Signature wrapping attacks are possible as the
   attacker jumbles the position of signed elements within the document
   exploiting the existing XML Signing algorithm that takes ID into
   consideration for referencing the elements being signed.The proposed
   algorithm suggests using "Absolute XPath" for referencing the signed
   elements as well as doping the elements subjected to be signed with
   it.  Hence, the digest of the signed element inside <DigestValue>
   node has a trace of the position of element; refer step-6 of
   algorithm in Section 3.1.  Hence, any changes in the position of
   signed elements by the attackers will invalidate the signature; refer
   step-12 of algorithm in Section 3.2,as the calculated digest during
   signature validation will not match with the digest contained in
   <DigestValue> the forged XML document.

6.  Conclusion

   XML Signature wrapping attacks try to inject forged elements into the
   XML document structure in such a way that the valid signature covers
   the unmodified elements, while forged elements are processed by the
   application logic.  This results in a scenario, where an attacker can
   perform arbitrary web service requests, while authenticating as a



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   legitimate user.  The proposed algorithm takes help of the absolute
   XPath as a "Positional Token" for identifying the signed elements and
   adding this to the elements being signed as an attribute before the
   canonicalization process has a trace of both content of signed
   element and its position in the XML document as well.  Hence, the
   proposed algorithm can solve the issue of XML signature wrapping
   attacks elegantly without much change in the current standard.

7.  IANA Considerations

   This memo includes no request to IANA.

8.  Security Considerations

   This draft proposes a modification to the existing algorithm of XML
   signature to counter XML Signature wrapping attacks.  However other
   forms of attack may be possible that could not be mitigated.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC2807]  Reagle, J., "XML Signature Requirements", RFC 2807,
              DOI 10.17487/RFC2807, July 2000,
              <https://www.rfc-editor.org/info/rfc2807>.

   [RFC3275]  Eastlake 3rd, D., Reagle, J., and D. Solo, "(Extensible
              Markup Language) XML-Signature Syntax and Processing",
              RFC 3275, DOI 10.17487/RFC3275, March 2002,
              <https://www.rfc-editor.org/info/rfc3275>.

9.2.  Informative References

   [I-D.narten-iana-considerations-rfc2434bis]
              Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", draft-narten-iana-
              considerations-rfc2434bis-09 (work in progress), March
              2008.

   [RFC2629]  Rose, M., "Writing I-Ds and RFCs using XML", RFC 2629,
              DOI 10.17487/RFC2629, June 1999,
              <https://www.rfc-editor.org/info/rfc2629>.




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   [RFC3552]  Rescorla, E. and B. Korver, "Guidelines for Writing RFC
              Text on Security Considerations", BCP 72, RFC 3552,
              DOI 10.17487/RFC3552, July 2003,
              <https://www.rfc-editor.org/info/rfc3552>.

   [wrapping_attack]
              McIntosh, Michael. and Paula. Austel, "XML signature
              element wrapping attacks and countermeasures", 2005,
              <https://dl.acm.org/citation.cfm?id=1103026>.

   [WS-Security]
              OASIS., "OASIS Web Services Security (WSS) TC", 2006,
              <https://www.oasis-open.org/committees/
              tc_home.php?wg_abbrev=wss>.

Authors' Addresses

   Jitendra Kumar
   C-DAC Bangalore
   #68, Electronics City Hosur Road
   Bangalore  560100
   India

   Email: jitendra@cdac.in


   Balaji Rajendran
   C-DAC Bangalore
   #68, Electronics City Hosur Road
   Bangalore  560100
   India

   Email: balaji@cdac.in


   Bindhumadhava BS
   C-DAC Bangalore
   Old Madras Road, Opposite Hal Aero Engine Division
   Bangalore  560038
   India

   Email: bindhu@cdac.in









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