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Securing XML Documents Through Merkle Hash Trees

Securing XML Documents Through Merkle Hash Trees. Bhavani Thuraisingham October 8, 2007 Lecture #19 (Guest Lecture). Outline. Motivation for Research on XML Security Connection to digital forensics Technical Details of the Research on XML Security Related work and Future Directions

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Securing XML Documents Through Merkle Hash Trees

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  1. Securing XML Documents Through Merkle Hash Trees Bhavani Thuraisingham October 8, 2007 Lecture #19 (Guest Lecture)

  2. Outline • Motivation for Research on XML Security • Connection to digital forensics • Technical Details of the Research on XML Security • Related work and Future Directions • Based on paper published in IEEE Transactions on Knowledge and Data Engineering, October 2004 (Bertino, Ferrari, Carminati, Thuraisingham)

  3. Motivation for Research on XML Security • XML (extensible Markup Language) Security • XML has become the standard document interchange language for the web (internal and external) • XML is a critical technology for the semantic web • RDF and other specifications are built on XML • XML documents must satisfy security and privacy policies • Challenges: Access Control, Secure publishing, Secure Web Services Applications, Securing RDF, Secure semantic web, Temporal models, Privacy, Handling evolving XML specifications • Outline of XML Security Presentation • Access Control • Example XML document, Policy Specification, Access Control Strategy and Architecture • Third Party Publication of XML Documents • Architecture • Interactions between Owner, Publisher and Subject • Checking for Authenticity and Completeness • Potential Attacks and Performance Issues • Integrating Confidentiality with Authenticity and Completeness • Application: Secure Web Services

  4. Connection to Digital Forensics • Law enforcement is using XML documents to exchange data through their internal webs; these documents have to be protected on a need to know basis • http://www.gcn.com/print/22_21/22967-1.html • Law enforcement gets XML standard for data exchange • “:A few years ago, there were no law enforcement standards for electronic data interchange, Gill said. Through the Justice XML Standards Task Force formed in August 2002, state, local, federal and tribal IT officials have been hashing out standards to make their systems talk to each other. “ • In general one can assume that the environment is trusted; however different law enforcement officials may have different needs; furthermore, their systems may be hacked • Digital Forensics service could be offered as a web service • Semantic web based infrastructure could be used to build a digital forensics system

  5. Year: 2003 Annual report Name: UTD Assets Patents Other Equipment Asset Asset Patent Dept Patent Expenses Books Author Cash CS ID Tot Date Tech-details Funds Dept Fund Amout Date Short-desc UTD Type 6/1/03 1m$ Example XML Document NSF

  6. Publishing Service WEB Users Publishing service: how it works • Anew class of information-centered applications based on Data dissemination • Possible scenarios: • Information commerce (Digital libraries, Electronic news, etc.) • Intra-company information systems • Push/Pull modes • Security requirements: • Confidentiality • Integrity • Authenticity • Completeness

  7. Subject Credentials, Protection Objects and Policy Base • Subjects are given access to XML documents or portions of documents depending on user ID and/or Credentials • Credential specification is based on credentials a subject has • Professor is a credential; Secretary is a credential • Protection objects are objects to which access is controlled • Entire XML documents or portions of XML documents • Policy base stores security policies for protecting the XML source contents

  8. Subject Credential Base Example <Professor credID=“9” subID = “16: CIssuer = “2”> <name> Alice Brown </name> <university> UTD <university/> <department> CS </department> <research-group> Security </research-group> </Professor> <Secretary credID=“12” subID = “4: CIssuer = “2”> <name> John James </name> <university> UTD <university/> <department>CS </department> <level> Senior </level> </Secretary>

  9. Policy Base Example <?xml version="1.0" encoding="UTF-8"?> <policy_base> ... <policy_spec ID=‘P1' cred_expr="//Professor[department='CS']" target="annual_report.xml" path="//Patent[@Dept='CS']//node()" priv="VIEW"/> <policy_spec ID=‘P2'cred_expr="//Professor[department='CS']" target="annual_report.xml" path="//Patent[@Dept='IST']/Short-descr/node() and //Patent[@Dept='IST']/authors" priv="VIEW"/> <policy_spec ID=‘P3'cred_expr="//Professor[department='IST'] " target="annual_report.xml" path="//Patent[@Dept='IST']//node()" priv="VIEW"/> <policy_spec ID=‘P4'cred_expr="//Professor[department='IST']" target="annual_report.xml" path="//Patent[@Dept='CS']/Short-descr/node() and //Patent[@Dept='CS']/authors" priv="VIEW"/> <policy_spec ID=‘P5'cred_expr="//secretary[department='CS' and level='junior']" target="annual_report.xml" path="//Asset[@Dept='CS']/node()" priv="VIEW "/> <policy_spec ID=‘P6'cred_expr="//secretary[department='CS' and level='senior']" target="annual_report.xml" path="//Asset[@Dept='IST']/Funds/@Type and //Asset[@Dept='IST']/Funds/@Funding-Date" priv="VIEW "/> <policy_spec ID=‘P7'cred_expr="//secretary[department='IST' and level='junior']" target="annual_report.xml" path="//Asset[@Dept='IST']/node()" priv="VIEW "/> ... </policy_base>

  10. Access Control Strategy • Subjects request access to XML documents under two modes: Browsing and authoring • With browsing access subject can read/navigate documents • Authoring access is needed to modify, delete, append documents • Access control module checks the policy based and applies policy specs • Views of the document are created based on credentials and policy specs • In case of conflict, least access privilege rule is enforced • Works for Push/Pull modes

  11. System Architecture for Access Control User Pull/Query Push/result X-Access X-Admin Admin Tools Credential base Policy base XML Documents

  12. Third-Party Architecture XML Source policy base Credential base • The Owner is the producer of informationIt specifies access control policies • The Publisher is responsible for managing (a portion of) the Owner information and answering subject queries • Goal: Untrusted Publisher with respect to Authenticity and Completeness checking SE-XML Owner Publisher Reply document credentials Query User/Subject

  13. Subject Owner Interaction • Subjects register with Owner during subscription phase; during this phase subject is assigned by owner credentials stored at the owner site • Owner returns to the subject the Subject Policy Configuration (policy identifiers) that apply to the subject signed with the private key of the owner • Example: If polices P1 and P2 apply to John (e.g. CS prof) and policy P6 applies to Jane (IST secretary), owner Joe sends John P1 and P2 and to Jane P6 signed with Joe’s private key

  14. SubjectPolicy Configuration <?xml version="1.0" encoding="UTF-8" ?> <SubjectPolicyConfiguration ID=“ProfessorCS" created="08-05-2002"> <owner> <name>owner1</name> <organization>CS</organization> <state>Texas</state> <uri>www.owner1.com</uri> <owner> <policy>VtaUBIxliHS1hzrqkKhYVTtYrafVSmCoJPkUVKYXCA7yVdc7a/ne5sgIg0tGGRe3 /D2Xg6Fbwp3SAKK/Ref1teZCpD0nlkx89GOIIcw8o9R3Mb2YY/slk5+Fu0xxWXlB YuWKWWNsXENKTkgiXL4mB1SUt4bmF6YG4lTxfxduVAw=</policy> </SubjectPolicyConfiguration> P1, P2

  15. Owner Publisher Interaction • For each document the owner sends the publisher the following information • Information on which subjects can access which portions of the document according to the policy base (I.e. access control policies) • Policy element which describes the policies for the document is also inserted • Also for each element e based on the policies applied to e, the owner inserts policy configuration (binary string) converted to hexadecimal representation • Merkle Signature of each document • The document together with the security information is called “Security Enhanced Document” (SE-XML) and will enable the subject to verify the authenticity of the document • Additional information encoded in the document called Secure Structure is used by the subject to verify completeness of the result

  16. Where m is the number of policies applied on document d Policy Configuration/Policy Element • Consider the element e of an XML document d • The policy configuration of e consists of a binary string, of m bits where the i-th bit is equal to: • 1 if the policies, whose identifier is i, is applied on e • 0 otherwise • SE-XML document has information about the set of the m policies applied to the document: • <Policy> 1, 2, 3, 4, 5, 6, 7 </Policy> • For each element, policy information in hexadecimal representation is also inserted

  17. Policy Configuration: example • Consider the short-descrelement of the CS dept: • The only policies applied to it are: 1st and 4th policy (i.e. P1, P4) • According to the Policy element, these policies are located in the first and fourth position of the policy configuration The policy configuration associated with short-descrelement is 1001000 short-descr’s PC value is 90 (hexadecimal representation)

  18. Publisher Policy evaluation: example 1. Decrypt with the Public key of the Owner the policies applied to the subject (i.e., the Subject Policy Configuration) P1, P2 2. Match the obtained policy identifiers with the identifiers contained in the Policy element i.e. match P1, P2 with P1, P2, P3, P4, P5, P6, P7 3. Find the policies applicable to the element specified in the query (e.g., short description element) P1, P4 4. Find the matching policies from steps 2 and 3 P1 The only policies applied on Short-descr element is “P1”

  19. Security Enhanced XML document XML Document • Policy Information • Merkle Signature SE-XML Document

  20. title Newspaper date Frontpage Literary_page Politic_page Sport_page Leading Paragraphs Politic Article news news Author title paragraph topic topic Author title Author title topic topic Author title Author title Author title paragraph Author title Merkle Signature Example MhX(Author)=h(h(Author)||h(Author.value)) MhX(title)=h(h(title)||h(title.value)) MhX(paragraph)=h(h(paragraph)||h(paragraph.content)|| MhX(Author)||MhX(title))

  21. title Newspaper date Frontpage Literary_page Politic_page Sport_page Leading Paragraphs Politic Article news news Author title paragraph topic topic Author title Author title topic topic Author title Author title title Author paragraph Author title Merkle Signature of Newspaper XML file MhX(Newspaper) Merkle Signature Example MhX(Newspaper)=h(h(Newspaper)||h(Newspaper.content)|| MhX ()……||MhX()||…||MhX())

  22. Security Enhanced XML document: Example <?xml version="1.0" encoding="UTF-8"?> <Annual Report Year = 2003 Name = CS“CG4g3D8/,mPVV/t+T2O1kZRFhdio="> <Policy>1,2,3,4,5,6,7 </Policy> <Assets> <Asset Dept = “CS"> <Expense Tot="...." PC_ATTR="08"/> <Funds> <Fund Funding Date=“6/1/03" Type=“NSF" Amount="..." PC_ATTR="080808"/> </Funds> </Asset> <Asset Dept=“IST"> <Expenses Tot="...." PC_ATTR="02"/> <Funds> <Fund Funding Date=“10/1/03" Type=“DoD" Amount="..." PC_ATTR="060602"/> <Fund Funding Date=“4/1/03" Type=“CIA" Amount="..." PC_ATTR="060602"/> </Funds> </Asset> </Assets> - - - - - - - - - /AnnualReport>

  23. Secure Structure Example <?xml version="1.0" encoding="UTF-8"?> <x-82592553 x-4639474="rVR5DQ" x-67303774="QTQXS“Sign="OD2mc9aVV/tP4g3TG+1kr4sFhdio="> <Policy>1,2,3,4,5,6,7 </Policy> <x-1915689488> <x-785490824 x-40276037="PlcZUo"> <x-590292021 x-57205665="...." PC_ATTR="08"/> <x-13947931> <x-1037159472 x-122584813="fNhtL" x-0260379="hgKID" x-93640287="..." PC_ATTR="080808"/> </x-13947931> </x-785490824> <x-785490824 x-40276037="pKGEs"> <x-590292021 x-57205665="...." PC_ATTR="02"/> <x-13947931> <x-1037159472 x-122584813="gPd39" x-0260379="hgKID" x-93640287="..." PC_ATTR="060602"/> <x-1037159472 x-122584813="o4GpM" x-0260379="yr0QjJ" x-93640287="..." PC_ATTR="060602"/> </x-13947931> </x-785490824> </x-1915689488> /x-82592553>

  24. Subject Publisher Interaction • The subject submits queries to publisher; it also sends its subject policy configuration • Publisher computes a view of the requested documents based on access control policies for the subject set by the owner • To verify the authenticity of the answer, subject must recompute the same bottom up hash value signed by owner (i.e. Merkle signature) and compare it with the Merkle signature generated by the owner and inserted by the publisher • Subject may not get the entire document; therefore publisher sends to the subject additional hash values that refer to the missing portions of the document • Subject verifies the authenticity of the document

  25. v v v 1 1 1 2 3 2 3 2 3 w w w 4 5 7 4 5 4 5 7 7 6 6 6 8 9 8 8 9 9 10 11 12 13 10 11 12 13 10 11 12 13 14 15 16 17 14 15 16 17 14 15 16 17 MhPath(4,1) MhPath(7,1) MhPath(5,1) Merkle Hash Paths

  26. Completeness Verification <?xml version="1.0" encoding="UTF-8"?> <x-82592553 x-4639474="rVR5DQ" x-67303774="QTQXS“Sign="OD2mc9aVV/tP4g3TG+1kr4sFhdio="> <Policy>1,2,3,4,5,6,7 </Policy> <x-1915689488> <x-785490824 x-40276037="PlcZUo"> <x-590292021 x-57205665="...." PC_ATTR="08"/> <x-13947931> <x-1037159472 x-122584813="fNhtL" x-0260379="hgKID" x-93640287="..." PC_ATTR="080808"/> </x-13947931> </x-785490824> <x-785490824 x-40276037="pKGEs"> <x-590292021 x-57205665="...." PC_ATTR="02"/> <x-13947931> <x-1037159472 x-122584813="gPd39" x-0260379="hgKID" x-93640287="..." PC_ATTR="060602"/> <x-1037159472 x-122584813="o4GpM" x-0260379="yr0QjJ" x-93640287="..." PC_ATTR="060602"/> </x-13947931> </x-785490824> </x-1915689488> /x-82592553> • Verify authenticity and integrity of ST • by using the Merkle Signature • Verify the completness • Translate the submitted query • Evaluate the obtained query on the secure structure • Check the policy configuration • Hash the result answer received by thePublisher • Match it with the obtained node-set. /Asset[@Dept=‘IST’]/Funds/* //x-785490824[@x-40276037=`pKGEs']/ x-13947931/*

  27. Reply Document Generation Algorithm • Reply Generator Algorithm • Input: SE-XML version of XML document d; A query q on d submitted by s; the policy configuration of s • Output: Reply document r • Evaluate q on d and return a well-formed XML document containing all and only the nodes satisfying q • Determine which access control policies apply to each node satisfying q • Remove from node-set the nodes that s is not authorized to see based on information in SE-XML for d • All the attributes in the resulting document are replaced with an AtteibuteElement element • An additional attribute called MhPath is inserted in each node to be returned to s • Insert Merkle signature of d • Rebuild document by taking the set of nodes returned from the above steps and transform into a well-formed document

  28. Example Reply Document • Reply document generated by Publisher to an IST Professor who requests all the patents of CS department • MhPath attribute associated with Short-descr contains all the hash values needed to computer the Merkle has value of Patent starting from Short-descr. Patent Sign Authors MhPath Short-descr MhPath MhPath

  29. Authentication: Authenticable Element • Document d = (Vd, vd, Ed, Fed) is defined as follows: Vd is the set of all element nodes and attribute nodes in d, vd is the node representing the document element called the document root, Ed is the set of edges in d, and FEd is the edge labeling function • Definition 1: Let d = (Vd, vd, Ed, FEd) be an XML document, let g = (Vg, vg, Eg, FEg) be the SE-XML version of d, and r = (Vr, vr, Er, FEr) be the reply document to a query on d submitted by s. Let VT be the set of terminal nodes of r. Let Vr,e be the set of element nodes in the reply document r. For each v in Vr, e, v is authenticable by s iff. there exists vt in VT with v in path(vt) such that it is possible through a recursive bottom up computation to compute the Merkle hash value of vd using only the values in {w.MhPath: w is in path(vt)} • Theorem 1: Let g = (Vg, vg, Eg, FEg) be the SE-XML version of an XML document d and r = (Vr, vr, Er, FEr) be the reply document corresponding to a query submitted on d by subject s. Each node in Vr,e is authenticable by s • Proof of Theorem is by Induction on the length of the relative path of w with respect to v where v is in VT, w is in Vd and v is in path(w)

  30. AuthenticationSubject Verification Algorithm • Subject Verification Algorithm: Input: Reply document r = (Vr, vr, Er, FEr) Output = True if all nodes in r are authentic. False otherwise • Starting from each terminal node in the reply document, the algorithm recomputes the Merkle hash value of the root of document d through a bottom-up computation that uses the values of attributes MhPath of each node belonging to the path connecting the terminal node to the root of the reply document • The obtained value is compared with the decryption of the Merkle Signature of d using the Owner’s public key • If the two values coincide then all the nodes belonging to the path are authentic, otherwise the algorithm terminates and returns false

  31. Authentication:Authentic Element • Definition 2: Let g = (Vg, vg, Eg, FEg) be the SE-XML version of d, and r = (Vr, vr, Er, FEr) be the reply document to a query on d submitted by s. Each node v in Vr, e is authentic iff. V is authenticable by s and the computed Merkle has value of vd is equal to the decryption of Sign.val using the Owner public key • Theorem 2: • Let s be a subject, q be a query submitted by s, and r be the reply document received by s as an answer to q. • Subject verification algorithm returns True iff. Each v in Vr,e is authentic where Vr,e is the set of element nodes in the reply document r • Proof: • By theorem 1 all nodes of a reply document correspond to elements are authenticable by s. Therefore bt definitions 1 and 2, there exists for each vt in VT a recursive bottom-up computation able to compute Merkle hash value of the root of the document d using only the values of MhPath attributes of all nodes belonging to path (vt), and this value coincides with the decryption of the value of the Sign attribute with the Owner public key • The proof is to show that this recursive bottom-up computation is implemented by the Subject Verification Algorithms

  32. Potential Attacks and Performance Issues • Attacks • Subject attacks • Subject Si eavesdropping during subscription phase of subject Sm • Subject attacking the secure structure and deducing sensitive attributes • Publisher attacks • Publisher changes the Sign element in SE-XML • Performance Issues • Update management • Modification of document implies changes to Merkle hash and SE-XML document • Storage complexity of security information • E.g, SE-XML and Secure Structure

  33. Challenge: Integrating Confidentiality and Authentication • Currently the portion of the Publisher that actually enforces the access control policy must be trusted • I.e. Trust with respect to Confidentiality • Challenge: How can we come up with a unified approach that ensures Confidentiality, Authenticity and Completeness • Directions: Apply access control policies to portions of the document and encrypt the views computers; only those authorized to see the views have the keys for decryption • Querying encrypted databases

  34. Application: Secure Web Services • How authenticity, confidentiality and integrity can be ensured in the presence of an untrusted UDDI? • Traditional techniques are not enough! • Possible solutions: • Integrity, confidentiality: selective encryption of the data managed by the UDDI according to the specified access control policies • Authenticity: Merkle hash trees • Additional security properties: • Completeness • Consistency

  35. Query UDDI Service requestor BusinessService Authenticity ….traditional digital signatures do not fit well in third-party architectures!! BusinessEntity <dsig:Signature> tModel BusinessService PublisherAssertion BindingTemplate Service provider

  36. Merkle Signature • An alternative way to sign an XML doc • By applying a unique digital • signature on an XML doc • it is possible to ensure the • authenticity of: • the whole document • any portion of it • It uses a different way to compute the digest of XML docs, based on the Merkle tree authentication mechanisms BusinessEntity <dsig:Signature> tModel BusinessService PublisherAssertion BindingTemplate

  37. Related Work and Directions on XML Security • Related Work • University of CA, Davis (started with relational databases and extended to XML) • Directions • Keep up with security as XML specifications evolve • How can we integrate confidentiality with authenticity and completeness without trusting the publisher? • Secure RDF models • Paper submitted to DEXA WebS Workshop • Temporal access control models for XML documents • Secure information interoperability • Secure semantic web • Computer Standards and Interface Journal, March 2005

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