1. Securing XML Documents Through Merkle Hash Trees
Bhavani Thuraisingham
October 8, 2007
Lecture #19 (Guest Lecture)
Good afternoon, my name is Barbara Carminati, and In this talk, I’m going to present the work carried out in secure Publishing Service of XML documents.
Due to the widespread use of XML as a standard for
document representation and exchange over the Web, the development
of a framework supporting secure publishing of XML documents is
becoming a crucial need.

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

6. Publishing service: how it works
A new class of information-centered applications based on Data dissemination
Possible scenarios:
Information commerce (Digital libraries, Electronic news, etc.)
Intra-company information systems
Users
Publishing Service
WEB
Push/Pull modes
Security requirements:
Confidentiality
Integrity
Authenticity
Completeness
Secure dissemination in a web environment entails addressing three main issues: authenticity, integrity, and confidentiality.
Ensuring document confidentiality means that the document contents can only be disclosed to users authorized according to the specified access control policyies.
Ensuring document integrity means ensuring that the contents of the document are not altered during its transmission from the source to the intended recipient. es.
Ensuring document authenticity means that the subject receiving a document is assured that the document contents come from the source it claims to be from.
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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>
<department>CS </department>
<level> Senior </level>
</Secretary>

9. Policy Base Example <?xml version="1.0" encoding="UTF-8"?>
...
<policy_spec ID=‘P1' cred_expr="//Professor[department='CS']" target="annual_report.xml" priv="VIEW"/>
<policy_spec ID=‘P2' cred_expr="//Professor[department='CS']" target="annual_report.xml" and priv="VIEW"/>
<policy_spec ID=‘P3' cred_expr="//Professor[department='IST'] " target="annual_report.xml" priv="VIEW"/>
<policy_spec ID=‘P4' cred_expr="//Professor[department='IST']" target="annual_report.xml" and priv="VIEW"/>
<policy_spec ID=‘P5' cred_expr="//secretary[department='CS' and level='junior']" target="annual_report.xml" priv="VIEW "/>
<policy_spec ID=‘P6' cred_expr="//secretary[department='CS' and level='senior']" target="annual_report.xml" and priv="VIEW "/>
<policy_spec ID=‘P7' cred_expr="//secretary[department='IST' and level='junior']" target="annual_report.xml" priv="VIEW "/>
</policy_base>
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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
Credential
base
policy base
The Owner is the producer of information It 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
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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. Subject Policy Configuration
<?xml version="1.0" encoding="UTF-8" ?>
<SubjectPolicyConfiguration ID=“ProfessorCS" created=" ">
<owner>
<name>owner1</name>
<organization>CS</organization>
<state>Texas</state>
<uri>
<policy>VtaUBIxliHS1hzrqkKhYVTtYrafVSmCoJPkUVKYXCA7yVdc7a/ne5sgIg0tGGRe3 /D2Xg6Fbwp3SAKK/Ref1teZCpD0nlkx89GOIIcw8o9R3Mb2YY/slk5+Fu0xxWXlB YuWKWWNsXENKTkgiXL4mB1SUt4bmF6YG4lTxfxduVAw=</policy>
</SubjectPolicyConfiguration>
P1, P2
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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. Policy Configuration/Policy Element
Where m is the number of policies applied on document d
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
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17. Policy Configuration: example
Consider the short-descr element 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-descr element is
short-descr’s PC value is 90 (hexadecimal representation)
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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”
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19. Security Enhanced XML document
Policy Information
Merkle Signature
SE-XML Document
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20. Merkle Signature Example
title
Author
paragraph
Politic_page
Literary_page
Paragraphs
date
topic
Article
Newspaper
Frontpage
Leading
Sport_page
news
Politic
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))
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21. Merkle Signature Example
title
Author
Politic_page
Literary_page
Paragraphs
date
topic
Article
Newspaper
Frontpage
Leading
Sport_page
news
Politic
paragraph
MhX(Newspaper)=h(h(Newspaper)||h(Newspaper.content)||
MhX ()……||MhX()||…||MhX())
MhX(Newspaper)
Merkle Signature
of Newspaper XML file
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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"/>
<Fund Funding Date=“10/1/03" Type=“DoD" Amount="..." PC_ATTR="060602"/>
<Fund Funding Date=“4/1/03" Type=“CIA" Amount="..." PC_ATTR="060602"/>
</Assets>
/AnnualReport>
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23. Secure Structure Example
<?xml version="1.0" encoding="UTF-8"?>
<x x ="rVR5DQ" x ="QTQXS“ Sign="OD2mc9aVV/tP4g3TG+1kr4sFhdio=">
<Policy>1,2,3,4,5,6,7 </Policy>
<x >
<x x ="PlcZUo">
<x x ="...." PC_ATTR="08"/>
<x >
<x x ="fNhtL" x ="hgKID" x ="..." PC_ATTR="080808"/>
</x >
</x >
<x x ="pKGEs">
<x x ="...." PC_ATTR="02"/>
<x x ="gPd39" x ="hgKID" x ="..." PC_ATTR="060602"/>
<x x ="o4GpM" x ="yr0QjJ" x ="..." PC_ATTR="060602"/>
</x >
/x >
Additionally, in order to prevent alterations by the Publisher, the Owner computes the Merkle Signature of the secure structure. Similarly to the SE-XML document, this signature is sent to Publishers together with the corresponding secure structure.
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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. Merkle Hash Paths 5 8 11 9 13 4 2 10 12 6 14 16 15 17 7 3 1 w v
MhPath(4,1)
MhPath(7,1)
MhPath(5,1)
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26. Completeness Verification
<?xml version="1.0" encoding="UTF-8"?>
<x x ="rVR5DQ"
x ="QTQXS“ Sign="OD2mc9aVV/tP4g3TG+1kr4sFhdio=">
<Policy>1,2,3,4,5,6,7 </Policy>
<x >
<x x ="PlcZUo">
<x x ="...." PC_ATTR="08"/>
<x >
<x x ="fNhtL"
x ="hgKID" x ="..." PC_ATTR="080808"/>
</x >
</x >
<x x ="pKGEs">
<x x ="...." PC_ATTR="02"/>
<x x ="gPd39"
x ="hgKID" x ="..." PC_ATTR="060602"/>
<x x ="o4GpM"
x ="yr0QjJ" x ="..." PC_ATTR="060602"/>
</x >
/x >
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 the Publisher
Match it with the obtained node-set.
x /*
Finally, the last step of the completeness verification is to hash the answer received by the Publisher, and match it with the obtained node-set. The junior secretary verifies that in the answer received by the Publisher an element is omitted, more preCSely, a Fund element.
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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
MhPath
Authors
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. Authentication Subject 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
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. <dsig:Signature>
Authenticity
….traditional digital signatures do not fit well in third-party architectures!!
BusinessEntity
UDDI
<dsig:Signature>
tModel
Query
BusinessService
BusinessService
PublisherAssertion
BindingTemplate
Service requestor
Service provider
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36. <dsig:Signature>
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
<dsig:Signature>
BusinessEntity
tModel
BusinessService
PublisherAssertion
BindingTemplate
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37. Related Work and Directions on XML Security
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