Secure Publishing of XML Documents
Bhavani Thuraisingham
October 29, 2010
Motivation Digital Evidence Represented in XML (eXtensivle Markup Language) XML documents have to be secured
XML has become the standard document interchange language for the web
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
Based on paper published in IEEE Transactions on Knowledge and Data Engineering, October 2004 (Bertino, Ferrari, Carminati, Thuraisingham)
Example XML Document
NSF
Patents
Asset
Year: 2003
Name: UTD
Expenses
Dept
Author
Short-desc
ID
Annual report
Assets
Asset
Equipment
Books
Patent
Other
Tot
Funds
Date
6/1/03Type
Amout
1m$
Date
Dept
UTD
Tech-details
Patent
CashCS
Fund
04/21/23 4
Users
Publishing Service
WEBWEB
Push/Pull modes Security requirements:
Confidentiality Integrity Authenticity Completeness
Publishing service: how it worksA new class of information-centered applications based on Data
dissemination
Possible scenarios:
Information commerce (Digital libraries, Electronic news, etc.)
Intra-company information systems
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
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>
04/21/23 7
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>
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
System Architecture for Access Control
UserPull/Query Push/result
XML Documents
X-Access X-AdminAdmin Tools
Policybase
Credentialbase
04/21/23 10
Third-Party Architecture
Credential base
policy baseXML Source
User/Subject
Owner
Publisher
Query
Reply documen
t
SE-XML
credentials
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
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
04/21/23 12
<?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>
Subject Policy Configuration
P1, P2
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
04/21/23 14
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
Where m is the number of policies applied on document d
04/21/23 15
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 1001000
short-descr’s PC value is 90 (hexadecimal representation)
04/21/23 16
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 elementi.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 3P1
The only policies applied on Short-descr element is “P1”
04/21/23 17
• Policy Information• Merkle Signature
XML Document
SE-XML Document
Security Enhanced XML document
04/21/23 18
MhX(Author)=h(h(Author)||h(Author.value))
MhX(title)=h(h(title)||h(title.value))
titletitleAuthor
Author
paragraph
Politic_page Literary_page
Paragraphs
title
date
titleAuthor
titleAuthortopictitleAuthortopictitleAuthortopic
titleAuthortopic
Article
Newspaper
Frontpage
Leading
Sport_page
news newsPolitic
paragraph
MhX(paragraph)=h(h(paragraph)||h(paragraph.content)|| MhX(Author)||MhX(title))
Merkle Signature Example
04/21/23 19
MhX(Newspaper)=h(h(Newspaper)||h(Newspaper.content)|| MhX ()……||MhX()||…||MhX())
titletitleAuthor
Author
Politic_page Literary_page
Paragraphs
title
date
titleAuthor
titleAuthortopictitleAuthortopictitleAuthortopic
titleAuthortopic
Article
Newspaper
Frontpage
Leading
Sport_page
news newsPolitic
paragraphparagraph
MhX(Newspaper)
Merkle Signature of Newspaper XML file
Merkle Signature Example
04/21/23 20
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>
04/21/23 21
<?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>
Secure Structure Example
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
04/21/23 23
Merkle Hash Paths
5
8
11
9
13
4
2
10 12
6
14 1615 17
7
3
1
8
11
9
13
4
2
10 12
6
14 1615 17
7
1
3
5
8
11
9
13
4
2
10 12
6
14 1615 17
7
1
5
3
w w w
v v v
MhPath(4,1) MhPath(7,1) MhPath(5,1)
04/21/23 24
<?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>
Completeness Verification
Verify authenticity and integrity of ST by using the Merkle Signature
Verify the completness1. Translate the submitted query
2. Evaluate the obtained query on the secure structure
3. Check the policy configuration4. Hash the result answer received by
the Publisher5. Match it with the obtained node-
set.
//x-785490824[@x-40276037=`pKGEs']/ x-13947931/*
/Asset[@Dept=‘IST’]/Funds/*
Applications to Digital Forensics
Owner is the FBI person who collects the forensics Publisher is the FBI web site that may be hacked (e.g.,
CENTCOM was hacked recently – 60 minutes CBS Nov 8, 2009) Subject is an FBI agent located anywhere in the US or in the
world Digital evidence represented as an XML document (XML is now
used for digital evidence) Not all agents have access to all evidence Policies set by owner will specify who has access to what
APPENDIX
Bhavani Thuraisingham
November 11, 2009
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
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
Authors
Short-descr
Sign
MhPath
MhPath MhPath
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)
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
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
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
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
Application: Secure Web ServicesHow 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
04/21/23 35
Authenticity
…….traditional digital signatures do not fit well in .traditional digital signatures do not fit well in third-party architectures!!third-party architectures!!
Service requestor
Service provider
UDDIQuery
BusinessEntity
BusinessService
BindingTemplate
<dsig:Signature>
BusinessService
tModel
PublisherAssertion
04/21/23 36
Merkle Signature• An alternative way to sign an XML doc
• By applying a unique digitalsignature on an XML docit 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
BusinessService
BindingTemplate
<dsig:Signature>
tModel
PublisherAssertion