A Structure Preserving Approach for Securing XML Documents

A Structure Preserving Approach
for Securing XML Documents
TrustCol-2007
The Department of Computer Science
Purdue University

Mohamed Nabeel
nabeel@cs.purdue.edu

Outline
• Introduction and Basic Concepts
• Annotation and Encoding Scheme
• Enforcing and Verifying Security
Requirements
• Experimental Results
• Conclusion and Future Work

Secure Sharing
• Hierarchical Data such as XML
• Correct Data
• Access Control
B

Bob
A
E F

B C D
K L

E F G H I J

D

K L
Alice
I J

Secure Sharing – Access Control

Apply Access
Control Policy
A

B
B C D

E F G H I J
E F Bob

K L
K L

Secure Sharing – Correct Data
Bob
Eve has modified B

the values
E X
A

Y L
B C D

Eve
E F G H I J
B

K L
E F

Eve has dropped
K L
elements

Why Preserving Structure
• Partial access to secured documents
• Applying content filters
• Querying secured documents

Late Processing High Scalability

Message Level Security
• P2P vs. E2E
– Transport level security (HTTPS, IPSec, etc)
is sufficient to provide P2P security
– But E2E requires more than TLS
– We need message level security
P2P

Source Intermediary Destination

E2E

Typical Distributed Setting
• Three-tier architecture
Document Source(s)

Intermediaries

Clients

Scalable Systems Message Level Security

XML Node Orderings
• Two types of ordering
1.Hierarchical ordering
2.Sibling ordering
• What orderings are significant?
• What is the relationship between them?
• How does schema validation tools treat
these orderings?

XML Node Orderings
• Is Hierarchical ordering significant?
– Yes, It is!
• Is Sibling ordering significant?
– Depends on the application

Two orderings Two-level structural integrity

XML Node Orderings
<Review> <Review>
Einstein is a Einstein is a
genius; ordinary;
ordinary genius
people may not understand his work. people may not understand his work.
</Review> </Review>

XSLT XSLT

Einstein is a genius; ordinary people may Einstein is a ordinary; genius people may
not understand his work. not understand his work.

Sibling ordering in document centric
applications is significant

XML Node Orderings
person table
<person> firstname country major
<firstname>nabeel</firstname> nabeel sri lanka cs
<country>sri lanka</country>
<major>cs</major>
<person>

<person> Class Person {
<country>sri lanka</country> String firstname;
<firstname>nabeel</firstname> String country;
<major>cs</major> String major;
<person> };

Sibling ordering in data centric
applications may not be significant

Information Leakage
Direct Leakage Indirect Leakage No Leakage

A

A Key K2 B

B C D
Key K1
B C D

E F

E F G H I J E F G H I J

K L

K L
K L

Bob only knows K1

Hiding the existence No Information Leakage

One Example
• Delta-publishing

Delta-Message at t2

First Message at t1 Second Message at t2

The smallest unit of change: An Element

Our Approach
• Recognize two level-ordering
• Provide E2E security for hierarchical data
• Reason about security at the smallest
possible change
• Minimal indirect information leakage

Next
Requirements
• Experimental Results

XML Document
• A Graph G = { V, v, E, f, g}
– V = Ve U Va U Vr where Ve = {x | x is an element}, Va = {x | x is
an attribute}, Vr = {x | x is a node not in Ve U Va}
– v = document root
– E = Ee U Ea U Er where Ee = {e | e is an edge representing an
element-element connection or a link} , Ea = {e | e is an edge
representing an element-attribute connection}, Er = {e | e is an
edge not in Ee U Ea but starts from an element}
– f:E  L where L = {l | l is a node name or an attribute name or
a pre-defined label}, f is called the labeling function
– g:(Ve, i)  Ver where g returns the ith child of Ve, Ver = Ve U Vr

XML Document
• Example
<?xml version=“1.0” encoding=“UTF-8” ?>
<quote type =„bid‟>
<market>NY</market> v

<price cur=„USD‟ size=5m>750</price>
quote
</quote> bid
type

USD
Circles – elements
market price
cur
Squares – attributes
Ellipse - other text text
size
5m

NY 750

Properties of the Annotation Scheme

• Two independent annotation schemes for
– Hierarchical ordering and
– Sibling ordering
• Time complexity = O( height of the XML
DOM tree)
• Provides the flexibility to incrementally
annotate

Hierarchical Ordering
• Should be able to unambiguously identify
parent-child relationships
• Annotate each element with its parent HID
• Element HID‟s need not be unique
• Example: using XPath as HID‟s
– Element x is the parent of y
– Annotate y with h(XPx || name of y), where h
is a collision-resistant hash function and XPx
is the XPath of x. XPath sequencing numbers
are not used to prevent indirect
Information leakage.

Sibling Ordering
• Maintain the following condition
– Given that elements x and y are siblings and x
is to the left of y, seqx < seqy where seqx and
seqy are secure random numbers assigned to
x and y respectively.

Secure random numbers make inferring
about hidden elements difficult, thus
preventing indirect information leakage.

Encoding Scheme
v
v

quote quote
bid bid
type type

market price USD USD
market price
cur NY
cur
content

size size
text text 5m content 5m
NY 750
750

Elements and non-elements Only elements

High reduction in |V| and |E| for document-centric
applications.

Encoding Scheme
• New Graph G‟ = { V‟, v, E‟, f’, g’}
• V‟ = V U {x | x is an attribute for ID, seq or
content} - Vr
• E‟ = E U {e | e is an attribute-element from
ID, seq or content} - Er
• f‟:V‟  L‟ where L‟ = L U {ID, seq, content}
• g‟:{Ve, i}  Ve where Ve consists only of
elements

Integrity
• Two types of integrity
– Structural integrity
– Content integrity
• Introduce a new attribute (signed)
• Attribute value = h(E.attrs || E.content)
– h – hash function
– E.attrs - concatination of attribute name-value pairs of
element E
– E.content – content of element E

• Merkle hash vs. Our approach

Integrity
A

Content Integrity is B

B C D
violated
E X

E F G H I J
Y L

Sibling Integrity is
K L
Bob receives.. violated
B

Completeness Hierarchical B

B

is violated Integrity is violated E F
L F

E F

L K
K E

K L

Confidentiality
• Content of each element is encrypted
• Introduce a new attribute (encrypted)
• Attribute value = keys(keyr||keyr (E.attrs ||
E.content || E.signed))
– keyr – a randomly generated key
– keys – shared key
– E.attrs – concatination of attribute name-value pairs
of element E
– E.content – content of element E
– E.signed – digital signature computed for E

Verifying and Updating
• Each element can be verified
independently
• Hierarchical and Sibling integrity can be
verified independently
• Each element can be updated
independently
• Structure can be updated without affecting
the existing values

Example: Updating
<?xml version=“1.0” encoding=“UTF-8” ?>
<quote type =„bid‟>
<market>NY</market>
<price cur=„USD‟ size=5m>765</price> v
</quote> X
quote
signed
Re-calculate signed and encrypted X

encrypted attributes only
for this element market price

signed
X encrypted signed encrypted

X X X

Global vs. Local Annotation
Local Annotation Global Annotation

400
Time taken to annotate (ms)

350

300
250

200

150

100

50
0
1 2 3 4 5 6 7 8
Number of Elements in the XML document (in 500)

Updating XML Document
Our Scheme W3C Scheme

800
700
600
Time taken (ms)

500
400
300
200
100
0
1 2 3 4 5
Percentage of the Document Updated

Division of Labor
encoding signing encrypting

45000
40000
35000
30000
Time Taken

25000
20000
15000
10000
5000
0
1 2 3 4 5 6 7 8
Number of Elements in the XML Document (in 500)

Outline
Requirements
• Implementation and Experimental Results

Conclusion and Future Work
• We presented an interesting approach to
secure XML documents while preserving
the structure
• We plan to extend the work presented to
– Explore ways to reduce the signing time
– Explore possible hybrid combinations of our
approach and the standard approach
• We are planning to publish the library
under ASF license

A Structure Preserving Approach for Securing XML Documents

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