Collaborative Enforcement of Firewall
Policies in Virtual Private Networks
Fei Chen
Dept. of Computer Science and Engineering
Michigan State University
Joint work with Prof. Alex X. Liu
Introduction
Virtual Private Network (VPN)
MSU
IBM
1.1.0.0/16
Malicious
websites
2.2.0.0/16
A secure hole
IBM
Representative
1.1.0.10
Firewall
VPN Server
Confidential
Database
2.2.0.1
2.2.0.25
Header
Encrypted
Payload
2.2.0.2
Src IP: 1.1.0.10
Dst IP: 2.2.0.1
Src IP: 2.2.0.25
Dst IP: 2.2.0.2
Src IP: 2.2.0.25
Dst IP: 2.2.0.2
Header
Payload
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
2/18
Motivation
• The problem: MSU firewall cannot know what traffic is inside VPN
– Viruses or worms can enter into MSU’s networks
• Two straight forward ways
MSU
Firewall
×
IBM
VPN Server
Firewall
packet
• Goal: MSU and IBM collaboratively enforce the firewall policy
without MSU knowing IBM packet and IBM knowing MSU firewall
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
3/18
Related Work
• Secure Function Evaluation (SFE) (Yao, 1982)
– Garbled circuits (Yao, 1986)
• Computation cost is O(2b)
• Oblivious Attribute Certificates (OACerts) (Li et al., 2005)
– Trusted third party
– Expensive PKI operation
• Cross-Domain Cooperative Firewall (CDCF) (Cheng et
al., 2007)
– CDCF is insecure
• MSU knows which rule matches which packet
– CDCF is inefficient
• It uses commutative encryption functions
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
4/18
Our Approach: VGuard
MSU
Firewall
f1(Firewall)
IBM
VPN Server
f2(f1(Firewall))
1. Compute f1(f2(packet))
2. Search f1(f2(packet)) in
Bootstrapping Protocol
f2(packet)
decision
Filtering Protocol
f2(f1(Firewall))
• Key idea I: We propose Xhash protocol for oblivious comparison
– Three orders faster than the commutative encryption
• Key idea II: We uses Firewall Decision Diagrams (FDD)
– For security purpose, FDD can help to prevent MSU from knowing which
rule matches which packet
– For efficiency purpose, processing packets using FDD is much more
efficient than using linear search
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
5/18
Oblivious Comparison
• Oblivious comparison problem
c1
IBM
MSU
c2
c1 ?= c2
If c1 ≠ c2, no party should learn the value of the other party
• Xhash protocol
MSU(K1)
Compute h(c2⊕K2⊕K1)
c1⊕K1
IBM(K2)
h(c1⊕K1⊕K2)
c2⊕K2
Compare with h(c1⊕K1⊕K2)
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
6/18
Membership Query
• Membership query problem
[a, b]
c
IBM
MSU
Does c in the range [a,b]?
No party should learn the value of the other party
• Solutions using Xhash protocol
[3, 7]
Prefix format
{011, 1**}
5
Prefix family
PF(5)={101, 10*,1**,***}
1**
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
1**
Liu, Chen
7/18
How to check the intersection of two sets in a privacy
preserving manner ?
MSU
[3, 7]
IBM
5
Prefix format
Prefix family
{011, 1**}
PF(5)={101, 10*,1**,***}
Prefix numericalization
{01100, 10010}
{01100⊕K1, 10010⊕K1}
Prefix numericalization
{h(01100⊕K1⊕K2),
h(10010⊕K1⊕K2)}
Store
{h(01100⊕K1⊕K2), h(10010⊕K1⊕K2)}
10111⊕K2, …,00011⊕K2
{10111, 10010,10001,00011}
Compute
{h(10111⊕K2⊕K1), …, h(00011⊕K2⊕K1)}
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
8/18
The Bootstrapping Protocol (1/3)
•
Why to prevent MSU from knowing which rule matches which packet?
f1(Firewall)
Bootstrapping Protocol
f2(f1(Firewall))
1. Compute f1(f2(packet))
f2(packet)
2. Search f1(f2(packet)) in
decision
f2(f1(Firewall))
MSU may figure out packet by knowing the
original rule that matches packet
Filtering Protocol
IBM cannot figure out
Firewall by using f1(Firewall)
F1 [1,4]  F2 [5,8]  accept match f2(packet)
change by MSU
F1 [1,2]  F2 [5,6]  accept
F1(packet) is in [1, 2]
F2(packet) is not in [5, 6]
F1 [1,2]  F2 [7,8]  accept
……
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
9/18
The Bootstrapping Protocol (2/3)
• Convert overlapping rules to non-overlapping rules
F1  [4,11]  F2  [0,5]  accept
F1  [0,3]
 F2  [2,7]  accept
FDD
construction
Prefix
generation
F1  [12,15]  F2  [2,7]  accept
F1  [0,15]  F2  [0,15]  discard
Overlapping rules
Prefix
numericalization
Non-Overlapping rules
Rule
(0100010⊕K1, 0000010⊕K1) → a generation
(0100010⊕K1, 0100011⊕K1) → a
(1000010⊕K1, 0000010⊕K1) → a
(1000010⊕K1, 0100011⊕K1) → a
(0100010⊕K1, 0110011⊕K1) → d
XOR by MSU
……
The order of non-overlapping rules does not affect their function
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
10/18
The Bootstrapping Protocol (3/3)
(0100010⊕K1, 0000010⊕K1) → a
(0100010⊕K1, 0100011⊕K1) → a
(1000010⊕K1, 0000010⊕K1) → a
(1000010⊕K1, 0100011⊕K1) → a
(0100010⊕K1, 0110011⊕K1) → d
Send to IBM
……
(h (0100010⊕K1⊕K2), h(0000010⊕K1⊕K2)) → a
(h(0100010⊕K1⊕K2), h(0100011⊕K1⊕K2)) → a
(h(1000010⊕K1⊕K2), h(0000010⊕K1⊕K2)) → a
(h(1000010⊕K1⊕K2), h(0100011⊕K1⊕K2)) → a
(h(0100010⊕K1⊕K2), h(0110011⊕K1⊕K2)) → d
……
IBM shuffles rules
MSU statistically analyze Send back to MSU
1. Frequency of values
2. Frequency of decisions
IBM add
dummy rules
(h(0100010⊕K1⊕K2),
(h(0100010⊕K1⊕K2),
(h(1000010⊕K1⊕K2),
(h(0100010⊕K1⊕K2),
(h(1000010⊕K1⊕K2),
h(0000010⊕K1⊕K2)) → a
h(0100011⊕K1⊕K2)) → a
h(0000010⊕K1⊕K2)) → a
h(0110011⊕K1⊕K2)) → d
h(0100011⊕K1⊕K2)) → a
(h(0100010⊕K1⊕K2), h(dummy1⊕K2)) → d
……
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
11/18
The Filtering Protocol (1/2)
A Packet:
Prefix family
generation
(0101, 0011)
0101 0011
010* 001*
01** 00**
0***
0***
****
****
Prefix
Numericalizaiton
0101100
0100011
0100010
0000001
0000000
0011100
0010011
0000010
0000001
0000000
XOR by IBM
h(0101100⊕K2⊕K1)
h(0100011⊕K2⊕K1)
h(0100010⊕K2⊕K1)
h(0000001⊕K2⊕K1)
h(0000000⊕K2⊕K1)
h(0011100⊕K2⊕K1)
0101100⊕K2
HMAC
h(0010011⊕K2⊕K1) XORByand
0100011⊕K2
MSU
h(0000010⊕K2⊕K1)
0100010⊕K2
h(0000001⊕K2⊕K1)
0000001⊕K2
h(0000000⊕K2⊕K1)
0000000⊕K2
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
0011100⊕K2
0010011⊕K2
0000010⊕K2
0000001⊕K2
0000000⊕K2
Liu, Chen
12/18
The Filtering Protocol (2/2)
•
To improve search efficiency, MSU can convert non-overlapping
rules to a FDD
(h(0100010⊕K1⊕K2),
(h(0100010⊕K1⊕K2),
(h(1000010⊕K1⊕K2),
(h(1000010⊕K1⊕K2),
(h(0100010⊕K1⊕K2),
(h(dummy5⊕K1⊕K2),
h(0000010⊕K1⊕K2)) → d
h(0100011⊕K1⊕K2)) → d
h(0110011⊕K1⊕K2)) → a
h(0100011⊕K1⊕K2)) → d
h(0110011⊕K1⊕K2)) → a
h(dummy7⊕K1⊕K2)) → d
……
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
13/18
Experimental Results (1/3)
• For real-life firewalls in bootstrapping protocol
– Bootstrapping cost of VGuard is lower than that of CDCF for
most firewalls
MSU
IBM
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
14/18
Experimental Results (2/3)
• For real-life firewalls in filtering protocol
– VGuard is 552 times faster than CDCF on the MSU side
– VGuard is 5035 times faster than CDCF on the IBM side
(Log scale)
(Log scale)
MSU
• Two intuitive reasons for the better performance
IBM
– Xhash is three orders faster than commutative encryption
– FDD is much efficient to search the decision of a given packet
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
15/18
Experimental Results (3/3)
• For synthetic firewall policies in filtering protocol
– VGuard is 252 times faster than CDCF on the MSU side
– VGuard is 5529 times faster than CDCF on the IBM side
(Log scale)
(Log scale)
MSU
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
IBM
Liu, Chen
16/18
Concluding Remarks
• VGuard is secure
– VGuard prevents MSU from identifying which rule matches the
given packet
• VGuard is efficient
– Xhash is three orders faster than the commutative encryption
– VGuard uses firewall decision diagrams for processing packets
• Xhash is very efficient for oblivious comparison and can
be used for other applications
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
17/18
Questions?
Thank you!
Collaborative Enforcement of Firewall Policies in Virtual Private Networks
Liu, Chen
18/18