Secure Ad-Hoc Network
Eunjin Jung
[email protected]
1
What is Ad-Hoc Network?
 Ad-Hoc Network
– Subset of peer-to-peer computing problem
– Sensor network
– Wireless and mobile
– Physically neighboring participants
– No infrastructure
2
Truth is…
 Ad-Hoc Network relies on
– Base Station
– Offline configuration
 Potential
– Military operation use
– Sensor network
– Pervasive, ubiquitous computing
3
Challenges in Ad-Hoc Network
 Mobility
– Restricted computing resource
– Restricted power resource
– Unreliable communication
 Ad-Hoc
– Transient states
– No trustworthy third party
– Often security protocol integrated with others
4
Security in Ad-Hoc Network
 Availability
– Sleep Deprivation Torture
• Power consumption is worse than computing or
network resource consumption, because the device
cannot recover as soon as the attack finishes
– Jamming
• Spectrum Spread, Frequency Hopping
5
Security in Ad-Hoc Network
 Confidentiality
– Easier to passively eavesdrop
– Cannot rely on expensive cryptosystem
– Symmetric key cryptography is used
– Small key, frequent update vs. large key,
intermittent update
6
Security in Ad-Hoc Network
 Authorization
– Network resource
• Inherently vulnerable to bandwidth stealing
• Should reject routing unauthorized packet
– Transient states
• Security associations between principals are
transient
• Static authorization policy is unfeasible
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Security in Ad-Hoc Network
 Authentication
– Cannot rely on central server
– Neither on public key cryptography
– Should be adaptive to transient authorization
policy
– Should be swift to renew symmetric key
– Pre-computed certificate
– Threshold cryptography
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Security in Ad-Hoc Network
 Integrity
– Similar to any communication
– Use traditional solution based on symmetric
key
 Non-Repudiation
– Based on public/private key cryptography
– Hard to achieve with limited computing
resource
– Content with certificates
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Security in Ad-Hoc Network
 Tamper-Resistance
– Security not only on communication, but also
on its physical status
 Intrusion Detection
– Shares have to be revoked and renewed when
compromised
 Anonymity
– Hide the identity of the senders and receivers
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Security in mobile network
 AAA properties
– Authentication
– Authorization
– Accounting
 Standard in CDMA2000 packet core
network
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Everything comes to…
 Proper authentication scheme is the key to
solve security problem in ad-hoc network
 Hierarchical authentication scheme
– Less mobility, higher in hierarchy
 Multilevel authentication scheme
– Link layer[BT01]
– Routing layer[PSWCT01]
– Application layer
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Traditional ways do not work
 Indirect Kerberos[FG96]
– Assuming application-level proxy to delegate
public key operations
– Base station can do the job if there is one
 Duplicated servers
– Tradeoff between mobility and cost
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Early works may not either…
 Authentication protocols for PCS [LH95]
– offer even non-repudiation
– Assumption of static and high-capability
HOME base station; works with mobile-IP
– Assumption of reliable communication between
home base station and current one
– Frequent cryptographic operation including
public key operation on the subscriber’s side
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SPINS – authenticated routing
 TESLA : streaming authentication protocol
– Two-party key agreement protocol
 SNEP(Secure Network Encryption
Protocol)
– data confidentiality, two-party data
authentication, and data freshness
 Key from TESLA , further operation on
SNEP
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SPINS – authenticated routing
 Problem
– Assumption on the functionality of base station
– Lack of local operation
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Decentralized solutions
 Emulations of Certificate Authority
 Key agreement based on prior context or
offline agreement
 Self-organized public key infrastructure
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Shamir’s secret sharing scheme
 Interpolating scheme (m>1)
f ( x)  a0  a1 x    am1 x
m 1
F ( x)  a0  a1 x    am1 x m1
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What is threshold cryptography?
 (m, n) – threshold scheme
– m-out-of-n scheme, secret sharing scheme
– 1 sender(dealer) distributes partial
secret(shares, shadows) to n participants
– Any m parts put together can retrieve the secret,
but not less than m
– Perfect for any group of at most m-1
participants
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Threshold Scheme
 Tradeoff between security and reliability
according to the choice of m and n
– Reliability measure
• Target of denial of service attack : n-m+1
– Security measure
• Target of compromising : m
 Good for distributed authentication
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Emulation of Certificate
Authority
 Each entity has a share of group key
 More than m entities can act as a certificate
authority – local operation
 Each entity computes partial certificate out
of partial secret
 Proactively update shares, and actively
revoke any compromised ones
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Still problem remains…
 Requires collaborative users – have to
respond the partial certificate request
anytime.
 Who can be a dealer?
– Shares are given to principals in bootstrap
phase (still base station?)
22
Password based public key
infrastructure
 Prior context is assumed, so all participants
share a weak secret.
 Extending Diffie-Hellman method to agree
on stronger symmetric key among multiparties.
23
Password based public key
infrastructure
 O(n) steps
c1^S4
m1
g^S1
P(c1=g^S1bs2S3)
g^S1S2S3
m2
m4
g^S1S2S3
g^S1S2S3
g^S1S2
m3
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Password based public key
infrastructure
 Need to communicate with all group
members and select a leader
 Static group assumption
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Self-organized public-key
infrastructure
 Each user publishes its own certificate and
some for others
 Each user maintains certificate repository,
some issued by itself, rest by others.
 Trust graph : each user is a node, and an
edge (u,v) denotes user u published
certificate to v.
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Self-organized public-key
infrastructure
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Self-organized public-key
infrastructure
 How many certificates should be stored in
the repository to cover all pairs in the ad
hoc network? 2 n covers 95%
 Certificate neighbor may not be available at
the trust graph construction time
 Tested on PGP trust graphs – does that
represent ad hoc network properly?
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No scheme is perfect yet
 Security issues in ad-hoc networks are
converged into authentication problem
without infrastructure, in peer-to-peer
manner.
 The burden of CA is reduced, but still we
need co-ordination
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