Key Management in Mobile Ad Hoc Networks

1
Key Management in Mobile Ad Hoc Networks

• Presented by Edith Ngai
• Spring 2003

2
Outline

• Introduction
• Ad hoc network security
• Key management in ad hoc networks
• Fully distributed CA
• Trust-based and dynamic fully distributed CA
• Future work
• Conclusion

3
Introduction

• An ad hoc network is a collection of nodes that
  do not need to rely on predefined infrastructure
  to keep the network connected.
• Nodes of ad hoc networks are often mobile, apply
  wireless communication
• MANET (mobile ad hoc network)
• Applications
• Personal area networks
• Collaborative networks
• Military communications
• Sensor networks
• Disaster area networks

4
Characteristics

• Dynamic network topology
• Limited physical security
• Limited bandwidth
• Energy constrained nodes
• Natures of ad hoc networks makes them vulnerable
  to security attacks
• Passive eavesdropping
• Denial of service attacks by malicious nodes
• Attacks from compromised entities or stolen
  devices

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Security wired network vs Ad hoc network

• Wired network
• Adversary must gain physical access to wired link
• Adversary has to sneak through security holes at
  firewalls or routers
• Ad hoc network
• Infrastructureless network does not have a clear
  line of defense
• Wireless attacks may come from all directions
• Every node must be prepared to encounter with an
  adversary

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Key management in ad hoc networks

• Security in networking is in many cases dependent
  on proper key management
• A centralized approach in key management may not
  be available
• Centralized approaches are vulnerable as single
  point of failures
• Distributed approach is used
• Partially distributed certificate authority
• Fully distributed certificate authority

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Fully distributed certificate authority

• It uses a (k,n) threshold scheme to distribute an
  RSA certificate signing key to all nodes in the
  network
• Any operation requiring the CAs private key SKCA
  can only be performed by a coalition of k or more
  nodes
• Certificate renewal and revocation
• Share initialization for incorporating joining
  nodes into the CA

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Fully distributed certificate authority

• Polynomial secret sharing
• f(x) SK f1x fk-1xk-1 is the secret
  polynomial, where SK is the certificate signing
  key, PK is the certificate verification key
  assumed to be well-known
• Each node holds a polynomial share
• Pvi f(vi) mod N
• Node vi firstly chooses a coalition of k nodes
  from its neighborhood.

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Fully distributed certificate authority

• Let the coalition be Bv1, v2, , vk,vi
  broadcast the certificate renewal request
• The node vj returns a partial certificate CERTvj
• Node vi then converts each of them according to
  the IDs of these k responding nodes
• vi then combine the certificates received
• By k-bounded coalition offsetting algorithm, vi
  can recovers its new certificate CERT

10
Fully distributed certificate authority

• An initialized node is defined as the node that
  possesses a valid polynomial share of SK
• The initialized nodes collaborately initialize
  the other nodes
• When vi requests for initialization, each vj can
  calculate its partial share by
• By Lagrange interpolation, vi can obtain its
  partial secret key

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Trust-based and dynamic fully distributed CA

• Different assumptions
• Each node maintains a trust value to its
  neighbours.
• Each node holds c partial secret keys, instead of
  one in the old model
• Each node signs out different number of partial
  certificates according to the trust level of the
  requesting node.

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Trust-based and dynamic fully distributed CA

• Trust model
• The trust value from node vj to node vi represent
  the different levels of trust that node vj
  towards node vi according to vjs observation on
  the behaviour of node vi at that moment.
• There is a number of trust models proposed in the
  past. Our system can work with different trust
  models, no matter with continues or discrete, and
  different ranges of trust values.

13
Trust-based and dynamic fully distributed CA

• Each node holds a number of partial keys that can
  be used to sign certificates for its neighbours.
• We define c be the number of partial keys that a
  node holds.
• Each node has its unique ID, and this node ID
  will be used to generate the unique partial key
  IDs that the node holds.

Node ID Partial Key IDs
1 1, 2, , c
2 c1, c2, , 2c
3 2c1, 2c2, , 3c

k (k-1)c1, (k-1)c2, , kc

n (n-1)c1, (n-1)c2, , nc
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Trust-based and dynamic fully distributed CA

• A node vi broadcasts its request for certificate
  renewal among its neighbourhood.
• A neighbouring node vj receives the request will
  return a number of its partial certificates
  according to the trust value it gives to vi.
• Define range of trust value is w1, w2. It
  should be noted that the smaller the trust value
  represents the less trust from vj to vi and vice
  versa.
• Let x be the trust value that vj towards vi,
• (no. of partial certificates vj sends vi) nj

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Trust-based and dynamic fully distributed CA

• Upon receiving at least k such partial
  certificates, node vi picks k to form the
  coalition B. Suppose, vi chooses CERTa1, CERTa2,
  , CERTak, where a1, a2, , ak are the IDs of
  the k partial keys.
• CERTaj (CERTaj)Laj(0) mod N,
• where
• vi then multiples CERTa1, CERTa2, , CERTak
  together to generate the candidate certificate
  CERT
• CERT mod N
• Then, vi can employ the k-bounded coalition
  offsetting algorithm to recover its new
  certificate CERT.

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Trust-based and dynamic fully distributed CA

• The threshold k originally represents the number
  of neighbors required, now is dynamic base on the
  trust of the requesting node
• In our system, a node vi may not need k neighbors
  to sign a certificate if it has high trust value
• Nodes can sign certificate according to a
  quantitative trust value with our system

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Future Work

• Design the algorithm and protocol for
  initialization of incorporating joining nodes in
  the trust-based and dynamic fully distributed CA
• Consider the number of partial keys per node to
  be also dynamic
• Do performance evaluation on the proposed
  algorithm and protocol
• Increase the performance of the current design

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Conclusion

• We studied the current security issues in ad hoc
  networks
• We reviewed the key management techniques in ad
  hoc networks
• We proposed a system of trust-based and dynamic
  fully distributed CA
• We designed algorithm and protocol for
  certificate renewal or revocation in our system