Efficient Public Key Infrastructure
Implementation in
Wireless Sensor Networks
Wireless Communication and Sensor Computing, 2010.
ICWCSC 2010. International Conference on
報告者：林昌宏
Outline
Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

Introduction

Security is an important issue when designing
network or protocol, but taking into account the
specificity of WSN, it haven’t given the necessary
attention to security.

The problem of security is regarding
 the limitation of sensors
 the deployed environment
small memory
weak processor
limited battery power of sensor nodes
Introduction(cont.)
The proposed schemes in literature aren’t secure.
 using some simplified techniques, like symmetric
encryption, to ensure all security services.


The author proposes an implementation of a
combination of symmetric and asymmetric
encryption.
Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

Security In Wireless Sensor
Networks
A.
Security services
 Confidentiality
 Integrity
 Authentication
B.
Public key cryptography
Confidentiality
 Ensuring
that the exchanged data is kept secret
from any unauthorized entities over the network.
 Considering
the consumption of devices resources,
symmetric encryption is more efficient.
 It
must also protect information from long term
eavesdropping by using periodic key update.
Integrity
 The
message should be un-altered during its
transmission from a source to destination by any
intermediate sensor or malicious node.
 MAC(Message Authentication Code)
 Digital signatures
Authentication
 The
process of identification that a receiving entity
is sure that the message comes from a legitimate
source.
 using Public Key Infrastructure.
 In WSN, however, it
is usually done by predistributing some bootstrapping information used
after to authenticate sensors by the base station.
Public key cryptography
 It
uses two keys, public key and private key, to do
encryption and decryption.
 Public key：publicly known by each entity.
 Private key：kept secret by it holder.
 However, PKI
is omitted from the use in WSN,
because of its great consumption of energy and
bandwidth which are very crucial in sensor
network.
Public key cryptography(cont.)
 Elliptic
Curve Cryptography (ECC), is the most
one of new cryptographic algorithms.
 Having more energy efficient for sensors.
 Giving the same threshold of security as the
conventional algorithms with much smaller key sizes to
save more memory.
 This
paper presents a lightweight public key
infrastructure for WSN called micro PKI.
Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

State Of The Art
A.
Symmetric encryption based schemes
 Shared key
 Pre-distributed keys
B.
Public key based schemes
 Simplified Kerberos protocol
Shared key
This solution is the simplest way for securing
WSN.
 It uses a single shared key to encrypt traffic over
the network, and this key may be periodically
updated to ensure more security against
eavesdropping.
 But it is vulnerable against capture attack which
can compromise the shared key and then the
whole network.

Pre-distributed keys
 An
off-line dealer distributes a set of symmetric
keys to sensors before their deployment.
a. A random key pre-distribution scheme for
WSN in which sensor obtains a subset of
symmetric keys from a large key pool.
b. After deployment, each sensor tries to find a
shared key with each of its neighbors to secure
the links with them.
c. Managing how to obtain the session key
between sensors and the base station.
Simplified Kerberos protocol
 Setup
a session key between each communicating
pair of sensors by contacting a trusted third party
(the base station).
 There
is a long term key shared between each
node and the base station, and the base station
generates the secret key for each pair of sensors.
Simplified Kerberos protocol(cont.)
 Disadvantage：
it is vulnerable against capture attacks to exposed sensor.
the handshaking is not energy saving.
it may consume lot of network resources if the base
station is far from the pair of nodes.
Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

Encryption Algorithms
A.
Elliptic Curve Cryptography
B.
Message Authentication Codes
Elliptic Curve Cryptography
 The
ECC algorithm can be classified as the one of
the most efficient asymmetric algorithms regarding
its energy cost and its encryption speed.
Algorithm
Sign
MIPS Years to
Attack
RSA-1024
304
1012
ECC-160
22.82
RSA-2048
2302.7
ECC-224
61.54
1024
Energy cost of digital signature (mJ)
Message Authentication Codes
 MACs
is the common solution to ensure integrity
and authentication of messages in conventional
networks.
A
MAC can be viewed as hash function applied on
data packets, and is encrypted by the session key.
A
receiver sharing the same session key can
verifies the integrity of the message by computing
MAC value and comparing it with the received one.
Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

Micro PKI For WSN
 Micro
PKI is a lightweight implementation of PKI
for WSN since it only implements a subset of a
conventional PKI services.
A. Network Architecture
B. Micro PKI System Bootstrapping
C. Base Station To Sensor Nodes Handshake
D. Sensor To Sensor Handshake
E. Micro PKI Functioning
F. Micro PKI Key Update
G. Joining The Network
Network Architecture
 The
base station have more computational and
energy power compared to sensors.
 The base station has a pair of keys(private and
public key).
 Each sensor is capable to use symmetric and
asymmetric encryption.
 Each sensor has the capacity to save at least the
public key of the base station and a session key
used for data encryption.
 Each sensor node gets the public key of the base
station before deployment from an off-line dealer.
Micro PKI System Bootstrapping
 Before
the deployment of the WSN, an off-line
dealer distributes the public key of the base station
to each sensors in the network.
 This
public key is used after in the handshake
between the base station and sensors to encrypt
the symmetric session key.
Base Station To Sensor Nodes
Handshake
Sensor generates a random session key, encrypts
it with the public key of the base station, and
then sends the message embedded the encrypted
key to the base station.
2. The base station decrypts this message using its
private key and saves the session key in a global
table which has all session keys corresponding to
each sensor in the network.
3. The base station encrypts an OK message using
the established session key, and sends to sensor
to ensure that the session key setup is successful.
1.
Base Station To Sensor Nodes
Handshake(cont.)
 Micro
PKI handshake ensures a great level of
security, since it uses both symmetric and
asymmetric encryption to secure the session key.
 After
the establishment of the session key,
the sensor and the base station begin to use
it for data encryption until the next key
update.
Sensor To Sensor Handshake
 After
the establishment of the base station to
sensor nodes, sensors can establish a secure tunnel
between them for any purpose.
1. One of the two sensors sends a request which
contains the identifier of the corresponding
sensors to the base station.
2. The base station generates a random key for this
propose, and saves the pairs of sensors’
identifier and corresponding session key in the
global table.
Sensor To Sensor Handshake(cont.)
The base station encrypts the requested session
key by using the corresponding key between the
base station and the sensor.
4. When receiving the new session key by sensors,
they begin to use it to secure the data
transmission between themselves.
3.
Micro PKI Functioning
 In
order to guaranty the integrity and the
authenticity of the exchanged between each
communicating parties, a MAC encrypted by
session key is embedded to the packet.
 By verifying the joined MAC, if the verification fails,
this means that there may have an attacker which
has altered this packet.
 Using a mechanism like multi-path routing to avoid
this attacker, otherwise the base station use any
mechanism to detect and exclude this attacker
from the network, if it exists.
Micro PKI Key Update
A
key update tries to prevent long term attack
aiming to extract the encrypting keys by analyzing
the encrypted traffic over the network for long
time.
 In a WSN, an automatic key update must be
defined, since a network can be deployed for many
days or months.
 The key update is initiated by the sensor node by
launching new handshake, and the period time is
relative to the key length and the complexity of
the used algorithm.
Joining The Network
 If
a new node wants to join the network, the
administrator must load the public key of
the base station into this node.
 After
getting the public key, the new sensor
can automatically launch a handshake and
join the network.
Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

Analysis
A.
Security services




B.
Scalability
Confidentiality
Authentication
Integrity
Energy cost analysis of micro PKI
Security services

Scalability
Micro PKI manages the increasing number of sensor
nodes by new handshake and a new entry is created in
the global table of the base station to manage connection.

Confidentiality
The use of symmetric encryption to encrypt the
exchange data between the base station and sensors, and
using periodic key update to prevent long term attacks.
Security services(cont.)

Authentication
By pre-installing the public key of base station in each
deployed sensor, the authenticity of the base station can
be authenticated by sensors.

Integrity
Computing and joining MAC to each packet between the
base station and any sensor over the network as well as
between sensors.
Energy cost analysis of micro PKI
Operations
Sensor
to
Base station
handshake
Sensor to Sensor
handshake
Energy cost of sensor (mJ)
Encrypt session key
22.82
Send a packet
3.78
Receive a packet
1.83
Decrypt OK message
0.039
Total
28.46
Send request message and
Receive session key message
3.66
Decrypt message
0.039
Total
3.70
Total energy cost of micro PKI
32.16
Energy cost of the simplified Kerberos
39.6 ~ 47.6
Energy cost of the simplified SSL
93.9
Introduction
 Security In Wireless Sensor Networks
 State Of The Art
 Encryption Algorithms
 Micro PKI For WSN
 Analysis
 Conclusion

Conclusion
 Micro
PKI implement a combination of symmetric
and asymmetric encryption which tries to solve
the problem of security in WSN.
 By
the use of public key cryptography as a tool, it
ensures the authenticity of the base station.
 Micro
PKI is composed of two phases
Sensor to base station handshake
Sensor to sensor handshake
Conclusion(cont.)
 Ensuring
the confidentiality and integrity of
the exchanged data using the MAC joined to
each packet.
 For
more security, a periodic key update is
defined for the session key
 Micro
PKI is energy efficient and gives a
considerable threshold of security.