13

CHAPTER II

LITERATURE SURVEY

2.1 Introduction

Latest developments in VLSI, wireless communications, and

biomedical sensing devices, allow very small, lightweight, low power,

intelligent sensing devices called biosensors.

A set of these devices can be integrated into a Wireless Biomedical

Sensor Network (WBSN), a new breakthrough technology used in

telemedicine for monitoring the physiological condition of an individual.

Considering the sensitivity of information in WBSN, security and patient

data privacy is provided as it is an important issue in the design of such

systems. The deployment of security mechanism may require more

amount of resources in terms of memory capacity and battery power.

But the biomedical sensor nodes in WBAN has got resource limitations

in terms of battery lifetime, CPU processing capability, and memory

capacity. Replacement or recharging of batteries in biomedical sensor

nodes is quiet difficult or too costly. Hence an energy efficient security

protocol for WBAN is proposed in this work. In this chapter several

ongoing projects in WBSN and the security architectures used in these

projects have been discussed. And also existing security protocols for

14

authentication of patient to the hospital server is discussed in this

chapter.

2.2 Security in Wireless Sensor Networks

Mohamed Youssef et al [18] has given research vs. reality design and

deployment issues of wireless sensor networks. In this, the author has

also given the security challenges in WSN. As WSNs are deployed for

monitoring, data gathering, collaborative communication and computing,

these networks must be able to provide authentic information. But the

wireless networks and their flexibility to form ad hoc networks with

minimal or no prior infrastructure is vulnerable to unwanted

eavesdropping and other attacks such as wormholes.

John Paul Walters et al in their book [19] on “Wireless Sensor

Network” Security explain the obstacles and the requirements in the

sensor security, classify many of the current attacks, and finally list their

corresponding defensive measures. As wireless sensor networks continue

to grow, the need for effective security mechanisms also grows. Because

sensor networks may interact with sensitive data and/or operate in

hostile unattended environments, it is important that these security

concerns be addressed from the beginning of the system design.

However, due to inherent resource and computing constraints, the

security in sensor networks poses different challenges than traditional

network security. A wireless sensor network is a special network which

has many constraints compared to a traditional computer network. Due

15

to these constraints it is difficult to directly employ the existing security

approaches to Wireless Sensor Networks(WSN). Dimitriou [16] et al.

discuss the security issues in wireless biomedical sensor networks which

is a special purpose WSN used in telemedicine applications and the

network for which a biometric based security architecture is proposed in

this research work. The existing security architectures for WBSN are the

given in section 2.3.

2.3. Related work

2.3.1 Cryptographic Security Mechanisms

Several security solutions have been proposed in protecting

biomedical sensor network‟s link layer communication, which constitutes

the bottom layer of the sensor network protocol stack. More attention

has been given to robust and efficient key management schemes, which

serve as the fundamental requirement in encryption and authentication.

The different types of security protocols using cryptographic mechanisms

are the following.

2.3.1.1 TinySec

Karlof et al.[ ] designed the replacement for the unfinished SNEP[21],

of WSN is known as TinySec [15][16]. TinySec is proposed as a solution

to achieve link-layer encryption and authentication of data in biomedical

sensor networks as part of Tiny OS release. Inherently it also provides

security services like message integrity and replay protection. A major

difference between TinySec and SNEP is that there are no counters used

16

in TinySec. For encryption, it uses CBC mode with cipher text stealing,

and for authentication, CBC-MAC is used. TinySec XORs the encryption

of the message length with the first plaintext block in order to make the

CBC-MAC secure for variably sized messages. There are two packet

formats defined by TinySec. These are TinySec-Auth, for authenticated

messages, and TinySec-AE, for authenticated and encrypted messages.

For the TinySec-AE packet, a payload of up to 29 Bytes is specified, with

a packet header of 8 Bytes in length. Encryption of the payload is

necessary to provide confidentiality security service, but the MAC is

computed over the payload and also the header. The TinySec-Auth

packet can carry up to 29 Bytes of payload. The MAC is computed over

the payload and the packet header, which is 4 Bytes long. Generally, the

security of CBC-MAC is directly related to the length of the MAC. TinySec

specifies a MAC of 4 Bytes, much less than the conventional 8 or 16

Bytes of WSN security protocols.

The drawbacks in this security architecture are: By default relies on a

single key manually programmed into the sensor nodes before

deployment. This network-wide shared key provides only a baseline level

of security. It cannot protect the network against node capture attacks. If

an adversary compromises a single node or learns the secret key, he/she

can gain access on the information anywhere in the network, as well as

inject his/her own packets. This is a major disadvantage in Tinysec

security protocol for WBAN.

17

2.3.1.2 Hardware Encryption

As an alternative to TinySec, hardware encryption[16] supported by

the ChipCon 2420 ZigBee complaint RF transceiver based on AES

encryption using 128-bit keys has been designed. The CC2420 can

perform IEEE 802.15.4 MAC security operations, including counter (CTR)

mode encryption and decryption, CBC-MAC authentication and

encryption with authentication.

One limitation of this method is that AES decryption is not allowed in

the sensor nodes, so transmitted information cannot be accessed by

intermediate nodes if needed (e.g. for aggregation purposes). Any

decryption can be performed only at the base station. Another drawback

of the method is that it is highly dependent on the specific platform.

Other sensor node hardware do not offer hardware encryption support,

so a different approach has to be taken in this case.

2.3.1.3 Elliptic Curve Cryptography

Recently, elliptic curve cryptography (ECC)[16] has emerged as a

promising alternative to RSA-based algorithms, as the typical size of ECC

keys is much shorter for the same level of security. Uhsadel et al. [22]

propose an efficient implementation of ECC and Liu et al. [23] developed

TinyECC, an ECC library that provides elliptic curve arithmetic over

18

prime fields and uses inline assembly code to speed up critical

operations on the ATmega128 processor of sensor node.

Also lately, Szczechowiak et al. presented NanoECC [24], which is

relatively fast compared with other existing ECC implementations,

although it requires a heavy amount of ROM and RAM sizes. Even

though elliptic curve cryptography is feasible on sensor nodes, its energy

requirements are still orders of magnitude higher compared to that of

symmetric cryptosystems. Due to the strength ECC can be used for

operations, like key establishment during the initial configuration of the

wireless biomedical sensor network [25][26].

2.3.2. Biometric Based Security Mechanisms

Recently the key establishment method to secure communications in

biomedical sensor networks has emerged to be biometrics[27]. It

advocates the use of the body itself as a means of managing

cryptographic keys for symmetric cryptography. For sensors attached on

the same body, if they measure a previously agreed physiological value

simultaneously and use this value to generate a pseudo-random number,

this number will be the same if they are generated simultaneously. Then

it can be used to encrypt and decrypt the symmetric key or data.

The physiological value to be used should be chosen carefully, as it

exhibits proper time variance and randomness. For example, blood

glucose, blood pressure or heart rate are not appropriate. On the other

hand, ECG (electrocardiogram) has been shown to be appropriate and is

19

used for resource efficient key management system in WBSN[28] The

electrocardiogram (ECG) is a technique of recording bioelectric currents

generated by the heart. Clinicians can evaluate the conditions of a

patient's heart from the ECG and perform further diagnosis. ECG records

are obtained by sampling the bioelectric currents sensed by several

electrodes, known as leads. Shu dio et al[4] has proposed the usage of

PPG for entity authentication of sensor nodes.

.

2.4 Ongoing Projects In WBSN

The architecture and design of biomedical sensor networks depend

greatly on the specific application and deployment environment. In this

section some of the latest projects done in such networks. Code Blue [29]

is a sensor network based medical research project being developed at

Harvard. It is mainly developed for usage in pre-hospital and in-hospital

emergency care, disaster response and stroke patient rehabilitation. The

sensor nodes collects heart rate (HR), oxygen saturation (SpO2), and ECG

data from the patient. These data are then relayed over a short-range

wireless network to any number of base stations like PDAs, laptops, or

ambulance-based terminals. The hardware platform used for this project

is Mica2 and the security scheme used are ECC and Tinysec.

ALARM-NET [30] is a wireless sensor network that consists of

biomedical and environmental sensors to form a heterogeneous

architecture. This project is mainly developed for pervasive, adaptive

health care applications. For data aggregation a query protocol is used.

20

This protocol allows real-time collection and processing of sensor data for

authorized care providers. And this data can be given to analysis

programs for real time analysis. The hardware platform used for this

project is Tmote Sky and the security scheme used is Hardware

encryption.

SNAP [31] is an architecture for wireless biomedical sensor networks

that focuses mainly on security. This approach does not address routing,

mobility or congestion problems in the network. In this architecture, one

or more biomedical sensors are attached to each patient. The transmitted

data are forwarded by a number of wireless relay nodes throughout the

hospital area. These nodes are categorized into unlimited-powered and

limited powered nodes. The hardware platform used for this project is

Tmote Sky and the security scheme used is TinyECC.

Another WBSN project is a Nordic project BWSN[32] that was

developed, implemented and tested at the Norwegian National Hospital.

The hardware platform used is Tmote Sky with integrated sensors, like

invasive arterial blood pressure, ECG, epicardial accelerometer and a

digital intra pleural drainage system. This system does not address

security.

Finally, the WBAN group [33] is developing wearable health

monitoring systems using off-the-shelf ZigBee wireless sensor platforms,

custom signal conditioning boards, with the TinyOS software

environment. Sensor nodes are strategically placed on the users body

21

and sample, process, and store information about users physiological

signals. The hardware platform used for this project is Tmote Sky and

the security scheme used is Hardware encryption. During session

initialization the personal server shares the encryption key with all of the

sensors.

TABLE 2.1

Security schemes used in different WBAN architectures

Name of the

System

Architecture

Hardware

platform

Security

scheme

Overhea

d

MAC

Used

Key

Agreement

CodeBlue Mica2 ECC &

Tinysec

8 Bytes Yes Predeployed

variable

ALARM-NET Tmote

Sky

Hardware

Encrytpio

n

Variable Yes Predeployed

Variable

SNAP Tmote

Sky

TinyECC Variable Yes Predeployed

Variable

BWSN Tmote

Sky

None 0 Bytes No ----

WBAN Tmote

Sky

Hardware

Encrytpio

n

4,8,16

Bytes

Yes Predeployed

Variable

22

The other security related issues like key generation and distribution

are focused on two major things. First one is for providing higher end

hardware support which can allow strong cryptographical algorithms to

run efficiently on these tiny devices. Next is to develop good random key

distribution protocols which can maximize the associated link

probabilities.

2.5 Security issues in WBSN

When designing a new security architecture for WBSN, the threats to

WBSN security, its security requirements and constraints in the network

for providing security mechanisms must be taken into account. All these

issues are discussed below.

2.5.1 Security attacks

The security threats[16] in the WBSN can be put under two major

categories (i) insider attack and (ii)outsider attack.

2.5.1.1 Insider attack

In this kind of attack[16] an attacker will try to capture the node

physically and read its memory contents get the key material and forge

node messages. After getting the keys the attacker can easily launch the

following kinds of attacks

Unauthorized access to the physiological data.

False data injection

Selective reporting

Modification of data leading to wrong diagnosis.

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2.5.1.2 Outsider Attack

Outsider attack [16] is also called as intruder node attack where

the attacker node is an unauthorized participant of the sensor network.

In this kind of attack the adversary is able to launch only the following

type of passive attacks.

Eavesdropping

Denial of service attacks

Replay attacks

2.5.2 Security requirements

A sensor network is a special purpose network. The security

requirements are similar to common network. In biomedical sensor

networks there may be multiple base stations like mobile phone, PDA or

PC that communicates with the IP network. Hence base station act as a

gateway. Base stations are said to be trustworthy because they are

physically protected or they must be of tamper resistant hardware. The

following are the security requirements in wireless biomedical sensor

network.

2.5.2.1 Data Confidentiality

Data confidentiality[15] is the most important security requirement in

WBSN. In sensor networks, the confidentiality relates to the following.

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Sensor readings in a sensor network should not be leaked to

its neighbor sensors. Especially in a medical application, the

data stored in the sensor node may be highly sensitive,

hence confidentiality security service is vital in WBAN.

In many security protocols highly sensitive data like key is

transmitted, therefore it is extremely important to build a

secure channel in a wireless sensor network.

Public sensor information, such as sensor identities and

public keys, should also be encrypted to some extent to

protect against traffic analysis attacks.

The common approach to provide confidentiality security service is to

encrypt the data with a session key that only intended receiver possess,

thereby achieving confidentiality.

2.5.2.2 Data Integrity

By providing confidentiality the data in a sensor network can be

protected against traffic analysis kind of attacks. But it does not ensure

that the data transmitted is received by the receiver as such. For

example, an attacker may be a malicious node that may insert some data

fragments. This modified packet will be sent to the receiver. Data

integrity[15] may also be lost due to the damage of packets. Thus, data

integrity ensures that any received data has not been altered in transit.

This requirement ensures that no data packet has undergone data

modification attack.

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2.5.2.3 Data Freshness

Even if confidentiality and data integrity are achieved, the

freshness[15] of each message in WBSN need to be ensured. Data

freshness tells us that the physiological data is recent so that proper

analysis is done, and it ensures that no old messages have been

replayed. This security requirement is important when WSN makes use

of shared-key strategies . Typically shared keys are changed periodically

over time. However, new shared keys that need to be propagated will take

some time interval. During this time it will be easy for an attacker to

perform replay attack. To overcome this problem a nonce which may be a

time related counter, can be included into the packet to ensure data

freshness.

2.5.2.4 Availability

The implementation of traditional cryptographic algorithms in WBSN

have high operational costs. But the approaches weaken the availability

of a sensor and sensor network for the following reasons:

Additional computation consumes additional energy. If no more

energy exists, the data will no longer be available.

Additional communication also consumes more energy. Conflict

may also occur if there is more additional communication.

26

A single point failure will be introduced if the central point

management is used. This greatly threatens the availability of the

network. The requirement of security not only affects the operation

of the network, but is also highly important in maintaining the

availability of the whole network.

Availability ensures that the DoS attack is overcome.

2.5.2.5 Authentication

An adversary is not just limited to modifying the data packet.

He/she can change the whole packet stream by injecting additional

unwanted packets. So the receiver needs to ensure that the data used in

any decision-making process originates from the intended source. The

message authentication is important for many applications in sensor

networks. Informally, data authentication allows a receiver to verify that

the data really is sent by the claimed sender. In the case of two-party

communication, data authentication can be achieved through a purely

symmetric mechanism: the sender and the receiver share a secret key to

compute the Message Authentication Code(MAC)[15] of all communicated

data.

2.5.2.6 Self-Organization

A wireless sensor network is typically an ad hoc network, which

requires every sensor node be independent and flexible enough to be self-

organizing and self-healing according to different situations. There is no

fixed infrastructure available for the purpose of network management in

27

a sensor network. This inherent feature brings a great challenge to

wireless sensor network security. The dynamics of the whole network

inhibits the idea of pre-installation of a shared key between the base

station and all sensors. Several random key pre distribution schemes

have been proposed[18][19] in the context of symmetric encryption

techniques. In the context of applying public-key cryptography

techniques in sensor networks, an efficient mechanism for public-key

distribution is necessary.

2.5.2.7 Other Security Requirements for WBSN

When designing security mechanisms that address the above

security requirements, the specific factors that are related to WBSN,

which are not required for other kinds of wireless sensor networks need

to be considered. Hence WBSN has some additional requirements that

are given below.

Multiple users in different roles i.e) the user may be a

patient/doctor/nurse/other clinicians must be supported where

each user has different privacy interests and decision making

power.

Mobility of the patient must be supported, therefore security

mechanisms should adapt quickly to dynamic topologies.

The proposed security protocol must add a low communication

overhead to the existing, since throughput is very important for such

networks. Medical data are of higher data rates, e.g. ECG data are

28

normally sampled at 250 Hz and blood pressure at 100 Hz[20].

Since the physiological data are continuously monitored, the traffic

in WBSN is also more.

Hence these security requirements must be addressed with a tradeoff

between the security of physiological data and the computational

complexity. Usage of public key (Asymmetric) cryptography is

computationally expensive and cannot be used much in the WBSN for

providing security. If applied, their computational complexity need to be

reduced. Instead of public key cryptographic algorithms, symmetric

encryption/decryption algorithms and hash functions can be used in

WBSN for providing security. However, symmetric key cryptography can

complicate the design of security architectures because they are not as

versatile as public key cryptography.

2.5.3 Security Constraints in WBSN

A typical Mica sensor node[18] processor is of 4-8 MHz, having 4KB of

RAM, 128KB flash and ideally 916 MHz of radio frequency.

Heterogeneous nature of sensor nodes is an additional limitation which

prevents one common security solution to all the nodes. In the case of

deployment nature, sensor nodes would be deployed in environments

where they would be highly prone to physical attacks. Beside node

limitations, sensor networks bring all the limitations of a mobile ad hoc

network where they lack physical infrastructure, and they rely on

insecure wireless media.

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2.5.3.1 Very Limited Resources

All kinds of security mechanisms require a certain amount of

computer resources for their implementation. The resources include

data and program memory, and battery power of the sensor. However,

currently these resources are very limited in a tiny wireless sensor.

2.5.3.1.1 Limited Memory and Storage Space

A sensor is a tiny device with only a small amount of memory and

storage space for the code. A Berkeley Mica2 sensor Mote[15] has a tiny

Atmega Microprocessor and 128 KB of programmable flash memory.

Hence, implementation of computationally intensive cryptographic

algorithms over sensor node with limited resources is infeasible. Hence

computationally intensive public key cryptographic algorithms cannot be

used for serving security in WBSNs. In order to build an effective security

mechanism, it is necessary to have a cryptographic algorithm with less

code space providing greater security.

2.5.3.1.2 Power Limitation Energy

This is the major constraint in the wireless sensor network. Since the

sensor nodes are deployed remotely, they cannot be easily replaced or

recharged because of the high operating cost. Therefore, the battery

charge taken with them to the field must be conserved to extend the life

of the individual sensor node and the entire sensor network. When

implementing a cryptographic function or protocol within a sensor node,

the energy impact of the added security code must also be considered.

30

When providing security to a sensor node, the mechanism should take

care of the battery life of the sensor. The extra power in a secured sensor

node is consumed by security functions like encryption, decryption,

digital signature generation and its verification. The additional energy

required in a secured WSN is the energy required to transmit the

security related data e.g., initialization vectors needed for

encryption/decryption, and the energy required to store security

parameters in a secure manner (e.g., cryptographic key storage).

2.5.3.2 Unreliable Communication

Since the mode of transmission in WSN is wireless, the

communication in WSN is said to be unreliable. Unreliable

communication is another threat to wireless sensor network security.

The security of the network is finally incorporated on a defined protocol,

which in turn depends on communication.

2.5.3.2.1 Unreliable Transfer

WSN does packet based connectionless routing which is unreliable.

The packets may be lost at congested nodes or get damaged because of

channel errors. The protocol in the network should have error detection

and correction mechanisms that may include a cryptographic key.

2.5.3.2.2 Conflicts

Due to the broadcast nature of the wireless sensor network the

channel may be still unreliable. Conflicts will occur if the transmitted

31

packets meet in the middle. This may be a common problem in a WSN

consisting of densely deployed sensor nodes.

2.5.3.2.3 Latency

Latency is also one of the major problems in the WSN due to

processing in the node level, congestion and multihop routing. Hence

synchronization becomes very difficult in the case of WSN.

Synchronization in WSN is very important as it is needed for secure key

management in providing the security service.

2.5.3.3 Unattended Operation

Most of the time sensor nodes are deployed in remote fields. The

sensor nodes are unattended for a long period of time which may lead to

a physical attack.

2.5.3.3.1 Exposure to Physical Attacks

The sensor may be deployed in an environment open to

adversaries, bad weather, and so on. The likelihood that a sensor suffers

a physical attack in such an environment is therefore much higher than

the typical PCs, which is located in a secure place and mainly faces

attacks from a network.

2.5.3.3.2 Managed Remotely

Since the sensor nodes are deployed in remote fields they are

managed remotely. This makes it very difficult to detect physical attacks

32

like physical tampering which is done through tamperproof seals and if

the battery goes down it cannot be replaced with greater ease.

2.5.3.3.3 No Central Management Point

A sensor network is a kind of distributed network without a central

management point. This will increase the vitality of the sensor network.

However, if designed correctly, it will make the organization of the

network robust and efficient.

2.6 Security Issues In Authentication Of The Patient To The

Hospital Server.

In a distributed computing environment, the services are dynamically

made available from the servers to the clients. In the distributed

environment every service to be provided by the server is not exposed to

the client. Instead a service agent is used to keep track of clients and to

direct the clients to get the service from the respective server. For doing

this client initially contacts the service agent by using a purchase

protocol[34]. This protocol performs the required mutual authentication

before granting the ticket to access the corresponding server to get the

service. After this step the client uses the ticket to redeem services from

the actual server using a redemption protocol[34]. Authentication

performed by the purchase protocol is the same as the peer to peer

authentication protocol. But in the redemption protocol authentication is

33

based upon possession of a ticket and knowledge of some information

recorded in the ticket. Such a ticket contains the names of the client and

the server, a key and a timestamp to indicate lifetime (similar to a login

certificate). A ticket can be used only between the specified client and

server. An example of this approach is the Kerberos [34] authentication

which makes use of password.

2.6.1 Password Based Authentication

Password based authentication is a simple and easy way of doing

authentication, but the use of password has an intrinsic weakness[7].

i.e) The human-user-chosen passwords are inherently weak owing to the

fact that most users choose short and easy to remember passwords. In

particular passwords are normally drawn from a relatively small

dictionary, so it is prone to dictionary attack. There are two types of

dictionary attacks known as online and offline[8]. In an online dictionary

attack the attackers try to login into the server by trying all possible

passwords from the dictionary until they find a correct one. In an offline

dictionary attack the attacker record a past successful login session

between a user and a server and then check all the passwords in the

dictionary against the login transcript. Offline dictionary attacks are very

difficult to deal with. As a result, tremendous effort has been dedicated to

countering offline dictionary attacks in password systems.

It has been proved that the public key techniques are absolutely

necessary to make password systems secure against offline dictionary

34

attacks, whereas the involvement of public key cryptosystems under a

PKI is not essential. This observation differentiates two separate

approaches to the development of secure password systems, one is

combined use of a password and public key cryptosystem under a PKI,

and the other is password only approach. The former takes into account

the asymmetry of capabilities between users and servers, so a user only

uses a password while the server has a public/private key pair at its

disposal. Most of the existing password systems were designed over a

single authentication server. These systems are essentially intended to

defeat offline dictionary attacks by outside attackers and assume that

the server is completely trusted in protecting the user password

database. Once this single authentication server is compromised, all the

user passwords fall into the hands of the attackers, who are definitively

effective in offline dictionary attacks against the user passwords. To

eliminate this single point of vulnerability inherent in the single server

systems, password systems based on multiple servers were proposed. In

this system the password is distributed among the multiple servers and

the attacker needs to compromise the multiple servers to be successful

in offline dictionary attacks. Brainard et al.[9] proposed a two-server

password system in which one server exposes itself to users and the

other is hidden from the public. While this two server setting is

interesting, it is not a password only system. Both servers need to have

35

public keys to protect the communication channels from users to servers

which makes it difficult to fully enjoy the benefits of a password system.

2.6.1.1 Password Based Single Server Authentication

In the single server model, as shown in figure 2.1, only one server is

involved and it keeps a database of users passwords. Most of the existing

systems make use of single server model, but the single server results in

single point of vulnerability in terms of offline dictionary attacks.

Pointcheval et al [35] has proposed a security for the systems that make

use of password authentication by a single server to overcome offline

dictionary attacks.

Fig 2.1 Single Server Password Authentication

In this a mobile user, who wish to access a network from a client

terminal, is authenticated by an authentication server using a password.

After authentication the user is provided with a secure environment for

accessing the network. However, the conventional authentication server

User 1

User n

Server

Passwd DBS

36

designs are vulnerable to password guessing attack at the server side.

Hence Ford et al.[36] proposed a new authentication server model and a

security protocol that withstands the password guessing attack. The

protocol deals with securely generating a strong secret from a password

which is considered to be a weak secret. The same authors have

proposed a new protocol called as password hardening protocol. Using

this protocol a server can interact with the user and the user‟s password

is hardened into a strong secret. Here the user‟s password or hardened

result are not exposed to the outsiders. Additional strong secrets can be

generated from the hardened passwords and these additional strong

secrets cannot be determined by a single server. The advantage of this

password hardening protocol is that a possible attacker cannot feasibly

compute the strong secret. Even if he or she has access to all information

and has control over some of the servers, but will unable to have a

control over all the servers. The drawback of this protocol is that if the

attacker comes to know the algorithm for generation of hardened

password, he or she could very well mount an offline dictionary attacks

on the server.

Pointcheval et. al [35] has proposed an Authenticated Key Exchange

System secure against dictionary attacks. Normally passwords are drawn

from a small space and the attacker could offline try to enumerate all

possible passwords. AKE is designed in such a way that it works even in

the above environment. The type of authentication used in AKE is

37

implicit authentication. The correctness of AKE is proved with the

Encrypted Key-Exchange (EKE) protocol of Bellovin and Merritt[8].

2.6.1.2 Password Based Two server and Multi Server Authentication

Fig 2.2 Two Server Password Authentication Model

In the two-server model, proposed by Yang et al.[10] consists of two

servers at the server side, one of which is a public server exposing itself

to users and the other is a back-end server as shown in figure 2.2. The

users contact only the front end server. In this model the user ends up

establishing a session key only with the public server, and the role of the

back-end server as shown in figure 2.2 is merely to assist the public

server in user authentication, wherein multi server model the user

establishes a session key with each of the servers. In the multi server

model the servers are equally exposed to outside attackers thereby the

attackers can do offline dictionary attack, while in the two-server

model[10], only the public sever is exposed to outsiders. In the two-server

User 1

User n

Service

Server SS Control

Server CS

SS

DBS

CS

DBS

38

model proposed by Yang[10], completely removes the usage of PKI to

authenticate the user. In this system the password is transformed into

two long secrets, and are stored into the front-end server known as

service server and in the back-end server known as control server. In this

system in order to uncover the passwords by offline dictionary attack

the adversary has to compromise both the control server and the service

server which is almost impossible.

In plain multi server model, the server side comprises of multiple

servers as shown in figure 2.3. Hence the single point of vulnerability is

removed by having multiple servers. In this model all the servers are

equally exposed to users and the user has to communicate in parallel

with several or all servers take part in authentication. The disadvantage

of this approach is the demand on communication bandwidth.

Fig 2.3 Multi Server Password Authentication

The gateway augmented multi server model [36] [37] is a variation on

multi server model where a gateway is kept as a relaying point between

User 1

User n

Server 1

Password

dBS Server i

Server m

39

servers and the user only needs to contact the gateway. The

disadvantage of this model is the augmented gateway which is a

overhead that just relays the messages between users and servers. This

augmented gateway does not provide any service or does not do

authentication. In the case of security architecture if there are more

components involved in the security architecture, that will be more

vulnerable points of security attacks.

2.6.2 Biometric Based Authentication

2.6.2.1 Using Single Modal Biometrics

Many of the drawbacks of the password based authentication can be

overcome by incorporation of better methods of user authentication. The

interest in biometrics [11][38] for providing authentication and

information assurance has never been greater than as it is today.

Increasingly public and private sectors are choosing biometrics to secure

their physical facilities, electronic data and computer networks.

Biometric technology is being used in a wide variety of applications

including access control, forensic investigation, identity verification,

information protection and security monitoring. Biometric solutions

identify or verify an individual‟s identity by measuring either

physiological or behavioral characteristics. In traditional cryptosystems

for providing security, user authentication is based on possession of

secret keys, which fails if the keys are not kept as secret. Further keys

40

can be stolen or forgotten or lost and thus cannot provide non-

repudiation.

Current biometric authentication systems based on physiological and

behavioral characteristics of persons (known as biometrics) such as

fingerprints, provide solutions to many of the above problems and

replace the authentication component of the traditional cryptosystems.

In providing authentication in the traditional cryptosystems the

decrypting key is sufficient to establish user authenticity. Since

cryptographic keys used in the current symmetric cryptographic

algorithms like AES[39] are long and random i.e)128 bits, hence they are

difficult to memorize. Biometrics are extremely difficult to copy, share

and distribute and require the person being authenticated to be present

at the time and point of authentication. It is very difficult to forge

biometrics and it is unlikely for an user to repudiate having accessed the

digital content using biometrics. Thus biometric based authentication is

a potential candidate to replace password based authentication either by

providing complete authentication mechanism by using pattern matching

technique or by generating traditional cryptographic keys from the

biometric.

TABLE 2.2

Comparison Of Various Biometrics [14]

Biometric Identifier Un Di Pm Co Pf Ac Ci

41

Face H L M H L H H

Finger print M H H M H M M

Hand Geometry M M M H M M M

Iris H H H M H L L

Key stroke L L L M L M M

Signature L L L M L H H

Un- Universality Pf – Performance Di– Distinct

Ci– Circumvention Pm– Permanence L – Low

Co – Collectability M – Medium H- High

There are many kinds of biometrics that are used in various types of

applications. Each biometric has its own strengths and weaknesses, and

the selection of biometric is done based on the requirement of specific

application. The comparison between various biometric features[14] is

shown in table 2.2. The basic idea behind biometric based security is

that the biometric component performs user authentication. Biometric

based authentication are of two types. In the first type of biometric based

authentication, the biometric matching is done. If a legitimate user wants

to access the digital content the user offers his biometric sample to the

system. The biometric matcher tries to match this sample with the

registered biometric template. If the sample successfully matches the

42

biometric template of user then a cryptographic key is released as shown

in the figure 2.4. This key can be used in an digital signature algorithm

to provide authentication security service. In the second type, the feature

that is extracted from the biometric can be used for generating keys[22]

which could be used in a digital signature generation algorithm for

providing authentication security service. This is shown in figure 2.5. For

example the minutia that can be extracted from the fingerprint biometric

[40] are put in the form of a template and can be used as a private key

to generate the digital signature[41] [42] using Diffie-Hellman [41]

algorithm.

Fig 2.4 Biometric Based Key Release

Biometric

Sensor

Featur

e Extrac

tor

Matcher Decision

Rejected if

no match

Key

release if matches

Biometric

sensor

Feature

Extracto

r

Key

generation

Digital

signature

algorithm

Digital

signatur

e

43

Figure 2.5 Biometric Based Digital Signature

2.6.2.2 Using Multimodal Biometrics

Since the biometrics are available in the form of signals, variations

can occur due to background noise, signal distortion, feature changes,

and environment variations. For example facial biometric can vary with

changes in facial expressions and ambient light, and fingerprint

biometrics can vary with pressure and moisture. Hence the recognition

based on a single biometric is not sufficient to provide authentication.

Hence more than one biometric can be fused to form multimodal

biometrics[38] to provide proper authentication. In multimodal

biometrics the samples are taken from multiple biometric traits using

multiple sensor technologies and are combined using fusion technology

to obtain a more reliable and accurate result. This is shown in figure 2.8.

Biometric systems are designed to make binary decisions accepting the

authorized personnel and rejecting the impostors. Two types of errors

accompany biometric systems[38] false acceptance (FA) errors, letting the

impostor in, and false rejection (FR) errors, keeping the authorized

personnel out.

Biometric

Sensor1

Feature

Extractor1 Matcher1 Decision1

Sensor

Fusion

Feature

fusion

Matcher

fusion

Decision

Fusion

44

Fig 2.6 Multimodal Biometric Fusion

The disadvantages of password based systems can be overcome by

means of biometric based security.

2.6.2.3 Biometric Based Authentication and Key Exchange

Biometrics are automated methods of identity verification or

identification based on the principle of measurable physiological or

behavioral characteristics such as a fingerprint, an iris pattern or a voice

sample. Biometric characteristics are unique and not duplicable or

transferable. Because of robust and efficient authentication biometric

information for mutual authentication and key generation is proposed.

The biometric based key generation is unforgeable to a certain extent as

biometric identities like finger print are unique to each and every

individual. This system is a biometric-only system in the sense that it

requires no users key cryptosystem and, thus, no Public Key

Infrastructure(PKI). This makes the proposed system very attractive

considering PKIs are proven notoriously expensive to deploy in real

45

world. Moreover, the proposed system is particularly suitable for online

web applications due to its efficiency in terms of both computation and

communication.

2.7 Conclusion

The security issues in wireless biomedical sensor networks and

different types of authentication and key exchange systems have been

discussed in detail in this chapter.

CHAPTER III

Energy efficient ECG based BAKE protocol for Wireless Body Area

Networks

3.1 Introduction

To make wireless biomedical sensor network infrastructure ubiquitous

and affordable, a number of challenging issues in the security of

biomedical sensors, and privacy of physiological data in the network

should be resolved. In the resource constrained WBSN the base station

collect the physiological data from the mobile patients through

biomedical sensors and then transmit it to the healthcare provider for

health monitoring [4]. This data is subjected to security attacks both at

the sensor side and also in the transit when the data is transmitted from

the sensor node to the base station. To overcome the attacks this work