The traditional face-to-face
transactions require only minimal interaction and normally do not necessitate
the use of other security and integrity mechanisms. However, for e-commerce on
the Internet, additional security and integrity mechanisms become necessary.
Security is important when data is either confidential or commercial. However,
many networks are not secure, so an eavesdropper can conveniently intercept and
capture the sensitive and valuable data that are moving in an insecure channel.
In general, the security of data
against unauthorized access can be accomplished by several methods, the first
method is based on symmetric cryptography, which provides the confidentiality (providing
the secrecy and privacy of data) and the integrity (ensuring that data cannot
be corrupted or modified, and transactions cannot be altered) of the data
(hash functions and digital signature). The second method is public key
cryptography, which provides in addition to confidentiality and the integrity
the authentication (verifying that the identity of entities is provided using
public key certificates and digital signature) and non-repudiation (ensuring
that data, cannot be renounced or a transaction denied).
These four security requirements are
provided by the Public Key Infrastructure (PKI), which is the name given to the
combination of hardware, software, people and policies with aim to manage
digital certificates (create, issue, modify, store and remove digital
certificates). A Digital Certificate associates an identity with the
private-public key pair of the owner of the identity. The main role of the PKI
is to provide a system for distributing and managing digital certificates, to
enable users of an insecure public network (such as the Internet) to securely
and privately exchange data using a public and a private cryptographic key pair
that is obtained and shared through a trusted authority.
A general purpose of the PKI raised
from the simple fact that, in order to use a public key, one should have a
guarantee that the public key is truly belonging to the entity that claims to
own it. This guarantee of authenticity is achieved by means of a certificate,
i.e., a digitally signed document binding the identity of the keyholder to its
Even though, the digital certificates are considered as the best form of
authentication, but they are hard to manage, especially in terms of certificate
validation and revocation problems. When the certificate is to be revoked, then
third parties cannot rely on that certificate unless the CA distributes
certificate status information indicating whether the certificate is currently
valid. Certificate revocation problem becomes harder when the number of PKI
users becomes large, and this problem is termed as the scalability problem.
In addition, solving this problem requires a lot of infrastructure, and the
need for this infrastructure taken as a reason against widespread
implementation of public-key cryptography and the PKI 9.
As we stated in , the currently existed PKI technologies suffer the scalability
and certificate management, making the authentication service
inefficient, particularly with devices, which are limited in their resources.
Furthermore, the implementation of PKI requires a lot of infrastructure and
high transmission costs to be operated and managed in an environment such as
the mobile banking.
Identity-based Public Key Cryptography (ID-PKC) 4 came to address these two
problems, but could not offer true non-repudiation due to the key escrow problem
3,5. In ID-PKC, an entity’s public key is derived directly from certain
aspects of its identity, for example, an IP address belonging to a network
host, or an email address associated with a user. Private keys are generated
for entities by a trusted third party called a private key generator (PKG). The
first fully practical and secure identity-based public key encryption scheme
was presented in 6. Since then, rapid development of ID-PKC has taken place. The
ID-PKC suffers from key escrow problem
that the PKG knows all users’ private keys in the system and furthermore cannot
offer true non-repudiation.
2003 Al-Riyami and Paterson 3 introduced the concept of Certificateless
Public Key Cryptography (CL-PKC) to overcome the key escrow limitation of the
identity-based public key cryptography (ID-PKC). In CL-PKC a trusted third
party called Key Generation Center (KGC) supplies a user with a partial private
key. Then, the user combines the partial private key with a secret value (that
is unknown to the KGC) to obtain his full private key. In this way the KGC does
not know the user’s private key. Then the user combines his secret value with
the KGC’s public parameters to compute his public key.
certificateless cryptography is considered a combination between PKI and identity-based
cryptography 3. It combines the best features of the PKI and ID-PKC, such as
lack of certificates, no key escrow property, reasonable trust to trust
authority and lightweight infrastructure 16. It provides a solution to the
non-repudiation problem, through enabling a user to generate his/her full
long-term private key, where the trusted third party is unable to impersonate
the user. The use of certificateless cryptography schemes have appeared in
literature, this includes the uses of certificateless encryption 5, 17;
certificateless signatures 18, 19 and 20 and certificateless signcryption
21, 22 and 23.
all the CLPKC schemes found in the literature focus on algorithms of public
parameters generation, public/private key generation of system’s parties,
encryption and decryption processes, but leaves many key problems without clear
solutions. Such problems like how the system parameters are published and
where, what the authentication method that can be used between the users and
the KGC server, what the users shall do if the KGC updates its parameters and
how they can be notified, what is the format of the elements of the CLPKC
system, and so forth. Also, there are other challenges regarding trust models,
such as to determining whether the traditional PKI trust models can be applied
to CL-PKI, whether a PKI can be migrated to CL-PKI, and whether an existing
PKI-based system can be integrated with another CL-PKI-based system. In this
chapter, an integrated model of Certificateless Public Key Infrastructure
(CL-PKI) is studied. It is assumed that there exists a Registration Authority
(RA) which is responsible for user’s registration in the system, and a Key
Generation Center (KGC) that is used to generate the system parameters and
master secret and publish the system parameters on the public directory (PD)
and keep the master secret secure.