Datasheet

32 Part I  Overview of Commerce
for the credentials sent by the user. There are two types of overheads
associated with such systems. There is an overhead of generating the
CRLs to send to the access controller. However, the more significant
overhead is that each time the access controller performs an access
check it needs to see whether there is a CRL associated with each user
credential. This may lead to a loss of scalability, especially if there are
a huge number of users.
Passive Revocation Mechanism:

This type of revocation mechanism is
accomplished through expiration times provided in most certificates.
During the generation of certificates, an expiration time is provided
after which the certificate is deemed invalid. In terms of scalability,
the passive revocation mechanisms are better than their active coun-
terparts. However, the scalability comes at a cost. Let’s assume that
the certificate is issued at time T and expires at time T + t. Now let’s
assume that the user is compromised just after T. Then for a period of
t, the adversary is capable of compromising the system further. If t is
small, then the system is more secure. However, smaller t also indi-
cates that more authorizations are required, reducing the scalability of
the system. Therefore, there is a trade-off between the scalability and
security, which is tuned by the choice of the time t.
Threats
One school of thought defines security purely based on the types and categories
of threats it should protect against. One of the most important security threats
existing in any infrastructure is the malicious service disruption created by adver-
saries. Many examples exist in the Internet space where servers and networks
are brought down by a huge amount of network traffic, and users are denied
access to a service. Because cloud computing deployment has not reached the
“critical mass” yet (that is, not every single service provider on the Internet is
deploying cloud computing), the service level attacks are still limited. However,
with cloud computing poised for a huge growth in the next few years, this area
is of the utmost concern. The cloud service level security issues can be further
subdivided into two main types: denial of service (DoS) and Distributed Denial
of Service (DDoS) issues; and Quality of Service (QoS) violation issues, which in
turn is the forced QoS violation by the adversary through congestion, delay-
ing or dropping packets, or through resource hacking.. The D/DoS is the more
dangerous of the two where access to a certain service is denied.
DoS
The solutions proposed for DoS (denial of service) attacks can be categorized
into two primary types: preventive solutions and reactive solutions. Preventive
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