Security and reliability of cloud computing services remain among the dominant concerns inhibiting their pervasive adaptation. The distributed and the multi-tenancy nature of the cloud computing paradigm can be considered as the root causes for their increased risks and vulnerabilities. Resource sharing and virtualization can also be mentioned as additional main factors contributing to or augmenting cross-site scripting and other cloud vulnerabilities. Cloud are also exposed to the risks and liabilities faced by other networked systems. Poorly designed APIs that may cause security problems or distributed denial of services attacks are the examples of this category that are considered in this paper. Public key infrastructure provides the foundations for provision of some essential security services. These include services such as confidentiality, authentication, and privacy that are of vital importance for establishing trust and confidence between the cloud providers and their clients. In this work, we will discuss the potential flaws of this infrastructure and examine how they may deteriorate the security and reliability levels of the cloud environments. To enable a comprehensive study of the challenges in security and reliability of the cloud computing environments, we categorize the risks and vulnerabilities they face. Traditional techniques, based on cryptography, can address some of these challenges to a certain degree. We will argue that they may not be efficient for use in cloud environments. We then focus on data-centric and homomorphic encryption methods that may provide more appropriate solutions in addressing the challenges in cloud computing security and reliability