Because of the small size, fast read write time, shock resistance and low power consumption nature of flash memory storage systems, they are preferred over traditional data storage mechanisms for permanent storage. Though, there is a high risk of losing data to unauthorized users since they are easy to transport and ubiquitous. There have been a number of researches conducted to secure the content and the access of these devices, but on the other hand an encryption system specifically designed for flash memory systems is a relatively new area of interest in computing community. We proposed a bit wise partial encryption scheme which is specifically adopted to FAST FTL. The scheme presents a light weight encryption mechanism for the protection of the contents of flash memory storage systems. In this paper we have described the detailed algorithm of the scheme, and available options for key management. We intentionally left the freedom for the implementers to decide the key length which affects the security level. Finally we have analytically determined the security level and the performance of the proposed scheme.