A feature that has become desirable for low-power mobile devices with limited computing and energy resources is the ability to select a security configuration in order to create a trade-off between security and other important parameters such as performance and energy consumption. Selective encryption can be used to create this trade-off by only encrypting chosen units of the information. In this paper, we continue the investigation of the confidentiality implications of selective encryption by applying entropy on a generic selective encryption scheme. By using the concept of run-length vector from run-length encoding theory, an expression is derived for entropy of selectively encrypted strings when the number of encrypted substrings, containing one symbol, and the order of the language change.