Short Message Service (SMS) is a text messaging service component of mobile communication systems. It uses standardized communications protocols to exchange short text between mobile devices. SMS does not have any built-in procedure to offer security for the text transmitted as data. Most of the applications for mobile devices are designed and developed without taking security into consideration. In practical use, SMS messages are not encrypted by default during transmission. Therefore, a data confidentiality approach to SMS on Android will be developed in the paper. It includes design, implementation and confidentiality measurement of RC4 stream cipher for SMS data confidentiality on mobile networks.