Secure transmission of multimedia contents is computationally challenging due to the sheer volume of data involved, and achieving this by fully encrypting multimedia contents is not a viable solution for mobile devices, as they run on limited battery power and employ relatively slower processors compared to their desktop counterparts. Also, in some particular application scenarios, the value, secrecy, and/or privacy of the information is time dependent and short lived. This paper presents a highly scalable approach for securing multimedia streams on mobile devices by encrypting intelligently only selected portions of the bit stream. More importantly, the proposed solution works mainly on compressed bit stream and does not require any media decoding. It encrypts on the average less than 3% of a packet load and provides robust security equivalent to that of a fully encrypted bit stream. This solution has been implemented on desktop, laptop, netbook and Nokia N series platforms. The proposed scheme is approximately 15 times faster (on average) and processes 8 to 12 times more information for a given battery life when compared to a state of the art full encryption based solutions.