Supachai Tangwongsan and  Sathaporn Kassuvan,   A Security Model of Voice Eavesdropping Protection over Digital Networks .   journal   = {International Journal of Electrical and Computer Engineering}, [online]. World Academy of Science, Engineering and Technology.
    February 2007, vol. 2(2). 191 - 195
    [viewed 03 June 2020]. Available from: https://publications.waset.org/pdf/3240.