sentence: operating environment zrtp protocol has been implemented and used on the following platforms : windows , linux , mac os x , iphone , symbian , blackberry os , android zrtp protocol has been implemented in the following languages : c , c++ , java zrtp protocol has been used successfully on the following transport media : wifi , umts , edge , gprs , satellite ip modem , gsm csd , isdn implementations zrtp has been implemented as gnu zrtp which is used in twinkle and sflphone gnu zrtp4j which is used in jitsi ( formerly sip communicator ) .