Please can you tell me, if Kiax has some security mechanisms for media-stream and signaling data?
Kiax supports only MD5 based authentication. It uses IAX2 protocol using a modified version of iaxclient (iaxclient.sourceforge.net) that currently has no encryption mechanisms. In IAX2 signaling and media are passed through a single channel using UDP datagrams. Theoretically you should be able to implement encryption on packet level and use the same mechanisms in your asterisk server and the clients. Currently there is no standard way of doing this..
I noticed that:
As of asterisk version 1.2.4 (maybe before) there is a rather undocumented channel encryption feature included in chan_iax2. After successful authentication the whole channel including control data and voice data is encrypted with AES128.
As noted in:
Is this supported?
It is not. It is not supported by libiaxclient and until it gets there officially kiax will likely not support it..