Some fixes for minisip with respect to SRTP, RTP extensione headers and so on

[Werner Dittmann]

All,

as of today I had a first success with the ZRTP protocol. I got my
implementation to inter-operate with Phil Zimmermann's ZRTP software.
I had complete encrypted sessions, all keys were negotiated "on-the-fly",
no public key set up, no certificates required, etc. I'll check in the
stuff after some more tests and stabilization.

During these tests I found some issues with RTP extension header support
in the RTP/SRTP part of minisip and fixed it. Also the crypto operations
needed a fix to deal with the extension header. In addition the AES CTR
crypto mod used a 32bit counter where the SRTP RFC required a 16bit
counter. Fixed that as well. After these fixes minisip was able to work
with the SRTP protocol as provided by the libsrtp project (ZFONE ZRTP uses
this SRTP implementation, see http://srtp.sourceforge.net/srtp.html for
more info about this lib. BTW, the code of this lib is IMHO not very readable).

Thus, if there are any problems with AES, crypto, SRTP in minisip please
e-mail. Thanks.

To have full support of ZRTP I need a way that the media handler (media stream)
can communicate with the GUI. Can this be done via the message handler? If yes
any hint and/or support is appreciated.

Regards,
Werner