Have a look at the RTMPE or RTMPS protocols. RTMPS use SSL-encryption. RTMPE is simpler than RTMPS since there's no need to acquire a SSL certificate and use industry standard Diffie-Hellman key exchange and HMACSHA256.
RTMPE put less stress on the FMS server vs RTMPS.
Utilizing SWF Verification in conjunction with one of the aforementioned protocols should also enhance security.
Check out this Adobe article for more details on SWF Verification and RTMPE
This is an old (but relevant) article from Adobe on securing streaming content.
*Not sure what issues you are going to run into at the device level in terms of protocol support. Some android devices support flash, so that may not be an issue. But these are the protocols specifically for securing streaming content.