I recently installed Asterisk 1.8.3 (the Asterisk team now provides pre-built Debian packages at http://packages.asterisk.org).
Unfortunately, that package came without the res_srtp SRTP module. (UPDATE: Starting in 1.8.4, it does come with it.) Because I didn’t feel like re-compiling the entire package, I just took the corresponding version of res_srtp.c from the SVN, added the following lines to the beginning of it:
#ifndef AST_MODULE
#define AST_MODULE "res_srtp"
#endif
and compiled and installed it using
gcc -shared res_srtp.c -o res_srtp.so -lsrtp
sudo cp res_srtp.so /usr/lib/asterisk/modules/
You’ll need to have libsrtp0-dev and asterisk-dev installed, otherwise the compile will fail.
Then, you can do sudo asterisk -r and load the module using module load res_srtp (or just restart Asterisk).
I’m still working on getting SRTP working flawlessly both incoming and outgoing and with stuff like transfers. Asterisk Secure Calling Specifics are a good starting point, but I’m also planning to write another post about this in the near future.