This looks like a bug/oversight as obviously 256 won't fit into an unsigned char:
[ 96%] Building CXX object test/CMakeFiles/uvgrtp_test.dir/test_5_srtp_zrtp.cpp.o
/home/vitaly/uvgRTP/test/test_5_srtp_zrtp.cpp:94:23: warning: result of comparison of constant 256 with expression of type 'uint8_t' (aka 'unsigned char') is always false [-Wtautological-constant-out-of-range-compare]
else if (key_size == 256)
~~~~~~~~ ^ ~~~
/home/vitaly/uvgRTP/test/test_5_srtp_zrtp.cpp:136:23: warning: result of comparison of constant 256 with expression of type 'uint8_t' (aka 'unsigned char') is always false [-Wtautological-constant-out-of-range-compare]
else if (key_size == 256)
~~~~~~~~ ^ ~~~
This looks like a bug/oversight as obviously 256 won't fit into an unsigned char: