Look at some of the narrow band radio standards. P25 standards are not freely available, but DMR standards can be freely downloaded from ETSI [1]. P25 with 56

Look at some of the narrow band radio standards. P25 standards are not freely available, but DMR standards can be freely downloaded from ETSI [1]. P25 with 56-bit DES is well and truly broken, but I gather 256-bit AES is still okay. Also look at the signal protocol? [1] https://www.etsi.org/technologies/mobile-radio https://www.etsi.org/technologies/mobile-radio