Satellites larger than cubesats usually use rad-hardened processors, e.g. in Europe the LEON processors are quite common. These are basically rad-hardened SPARC

Satellites larger than cubesats usually use rad-hardened processors, e.g. in Europe the LEON processors are quite common. These are basically rad-hardened SPARC v8 processors. They are also clocked quite low since they are often implemented using a FPGA. So, a rather old RISC architecture, lacking any cryptography extensions/intrinsics, running at a rather low clock frequency means that it is not that easy to fit authentication and cryptography in software (at least at the necessary rates). To get authentication/encryption in these systems you need a separate crypto unit or implement AES-GCM/AES-HMAC in FPGA (if you have room).