You misunderstand; if you can't even decrypt the binary, you can't disassemble, much less run a decompiler over it.
As someone who has done quite a bit of reverse engineering work, I have no idea how I'd identify and isolate a vulnerability found by fuzzing without the ability to even look at the machine code.
If it runs, it has to be decrypted (at a current level of cryptography); at most it is obfuscated and the access is blocked by some hardware tricks which may be costly to circumvent, but there is nothing fundamental stopping you.
