While fuzz testing is good and all, when it comes to cryptography, the input spaces is so large that chances of finding something are even worse than finding a needle in a hay stack.

For instance here the keys are going to be around 256 bits in a size, so if your fuzzer is just picking keys at random, your basically never likely to pick zero at random.

With cryptographic primitives you really should be testing all known invalid input parameters for the particular algorithm. A a random fuzzer is not going to know that. Additionally, you should be testing inputs that can cause overflows and are handled correctly ect...