crypto_kdf_derive_from_key segfaults on ARM

[encukou]

test_crypto_kdf_derive_from_key segfaults on the ARM architecture due to incorrect use of ctypes.

The information about argument/return types of C function is present in .h files that aren't available at run time. So, when using ctypes, you need to declare argument types (technically only if different from defaults – but it's better to do it always). See ctypes documentation.

For kdf_derive_from_key specifically, this should probably be:

    nacl.crypto_kdf_derive_from_key.argtypes = [
        ctypes.c_char_p, ctypes.c_size_t, ctypes.c_ulonglong, ctypes.c_char_p, ctypes.c_char_p,
    ]

Without that, subkey_id is treated as C int instead of uint64_t. This is most likely causing #122 (Cannot use 64bit subkey ID) and the segfault on ARM architecture.

Take the above fix with a grain of NaCl though. I'm not really a ctypes expert.
You probably know about tools like Cython or CFFI, which can use info in .h files to avoid problems like this.