def scrypt(password, salt, N=SCRYPT_N, r=SCRYPT_r, p=SCRYPT_p, olen=64):
    """Returns a key derived using the scrypt key-derivarion function

    N must be a power of two larger than 1 but no larger than 2 ** 63 (insane)
    r and p must be positive numbers such that r * p < 2 ** 30

    The default values are:
    N -- 2**14 (~16k)
    r -- 8
    p -- 1

    Memory usage is proportional to N*r. Defaults require about 16 MiB.
    Time taken is proportional to N*p. Defaults take <100ms of a recent x86.

    The last one differs from libscrypt defaults, but matches the 'interactive'
    work factor from the original paper. For long term storage where runtime of
    key derivation is not a problem, you could use 16 as in libscrypt or better
    yet increase N if memory is plentiful.
    """
    check_args(password, salt, N, r, p, olen)

    out = ctypes.create_string_buffer(olen)
    ret = _libscrypt_scrypt(password, len(password), salt, len(salt),
                          N, r, p, out, len(out))
    if ret:
        raise ValueError

    return out.raw