Not only the number of bits in the seed, but also their unpredictability. There are many things that can go wrong. Some examples: - the seed is timer-based an

Not only the number of bits in the seed, but also their unpredictability. There are many things that can go wrong. Some examples: - the seed is timer-based and has far less entropy (information) than its bit size - the seed is OK, but the generator is somehow weak (e.g., only made with linear components, or throws away parts of the seed, or outputs too much of its internal state) - Bugs, bugs everywhere (cute example: an RC4-based generator that uses the XOR trick to swap elements) No standard statistical test would catch those (it would catch the latter, actually), even though the security of the generator would be gravely diminished.