No, building random number generators is easy; the reason arc4random was (it's been fixed) insecure was because arc4 was insecure. If you have a secure stream

No, building random number generators is easy; the reason arc4random was (it's been fixed) insecure was because arc4 was insecure. If you have a secure stream cipher, you have a secure RNG. Not "can build", they're the same thing. If you only have block ciphers, the security proof is less trivial, but in practice "decrypt /dev/zero" still works. If you don't have a secure (symmetric) cipher, you have bigger crypographic problems than bad random numbers.