We think that cosmic background radiation and certain subatomic mechanics are truly random. If you attach extra hardware to a computer, you can observe these and send a stream of bits to the computer.<p>You don&#x27;t usually want to use truly random numbers for cryptography though; &#x2F;dev&#x2F;random is more appropriate. I&#x27;ve never understood why, exactly, but that&#x27;s generally the advice.