Hi!

I happen to have just given a talk on urandom internals at 32C3: https://media.ccc.de/v/32c3-7441-the_plain_simple_reality_of_entropy

The main takeaway about urandom is that it's a CSPRNG, just like the OpenSSL one. (Just better, because the kernel can protect the entropy pool memory space better, the kernel one is continuously reseeded, and has a better security track record--see Debian.) So there is no security advantage, or actually any theoretical difference in using OpenSSL. In practice, urandom is significantly more secure.

What Ruby currently does is chaining two CSPRNGs, urandom and OpenSSL's. This makes problems twice as likely (not half), because now if any of the two fail, the entire system is compromised. If you just read from urandom, there's only one thing that can go wrong.

Another bad consequence of this scheme is that now there is no reliable failure detection. Take the case of exhausted file descriptors, where opening urandom is impossible, and let's say there's no getrandom support. An exception should be raised, as there's no recovery. But the current code will first ignore the failure of raw_seed, then call into OpenSSL which will hide the failure (which I argue is a OpenSSL API shortcoming, but I digress). If you just read from urandom, it's very clear when the system failed. (By the way, why adding the nanotime and pid at all? Nanotime and pid alone are not enough, and hide brokenness Debian-style, while if you have a real 16 byte of seed, that's enough and you don't need anything else.)

Finally, complexity is toxic. I believe that pre-7104a473, on Windows, when OpenSSL is installed, securerandom was only seeded with the time. (Because OpenSSL would take priority over the syscall, and OpenSSL is documented to seed itself automatically only on platforms that offer urandom. http://linux.die.net/man/3/rand_add I haven't checked the source.) This would be spectacularly bad (and I would actually be dropping a vulnerability right now considering that the patch is only one month old), but I'm not even sure that's the case after staring at code for a good hour. It's significantly bad that a reasonably experienced developer can't be sure of securerandom correctness easily. If you just read from urandom, it takes less than 30 minutes to audit the code.

I realize that we are some Internet strangers arguing against the Linux man page (which for the record was already recently amended to be FUD'y on urandom), but since this is all Open Source, we can thankfully discuss on the actual implementation instead of on what is written about it. If instead authority is what you are looking for, this slide in my 32C3 presentation has plenty, academic and not: https://speakerdeck.com/filosottile/the-plain-simple-reality-of-entropy-at-32c3?slide=36

Finally, I invite you to check how other libraries like BoringSSL, Python's stdlib and Go's stdlib solve this problem. They all read from /dev/urandom.