[swift-evolution] [Proposal] Random Unification

Stephen Canon scanon at apple.com
Fri Sep 8 19:50:54 CDT 2017

> On Sep 8, 2017, at 8:09 PM, Xiaodi Wu via swift-evolution <swift-evolution at swift.org> wrote:
> This topic has been broached on Swift Evolution previously. It's interesting to me that Steve Canon is so certain that CSPRNGs are the way to go. I wasn't aware that hardware CSPRNGs have come such a long way and are so ubiquitous as to be feasible as a basis for Swift random numbers. If so, great.
> Otherwise, if there is any way that a software, non-cryptographically secure PRNG is going to outperform a CSPRNG, then I think it's worthwhile to have a (carefully documented) choice between the two. I would imagine that for many uses, such as an animation in which you need a plausible source of noise to render a flame, whether that is cryptographically secure or not is absolutely irrelevant but performance may be key.

Let me be precise: it is absolutely possible to outperform CSPRNGs. They have simply become fast enough that the performance gap doesn’t matter for most uses (let’s say amortized ten cycles per byte or less—whatever you are going to do with the random bitstream will be much more expensive than getting the bits was).

That said, yes, there should definitely be other options. It should be possible for users to get reproducible results from a stdlib random interface run-to-run, and also across platforms. That alone requires that at least one other option for a generator be present. There may also be a place for a very high-throughput generator like xorshiro128+.

All I’m really saying is that the *default* generator should be an os-provided unseeded CSPRNG, and we should be very careful about documenting any generator options.

– Steve
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