[swift-evolution] [Proposal] Random Unification

Xiaodi Wu xiaodi.wu at gmail.com
Sat Dec 2 20:02:01 CST 2017

On Sat, Dec 2, 2017 at 19:02 Greg Parker <gparker at apple.com> wrote:

> On Dec 2, 2017, at 1:09 PM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
> On Sat, Dec 2, 2017 at 6:00 AM, Brent Royal-Gordon <brent at architechies.com
> > wrote:
>> On Dec 1, 2017, at 10:37 PM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> That said, I am not sure that this proposal should give any pretense of
>> being suitable for cryptographic use. On implementation, the code will not
>> have been audited for that purpose, although the author very rightly
>> attempts to use the "best" possible sources of entropy available for each
>> platform. Perhaps explicitly _not_ supporting cryptographic operations is
>> the more Swifty way to go (in the sense that, where possible, Swift API
>> design aims to help users avoid footguns).
>> People will use it for crypto whether we want them to or not.
> People are going to do all sorts of unanticipated things, sure. But this
> doesn't mean that we shouldn't consider how we may best encourage users to
> avoid _unintentional_ common pitfalls.
> There are options to explore here. Consider, for example--and I'm not
> suggesting that we be this verbose, but it is illustrative of the
> trade-offs which are possible to keep in mind--if certain methods were very
> clear that the result is *pseudorandom* and potentially *insecure*:
> `(0..<9).insecurePseudorandomElement`. Clearly, fewer people would use this
> method for crypto.
> But what *should* they use instead of our API? The OS-provided CSPRNG is
> almost certainly going to be the most secure thing available in the absence
> of specialized hardware. We should not deliberately scare users away from
> our API if there is nothing better on offer.

It’s not about OS-provided CSPRNG. It’s about the design and implementation
of _this proposal_ on top of the CSPRNG.

Earlier, we discussed how this API should minimize the number of optional
return values to improve user ergonomics. Instead, the returned value
should be a reasonable best-effort at randomness. This is sensible for a
general-use API, but it is unsuitable for a crypto-oriented API.

David Waite criticizes GameplayKit as lacking in crypto-oriented
functions—the implication being that we ought to provide them here. I
disagree. My point is that our entire design process has been geared
towards a reasonable, best-effort general-use API: It is being designed by
community members who are not specialized in this particular subfield. It
is explicitly being design for common, general use cases in mind. And the
implementation will come with no guarantee as to suitability for crypto,
nor should it have to.

Therefore, I reason, we ought not to extend the design with functions that
are explicitly geared towards crypto using primitive operations that we
haven’t audited for such suitability. Instead, we ought to make clear to
users both the features and the limitations of this API, to encourage use
where suitable and to discourage use where unsuitable.
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