[swift-evolution] [Proposal] Random Unification

Xiaodi Wu xiaodi.wu at gmail.com
Wed Oct 4 20:14:34 CDT 2017

On Wed, Oct 4, 2017 at 18:42 Ben Cohen <ben_cohen at apple.com> wrote:

> On Oct 4, 2017, at 9:20 AM, Xiaodi Wu via swift-evolution <
> swift-evolution at swift.org> wrote:
> If trapping is OK, then surely returning Optional is superior; any user
> who is OK with trapping can make that decision for themselves by writing
> `random()!`. Everyone else can then see clearly that trapping is a
> possibility, which is important.
> It’s important not to underestimate both the extent to which using `!`
> scares people, _and_ the extent to which beginners use it inappropriately.
> Both of these mean that it’s very important the standard library not
> include anything that makes force-unwrapping a common/routine operation
> (looking at you, UnsafeBufferPointer.baseAddress).

I agree; however, if given the choice of `random()` potentially trapping or
`random()!` potentially trapping, I would argue that the latter is strictly
superior to the former. If the latter option scares people, the former
should scare people more, and its nicer appearance is a bug, not a feature.

Given that arc4random, the recommended source of random numbers on Darwin,
> is always successful, requiring a force-unwrap when the user is using the
> default source doesn’t seem like the right move.

Always successful for some values of successful. We cannot design our way
out of physics.

Chris has indicated—and I agree—that the default random facilities should
do the right thing in terms of cryptographic security. Since arc4random is
guaranteed to be successful but the underlying hardware cannot be
guaranteed to always have the entropy to make it secure, it must follow
that some invocations of arc4random produce results unsuitable for
cryptographic use.

To reiterate, this is one of several options. When there isn’t enough
entropy for secure random numbers, the function can:

- block
- return an insecure result
- return nil
- throw
- trap

This is an exhaustive list, as far as I can tell. None of these options are
entirely desirable, but it is inescapable that one of these is the default,
go-to design for Swift; of course, alternatives can be offered, but the
question is, what should we tell the novice user to reach for?

On Wed, Oct 4, 2017 at 11:09 David Waite <david at alkaline-solutions.com>
> wrote:
>> On Oct 4, 2017, at 4:05 AM, Xiaodi Wu via swift-evolution <
>> swift-evolution at swift.org> wrote:
>> I agree with Felix’s concern, which is why I brought up the question, but
>> ultimately the issue is unavoidable. It’s not down to global instance or
>> not. If your source of random numbers is unseedable and may mix in
>> additional entropy at any time, then it may fail at any time because when a
>> hardware restart might happen may be transparent to the process. The user
>> must know about this or else we are laying a trap (pun intended).
>> I'm of the mindset (which might be controversial) that we should attempt
>> to expose legacy and cryptographically secure random number generators,
>> even a mixed algorithmic/entropy source like /dev/urandom, but that we
>> should not expose /dev/random at all. If someone is trying to use
>> /dev/random legitimately (such as to generate one-time-pads) they will have
>> to take into account that systems like linux still use algorithmic entropy
>> to drive /dev/random. If someone really has this sort of use case, they
>> have exceeded the bounds of the system randomness protocol.
>> Without /dev/random support as a requirement, the only failure cases I
>> know of are reading too much random data in one operation (which could be
>> solved by repeated calls) or calling before sufficient entropy has been set
>> up in /dev/urandom (such as in a system startup process). I'd be fine with
>> the second one being a special case, and such systems needing to know use
>> of the /dev/urandom -backed generator before randomness has been set up
>> will trap or return predictable information on certain platforms.
>> -DW
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