[ruby-core:88737] [Ruby trunk Bug#15039] Random.urandom and SecureRandom arc4random use

Issue #15039 has been updated by Freaky (Thomas Hurst).


shyouhei (Shyouhei Urabe) wrote:
> Freaky (Thomas Hurst) wrote:
> > The other is clarifying the intent of `Random.urandom` and the priorities of `SecureRandom`.  Following #9569, *are* they meant to be avoiding using userspace CSPRNGs?  They do so on Linux and Windows, but don't on platforms with arc4random().
> 
> As far as I understand Random.urandom intends to be cryptographically secure; nothing more.

Doesn't the name rather waggle its eyebrows at you while glancing pointedly at /dev/urandom?  Why else call it that?

> My attempt when fixing #9569 was that arc4random() supposedly adequately fulfilled this property like OpenBSD's.  I did not see any reason to reject such thing.  Maybe is it getting hard for a "userspace CSPRNG" in general to make sense these days?  That is a possible situation and I have to change my mind then.

Userspace is always going to be a bit riskier - the state is stored in the process itself rather than isolated in the kernel, fork() can trigger difficult to detect edge-cases (hopefully nobody breaks minherit()!), and their implementations tend to have a more, er, "varied" history.

The payoff, of course, is some orders of magnitude better performance.  I'd like that option, but I'm not sure `SecureRandom` should be taking it by default.

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Bug #15039: Random.urandom and SecureRandom arc4random use
https://bugs.ruby-lang.org/issues/15039#change-73790

* Author: Freaky (Thomas Hurst)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
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Random.urandom defaults to arc4random() on a lot of platforms, including FreeBSD.

On all currently released versions of FreeBSD, arc4random() is, as the name suggests, a dubious ARC4-based userspace PRNG dating from circa 1997.  Given the entire point of #9569 was that using the userspace CSPRNG in OpenSSL over /dev/urandom or equivalent is a bad idea, this seems to mean it's regressed to an *even worse* state on these platforms.  Even in cases where it's using something more modern (FreeBSD 12, OpenBSD), it's still a userspace CSPRNG.

If that's fine, we might as well *pick a known-good one* and use that everywhere.  Like, say, OpenSSL's.

Since the conclusion of #9569 seems to have been otherwise, I'd suggest dropping arc4random() as a potential source for Random.urandom due to it not matching the desired semantics.

Rust's OsRng seems a good template for alternative _syscall implementations: https://docs.rs/rand/0.5.5/rand/rngs/struct.OsRng.html#platform-sources



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