[Windows] Replace CryptGenRandom with BCryptGenRandom

[AronParker]

CryptGenRandom is currently marked as deprecated:
https://msdn.microsoft.com/en-us/library/windows/desktop/aa379942(v=vs.85).aspx

It has been replaced with:
https://msdn.microsoft.com/en-us/library/windows/desktop/aa375458(v=vs.85).aspx

The new API is compatible with Vista+ and additionally since Rust officially only supports Windows 7 afaik, you could even take a little shortcut by passing the flag BCRYPT_USE_SYSTEM_PREFERRED_RNG, which enables omitting hAlgorithm and therefore requires no additional handles.

[dhardy]

I know nothing about the Windows APIs so I'll cc @retep998.

[retep998]

While the compiler itself has a minimum version of Windows 7, the standard library tries to run on Windows XP, even if not everything is usable. It would be unfortunate if rand could not be used by projects targeting Windows Vista/XP, given how rand is such a common transitive dependency.

[AronParker]

I see, fair enough. Hopefully BCryptGenRandom uses RtlGenRandom internally but I guess for staying with it for compatibilities sake is probably for the better.

[Phyllostachys]

Not that this matters much since this seems fairly settled but I was tinkering with this in C++, compiling with clang on Win10. Using BCryptGenRandom and RtlGenRandom resulted in binaries that are about the same size:

So, I'm thinking that maybe your hope is true. :)

[AronParker]

@Phyllostachys Cheers for letting me know! Stuff like this obviously still keeps me up at night (not exactly but still I care! hehe) but it's good to know, but yeah sadly we don't know for sure given that the sizes although it's good enough. :)

[dhardy]

I suppose the correct method may be to detect the OS version at runtime, then use the appropriate function. But personally this isn't going to keep me up at night ;-)

[xtqqczze]

I believe Rust 1.49 was the last version to support Windows XP, so this issue could be reopened.

See also: rust-lang/compiler-team#378

[newpavlov]

@xtqqczze
Windows XP support in gerandom was dropped long ago. See its readme for the currently used APIs on supported targets.

[xtqqczze]

@newpavlov Right, so there's nothing blocking replacing RtlGenRandom with BCryptGenRandom?

[xtqqczze]

Actually never mind, BCryptGenRandom has "stability issues".

[dhardy]

I believe Rust 1.49 was the last version to support Windows XP, so this issue could be reopened.

This issue certainly won't be re-opened since OS integration has moved to https://github.com/rust-random/getrandom/