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//! # Random key generation
//!
//! This module wraps the RNG provided by the OS. There are a few different
//! ways to interface with the OS RNG so it's worth exploring each of the options.
//! Note that at the time of writing these all go through the (undocumented)
//! `bcryptPrimitives.dll` but they use different route to get there.
//!
//! Originally we were using [`RtlGenRandom`], however that function is
//! deprecated and warns it "may be altered or unavailable in subsequent versions".
//!
//! So we switched to [`BCryptGenRandom`] with the `BCRYPT_USE_SYSTEM_PREFERRED_RNG`
//! flag to query and find the system configured RNG. However, this change caused a small
//! but significant number of users to experience panics caused by a failure of
//! this function. See [#94098].
//!
//! The current version falls back to using `BCryptOpenAlgorithmProvider` if
//! `BCRYPT_USE_SYSTEM_PREFERRED_RNG` fails for any reason.
//!
//! [#94098]: https://github.com/rust-lang/rust/issues/94098
//! [`RtlGenRandom`]: https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom
//! [`BCryptGenRandom`]: https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom
use crate::mem;
use crate::ptr;
use crate::sys::c;
/// Generates high quality secure random keys for use by [`HashMap`].
///
/// This is used to seed the default [`RandomState`].
///
/// [`HashMap`]: crate::collections::HashMap
/// [`RandomState`]: crate::collections::hash_map::RandomState
pub fn hashmap_random_keys() -> (u64, u64) {
Rng::SYSTEM.gen_random_keys().unwrap_or_else(fallback_rng)
}
struct Rng {
algorithm: c::BCRYPT_ALG_HANDLE,
flags: u32,
}
impl Rng {
const SYSTEM: Self = unsafe { Self::new(ptr::null_mut(), c::BCRYPT_USE_SYSTEM_PREFERRED_RNG) };
/// Create the RNG from an existing algorithm handle.
///
/// # Safety
///
/// The handle must either be null or a valid algorithm handle.
const unsafe fn new(algorithm: c::BCRYPT_ALG_HANDLE, flags: u32) -> Self {
Self { algorithm, flags }
}
/// Open a handle to the RNG algorithm.
fn open() -> Result<Self, c::NTSTATUS> {
use crate::sync::atomic::AtomicPtr;
use crate::sync::atomic::Ordering::{Acquire, Release};
// An atomic is used so we don't need to reopen the handle every time.
static HANDLE: AtomicPtr<crate::ffi::c_void> = AtomicPtr::new(ptr::null_mut());
let mut handle = HANDLE.load(Acquire);
if handle.is_null() {
let status = unsafe {
c::BCryptOpenAlgorithmProvider(
&mut handle,
c::BCRYPT_RNG_ALGORITHM.as_ptr(),
ptr::null(),
0,
)
};
if c::nt_success(status) {
// If another thread opens a handle first then use that handle instead.
let result = HANDLE.compare_exchange(ptr::null_mut(), handle, Release, Acquire);
if let Err(previous_handle) = result {
// Close our handle and return the previous one.
unsafe { c::BCryptCloseAlgorithmProvider(handle, 0) };
handle = previous_handle;
}
Ok(unsafe { Self::new(handle, 0) })
} else {
Err(status)
}
} else {
Ok(unsafe { Self::new(handle, 0) })
}
}
fn gen_random_keys(self) -> Result<(u64, u64), c::NTSTATUS> {
let mut v = (0, 0);
let status = unsafe {
let size = mem::size_of_val(&v).try_into().unwrap();
c::BCryptGenRandom(self.algorithm, ptr::addr_of_mut!(v).cast(), size, self.flags)
};
if c::nt_success(status) { Ok(v) } else { Err(status) }
}
}
/// Generate random numbers using the fallback RNG function
#[inline(never)]
fn fallback_rng(rng_status: c::NTSTATUS) -> (u64, u64) {
match Rng::open().and_then(|rng| rng.gen_random_keys()) {
Ok(keys) => keys,
Err(status) => {
panic!("RNG broken: {rng_status:#x}, fallback RNG broken: {status:#x}")
}
}
}
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