From b86c140126bb515d639ee079232ecf4ff2aa5d9f Mon Sep 17 00:00:00 2001
From: Palash Tyagi <23239946+Magnus167@users.noreply.github.com>
Date: Thu, 17 Jul 2025 23:58:36 +0100
Subject: [PATCH] Implement cryptographically secure RNG using OS entropy
 sources

---
 src/random/randomx_secure.rs | 132 +++++++++++++++++++++++++++++++++++
 1 file changed, 132 insertions(+)
 create mode 100644 src/random/randomx_secure.rs

diff --git a/src/random/randomx_secure.rs b/src/random/randomx_secure.rs
new file mode 100644
index 0000000..192faa9
--- /dev/null
+++ b/src/random/randomx_secure.rs
@@ -0,0 +1,132 @@
+//! randomx_secure.rs
+//!
+//! Cryptographically secure RNG built on operating system entropy sources.
+//! Reuses the generic API defined in `randomx.rs` via `RandomApi<OsEngine>`.
+//!
+//! Usage:
+//! ```
+//! use crate::random::randomx_secure::SecureRandomX;
+//! let mut rng = SecureRandomX::new().expect("secure rng");
+//! let x = rng.normal(0.0, 1.0);
+//! ```
+
+#![allow(dead_code)]
+
+use super::randomx::{Engine, RandomApi}; // reuse everything
+
+/* =============================================================================
+ * Platform-specific secure entropy
+ * ========================================================================== */
+
+const BUF_LEN: usize = 4096;
+
+pub struct OsEngine {
+    buf: [u8; BUF_LEN],
+    idx: usize,
+}
+
+impl core::fmt::Debug for OsEngine {
+    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
+        f.debug_struct("OsEngine")
+            .field("remaining", &((BUF_LEN - self.idx) as u64))
+            .finish()
+    }
+}
+
+impl OsEngine {
+    /// Create a new engine, prefilled with OS randomness.
+    pub fn new() -> std::io::Result<Self> {
+        let mut eng = Self {
+            buf: [0u8; BUF_LEN],
+            idx: BUF_LEN, // force immediate fill
+        };
+        eng.refill()?;
+        Ok(eng)
+    }
+
+    #[inline]
+    fn need_refill(&self) -> bool {
+        self.idx >= BUF_LEN
+    }
+
+    fn refill(&mut self) -> std::io::Result<()> {
+        #[cfg(unix)]
+        {
+            use std::fs::File;
+            use std::io::Read;
+            let mut f = File::open("/dev/urandom")?;
+            f.read_exact(&mut self.buf)?;
+        }
+        #[cfg(windows)]
+        {
+            // Call RtlGenRandom (SystemFunction036). Safe wrapper.
+            unsafe {
+                if !rtl_gen_random(self.buf.as_mut_ptr(), self.buf.len()) {
+                    return Err(std::io::Error::new(
+                        std::io::ErrorKind::Other,
+                        "RtlGenRandom failed",
+                    ));
+                }
+            }
+        }
+        self.idx = 0;
+        Ok(())
+    }
+}
+
+impl Engine for OsEngine {
+    fn next_u64(&mut self) -> u64 {
+        if self.need_refill() {
+            // Best effort: panic if refill fails; alternatively propagate error by redesigning Engine.
+            self.refill().expect("OS RNG refill failed");
+        }
+        // read 8 bytes little-endian
+        let bytes = &self.buf[self.idx..self.idx + 8];
+        self.idx += 8;
+        u64::from_le_bytes(bytes.try_into().unwrap())
+    }
+}
+
+/* ----- type alias & constructors for user convenience ----- */
+
+pub type SecureRandomX = RandomApi<OsEngine>;
+
+impl SecureRandomX {
+    /// Get a crypto-secure RNG seeded from the OS.
+    pub fn new() -> std::io::Result<Self> {
+        OsEngine::new().map(RandomApi::from_engine)
+    }
+}
+
+/* =============================================================================
+ * Windows FFI (no external crate)
+ * ========================================================================== */
+#[cfg(windows)]
+unsafe fn rtl_gen_random(buf: *mut u8, len: usize) -> bool {
+    // SystemFunction036 in advapi32 (undocumented alias RtlGenRandom).
+    // Signature: BOOLEAN SystemFunction036(PVOID RandomBuffer, ULONG RandomBufferLength);
+    #[link(name = "advapi32")]
+    extern "system" {
+        fn SystemFunction036(RandomBuffer: *mut core::ffi::c_void, RandomBufferLength: u32)
+            -> u8;
+    }
+    let ok = SystemFunction036(buf as *mut _, len as u32);
+    ok != 0
+}
+
+/* =============================================================================
+ * Tests (these will actually read OS randomness; mark ignore if needed)
+ * ========================================================================== */
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn secure_draw() {
+        let mut rng = SecureRandomX::new().unwrap();
+        // Just make sure it runs and varies
+        let a = rng.u64();
+        let b = rng.u64();
+        assert_ne!(a, b);
+    }
+}