Add OpenBSD support (Absolucy#40)

Author: n1000

README.md

@@ -73,6 +73,7 @@ _Listed in order of priority_
   we will attempt to source as much entropy as possible via our [`rdseed_entropy`](entropy::rdseed_entropy) function
 * Linux and Android will attempt to use the [`getrandom`](https://man7.org/linux/man-pages/man2/getrandom.2.html) syscall.
 * macOS and iOS (Darwin-based systems) will use Security.framework's [`SecRandomCopyBytes`](https://developer.apple.com/documentation/security/1399291-secrandomcopybytes).
+* OpenBSD will attempt to use the [`arc4random_buf`](https://man.openbsd.org/arc4random.3) function.
 * Windows
   * If we're targeting UWP, then the [`BCryptGenRandom`](https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom) is used with system-preferred RNG (`BCRYPT_USE_SYSTEM_PREFERRED_RNG`).
   * Otherwise, we'll use [`RtlGenRandom`](https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom).

src/entropy.rs

@@ -8,6 +8,8 @@ pub use darwin::entropy as system;
 	not(feature = "getrandom")
 ))]
 pub use linux::entropy as system;
+#[cfg(all(target_os = "openbsd", not(feature = "getrandom")))]
+pub use openbsd::entropy as system;
 #[cfg(all(windows, not(target_vendor = "uwp"), not(feature = "getrandom")))]
 pub use windows::entropy as system;
 #[cfg(all(windows, target_vendor = "uwp", not(feature = "getrandom")))]
@@ -32,6 +34,10 @@ pub mod windows_uwp;
 /// An entropy generator for Windows, using WinAPI's `RtlGenRandom` function.
 pub mod windows;
 
+#[cfg(all(target_os = "openbsd", not(feature = "getrandom")))]
+/// An entropy generator for OpenBSD, using libc's `arc4random_buf` function.
+pub mod openbsd;
+
 #[cfg(feature = "getrandom")]
 /// Pull in system entropy using the [`getrandom`](https://crates.io/crates/getrandom) crate.
 /// Uses backup entropy (rdseed and system time) if it fails.
@@ -47,6 +53,7 @@ pub fn system(out: &mut [u8]) {
 	feature = "getrandom",
 	target_os = "linux",
 	target_os = "android",
+	target_os = "openbsd",
 	target_vendor = "apple",
 	windows
 )))]

src/entropy/openbsd.rs

@@ -0,0 +1,13 @@
+use core::ffi::c_void;
+
+extern "C" {
+	fn arc4random_buf(buf: *mut c_void, nbytes: usize);
+}
+
+/// Obtain a series of random bytes.
+pub fn entropy(out: &mut [u8]) -> bool {
+	unsafe {
+		arc4random_buf(out.as_mut_ptr() as *mut c_void, out.len());
+		true
+	}
+}

src/lib.rs

@@ -78,6 +78,7 @@
 //!   we will attempt to source as much entropy as possible via our [`rdseed_entropy`](entropy::rdseed_entropy) function
 //! * Linux and Android will attempt to use the [`getrandom`](https://man7.org/linux/man-pages/man2/getrandom.2.html) syscall.
 //! * macOS and iOS (Darwin-based systems) will use Security.framework's [`SecRandomCopyBytes`](https://developer.apple.com/documentation/security/1399291-secrandomcopybytes).
+//! * OpenBSD will attempt to use the [`arc4random_buf`](https://man.openbsd.org/arc4random.3) function.
 //! * Windows
 //!   * If we're targeting UWP, then the [`BCryptGenRandom`](https://docs.microsoft.com/en-us/windows/win32/api/bcrypt/nf-bcrypt-bcryptgenrandom) is used with system-preferred RNG (`BCRYPT_USE_SYSTEM_PREFERRED_RNG`).
 //!   * Otherwise, we'll use [`RtlGenRandom`](https://docs.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom).