add bindings to RAND_bytes

Change-Id: I9ec56ae8d4ca0a28dccbf2a04c4c675791c053b8
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/58265
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>

rust/bssl-crypto/src/lib.rs

@@ -37,6 +37,9 @@ pub mod hkdf;
 /// BoringSSL implemented hmac operations.
 pub mod hmac;
 
+/// BoringSSL implemented cryptographically secure pseudo-random number generation.
+pub mod rand;
+
 #[cfg(test)]
 mod test_helpers;
 

rust/bssl-crypto/src/rand.rs

@@ -0,0 +1,41 @@
+/* Copyright (c) 2023, Google Inc.
+ *
+ * Permission to use, copy, modify, and/or distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
+ * SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION
+ * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
+ * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+use crate::CSliceMut;
+
+/// Fills buf with random bytes. In the event that sufficient random data can not be obtained,
+/// BoringSSL will abort, so the assert will never be hit.
+pub fn rand_bytes(buf: &mut [u8]) {
+    let mut ffi_buf = CSliceMut::from(buf);
+    let result = unsafe { bssl_sys::RAND_bytes(ffi_buf.as_mut_ptr(), ffi_buf.len()) };
+    assert_eq!(result, 1, "BoringSSL RAND_bytes API failed unexpectedly");
+}
+
+#[cfg(test)]
+mod tests {
+    use super::rand_bytes;
+
+    #[test]
+    fn test_rand_bytes() {
+        let mut buf = [0; 32];
+        rand_bytes(&mut buf);
+    }
+
+    #[test]
+    fn test_rand_bytes_empty() {
+        let mut buf = [];
+        rand_bytes(&mut buf);
+    }
+}