diff --git a/pvmfw/src/rand.rs b/pvmfw/src/rand.rs
index 2824cbd..a53cac6 100644
--- a/pvmfw/src/rand.rs
+++ b/pvmfw/src/rand.rs
@@ -12,6 +12,80 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+use crate::hvc;
+use core::fmt;
+use core::mem::size_of;
+
+pub enum Error {
+    /// Error during SMCCC TRNG call.
+    Trng(hvc::trng::Error),
+    /// Unsupported SMCCC TRNG version.
+    UnsupportedVersion((u16, u16)),
+}
+
+impl From<hvc::trng::Error> for Error {
+    fn from(e: hvc::trng::Error) -> Self {
+        Self::Trng(e)
+    }
+}
+
+pub type Result<T> = core::result::Result<T, Error>;
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        match self {
+            Self::Trng(e) => write!(f, "SMCCC TRNG error: {e}"),
+            Self::UnsupportedVersion((x, y)) => {
+                write!(f, "Unsupported SMCCC TRNG version v{x}.{y}")
+            }
+        }
+    }
+}
+
+/// Configure the source of entropy.
+pub fn init() -> Result<()> {
+    match hvc::trng_version()? {
+        (1, _) => Ok(()),
+        version => Err(Error::UnsupportedVersion(version)),
+    }
+}
+
+fn fill_with_entropy(s: &mut [u8]) -> Result<()> {
+    const MAX_BYTES_PER_CALL: usize = size_of::<hvc::TrngRng64Entropy>();
+    let bits = usize::try_from(u8::BITS).unwrap();
+
+    let (aligned, remainder) = s.split_at_mut(s.len() - s.len() % MAX_BYTES_PER_CALL);
+
+    for chunk in aligned.chunks_exact_mut(MAX_BYTES_PER_CALL) {
+        let (r, s, t) = hvc::trng_rnd64((chunk.len() * bits).try_into().unwrap())?;
+
+        let mut words = chunk.chunks_exact_mut(size_of::<u64>());
+        words.next().unwrap().clone_from_slice(&t.to_ne_bytes());
+        words.next().unwrap().clone_from_slice(&s.to_ne_bytes());
+        words.next().unwrap().clone_from_slice(&r.to_ne_bytes());
+    }
+
+    if !remainder.is_empty() {
+        let mut entropy = [0; MAX_BYTES_PER_CALL];
+        let (r, s, t) = hvc::trng_rnd64((remainder.len() * bits).try_into().unwrap())?;
+
+        let mut words = entropy.chunks_exact_mut(size_of::<u64>());
+        words.next().unwrap().clone_from_slice(&t.to_ne_bytes());
+        words.next().unwrap().clone_from_slice(&s.to_ne_bytes());
+        words.next().unwrap().clone_from_slice(&r.to_ne_bytes());
+
+        remainder.clone_from_slice(&entropy[..remainder.len()]);
+    }
+
+    Ok(())
+}
+
+pub fn random_array<const N: usize>() -> Result<[u8; N]> {
+    let mut arr = [0; N];
+    fill_with_entropy(&mut arr)?;
+    Ok(arr)
+}
+
 #[no_mangle]
 extern "C" fn CRYPTO_sysrand_for_seed(out: *mut u8, req: usize) {
     CRYPTO_sysrand(out, req)
@@ -19,5 +93,7 @@
 
 #[no_mangle]
 extern "C" fn CRYPTO_sysrand(out: *mut u8, req: usize) {
-    #![allow(unused_variables)] // TODO(b/262393451): Fill with actual entropy.
+    // SAFETY - We need to assume that out points to valid memory of size req.
+    let s = unsafe { core::slice::from_raw_parts_mut(out, req) };
+    let _ = fill_with_entropy(s);
 }