diff --git a/rust/cryptest/src/main.rs b/rust/cryptest/src/main.rs
index b607e1b..a496b2b 100644
--- a/rust/cryptest/src/main.rs
+++ b/rust/cryptest/src/main.rs
@@ -631,7 +631,7 @@ fn main() -> ! {
         assert!(o.next().unwrap() == 0x2e2b34ca);
     }
     // writeln!(stdout).unwrap();
-    o.finish(None);
+    core::mem::drop(o);
     let time_end = clock();
     writeln!(stdout, "({} kB/s)", (size as u64) * 1_000 / (time_end - time_start)).unwrap();
 
diff --git a/rust/k210-shared/src/soc/aes.rs b/rust/k210-shared/src/soc/aes.rs
index 62dc562..90e9ecf 100644
--- a/rust/k210-shared/src/soc/aes.rs
+++ b/rust/k210-shared/src/soc/aes.rs
@@ -184,6 +184,7 @@ pub struct OutIterator<'a, I>
     len: usize,
     iptr: usize,
     optr: usize,
+    finished: bool,
 }
 
 impl <'a, I> Iterator for OutIterator<'a, I>
@@ -215,6 +216,18 @@ impl <'a, I> OutIterator<'a, I>
     pub fn finish(&mut self, tag: Option<&mut [u8]>) {
         assert!(self.iptr >= self.len && self.optr >= self.len);
         finish(self.aes, self.cipher_mode, tag);
+        self.finished = true;
+    }
+}
+
+impl <'a, I> Drop for OutIterator<'a, I>
+    where I: Iterator<Item=u32> {
+    /** Implement drop so that the AES hardware operation is finished up when the iterator goes out
+     * of scope. */
+    fn drop(&mut self) {
+        if !self.finished {
+            self.finish(None);
+        }
     }
 }
 
@@ -241,5 +254,6 @@ pub fn run_iter32<'a, X>(
         len,
         iptr: 0,
         optr: 0,
+        finished: false,
     }
 }