fix: avoid mutating self in TagShare::add (#748)

Author: themighty1

crates/mpc-tls/src/record_layer/aead/ghash.rs

@@ -287,9 +287,12 @@ struct TagShare([u8; 16]);
 impl Add for TagShare {
     type Output = Vec<u8>;
 
-    fn add(mut self, rhs: Self) -> Self::Output {
-        self.0.iter_mut().zip(rhs.0).for_each(|(a, b)| *a ^= b);
-        self.0.to_vec()
+    fn add(self, rhs: Self) -> Self::Output {
+        self.0
+            .iter()
+            .zip(rhs.0)
+            .map(|(a, b)| *a ^ b)
+            .collect::<Vec<_>>()
     }
 }
 

crates/mpc-tls/src/record_layer/aead/ghash/compute.rs

@@ -95,7 +95,7 @@ impl Task for ComputeTags {
                 let tags = tag_shares
                     .into_iter()
                     .zip(follower_tag_shares)
-                    .map(|(a, b)| (a + b).to_vec())
+                    .map(|(a, b)| a + b)
                     .collect();
 
                 Some(tags)