refactor: add comments for all .zeroize() places

Author: vinhtc27

rln/src/circuit/mod.rs

@@ -250,15 +250,15 @@ impl IdSecret {
     pub(crate) fn to_u256(&self) -> U256 {
         let mut big_int = self.0.into_bigint();
         let res = U256::from_limbs(big_int.0);
-        big_int.zeroize();
+        big_int.zeroize(); // wipe the secret limbs after copying them into the leaked U256
         res
     }
 }
 
 impl From<&mut Fr> for IdSecret {
     fn from(value: &mut Fr) -> Self {
         let id_secret = Self(*value);
-        value.zeroize();
+        value.zeroize(); // clear the caller-owned source Fr after the secret moved into IdSecret
         id_secret
     }
 }

rln/src/protocol/keygen.rs

@@ -17,7 +17,7 @@ pub fn keygen() -> (IdSecret, Fr) {
     let identity_secret = IdSecret::rand(&mut rng);
     let mut to_hash = [*identity_secret.clone()];
     let id_commitment = poseidon_hash(&to_hash);
-    to_hash[0].zeroize();
+    to_hash[0].zeroize(); // wipe the identity secret copy from the stack buffer
     (identity_secret, id_commitment)
 }
 
@@ -33,7 +33,7 @@ pub fn extended_keygen() -> (Fr, Fr, Fr, Fr) {
     let identity_secret = poseidon_hash_pair(identity_trapdoor, identity_nullifier);
     let mut to_hash = [identity_secret];
     let id_commitment = poseidon_hash(&to_hash);
-    to_hash[0].zeroize();
+    to_hash[0].zeroize(); // wipe the identity secret copy from the stack buffer
     (
         identity_trapdoor,
         identity_nullifier,
@@ -58,7 +58,7 @@ pub fn seeded_keygen(signal: &[u8]) -> (Fr, Fr) {
     let identity_secret = Fr::rand(&mut rng);
     let mut to_hash = [identity_secret];
     let id_commitment = poseidon_hash(&to_hash);
-    to_hash[0].zeroize();
+    to_hash[0].zeroize(); // wipe the identity secret copy from the stack buffer
     (identity_secret, id_commitment)
 }
 
@@ -81,7 +81,7 @@ pub fn extended_seeded_keygen(signal: &[u8]) -> (Fr, Fr, Fr, Fr) {
     let identity_secret = poseidon_hash_pair(identity_trapdoor, identity_nullifier);
     let mut to_hash = [identity_secret];
     let id_commitment = poseidon_hash(&to_hash);
-    to_hash[0].zeroize();
+    to_hash[0].zeroize(); // wipe the identity secret copy from the stack buffer
     (
         identity_trapdoor,
         identity_nullifier,

rln/src/protocol/proof.rs

@@ -144,7 +144,7 @@ impl From<&RLNWitnessInputSingle> for RLNProofValuesSingle {
         let a_1 = poseidon_hash(&to_hash);
         let y = *(a_0.clone()) + w.x * a_1;
         let nullifier = poseidon_hash(&[a_1]);
-        to_hash[0].zeroize();
+        to_hash[0].zeroize(); // wipe the identity secret copy from the stack buffer
         RLNProofValuesSingle {
             y,
             root,
@@ -206,7 +206,7 @@ impl From<&RLNWitnessInputMulti> for RLNProofValuesMulti {
             let selector = Fr::from(selected);
             let y = (*w.identity_secret + w.x * a_1) * selector;
             let nullifier = poseidon_hash(&[a_1]) * selector;
-            to_hash[0].zeroize();
+            to_hash[0].zeroize(); // wipe the identity secret copy from the stack buffer
             ys.push(y);
             nullifiers.push(nullifier);
         }