addressed comments

key_protocol/src/key_management/key_tree/keys_public.rs

@@ -1,4 +1,4 @@
-use k256::elliptic_curve::{PrimeField as _};
+use k256::elliptic_curve::PrimeField as _;
 use serde::{Deserialize, Serialize};
 
 use crate::key_management::key_tree::traits::KeyTreeNode;
@@ -25,9 +25,11 @@ impl ChildKeysPublic {
                 .expect("hash_value is 64 bytes, must be safe to get first 32"),
         )
         .expect("Expect a valid Private Key");
-        let csk = nssa::PrivateKey::tweak(cssk.value()).unwrap();
+        let csk = nssa::PrivateKey::tweak(cssk.value()).expect("`key_protocol::key_management::keys_public::root()`: Invalid private key produced from `tweak`");
 
-        let ccc = *hash_value.last_chunk::<32>().unwrap();
+        let ccc = *hash_value
+            .last_chunk::<32>()
+            .expect("hash_value is 64 bytes, must be safe to get last 32");
         let cpk = nssa::PublicKey::new_from_private_key(&csk);
 
         Self {
@@ -43,22 +45,18 @@ impl ChildKeysPublic {
     pub fn nth_child(&self, cci: u32) -> Self {
         let hash_value = self.compute_hash_value(cci);
 
-        let cssk = nssa::PrivateKey::try_new(
-            (k256::Scalar::from_repr(
-                (*hash_value
-                    .first_chunk::<32>()
-                    .expect("hash_value is 64 bytes, must be safe to get first 32"))
-                .into(),
-            )
-            .expect("Expect a valid k256 scalar"))
-            .add(
-                &k256::Scalar::from_repr((*self.cssk.value()).into())
-                    .expect("Expect a valid k256 scalar"),
-            )
-            .to_bytes()
+        let lhs = k256::Scalar::from_repr(
+            (*hash_value
+                .first_chunk::<32>()
+                .expect("hash_value is 64 bytes, must be safe to get first 32"))
             .into(),
         )
-        .expect("Expect a valid private key");
+        .expect("Expect a valid k256 scalar");
+        let rhs = k256::Scalar::from_repr((*self.cssk.value()).into())
+            .expect("Expect a valid k256 scalar");
+
+        let cssk = nssa::PrivateKey::try_new(lhs.add(&rhs).to_bytes().into())
+            .expect("Expect a valid private key");
 
         let csk = nssa::PrivateKey::tweak(cssk.value()).expect("Expect a valid Private Key");
 
@@ -84,7 +82,9 @@ impl ChildKeysPublic {
 
     fn compute_hash_value(&self, cci: u32) -> [u8; 64] {
         let mut hash_input = vec![];
-        // Only support harden keys to maintain PQ resilience.
+        // Simplified key logic by only supporting harden keys.
+        // Non-harden keys would require access to untweaked public keys associated to `cssk`s.
+        // Thus, not PQ secure.
         hash_input.extend_from_slice(&[0_u8]);
         hash_input.extend_from_slice(self.cssk.value());
 

nssa/src/signature/private_key.rs

@@ -63,12 +63,16 @@ impl PrivateKey {
         &self.0
     }
 
+    /// `tweak` produces the "tweaked secret key" (`sk`) given a public account's `ssk`.
+    /// We use "tweaked keys" to shield the public accounts' `ssk` against quantum threats.
+    /// The "tweaked keys" are used for Schnorr Signatures (BIP-340).
+    /// The usage of these keys will be greatly reduced once LEE is upgraded to use a PQ signatures.
     pub fn tweak(value: &[u8; 32]) -> Result<Self, NssaError> {
         if !Self::is_valid_key(*value) {
             return Err(NssaError::InvalidPrivateKey);
         }
 
-        let sk = k256::SecretKey::from_slice(value).expect("Expect a valid secret key");
+        let sk = k256::SecretKey::from_slice(value).map_err(|_e| NssaError::InvalidPrivateKey)?;
 
         let hashed: [u8; 32] = Impl::hash_bytes(sk.public_key().to_encoded_point(true).as_bytes())
             .as_bytes()
@@ -77,11 +81,11 @@ impl PrivateKey {
 
         let sk = sk.to_nonzero_scalar();
 
-        Self::try_new(
-            sk.add(&k256::Scalar::from_repr((hashed).into()).expect("Expect a valid k256 scalar"))
-                .to_bytes()
-                .into(),
-        )
+        let scalar = k256::Scalar::from_repr(hashed.into())
+            .into_option()
+            .ok_or(NssaError::InvalidPrivateKey)?;
+
+        Self::try_new(sk.add(&scalar).to_bytes().into())
     }
 }
 
@@ -98,4 +102,33 @@ mod tests {
     fn produce_key() {
         let _key = PrivateKey::new_os_random();
     }
+
+    #[test]
+    fn tweak_rejects_zero_key() {
+        assert!(matches!(
+            PrivateKey::tweak(&[0_u8; 32]),
+            Err(NssaError::InvalidPrivateKey)
+        ));
+    }
+
+    // tweak: 0xFF…FF exceeds the secp256k1 curve order
+    #[test]
+    fn tweak_rejects_out_of_range_key() {
+        assert!(matches!(
+            PrivateKey::tweak(&[0xFF; 32]),
+            Err(NssaError::InvalidPrivateKey)
+        ));
+    }
+
+    #[test]
+    fn tweak_deterministic() {
+        let tweaked = PrivateKey::tweak(&[1_u8; 32]).unwrap();
+        assert_eq!(
+            tweaked.value(),
+            &[
+                242, 210, 33, 19, 65, 108, 136, 176, 179, 128, 110, 210, 107, 193, 168, 112, 206,
+                171, 86, 238, 131, 10, 39, 36, 44, 39, 246, 20, 46, 193, 204, 66
+            ]
+        );
+    }
 }