Fix check in signature verification

Resolves #7

CHANGELOG.md

@@ -11,13 +11,18 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Change the implementation for hashing a slice of bytes into a BlsScalar to `BlsScalar::hash_to_scalar` [#3]
 
+### Added
+
+- Add `is_valid` check for `PublicKey` [#7]
+
 ## [0.1.0] - 2024-01-08
 
 ### Added
 
 - Add initial commit, this package continues the development of [dusk-bls12_381-sign](https://github.com/dusk-network/bls12_381-sign/) at version `0.6.0` under the new name: `bls12_381-bls` and without the go related code.
 
 <!-- ISSUES -->
+[#7]: https://github.com/dusk-network/bls12_381-bls/issues/7
 [#3]: https://github.com/dusk-network/bls12_381-bls/issues/3
 
 <!-- VERSIONS -->

src/error.rs

@@ -15,6 +15,8 @@ pub enum Error {
     BytesError(DuskBytesError),
     /// Cryptographic invalidity
     InvalidSignature,
+    /// Invalid Point
+    InvalidPoint,
 }
 
 impl From<DuskBytesError> for Error {
@@ -30,6 +32,9 @@ impl fmt::Display for Error {
             Self::InvalidSignature => {
                 write!(f, "Invalid Signature")
             }
+            Self::InvalidPoint => {
+                write!(f, "Invalid Point")
+            }
         }
     }
 }

src/keys/public.rs

@@ -52,6 +52,9 @@ impl PublicKey {
     /// Verify a [`Signature`] by comparing the results of the two pairing
     /// operations: e(sig, g_2) == e(Hₒ(m), pk).
     pub fn verify(&self, sig: &Signature, msg: &[u8]) -> Result<(), Error> {
+        if !self.is_valid() || !sig.is_valid() {
+            return Err(Error::InvalidPoint);
+        }
         let h0m = h0(msg);
         let p1 = dusk_bls12_381::pairing(&sig.0, &G2Affine::generator());
         let p2 = dusk_bls12_381::pairing(&h0m, &self.0);

src/signature.rs

@@ -30,6 +30,36 @@ impl Signature {
             }),
         )
     }
+
+    /// Returns true if the inner point is free of an $h$-torsion component, and
+    /// so it exists within the $q$-order subgroup $\mathbb{G}_2$. This
+    /// should always return true unless an "unchecked" API was used.
+    pub fn is_torsion_free(&self) -> bool {
+        self.0.is_torsion_free().into()
+    }
+
+    /// Returns true if the inner point is on the curve. This should always
+    /// return true unless an "unchecked" API was used.
+    pub fn is_on_curve(&self) -> bool {
+        self.0.is_on_curve().into()
+    }
+
+    /// Returns true if the inner point is the identity (the point at infinity).
+    pub fn is_identity(&self) -> bool {
+        self.0.is_identity().into()
+    }
+
+    /// Returns true if the inner point is valid according to certain criteria.
+    ///
+    /// A [`Signature`] is considered valid if its inner point meets the
+    /// following conditions:
+    /// 1. It is free of an $h$-torsion component and exists within the
+    ///    $q$-order subgroup $\mathbb{G}_2$.
+    /// 2. It is on the curve.
+    /// 3. It is not the identity.
+    pub fn is_valid(&self) -> bool {
+        self.is_torsion_free() && self.is_on_curve() && !self.is_identity()
+    }
 }
 
 impl Serializable<48> for Signature {

tests/signature.rs

@@ -4,7 +4,8 @@
 //
 // Copyright (c) DUSK NETWORK. All rights reserved.
 
-use bls12_381_bls::{PublicKey, SecretKey, APK};
+use bls12_381_bls::{Error, PublicKey, SecretKey, APK};
+use dusk_bls12_381::BlsScalar;
 use rand::rngs::StdRng;
 use rand::{RngCore, SeedableRng};
 
@@ -178,6 +179,17 @@ fn sign_verify_aggregated_incorrect_apk() {
     assert!(apk.verify(&agg_sig, &msg).is_err());
 }
 
+#[test]
+fn sign_verify_identity_fails() {
+    let rng = &mut StdRng::seed_from_u64(0xbeef);
+    let msg = random_message(rng);
+    let sk = SecretKey::from(BlsScalar::zero());
+    let pk = PublicKey::from(&sk);
+    let sig = sk.sign_vulnerable(&msg);
+
+    assert_eq!(pk.verify(&sig, &msg).unwrap_err(), Error::InvalidPoint);
+}
+
 fn random_message(rng: &mut StdRng) -> [u8; 100] {
     let mut msg = [0u8; 100];