From 57436249489d825da2d97b8563cf1d5bac99a257 Mon Sep 17 00:00:00 2001
From: aniampio <anna.piotrowska.15@ucl.ac.uk>
Date: Fri, 9 Aug 2024 00:11:03 +0100
Subject: [PATCH] Add check for identity point in all required Coconut
 functions

---
 common/nymcoconut/src/scheme/aggregation.rs |  5 +++
 common/nymcoconut/src/scheme/issuance.rs    | 36 +++++++++++++++++++++
 common/nymcoconut/src/scheme/mod.rs         |  5 +++
 3 files changed, 46 insertions(+)

diff --git a/common/nymcoconut/src/scheme/aggregation.rs b/common/nymcoconut/src/scheme/aggregation.rs
index c97fd4b8ad..5338be2dfa 100644
--- a/common/nymcoconut/src/scheme/aggregation.rs
+++ b/common/nymcoconut/src/scheme/aggregation.rs
@@ -115,6 +115,11 @@ pub fn aggregate_signatures_and_verify(
         .map(|(&attr, beta_i)| beta_i * attr)
         .sum::<G2Projective>();
 
+    if bool::from(signature.0.is_identity()){
+        return Err(CoconutError::Aggregation(
+            "Verification of the aggregated signature failed - h is an identity point".to_string(),
+        ));
+    }
     if !check_bilinear_pairing(
         &signature.0.to_affine(),
         &G2Prepared::from((alpha + tmp).to_affine()),
diff --git a/common/nymcoconut/src/scheme/issuance.rs b/common/nymcoconut/src/scheme/issuance.rs
index 973b9ebe6e..f882962e8c 100644
--- a/common/nymcoconut/src/scheme/issuance.rs
+++ b/common/nymcoconut/src/scheme/issuance.rs
@@ -317,6 +317,11 @@ pub fn blind_sign(
 
     // Verify the commitment hash
     let h = compute_hash(blind_sign_request.commitment, public_attributes);
+    if bool::from(blind_sign_request.commitment_hash.is_identity()){
+        return Err(CoconutError::Issuance(
+            "Commitment hash should not be an identity point".to_string(),
+        ));
+    }
     if !(h == blind_sign_request.commitment_hash) {
         return Err(CoconutError::Issuance(
             "Failed to verify the commitment hash".to_string(),
@@ -385,6 +390,9 @@ pub fn verify_partial_blind_signature(
     if num_private_attributes + public_attributes.len() > partial_verification_key.beta_g2.len() {
         return false;
     }
+    if bool::from(blind_sig.0.is_identity()){
+        return false;
+    }
 
     // TODO: we're losing some memory here due to extra allocation,
     // but worst-case scenario (given SANE amount of attributes), it's just few kb at most
@@ -534,6 +542,34 @@ mod tests {
     }
 
 
+    #[test]
+    fn test_blind_sign_with_identity_commitment_hash() {
+        let params = Parameters::new(1).unwrap();
+        random_scalars_refs!(private_attributes, params, 1);
+        random_scalars_refs!(public_attributes, params, 0);
+
+        // Call the function to prepare the blind sign
+        let (_commitments_openings, blind_sign_request) = prepare_blind_sign(&params, &private_attributes, &public_attributes).unwrap();
+        let blind_sign_request = BlindSignRequest {
+            commitment_hash: G1Projective::identity(),
+            ..blind_sign_request // This copies the other fields from the existing instance
+        };
+
+        let signing_secret_key = SecretKey{
+            x: params.random_scalar(),
+            ys: vec![params.random_scalar()],
+        };
+
+        // Call blind_sign and ensure it returns an error due to identity commitment hash
+        let result = blind_sign(&params, &signing_secret_key, &blind_sign_request, &public_attributes);
+
+        // The result should be an error
+        assert!(
+            result.is_err(),
+            "blind_sign should return an error when commitment_hash is the identity point"
+        );
+    }
+
     #[test]
     fn successful_verify_partial_blind_signature() {
         let params = Parameters::new(4).unwrap();
diff --git a/common/nymcoconut/src/scheme/mod.rs b/common/nymcoconut/src/scheme/mod.rs
index 691a8dc064..ce33dbb694 100644
--- a/common/nymcoconut/src/scheme/mod.rs
+++ b/common/nymcoconut/src/scheme/mod.rs
@@ -103,6 +103,11 @@ impl Signature {
         commitment_hash: &G1Projective,
     ) -> Result<()> {
         // Verify the commitment hash
+        if bool::from(self.0.is_identity()){
+            return Err(CoconutError::Verification(
+                "Commitment hash should not be an identity point".to_string(),
+            ));
+        }
         if !(commitment_hash == &self.0) {
             return Err(CoconutError::Verification(
                 "Verification of commitment hash from signature failed".to_string(),