Add ECDSA and designated verifier proof support in BBS-sharp, test to…

… show BBS-sharp works with keyed-verification accumulator and relaxing the type of accumulator from Pairing to Affine

Signed-off-by: lovesh <[email protected]>

Cargo.toml

@@ -46,6 +46,7 @@ serde_with = { version = "1.10.0", default-features = false, features = ["macros
 zeroize = { version = "1.7.0", features = ["derive"] }
 blake2 = { version = "0.10", default-features = false }
 ark-bls12-381 = { version = "^0.4.0", default-features = false, features = [ "curve" ] }
+ark-secp256r1 = { version = "^0.4.0", default-features = false }
 itertools = "0.12.1"
 
 [profile.release]

kvac/Cargo.toml

@@ -20,14 +20,14 @@ serde_with.workspace = true
 itertools.workspace = true
 dock_crypto_utils = { version = "0.20.0", default-features = false, path = "../utils" }
 schnorr_pok = { version = "0.20.0", default-features = false, path = "../schnorr_pok" }
+ark-secp256r1.workspace = true
 
 [dev-dependencies]
 blake2.workspace = true
 ark-bls12-381.workspace = true
 ark-ed25519 = { version = "^0.4.0", default-features = false }
 ark-curve25519 = { version = "^0.4.0", default-features = false }
 ark-secp256k1 = { version = "^0.4.0", default-features = false }
-ark-secp256r1 = { version = "^0.4.0", default-features = false }
 sha2 = {version = "0.10.8", default-features = false}
 
 [features]

kvac/src/bbs_sharp/README.md

@@ -3,11 +3,13 @@ BBS# as described [here](https://github.com/user-attachments/files/15905230/BBS_
 This assumes that the messages/attributes have already been prepared before signing, i.e. attributes are hashed
 with public salts, etc and whats called `H_i` in the paper is already created.
 
-Assumes that a Schnorr Signature will be generated by the user's secure hardware.
+Implements support for both Schnorr signature and ECDSA (on secp256r1) generated by the user's secure hardware.
 
 Implements both the offline and half-offline (HOL) mode.  
 In the former, the verifier is either the signer (has the secret key) or can ask the signer to verify the proof without revealing any user-specific info.  
 In the latter, the user needs to communicate with the signer before creating a proof and get "some helper data"
 to create a proof which the verifier can check without needing the secret key or interacting with the issuer.
 For efficiency and avoiding correlation (when signer and verifier collude), the user gets a batch of
-"helper data" to let him create several proofs.
+"helper data" to let him create several proofs.
+
+Implements designated verifier proof for both issuer's signature (proof of validity of MAC) and user's proof of knowledge of MAC

kvac/src/bbs_sharp/ecdsa.rs

@@ -0,0 +1,73 @@
+use ark_ec::{AffineRepr, CurveGroup};
+use ark_ff::{Field, PrimeField};
+use ark_secp256r1::{Affine, Fr, G_GENERATOR_X, G_GENERATOR_Y};
+use ark_std::{rand::RngCore, UniformRand, Zero};
+
+/// ECDSA signature
+pub struct Signature {
+    pub rand_x_coord: Fr,
+    pub response: Fr,
+}
+
+impl Signature {
+    /// Create new signature given that the message has already been hashed into a scalar
+    pub fn new_prehashed<R: RngCore>(rng: &mut R, hashed_message: Fr, secret_key: Fr) -> Self {
+        let g = Self::generator();
+        // r = k * g
+        let mut r = Affine::zero();
+        // x coordinate of r
+        let mut rand_x_coord = Fr::zero();
+        // response = 1/k * (message + secret_key * rand_x_coord)
+        let mut response = Fr::zero();
+        // response should be invertible
+        while r.is_zero() || rand_x_coord.is_zero() || response.is_zero() {
+            let mut k = Fr::rand(rng);
+            // k should be invertible
+            while k.is_zero() {
+                k = Fr::rand(rng);
+            }
+            r = (g * k).into_affine();
+            rand_x_coord = Fr::from(r.x.into_bigint());
+            response = k.inverse().unwrap() * (hashed_message + secret_key * rand_x_coord);
+        }
+        Self {
+            rand_x_coord,
+            response,
+        }
+    }
+
+    /// Verify the signature given that the message has already been hashed into a scalar
+    pub fn verify_prehashed(&self, hashed_message: Fr, public_key: Affine) -> bool {
+        let g = Self::generator();
+        let resp_inv = if let Some(inv) = self.response.inverse() {
+            inv
+        } else {
+            return false;
+        };
+        let gc = g * (resp_inv * hashed_message);
+        let yr = public_key * (resp_inv * self.rand_x_coord);
+        self.rand_x_coord == Fr::from((gc + yr).into_affine().x.into_bigint())
+    }
+
+    /// Chosen generator of the group
+    pub fn generator() -> Affine {
+        Affine::new_unchecked(G_GENERATOR_X, G_GENERATOR_Y)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use ark_std::rand::{rngs::StdRng, SeedableRng};
+
+    #[test]
+    fn sig_verify() {
+        let mut rng = StdRng::seed_from_u64(0u64);
+        let message = Fr::rand(&mut rng);
+        let g = Signature::generator();
+        let sk = Fr::rand(&mut rng);
+        let pk = (g * sk).into_affine();
+        let sig = Signature::new_prehashed(&mut rng, message, sk);
+        assert!(sig.verify_prehashed(message, pk));
+    }
+}

kvac/src/bbs_sharp/hol.rs

@@ -1,4 +1,4 @@
-//! Protocol between the user and signer to create blinded tokens in half-offline mode. Follows protocol in
+//! Protocol between the user and signer to create blinded tokens in half-offline (HOL) mode. Follows protocol in
 //! Fig. 8 but this supports batching
 
 use super::setup::{MACParams, SecretKey, SignerPublicKey, UserPublicKey};

kvac/src/bbs_sharp/mac.rs

@@ -1,5 +1,7 @@
 use crate::{
-    bbs_sharp::setup::{MACParams, SecretKey, SignerPublicKey, UserPublicKey},
+    bbs_sharp::setup::{
+        DesignatedVerifierPoKOfPublicKey, MACParams, SecretKey, SignerPublicKey, UserPublicKey,
+    },
     error::KVACError,
 };
 use ark_ec::{AffineRepr, CurveGroup};
@@ -51,6 +53,8 @@ pub struct ProofOfValidityOfMAC<G: AffineRepr> {
     pub sc_B: PokDiscreteLog<G>,
     /// For proving knowledge of secret key, i.e. `pk = g_tilde * sk`
     pub sc_pk: PokDiscreteLog<G>,
+    /// If set, then its a designated verifier proof which only the user can verify
+    pub designated_verifier_pk_proof: Option<DesignatedVerifierPoKOfPublicKey<G>>,
 }
 
 impl<G: AffineRepr> MAC<G> {
@@ -111,12 +115,14 @@ impl<G: AffineRepr> MAC<G> {
 }
 
 impl<G: AffineRepr> ProofOfValidityOfMAC<G> {
+    /// If `user_public_key` is provided, then create a designated verifier proof which only the user can verify
     pub fn new<R: RngCore, D: Digest>(
         rng: &mut R,
         mac: &MAC<G>,
         secret_key: &SecretKey<G::ScalarField>,
         public_key: &SignerPublicKey<G>,
         params: impl AsRef<MACParams<G>>,
+        user_public_key: Option<&UserPublicKey<G>>,
     ) -> Self {
         let witness = secret_key.0;
         let blinding = G::ScalarField::rand(rng);
@@ -130,10 +136,17 @@ impl<G: AffineRepr> ProofOfValidityOfMAC<G> {
             .unwrap();
         p2.challenge_contribution(&params.g_tilde, &public_key.0, &mut challenge_bytes)
             .unwrap();
-        let challenge = compute_random_oracle_challenge::<G::ScalarField, D>(&challenge_bytes);
+        // Adjust challenge if creating designated verifier proof
+        let mut challenge = compute_random_oracle_challenge::<G::ScalarField, D>(&challenge_bytes);
+        let designated_verifier_pk_proof =
+            user_public_key.map(|pk| DesignatedVerifierPoKOfPublicKey::new(rng, &pk.0, &params.g));
+        if let Some(dvp) = &designated_verifier_pk_proof {
+            challenge = challenge - dvp.challenge;
+        }
         Self {
             sc_B: p1.gen_proof(&challenge),
             sc_pk: p2.gen_proof(&challenge),
+            designated_verifier_pk_proof,
         }
     }
 
@@ -160,7 +173,12 @@ impl<G: AffineRepr> ProofOfValidityOfMAC<G> {
         self.sc_pk
             .challenge_contribution(&params.g_tilde, &signer_public_key.0, &mut challenge_bytes)
             .unwrap();
-        let challenge = compute_random_oracle_challenge::<G::ScalarField, D>(&challenge_bytes);
+        let mut challenge = compute_random_oracle_challenge::<G::ScalarField, D>(&challenge_bytes);
+        // Adjust challenge if received designated verifier proof
+        if let Some(dvp) = &self.designated_verifier_pk_proof {
+            dvp.verify(&user_public_key.0, &params.g)?;
+            challenge = challenge - dvp.challenge
+        }
         if !self.sc_B.verify(&B, &mac.A, &challenge) {
             return Err(KVACError::InvalidMACProof);
         }
@@ -197,8 +215,44 @@ mod tests {
 
         // Signer sends the following 2 items to the user
         let mac = MAC::new(&mut rng, &messages, &user_pk, &signer_sk, &params).unwrap();
-        let proof =
-            ProofOfValidityOfMAC::new::<_, Sha256>(&mut rng, &mac, &signer_sk, &signer_pk, &params);
+        let proof = ProofOfValidityOfMAC::new::<_, Sha256>(
+            &mut rng, &mac, &signer_sk, &signer_pk, &params, None,
+        );
+        assert!(proof.designated_verifier_pk_proof.is_none());
+
+        // User verifies both
+        mac.verify(&messages, &user_pk, &signer_sk, &params)
+            .unwrap();
+        proof
+            .verify::<Sha256>(&mac, &messages, &user_pk, &signer_pk, params)
+            .unwrap();
+    }
+
+    #[test]
+    fn mac_verification_with_designated_verifier_proof_of_validity() {
+        let mut rng = StdRng::seed_from_u64(0u64);
+        let message_count = 10;
+        let messages = (0..message_count)
+            .map(|_| Fr::rand(&mut rng))
+            .collect::<Vec<_>>();
+        let params = MACParams::<Affine>::new::<Sha256>(b"test", message_count);
+        let signer_sk = SecretKey::new(&mut rng);
+        let signer_pk = SignerPublicKey::new_from_params(&signer_sk, &params);
+
+        let user_sk = SecretKey::new(&mut rng);
+        let user_pk = UserPublicKey::new_from_params(&user_sk, &params);
+
+        // Signer sends the following 2 items to the user
+        let mac = MAC::new(&mut rng, &messages, &user_pk, &signer_sk, &params).unwrap();
+        let proof = ProofOfValidityOfMAC::new::<_, Sha256>(
+            &mut rng,
+            &mac,
+            &signer_sk,
+            &signer_pk,
+            &params,
+            Some(&user_pk),
+        );
+        assert!(proof.designated_verifier_pk_proof.is_some());
 
         // User verifies both
         mac.verify(&messages, &user_pk, &signer_sk, &params)

kvac/src/bbs_sharp/mod.rs

@@ -3,15 +3,19 @@
 //! This assumes that the messages/attributes have already been prepared before signing, i.e. attributes are hashed
 //! with public salts, etc and whats called `H_i` in the paper is already created.
 //!
-//! Assumes that a Schnorr Signature will be generated by the user's secure hardware.
+//! Implements support for both Schnorr signature and ECDSA (on secp256r1) generated by the user's secure hardware.
 //!
 //! Implements both the offline and half-offline (HOL) mode. In the former, the verifier is either the
 //! signer (has the secret key) or can ask the signer to verify the proof without revealing any user-specific info
 //! In the latter, the user needs to communicate with the signer before creating a proof and get "some helper data"
 //! to create a proof which the verifier can check without needing the secret key or interacting with the issuer.
 //! For efficiency and avoiding correlation (when signer and verifier collude), the user gets a batch of
 //! "helper data" to let him create several proofs.
+//!
+//! Implements designated verifier proof for both issuer's signature (proof of validity of MAC) and user's proof of
+//! knowledge of MAC
 
+mod ecdsa;
 pub mod hol;
 pub mod mac;
 pub mod proof;