ml-dsa: Add support for external mu (#1023)

ml-dsa/src/lib.rs

@@ -373,12 +373,18 @@ impl<P: MlDsaParams> SigningKey<P> {
         // the concatenated M'.
         // XXX(RLB) Should the API represent this as an input?
         let mu = message_representative(&self.tr, Mp);
+        self.raw_sign_mu(&mu, rnd)
+    }
 
+    fn raw_sign_mu(&self, mu: &B64, rnd: &B32) -> Signature<P>
+    where
+        P: MlDsaParams,
+    {
         // Compute the private random seed
         let rhopp: B64 = H::default()
             .absorb(&self.K)
             .absorb(rnd)
-            .absorb(&mu)
+            .absorb(mu)
             .squeeze_new();
 
         // Rejection sampling loop
@@ -389,7 +395,7 @@ impl<P: MlDsaParams> SigningKey<P> {
 
             let w1_tilde = P::encode_w1(&w1);
             let c_tilde = H::default()
-                .absorb(&mu)
+                .absorb(mu)
                 .absorb(&w1_tilde)
                 .squeeze_new::<P::Lambda>();
             let c = sample_in_ball(&c_tilde, P::TAU);
@@ -448,6 +454,24 @@ impl<P: MlDsaParams> SigningKey<P> {
         Ok(self.sign_internal(Mp, &rnd))
     }
 
+    /// This method reflects the randomized ML-DSA.Sign algorithm with a pre-computed μ.
+    ///
+    /// # Errors
+    ///
+    /// This method can return an opaque error if it fails to get enough randomness.
+    // Algorithm 2 ML-DSA.Sign (optional pre-computed μ variant)
+    #[cfg(feature = "rand_core")]
+    pub fn sign_mu_randomized<R: TryCryptoRng + ?Sized>(
+        &self,
+        mu: &B64,
+        rng: &mut R,
+    ) -> Result<Signature<P>, Error> {
+        let mut rnd = B32::default();
+        rng.try_fill_bytes(&mut rnd).map_err(|_| Error::new())?;
+
+        Ok(self.raw_sign_mu(mu, &rnd))
+    }
+
     /// This method reflects the optional deterministic variant of the ML-DSA.Sign algorithm.
     ///
     /// # Errors
@@ -458,6 +482,14 @@ impl<P: MlDsaParams> SigningKey<P> {
         self.raw_sign_deterministic(&[M], ctx)
     }
 
+    /// This method reflects the optional deterministic variant of the ML-DSA.Sign algorithm with a
+    /// pre-computed μ.
+    // Algorithm 2 ML-DSA.Sign (optional deterministic and pre-computed μ variant)
+    pub fn sign_mu_deterministic(&self, mu: &B64) -> Signature<P> {
+        let rnd = B32::default();
+        self.raw_sign_mu(mu, &rnd)
+    }
+
     fn raw_sign_deterministic(&self, M: &[&[u8]], ctx: &[u8]) -> Result<Signature<P>, Error> {
         if ctx.len() > 255 {
             return Err(Error::new());
@@ -653,7 +685,13 @@ impl<P: MlDsaParams> VerifyingKey<P> {
     {
         // Compute the message representative
         let mu = message_representative(&self.tr, Mp);
+        self.raw_verify_mu(&mu, sigma)
+    }
 
+    fn raw_verify_mu(&self, mu: &B64, sigma: &Signature<P>) -> bool
+    where
+        P: MlDsaParams,
+    {
         // Reconstruct w
         let c = sample_in_ball(&sigma.c_tilde, P::TAU);
 
@@ -667,19 +705,25 @@ impl<P: MlDsaParams> VerifyingKey<P> {
 
         let w1p_tilde = P::encode_w1(&w1p);
         let cp_tilde = H::default()
-            .absorb(&mu)
+            .absorb(mu)
             .absorb(&w1p_tilde)
             .squeeze_new::<P::Lambda>();
 
         sigma.c_tilde == cp_tilde
     }
 
-    /// This algorithm reflect the ML-DSA.Verify algorithm from FIPS 204.
+    /// This algorithm reflects the ML-DSA.Verify algorithm from FIPS 204.
     // Algorithm 3 ML-DSA.Verify
     pub fn verify_with_context(&self, M: &[u8], ctx: &[u8], sigma: &Signature<P>) -> bool {
         self.raw_verify_with_context(&[M], ctx, sigma)
     }
 
+    /// This algorithm reflects the ML-DSA.Verify algorithm with a pre-computed μ from FIPS 204.
+    // Algorithm 3 ML-DSA.Verify (optional pre-computed μ variant)
+    pub fn verify_mu(&self, mu: &B64, sigma: &Signature<P>) -> bool {
+        self.raw_verify_mu(mu, sigma)
+    }
+
     fn raw_verify_with_context(&self, M: &[&[u8]], ctx: &[u8], sigma: &Signature<P>) -> bool {
         if ctx.len() > 255 {
             return false;
@@ -1062,4 +1106,70 @@ mod test {
         many_round_trip_test::<MlDsa65>();
         many_round_trip_test::<MlDsa87>();
     }
+
+    #[test]
+    fn sign_mu_verify_mu_round_trip() {
+        fn sign_mu_verify_mu<P>()
+        where
+            P: MlDsaParams,
+        {
+            let kp = P::key_gen_internal(&Array::default());
+            let sk = kp.signing_key;
+            let vk = kp.verifying_key;
+
+            let M = b"Hello world";
+            let rnd = Array([0u8; 32]);
+            let mu = message_representative(&sk.tr, &[&[M]]);
+            let sig = sk.raw_sign_mu(&mu, &rnd);
+
+            assert!(vk.raw_verify_mu(&mu, &sig));
+        }
+        sign_mu_verify_mu::<MlDsa44>();
+        sign_mu_verify_mu::<MlDsa65>();
+        sign_mu_verify_mu::<MlDsa87>();
+    }
+
+    #[test]
+    fn sign_mu_verify_internal_round_trip() {
+        fn sign_mu_verify_internal<P>()
+        where
+            P: MlDsaParams,
+        {
+            let kp = P::key_gen_internal(&Array::default());
+            let sk = kp.signing_key;
+            let vk = kp.verifying_key;
+
+            let M = b"Hello world";
+            let rnd = Array([0u8; 32]);
+            let mu = message_representative(&sk.tr, &[&[M]]);
+            let sig = sk.raw_sign_mu(&mu, &rnd);
+
+            assert!(vk.verify_internal(&[M], &sig));
+        }
+        sign_mu_verify_internal::<MlDsa44>();
+        sign_mu_verify_internal::<MlDsa65>();
+        sign_mu_verify_internal::<MlDsa87>();
+    }
+
+    #[test]
+    fn sign_internal_verify_mu_round_trip() {
+        fn sign_internal_verify_mu<P>()
+        where
+            P: MlDsaParams,
+        {
+            let kp = P::key_gen_internal(&Array::default());
+            let sk = kp.signing_key;
+            let vk = kp.verifying_key;
+
+            let M = b"Hello world";
+            let rnd = Array([0u8; 32]);
+            let mu = message_representative(&sk.tr, &[&[M]]);
+            let sig = sk.sign_internal(&[M], &rnd);
+
+            assert!(vk.raw_verify_mu(&mu, &sig));
+        }
+        sign_internal_verify_mu::<MlDsa44>();
+        sign_internal_verify_mu::<MlDsa65>();
+        sign_internal_verify_mu::<MlDsa87>();
+    }
 }