add comments on P equals Q case in add_mont function

Author: manishbista28

g16ckt/src/gadgets/bn254/g1.rs

@@ -156,8 +156,9 @@ impl G1Projective {
 
 impl G1Projective {
     // http://koclab.cs.ucsb.edu/teaching/ccs130h/2018/09projective.pdf
+    /// Caution: Use this function only if P != +- Q
     #[component]
-    pub fn add_montgomery<C: CircuitContext>(
+    pub(crate) fn add_montgomery<C: CircuitContext>(
         circuit: &mut C,
         p: &G1Projective,
         q: &G1Projective,
@@ -391,6 +392,9 @@ impl G1Projective {
             ));
         }
 
+        // given two bases M and N which were both known in compile time,
+        // user can not find non-zero scalars 'a' and 'b' that should give [a]M = [b]N
+        // The problem would be DLOG hard
         let mut acc = to_be_added[0].clone();
         for add in to_be_added.iter().skip(1) {
             let new_acc = Self::add_montgomery(circuit, &acc, add);

g16ckt/src/gadgets/bn254/g2.rs

@@ -207,6 +207,7 @@ impl G2Projective {
 
 impl G2Projective {
     // http://koclab.cs.ucsb.edu/teaching/ccs130h/2018/09projective.pdf
+    /// Caution: Use this function only if P != +- Q
     #[component]
     pub fn add_montgomery<C: CircuitContext>(
         circuit: &mut C,

g16ckt/src/gadgets/groth16.rs

@@ -80,6 +80,8 @@ pub fn groth16_verify<C: CircuitContext>(
 
     // Add the constant term gamma_abc_g1[0] in Montgomery form
     let gamma0_m = G1Projective::as_montgomery(vk.gamma_abc_g1[0].into_group());
+    // gamma0_m is known in compile time, user can not find a value of `msm_temp` that's equal to `gamma0_m`
+    // The problem would be DLOG hard
     let msm =
         G1Projective::add_montgomery(circuit, &msm_temp, &G1Projective::new_constant(&gamma0_m));