G1/G2 rand() should produce elements of unknown exponent.

src/bls12_381/ec.rs

@@ -85,7 +85,6 @@ macro_rules! curve_impl {
         }
 
         impl $affine {
-
             fn mul_bits<S: AsRef<[u64]>>(&self, bits: BitIterator<S>) -> $projective {
                 let mut res = $projective::zero();
                 for i in bits {
@@ -95,7 +94,6 @@ macro_rules! curve_impl {
                 res
             }
 
-
             /// Attempts to construct an affine point given an x-coordinate. The
             /// point is not guaranteed to be in the prime order subgroup.
             ///
@@ -200,7 +198,18 @@ macro_rules! curve_impl {
 
         impl Rand for $projective {
             fn rand<R: Rng>(rng: &mut R) -> Self {
-                $affine::one().mul($scalarfield::rand(rng))
+                loop {
+                    let x = rng.gen();
+                    let greatest = rng.gen();
+
+                    if let Some(p) = $affine::get_point_from_x(x, greatest) {
+                        let p = p.scale_by_cofactor();
+
+                        if !p.is_zero() {
+                            return p;
+                        }
+                    }
+                }
             }
         }
 
@@ -845,7 +854,6 @@ pub mod g1 {
     }
 
     impl G1Affine {
-        #[allow(dead_code)]
         fn scale_by_cofactor(&self) -> G1 {
             // G1 cofactor = (x - 1)^2 / 3  = 76329603384216526031706109802092473003
             let cofactor = BitIterator::new([0x8c00aaab0000aaab, 0x396c8c005555e156]);
@@ -1359,7 +1367,6 @@ pub mod g2 {
             }
         }
 
-        #[allow(dead_code)]
         fn scale_by_cofactor(&self) -> G2 {
             // G2 cofactor = (x^8 - 4 x^7 + 5 x^6) - (4 x^4 + 6 x^3 - 4 x^2 - 4 x + 13) // 9
             // 0x5d543a95414e7f1091d50792876a202cd91de4547085abaa68a205b2e5a7ddfa628f1cb4d9e82ef21537e293a6691ae1616ec6e786f0c70cf1c38e31c7238e5