In pcurves point mul, handle a missing/unseeded RNG

In certain contexts we may not have an RNG available and yet still need to perform a point multiplication.
randombit · Jul 12, 2024 · fb67ece · fb67ece
1 parent 1af5545
commit fb67ece
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Showing 2 changed files with 29 additions and 3 deletions.
diff --git a/src/lib/math/pcurves/pcurves_impl.h b/src/lib/math/pcurves/pcurves_impl.h
@@ -815,6 +815,10 @@ class ProjectiveCurvePoint {
       }
 
       void randomize_rep(RandomNumberGenerator& rng) {
+         if(!rng.is_seeded()) {
+            return;
+         }
+
          auto r = FieldElement::random(rng);
 
          auto r2 = r.square();
@@ -966,7 +970,22 @@ class BlindedScalarBits final {
             constexpr size_t n_words = C::NW.size();
 
             uint8_t maskb[mask_bytes] = {0};
-            rng.randomize(maskb, mask_bytes);
+            if(rng.is_seeded()) {
+               rng.randomize(maskb, mask_bytes);
+            } else {
+               // If we don't have an RNG we don't have many good options. We
+               // could just omit the blinding entirely, but this changes the
+               // size of the blinded scalar, which we're expecting otherwise is
+               // knowable at compile time. So generate a mask by XORing the
+               // bytes of the scalar together. At worst, it's equivalent to
+               // omitting the blinding entirely.
+
+               std::array<uint8_t, C::Scalar::BYTES> sbytes;
+               scalar.serialize_to(sbytes);
+               for(size_t i = 0; i != sbytes.size(); ++i) {
+                  maskb[i % mask_bytes] ^= sbytes[i];
+               }
+            }
 
             W mask[n_words] = {0};
             load_le(mask, maskb, mask_words);

diff --git a/src/tests/test_pcurves.cpp b/src/tests/test_pcurves.cpp
@@ -29,15 +29,22 @@ class Pcurve_Basemul_Tests final : public Text_Based_Test {
          const auto P_bytes = vars.get_req_bin("P");
 
          auto& rng = Test::rng();
+         Botan::Null_RNG null_rng;
 
          if(auto curve = Botan::PCurve::PrimeOrderCurve::from_name(group_id)) {
             if(auto scalar = curve->deserialize_scalar(k_bytes)) {
                auto pt2 = curve->mul_by_g(scalar.value(), rng).to_affine().serialize();
                result.test_eq("mul_by_g correct", pt2, P_bytes);
 
+               auto pt3 = curve->mul_by_g(scalar.value(), null_rng).to_affine().serialize();
+               result.test_eq("mul_by_g (Null_RNG) correct", pt3, P_bytes);
+
                auto g = curve->generator();
-               auto pt3 = curve->mul(g, scalar.value(), rng).to_affine().serialize();
-               result.test_eq("mul correct", pt3, P_bytes);
+               auto pt4 = curve->mul(g, scalar.value(), rng).to_affine().serialize();
+               result.test_eq("mul correct", pt4, P_bytes);
+
+               auto pt5 = curve->mul(g, scalar.value(), null_rng).to_affine().serialize();
+               result.test_eq("mul correct", pt5, P_bytes);
             } else {
                result.test_failure("Curve rejected scalar input");
             }