fix: ensure dummy values are on the curve for MSM (#7653)

This PR fix an issue with MSM when trying to verify a circuit having MSM
blackbox, because dummy values are not on the curve.

I also added a BB unit test which ensure non-constant inputs are
properly handled in the 'no witness assignment' case

barretenberg/cpp/src/barretenberg/dsl/acir_format/acir_format.cpp

@@ -176,7 +176,7 @@ void build_constraints(Builder& builder,
     // Add multi scalar mul constraints
     for (size_t i = 0; i < constraint_system.multi_scalar_mul_constraints.size(); ++i) {
         const auto& constraint = constraint_system.multi_scalar_mul_constraints.at(i);
-        create_multi_scalar_mul_constraint(builder, constraint);
+        create_multi_scalar_mul_constraint(builder, constraint, has_valid_witness_assignments);
         track_gate_diff(constraint_system.gates_per_opcode,
                         constraint_system.original_opcode_indices.multi_scalar_mul_constraints.at(i));
     }

barretenberg/cpp/src/barretenberg/dsl/acir_format/multi_scalar_mul.cpp

@@ -10,7 +10,10 @@ namespace acir_format {
 
 using namespace bb;
 
-template <typename Builder> void create_multi_scalar_mul_constraint(Builder& builder, const MultiScalarMul& input)
+template <typename Builder>
+void create_multi_scalar_mul_constraint(Builder& builder,
+                                        const MultiScalarMul& input,
+                                        bool has_valid_witness_assignments)
 {
     using cycle_group_ct = stdlib::cycle_group<Builder>;
     using cycle_scalar_ct = typename stdlib::cycle_group<Builder>::cycle_scalar;
@@ -25,9 +28,26 @@ template <typename Builder> void create_multi_scalar_mul_constraint(Builder& bui
         field_ct point_x;
         field_ct point_y;
         bool_ct infinite;
+
         point_x = to_field_ct(input.points[i], builder);
         point_y = to_field_ct(input.points[i + 1], builder);
         infinite = bool_ct(to_field_ct(input.points[i + 2], builder));
+
+        // When we do not have the witness assignments, we set is_infinite value to true if it is not constant
+        // else default values would give a point which is not on the curve and this will fail verification
+        if (!has_valid_witness_assignments) {
+            if (!input.points[i + 2].is_constant) {
+                builder.variables[input.points[i + 2].index] = fr(1);
+            } else if (input.points[i + 2].value == fr::zero() &&
+                       !(input.points[i].is_constant || input.points[i + 1].is_constant)) {
+                // else, if is_infinite is false, but the coordinates (x, y) are witness
+                // then we set their value so to a curve point.
+                auto g1 = bb::grumpkin::g1::affine_one;
+                builder.variables[input.points[i].index] = g1.x;
+                builder.variables[input.points[i + 1].index] = g1.y;
+            }
+        }
+
         cycle_group_ct input_point(point_x, point_y, infinite);
         // Reconstruct the scalar from the low and high limbs
         field_ct scalar_low_as_field;
@@ -69,8 +89,10 @@ template <typename Builder> void create_multi_scalar_mul_constraint(Builder& bui
 }
 
 template void create_multi_scalar_mul_constraint<UltraCircuitBuilder>(UltraCircuitBuilder& builder,
-                                                                      const MultiScalarMul& input);
+                                                                      const MultiScalarMul& input,
+                                                                      bool has_valid_witness_assignments);
 template void create_multi_scalar_mul_constraint<MegaCircuitBuilder>(MegaCircuitBuilder& builder,
-                                                                     const MultiScalarMul& input);
+                                                                     const MultiScalarMul& input,
+                                                                     bool has_valid_witness_assignments);
 
 } // namespace acir_format

barretenberg/cpp/src/barretenberg/dsl/acir_format/multi_scalar_mul.hpp

@@ -21,6 +21,9 @@ struct MultiScalarMul {
     friend bool operator==(MultiScalarMul const& lhs, MultiScalarMul const& rhs) = default;
 };
 
-template <typename Builder> void create_multi_scalar_mul_constraint(Builder& builder, const MultiScalarMul& input);
+template <typename Builder>
+void create_multi_scalar_mul_constraint(Builder& builder,
+                                        const MultiScalarMul& input,
+                                        bool has_valid_witness_assignments);
 
 } // namespace acir_format

barretenberg/cpp/src/barretenberg/dsl/acir_format/multi_scalar_mul.test.cpp

@@ -0,0 +1,116 @@
+#include "multi_scalar_mul.hpp"
+#include "acir_format.hpp"
+#include "acir_format_mocks.hpp"
+#include "acir_to_constraint_buf.hpp"
+#include "barretenberg/numeric/uint256/uint256.hpp"
+#include "barretenberg/plonk/composer/ultra_composer.hpp"
+#include "barretenberg/plonk/proof_system/types/proof.hpp"
+#include "barretenberg/plonk/proof_system/verification_key/verification_key.hpp"
+
+#include <cstdint>
+#include <gtest/gtest.h>
+#include <vector>
+
+namespace acir_format::tests {
+
+using namespace bb;
+using Composer = plonk::UltraComposer;
+
+class MSMTests : public ::testing::Test {
+  protected:
+    static void SetUpTestSuite() { srs::init_crs_factory("../srs_db/ignition"); }
+};
+using fr = field<Bn254FrParams>;
+
+/**
+ * @brief Create a circuit testing the a simple scalar mul with a constant generator
+ *
+ */
+TEST_F(MSMTests, TestMSM)
+{
+    MultiScalarMul msm_constrain{
+        .points = { WitnessOrConstant<fr>{
+                        .index = 0,
+                        .value = fr(1),
+                        .is_constant = true,
+                    },
+                    WitnessOrConstant<fr>{
+                        .index = 0,
+                        .value = fr("0x0000000000000002cf135e7506a45d632d270d45f1181294833fc48d823f272c"),
+                        .is_constant = true,
+                    },
+                    WitnessOrConstant<fr>{
+                        .index = 0,
+                        .value = fr(0),
+                        .is_constant = true,
+                    } },
+        .scalars = { WitnessOrConstant<fr>{
+                         .index = 0,
+                         .value = fr(std::string("0x000000000000000000000000000000000000000000000000000000616c696365")),
+                         .is_constant = false,
+                     },
+                     WitnessOrConstant<fr>{
+                         .index = 0,
+                         .value = fr(0),
+                         .is_constant = true,
+                     } },
+
+        .out_point_x = 1,
+        .out_point_y = 2,
+        .out_point_is_infinite = 3,
+    };
+
+    AcirFormat constraint_system{
+        .varnum = 9,
+        .recursive = false,
+        .num_acir_opcodes = 1,
+        .public_inputs = {},
+        .logic_constraints = {},
+        .range_constraints = {},
+        .aes128_constraints = {},
+        .sha256_constraints = {},
+        .sha256_compression = {},
+        .schnorr_constraints = {},
+        .ecdsa_k1_constraints = {},
+        .ecdsa_r1_constraints = {},
+        .blake2s_constraints = {},
+        .blake3_constraints = {},
+        .keccak_constraints = {},
+        .keccak_permutations = {},
+        .pedersen_constraints = {},
+        .pedersen_hash_constraints = {},
+        .poseidon2_constraints = {},
+        .multi_scalar_mul_constraints = { msm_constrain },
+        .ec_add_constraints = {},
+        .recursion_constraints = {},
+        .honk_recursion_constraints = {},
+        .bigint_from_le_bytes_constraints = {},
+        .bigint_to_le_bytes_constraints = {},
+        .bigint_operations = {},
+        .poly_triple_constraints = {},
+        .quad_constraints = {},
+        .block_constraints = {},
+        .original_opcode_indices = create_empty_original_opcode_indices(),
+    };
+    mock_opcode_indices(constraint_system);
+
+    WitnessVector witness{
+        fr("0x000000000000000000000000000000000000000000000000000000616c696365"),
+        fr("0x0bff8247aa94b08d1c680d7a3e10831bd8c8cf2ea2c756b0d1d89acdcad877ad"),
+        fr("0x2a5d7253a6ed48462fedb2d350cc768d13956310f54e73a8a47914f34a34c5c4"),
+        fr(0),
+    };
+
+    auto builder = create_circuit(constraint_system, /*size_hint=*/0, witness);
+    auto composer = Composer();
+    auto prover = composer.create_ultra_with_keccak_prover(builder);
+    auto proof = prover.construct_proof();
+
+    auto builder2 = create_circuit(constraint_system, /*size_hint=*/0, {});
+    auto composer2 = Composer();
+    auto verifier = composer2.create_ultra_with_keccak_verifier(builder2);
+
+    EXPECT_EQ(verifier.verify_proof(proof), true);
+}
+
+} // namespace acir_format::tests