feat: fixed number of pub inputs for databus commitment propagation

barretenberg/cpp/src/barretenberg/bb/main.cpp

@@ -598,8 +598,8 @@ void prove_tube(const std::string& output_path)
     // these public inputs by turning proof into witnesses and call
     // set_public on each witness
     auto num_public_inputs = static_cast<uint32_t>(static_cast<uint256_t>(proof.folding_proof[1]));
-    num_public_inputs -= bb::AGGREGATION_OBJECT_SIZE; // don't add the agg object
-    num_public_inputs -= 1 * 8; // TODO(https://github.com/AztecProtocol/barretenberg/issues/1125) Make this dynamic
+    num_public_inputs -= bb::AGGREGATION_OBJECT_SIZE;             // don't add the agg object
+    num_public_inputs -= bb::PROPAGATED_DATABUS_COMMITMENTS_SIZE; // exclude propagated databus commitments
     for (size_t i = 0; i < num_public_inputs; i++) {
         auto offset = acir_format::HONK_RECURSION_PUBLIC_INPUT_OFFSET;
         builder->add_public_variable(proof.folding_proof[i + offset]);

barretenberg/cpp/src/barretenberg/client_ivc/client_ivc.cpp

@@ -54,6 +54,9 @@ void ClientIVC::perform_recursive_verification_and_databus_consistency_checks(
     const std::shared_ptr<RecursiveVerificationKey>& vkey,
     const QUEUE_TYPE type)
 {
+    // Store the decider vk for the incoming circuit; its data is used in the databus consistency checks below
+    std::shared_ptr<RecursiveDeciderVerificationKey> decider_vk;
+
     switch (type) {
     case QUEUE_TYPE::PG: {
         // Construct stdlib verifier accumulator from the native counterpart computed on a previous round
@@ -66,10 +69,8 @@ void ClientIVC::perform_recursive_verification_and_databus_consistency_checks(
         // Extract native verifier accumulator from the stdlib accum for use on the next round
         verifier_accumulator = std::make_shared<DeciderVerificationKey>(verifier_accum->get_value());
 
-        // Perform databus commitment consistency checks and propagate return data commitments via public inputs
-        bus_depot.execute(verifier.keys_to_fold[1]->witness_commitments,
-                          verifier.keys_to_fold[1]->public_inputs,
-                          verifier.keys_to_fold[1]->verification_key->databus_propagation_data);
+        decider_vk = verifier.keys_to_fold[1]; // decider vk for the incoming circuit
+
         break;
     }
     case QUEUE_TYPE::OINK: {
@@ -86,14 +87,20 @@ void ClientIVC::perform_recursive_verification_and_databus_consistency_checks(
         // Initialize the gate challenges to zero for use in first round of folding
         verifier_accumulator->gate_challenges = std::vector<FF>(CONST_PG_LOG_N, 0);
 
-        // Perform databus commitment consistency checks and propagate return data commitments via public inputs
-        bus_depot.execute(verifier_accum->witness_commitments,
-                          verifier_accum->public_inputs,
-                          verifier_accum->verification_key->databus_propagation_data);
+        decider_vk = verifier_accum; // decider vk for the incoming circuit
 
         break;
     }
     }
+
+    // Set the return data commitment to be propagated on the public inputs of the present kernel and peform consistency
+    // checks between the calldata commitments and the return data commitments contained within the public inputs
+    bus_depot.set_return_data_to_be_propagated_and_perform_consistency_checks(
+        decider_vk->witness_commitments.return_data,
+        decider_vk->witness_commitments.calldata,
+        decider_vk->witness_commitments.secondary_calldata,
+        decider_vk->public_inputs,
+        decider_vk->verification_key->databus_propagation_data);
 }
 
 /**
@@ -133,6 +140,9 @@ void ClientIVC::complete_kernel_circuit_logic(ClientCircuit& circuit)
     }
     stdlib_verification_queue.clear();
 
+    // Propagate return data commitments via the public inputs for use in databus consistency checks
+    bus_depot.propagate_return_data_commitments(circuit);
+
     // Perform recursive merge verification for every merge proof in the queue
     process_recursive_merge_verification_queue(circuit);
 }

barretenberg/cpp/src/barretenberg/client_ivc/client_ivc_integration.test.cpp

@@ -93,10 +93,14 @@ TEST_F(ClientIVCIntegrationTests, BenchmarkCasePrecomputedVKs)
 
     size_t NUM_CIRCUITS = 6;
 
-    MockCircuitProducer circuit_producer;
-
-    auto precomputed_vks = circuit_producer.precompute_verification_keys(NUM_CIRCUITS, ivc.trace_structure);
+    // Precompute the verification keys for each circuit in the IVC
+    std::vector<std::shared_ptr<VerificationKey>> precomputed_vks;
+    {
+        MockCircuitProducer circuit_producer;
+        precomputed_vks = circuit_producer.precompute_verification_keys(NUM_CIRCUITS, ivc.trace_structure);
+    }
 
+    MockCircuitProducer circuit_producer;
     // Construct and accumulate a series of mocked private function execution circuits
     for (size_t idx = 0; idx < NUM_CIRCUITS; ++idx) {
         Builder circuit = circuit_producer.create_next_circuit(ivc);

barretenberg/cpp/src/barretenberg/stdlib/primitives/databus/databus.hpp

@@ -68,6 +68,8 @@ template <class Builder> class DataBusDepot {
     using Commitment = typename Curve::Group;
     using Fr = typename Curve::ScalarField;
     using Fq = typename Curve::BaseField;
+    using CommitmentNative = typename Curve::AffineElementNative;
+    using FrNative = typename Curve::ScalarFieldNative;
 
     using RecursiveFlavor = MegaRecursiveFlavor_<Builder>;
     using RecursiveDeciderVerificationKeys =
@@ -77,54 +79,95 @@ template <class Builder> class DataBusDepot {
     static constexpr size_t NUM_FR_LIMBS_PER_FQ = Fq::NUM_LIMBS;
     static constexpr size_t NUM_FR_LIMBS_PER_COMMITMENT = NUM_FR_LIMBS_PER_FQ * 2;
 
+    Commitment app_return_data_commitment;
+    Commitment kernel_return_data_commitment;
+    bool app_return_data_commitment_exists = false;
+    bool kernel_return_data_commitment_exists = false;
+
     /**
      * @brief Execute circuit logic to establish proper transfer of databus data between circuits
      * @details The databus mechanism establishes the transfer of data between two circuits (i-1 and i) in a third
      * circuit (i+1) via commitment equality checks of the form [R_{i-1}] = [C_i], where R and C represent return data
      * and calldata, respectively. In practice, circuit (i+1) is given access to [R_{i-1}] via the public inputs of
      * \pi_i, and it has access to [C_i] directly from \pi_i. The consistency checks in circuit (i+1) are thus of the
-     * form \pi_i.public_inputs.[R_{i-1}] = \pi_i.[C_i]. This method peforms the two primary operations required for
-     * these checks: (1) extract commitments [R] from proofs received as private witnesses and propagate them to the
-     * next circuit via adding them to the public inputs. (2) Assert equality of commitments.
+     * form \pi_i.public_inputs.[R_{i-1}] = \pi_i.[C_i]. This method performs these consistency checks. It also prepares
+     * return data commitments [R] to be propagated via the public inputs of the present circuit.
      *
-     * In Aztec private function execution, this mechanism is used as follows. Kernel circuit K_{i+1} must in general
-     * perform two databus consistency checks: (1) that the return_data of app circuit A_{i} was secondary calldata to
-     * K_{i}, and (2) that the return_data of K_{i-1} was calldata to K_{i}.
+     * @note In Aztec private function execution, this mechanism is used as follows. Kernel circuit K_{i+1} must in
+     * general perform two databus consistency checks: (1) that the return_data of app circuit A_{i} was secondary
+     * calldata to K_{i}, and (2) that the return_data of K_{i-1} was calldata to K_{i}.
      *
-     * @param commitments Witness polynomial commitments for an key that has been accumulated
-     * @param public_inputs The public inputs of that key
-     * @param propagation_data Data about the presence of databus commitments on the public inputs of the key.
+     * @param return_data Return data from either an app or a kernel
+     * @param calldata Calldata corresponding to return data from a previous kernel
+     * @param secondary_calldata Calldata corresponding to some app return data
+     * @param public_inputs Public inputs of a kernel proof which contain propagated return data commitments
+     * @param propagation_data Info about the return data commitments stored in the provided public inputs
      */
-    void execute(const WitnessCommitments& commitments,
-                 const std::vector<Fr>& public_inputs,
-                 const DatabusPropagationData& propagation_data)
+    void set_return_data_to_be_propagated_and_perform_consistency_checks(const Commitment& return_data,
+                                                                         const Commitment& calldata,
+                                                                         const Commitment& secondary_calldata,
+                                                                         const std::vector<Fr>& public_inputs,
+                                                                         const DatabusPropagationData& propagation_data)
     {
-        // Flag indicating whether the input data corresponds to a kernel decider proving key (else, an app decider
-        // proving key). This is used to indicate whether the return data commitment being propagated belongs to a
-        // kernel or an app so that it can be checked against the appropriate calldata commitment in a subsequent round.
-        bool is_kernel_data = propagation_data.is_kernel;
-
-        // Assert equality between return data commitments propagated via the public inputs and the corresponding
-        // calldata commitment
-        if (propagation_data.contains_app_return_data_commitment) { // public inputs contain [R]_app
-            ASSERT(is_kernel_data); // Only kernels should contain databus commitments in their public inputs
-            size_t start_idx = propagation_data.app_return_data_public_input_idx;
-            Commitment app_return_data = reconstruct_commitment_from_public_inputs(public_inputs, start_idx);
-            // App return data should correspond to the secondary calldata of the subsequent kernel
-            assert_equality_of_commitments(app_return_data, commitments.secondary_calldata);
+        // Set the kernel/app return data commitment to be propagated via the public inputs
+        if (propagation_data.is_kernel) {
+            kernel_return_data_commitment = return_data;
+            kernel_return_data_commitment_exists = true;
+        } else {
+            app_return_data_commitment = return_data;
+            app_return_data_commitment_exists = true;
         }
 
-        if (propagation_data.contains_kernel_return_data_commitment) { // pub inputs contain [R]_kernel
-            ASSERT(is_kernel_data); // Only kernels should contain databus commitments in their public inputs
+        // If the input data corresponds to a kernel, perform consistency checks between the provided calldata
+        // commitments and the return data commitments stored in the provided kernel proof public inputs
+        if (propagation_data.is_kernel) {
+            // Reconstruct the kernel and app return data commitments stored in the public inputs of the kernel proof
             size_t start_idx = propagation_data.kernel_return_data_public_input_idx;
             Commitment kernel_return_data = reconstruct_commitment_from_public_inputs(public_inputs, start_idx);
-            // Previous kernel return data should correspond to the calldata of the subsequent kernel
-            assert_equality_of_commitments(kernel_return_data, commitments.calldata);
+            start_idx = propagation_data.app_return_data_public_input_idx;
+            Commitment app_return_data = reconstruct_commitment_from_public_inputs(public_inputs, start_idx);
+
+            // Assert equality between the corresponding calldata and return data commitments
+            assert_equality_of_commitments(kernel_return_data, calldata);
+            assert_equality_of_commitments(app_return_data, secondary_calldata);
         }
+    }
 
-        // Propagate the return data commitment via the public inputs mechanism
-        propagate_commitment_via_public_inputs(commitments.return_data, is_kernel_data);
-    };
+    /**
+     * @brief Propagate the existing return data commitments via the public inputs of the provided circuit
+     * @details For consistent behavior across kernels, every kernel propagates two return data commitments via its
+     * public inputs. If one of either the app or kernel return data does not exist, it is populated with a default
+     * value that will satisfy the consistency check on the next cycle. For example, the first kernel has no previous
+     * kernel to verify and thus neither receives a previous kernel return data commitment nor a calldata input
+     * corresponding to a previous kernel. The commitment to the "empty" calldata will take a default value and thus we
+     * set the same value for the missing return data so that the consistency check will be satisfied. See issue
+     * https://github.com/AztecProtocol/barretenberg/issues/1138 for more discussion.
+     * @note The ordering of the kernel/app return data commitments within the public inputs is arbitrary but must be
+     * consistent across all kernels in order for the corresponding conistency check constraints to be consistent.
+     *
+     * @param builder
+     */
+    void propagate_return_data_commitments(Builder& builder)
+    {
+        // Set default commitment value to be used in the absence of one or the other return_data commitment
+        CommitmentNative default_commitment_val = CommitmentNative::one() * FrNative(BusVector::DEFAULT_VALUE);
+        if (kernel_return_data_commitment_exists) {
+            propagate_commitment_via_public_inputs(kernel_return_data_commitment, /*is_kernel=*/true);
+        } else {
+            Commitment default_commitment = Commitment::from_witness(&builder, default_commitment_val);
+            propagate_commitment_via_public_inputs(default_commitment, /*is_kernel=*/true);
+        }
+
+        if (app_return_data_commitment_exists) {
+            propagate_commitment_via_public_inputs(app_return_data_commitment, /*is_kernel=*/false);
+        } else {
+            Commitment default_commitment = Commitment::from_witness(&builder, default_commitment_val);
+            propagate_commitment_via_public_inputs(default_commitment, /*is_kernel=*/false);
+        }
+        // Reset flags indicating existence of return data commitments
+        kernel_return_data_commitment_exists = false;
+        app_return_data_commitment_exists = false;
+    }
 
     /**
      * @brief Set the witness indices for a commitment to public
@@ -142,10 +185,8 @@ template <class Builder> class DataBusDepot {
         // Set flag indicating propagation of return data; save the index at which it will be stored in public inputs
         auto start_idx = static_cast<uint32_t>(context->public_inputs.size());
         if (is_kernel) {
-            context->databus_propagation_data.contains_kernel_return_data_commitment = true;
             context->databus_propagation_data.kernel_return_data_public_input_idx = start_idx;
         } else {
-            context->databus_propagation_data.contains_app_return_data_commitment = true;
             context->databus_propagation_data.app_return_data_public_input_idx = start_idx;
         }
 

barretenberg/cpp/src/barretenberg/stdlib_circuit_builders/databus.hpp

@@ -1,14 +1,22 @@
 #pragma once
 
+#include "barretenberg/ecc/curves/bn254/fr.hpp"
 #include <cstdint>
 namespace bb {
 
+// We assume all kernels have space for two return data commitments on their public inputs
+constexpr uint32_t PROPAGATED_DATABUS_COMMITMENTS_SIZE = 16;
+
 /**
  * @brief A DataBus column
  *
  */
 struct BusVector {
 
+    // TODO(https://github.com/AztecProtocol/barretenberg/issues/1138): A default value added to every databus column to
+    // avoid the point at infinity commitment and to ensure the validity of the databus commitment consistency checks.
+    static constexpr bb::fr DEFAULT_VALUE = 25;
+
     /**
      * @brief Add an element to the data defining this bus column
      *
@@ -70,10 +78,6 @@ enum class BusId { CALLDATA, SECONDARY_CALLDATA, RETURNDATA };
 struct DatabusPropagationData {
     bool operator==(const DatabusPropagationData&) const = default;
 
-    // Flags indicating whether the public inputs contain commitment(s) to app/kernel return data
-    bool contains_app_return_data_commitment = false;
-    bool contains_kernel_return_data_commitment = false;
-
     // The start index of the return data commitments (if present) in the public inputs. Note: a start index is all
     // that's needed here since the commitents are represented by a fixed number of witnesses and are contiguous in the
     // public inputs by construction.
@@ -85,19 +89,13 @@ struct DatabusPropagationData {
 
     friend std::ostream& operator<<(std::ostream& os, DatabusPropagationData const& data)
     {
-        os << data.contains_app_return_data_commitment << ",\n"
-           << data.contains_kernel_return_data_commitment << ",\n"
-           << data.app_return_data_public_input_idx << ",\n"
+        os << data.app_return_data_public_input_idx << ",\n"
            << data.kernel_return_data_public_input_idx << ",\n"
            << data.is_kernel << "\n";
         return os;
     };
 
-    MSGPACK_FIELDS(contains_app_return_data_commitment,
-                   contains_kernel_return_data_commitment,
-                   app_return_data_public_input_idx,
-                   kernel_return_data_public_input_idx,
-                   is_kernel);
+    MSGPACK_FIELDS(app_return_data_public_input_idx, kernel_return_data_public_input_idx, is_kernel);
 };
 
 } // namespace bb

barretenberg/cpp/src/barretenberg/stdlib_circuit_builders/flavor_serialization.test.cpp

@@ -49,8 +49,6 @@ TYPED_TEST(FlavorSerializationTests, VerificationKeySerialization)
 
     // Populate some non-zero values in the databus_propagation_data to ensure its being handled
     if constexpr (IsMegaBuilder<Builder>) {
-        original_vkey.databus_propagation_data.contains_app_return_data_commitment = 1;
-        original_vkey.databus_propagation_data.contains_kernel_return_data_commitment = 1;
         original_vkey.databus_propagation_data.app_return_data_public_input_idx = 2;
         original_vkey.databus_propagation_data.kernel_return_data_public_input_idx = 4;
         original_vkey.databus_propagation_data.is_kernel = 1;