Add Finalize and Decrypt functions to ServerAccumulator, expand ShellTestingDecryptor to handle partial decryption requests.

willow/proto/willow/BUILD

@@ -94,6 +94,7 @@ proto_library(
     deps = [
         ":aggregation_config_proto",
         ":messages_proto",
+        "//willow/proto/shell:shell_ciphertexts_proto",
     ],
 )
 

willow/proto/willow/messages.proto

@@ -65,3 +65,15 @@ message VerifierStateProto {
   bytes nonce_lower_bound = 2;
   bytes nonce_upper_bound = 3;
 }
+
+// Result of finalizing an accumulator.
+message FinalizedAccumulatorResult {
+  // Serialized decryption request to include in a DecryptRequest to be sent to
+  // the decryptor service.
+  bytes decryption_request = 1;
+
+  // Serialized state for creating a final result decryptor, which will handle
+  // the response from the decryptor service and produce a plaintext aggregation
+  // result.
+  bytes final_result_decryptor_state = 2;
+}

willow/proto/willow/server_accumulator.proto

@@ -26,7 +26,7 @@ message ServerAccumulatorState {
   ServerStateProto server_state = 1;
   AggregationConfigProto aggregation_config = 3;
   // We have one verifier state per range of nonces processed by this
-  // accumulator. States gat merged when adjacent ranges are processed.
+  // accumulator. States get merged when adjacent ranges are processed.
   repeated VerifierStateProto verifier_states = 2;
   // The ranges of nonces processed by this accumulator. In the same order as
   // the corresponding verifier states.
@@ -43,3 +43,9 @@ message NonceRange {
   bytes start = 1;  // Inclusive.
   bytes end = 2;    // Exclusive.
 }
+
+// State for creating a FinalResultDecryptor.
+message FinalResultDecryptorState {
+  ServerStateProto server_state = 1;
+  AggregationConfigProto aggregation_config = 2;
+}

willow/src/api/BUILD

@@ -61,7 +61,9 @@ cc_library(
         "@abseil-cpp//absl/strings",
         "@cxx.rs//:core",
         "//willow/proto/willow:aggregation_config_cc_proto",
+        "//willow/proto/willow:messages_cc_proto",
         "//willow/proto/willow:server_accumulator_cc_proto",
+        "//willow/src/input_encoding:codec",
     ],
 )
 
@@ -77,6 +79,7 @@ cc_test(
         "@abseil-cpp//absl/status:statusor",
         "//shell_wrapper:status_matchers",
         "//willow/proto/willow:aggregation_config_cc_proto",
+        "//willow/proto/willow:messages_cc_proto",
         "//willow/proto/willow:server_accumulator_cc_proto",
         "//willow/src/input_encoding:codec",
         "//willow/src/testing_utils:shell_testing_decryptor_cc",
@@ -102,6 +105,8 @@ rust_library(
         "//shell_wrapper:status",
         "//willow/proto/willow:aggregation_config_rust_proto",
         "//willow/proto/willow:server_accumulator_rust_proto",
+        "//willow/proto/shell:shell_ciphertexts_rust_proto",
+        "//willow/proto/willow:messages_rust_proto",
         "//willow/src/shell:kahe_shell",
         "//willow/src/shell:parameters_shell",
         "//willow/src/shell:vahe_shell",

willow/src/api/client_test.cc

@@ -35,7 +35,9 @@ namespace willow {
 namespace {
 
 using secure_aggregation::secagg_internal::StatusIs;
+using secure_aggregation::testing::ShellTestingDecryptor;
 using ::testing::ElementsAre;
+using ::testing::ElementsAreArray;
 using ::testing::Pair;
 using ::testing::UnorderedElementsAre;
 
@@ -108,7 +110,7 @@ TEST(WillowShellClientTest, InitializeAndGenerateContribution) {
   for (const auto& [name, values] : encoded_data) {
     EXPECT_TRUE(decrypted_encoded_data.contains(name));
     const auto& decrypted_values = decrypted_encoded_data[name];
-    EXPECT_THAT(decrypted_values, testing::ElementsAreArray(values));
+    EXPECT_THAT(decrypted_values, ElementsAreArray(values));
   }
 
   // Decode decrypted data.

willow/src/api/server_accumulator.cc

@@ -18,6 +18,7 @@
 #include <memory>
 #include <string>
 #include <utility>
+#include <vector>
 
 #include "absl/memory/memory.h"
 #include "absl/status/status.h"
@@ -26,12 +27,13 @@
 #include "willow/proto/willow/aggregation_config.pb.h"
 #include "willow/proto/willow/server_accumulator.pb.h"
 #include "willow/src/api/server_accumulator.rs.h"
+#include "willow/src/input_encoding/codec.h"
 
 namespace secure_aggregation {
+namespace willow {
 
-absl::StatusOr<std::unique_ptr<WillowShellServerAccumulator>>
-WillowShellServerAccumulator::Create(
-    const willow::AggregationConfigProto& aggregation_config) {
+absl::StatusOr<std::unique_ptr<ServerAccumulator>> ServerAccumulator::Create(
+    const AggregationConfigProto& aggregation_config) {
   secure_aggregation::ServerAccumulator* out;
   std::unique_ptr<std::string> status_message;
   int status_code =
@@ -41,12 +43,11 @@ WillowShellServerAccumulator::Create(
   if (status_code != 0) {
     return absl::Status(absl::StatusCode(status_code), *status_message);
   }
-  return absl::WrapUnique(new WillowShellServerAccumulator(IntoBox(out)));
+  return absl::WrapUnique(new ServerAccumulator(IntoBox(out)));
 }
 
-absl::StatusOr<std::unique_ptr<WillowShellServerAccumulator>>
-WillowShellServerAccumulator::CreateFromSerializedState(
-    std::string serialized_state) {
+absl::StatusOr<std::unique_ptr<ServerAccumulator>>
+ServerAccumulator::CreateFromSerializedState(std::string serialized_state) {
   secure_aggregation::ServerAccumulator* out;
   std::unique_ptr<std::string> status_message;
   int status_code = secure_aggregation::NewServerAccumulatorFromSerializedState(
@@ -55,17 +56,17 @@ WillowShellServerAccumulator::CreateFromSerializedState(
   if (status_code != 0) {
     return absl::Status(absl::StatusCode(status_code), *status_message);
   }
-  return absl::WrapUnique(new WillowShellServerAccumulator(IntoBox(out)));
+  return absl::WrapUnique(new ServerAccumulator(IntoBox(out)));
 }
 
-absl::Status WillowShellServerAccumulator::ProcessClientMessages(
-    willow::ClientMessageRange client_messages) {
+absl::Status ServerAccumulator::ProcessClientMessages(
+    ClientMessageRange client_messages) {
   auto serialized_client_messages = client_messages.SerializeAsString();
   client_messages.Clear();
   return ProcessClientMessages(std::move(serialized_client_messages));
 }
 
-absl::Status WillowShellServerAccumulator::ProcessClientMessages(
+absl::Status ServerAccumulator::ProcessClientMessages(
     std::string serialized_client_messages) {
   std::unique_ptr<std::string> status_message;
   int status_code = accumulator_->ProcessClientMessages(
@@ -77,8 +78,8 @@ absl::Status WillowShellServerAccumulator::ProcessClientMessages(
   return absl::OkStatus();
 }
 
-absl::Status WillowShellServerAccumulator::Merge(
-    std::unique_ptr<WillowShellServerAccumulator> other) {
+absl::Status ServerAccumulator::Merge(
+    std::unique_ptr<ServerAccumulator> other) {
   std::unique_ptr<std::string> status_message;
   int status_code =
       accumulator_->Merge(std::move(other->accumulator_), &status_message);
@@ -88,7 +89,7 @@ absl::Status WillowShellServerAccumulator::Merge(
   return absl::OkStatus();
 }
 
-absl::StatusOr<std::string> WillowShellServerAccumulator::ToSerializedState() {
+absl::StatusOr<std::string> ServerAccumulator::ToSerializedState() {
   rust::Vec<uint8_t> serialized_state;
   std::unique_ptr<std::string> status_message;
   int status_code =
@@ -100,4 +101,70 @@ absl::StatusOr<std::string> WillowShellServerAccumulator::ToSerializedState() {
                      serialized_state.size());
 }
 
+absl::StatusOr<FinalizedAccumulatorResult> ServerAccumulator::Finalize() && {
+  // Finalize accumulator in Rust and store the serialized results.
+  rust::Vec<uint8_t> decryption_request;
+  rust::Vec<uint8_t> final_result_decryptor_state;
+  std::unique_ptr<std::string> status_message;
+  int status_code = secure_aggregation::FinalizeServerAccumulator(
+      std::move(accumulator_), &decryption_request,
+      &final_result_decryptor_state, &status_message);
+  if (status_code != 0) {
+    return absl::Status(absl::StatusCode(status_code), *status_message);
+  }
+
+  // Pack the two serialized results into a single proto.
+  FinalizedAccumulatorResult result_proto;
+  result_proto.set_decryption_request(
+      std::string(reinterpret_cast<const char*>(decryption_request.data()),
+                  decryption_request.size()));
+  result_proto.set_final_result_decryptor_state(std::string(
+      reinterpret_cast<const char*>(final_result_decryptor_state.data()),
+      final_result_decryptor_state.size()));
+
+  return result_proto;
+}
+
+absl::StatusOr<std::unique_ptr<FinalResultDecryptor>>
+FinalResultDecryptor::CreateFromSerialized(
+    std::string final_result_decryptor_state) {
+  secure_aggregation::FinalResultDecryptor* out;
+  std::unique_ptr<std::string> status_message;
+  int status_code =
+      secure_aggregation::CreateFinalResultDecryptorFromSerialized(
+          std::make_unique<std::string>(
+              std::move(final_result_decryptor_state)),
+          &out, &status_message);
+  if (status_code != 0) {
+    return absl::Status(absl::StatusCode(status_code), *status_message);
+  }
+  return absl::WrapUnique(new FinalResultDecryptor(
+      secure_aggregation::FinalResultDecryptorIntoBox(out)));
+}
+
+absl::StatusOr<EncodedData> FinalResultDecryptor::Decrypt(
+    std::string serialized_partial_decryption_response) {
+  rust::Vec<EncodedDataEntry> out;
+  std::unique_ptr<std::string> status_message;
+  int status_code = aggregated_ciphertexts_->Decrypt(
+      std::make_unique<std::string>(
+          std::move(serialized_partial_decryption_response)),
+      &out, &status_message);
+  if (status_code != 0) {
+    return absl::Status(absl::StatusCode(status_code), *status_message);
+  }
+  EncodedData encoded_data;
+  for (const auto& rust_entry : out) {
+    std::string key(rust_entry.key);
+    std::vector<int64_t> val;
+    val.reserve(rust_entry.values.size());
+    for (auto v : rust_entry.values) {
+      val.push_back(static_cast<int64_t>(v));
+    }
+    encoded_data[std::move(key)] = std::move(val);
+  }
+  return encoded_data;
+}
+
+}  // namespace willow
 }  // namespace secure_aggregation

willow/src/api/server_accumulator.h

@@ -23,53 +23,84 @@
 
 #include "absl/status/status.h"
 #include "absl/status/statusor.h"
-#include "absl/strings/string_view.h"
 #include "include/cxx.h"
 #include "willow/proto/willow/aggregation_config.pb.h"
+#include "willow/proto/willow/messages.pb.h"
 #include "willow/proto/willow/server_accumulator.pb.h"
 #include "willow/src/api/server_accumulator.rs.h"
+#include "willow/src/input_encoding/codec.h"
 
 namespace secure_aggregation {
+namespace willow {
+
+// Holds the relevant state from a finalized accumulation, and can decrypt the
+// final result using the response from the decryptor service.
+class FinalResultDecryptor {
+ public:
+  // Creates a new final result decryptor from the given serialized
+  // state, likely coming from a FinalizedAccumulatorResult.
+  static absl::StatusOr<std::unique_ptr<FinalResultDecryptor>>
+  CreateFromSerialized(std::string final_result_decryptor_state);
+
+  // Decrypts final result using the given partial decryption
+  // response.
+  absl::StatusOr<EncodedData> Decrypt(
+      std::string serialized_partial_decryption_response);
+
+ private:
+  explicit FinalResultDecryptor(
+      rust::Box<secure_aggregation::FinalResultDecryptor>
+          aggregated_ciphertexts)
+      : aggregated_ciphertexts_(std::move(aggregated_ciphertexts)) {}
+
+  rust::Box<secure_aggregation::FinalResultDecryptor> aggregated_ciphertexts_;
+};
 
 // Implements an accumulator class intended to be used by a batch processing
 // system. Combines both the server and the verifier functionality of willow_v1,
 // using SHELL for the underlying cryptography.
-class WillowShellServerAccumulator {
+class ServerAccumulator {
  public:
   // Creates a new accumulator with the given aggregation_config and empty
   // state.
-  static absl::StatusOr<std::unique_ptr<WillowShellServerAccumulator>> Create(
-      const willow::AggregationConfigProto& aggregation_config);
+  static absl::StatusOr<std::unique_ptr<ServerAccumulator>> Create(
+      const AggregationConfigProto& aggregation_config);
 
   // Creates a new accumulator from the given serialized state, which must
   // correspond to a serialized ServerAccumulatorState proto.
-  static absl::StatusOr<std::unique_ptr<WillowShellServerAccumulator>>
+  static absl::StatusOr<std::unique_ptr<ServerAccumulator>>
   CreateFromSerializedState(std::string serialized_state);
 
   // Processes a list of client messages. If an invalid message is encountered,
   // an error is logged and processing continues.
-  absl::Status ProcessClientMessages(
-      willow::ClientMessageRange client_messages);
+  absl::Status ProcessClientMessages(ClientMessageRange client_messages);
 
   // Processes a list of client messages, given as a serialized
   // ClientMessageList proto.
   absl::Status ProcessClientMessages(std::string serialized_client_messages);
 
   // Merges the state of `other` into the current accumulator.
-  absl::Status Merge(std::unique_ptr<WillowShellServerAccumulator> other);
+  absl::Status Merge(std::unique_ptr<ServerAccumulator> other);
 
   // Converts the current state of the accumulator to a serialized
   // ServerAccumulatorState proto.
   absl::StatusOr<std::string> ToSerializedState();
 
+  // Finalizes the accumulator and returns a proto that holds the serialized
+  // decryption request (to be sent to the decryptor service) and the
+  // serialized decryptor state (to create a FinalResultDecryptor). This
+  // consumes the accumulator.
+  absl::StatusOr<FinalizedAccumulatorResult> Finalize() &&;
+
  private:
-  explicit WillowShellServerAccumulator(
+  explicit ServerAccumulator(
       rust::Box<secure_aggregation::ServerAccumulator> accumulator)
       : accumulator_(std::move(accumulator)) {}
 
   rust::Box<secure_aggregation::ServerAccumulator> accumulator_;
 };
 
+}  // namespace willow
 }  // namespace secure_aggregation
 
 #endif  // SECURE_AGGREGATION_WILLOW_SRC_API_SERVER_ACCUMULATOR_H_