Fix accumulator to use same context string as client; add test on verification.

PiperOrigin-RevId: 856362599

Author: tholop

willow/src/api/server_accumulator.rs

@@ -112,12 +112,12 @@ pub struct ServerAccumulator {
 
 impl ServerAccumulator {
     fn new(aggregation_config: AggregationConfig) -> Result<Self, StatusError> {
-        let context_string = aggregation_config.session_id.as_bytes();
+        let context_string = aggregation_config.compute_context_bytes()?;
         let vahe_config = create_shell_ahe_config(aggregation_config.max_number_of_decryptors)?;
         let kahe_config = create_shell_kahe_config(&aggregation_config)?;
-        let server_kahe = ShellKahe::new(kahe_config, context_string)?;
-        let server_vahe = ShellVahe::new(vahe_config.clone(), context_string)?;
-        let verifier_vahe = ShellVahe::new(vahe_config, context_string)?;
+        let server_kahe = ShellKahe::new(kahe_config, &context_string)?;
+        let server_vahe = ShellVahe::new(vahe_config.clone(), &context_string)?;
+        let verifier_vahe = ShellVahe::new(vahe_config, &context_string)?;
         let server = WillowV1Server { kahe: server_kahe, vahe: server_vahe };
         let verifier = WillowV1Verifier { vahe: verifier_vahe };
         Ok(Self {

willow/src/api/server_accumulator_test.cc

@@ -458,5 +458,31 @@ TEST_F(ServerAccumulatorTest, ProcessClientMessagesMergesThreeRanges) {
   }
 }
 
+TEST_F(ServerAccumulatorTest, VerifiesCorrectly) {
+  willow::EncodedData encoded_data = {
+      {"test_vector", {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}}};
+  std::string nonce = "nonce1";
+  SECAGG_ASSERT_OK_AND_ASSIGN(
+      auto client_message,
+      GenerateClientContribution(config_, encoded_data, public_key_, nonce));
+  ClientMessageRange messages;
+  *messages.add_client_messages() = client_message;
+  messages.mutable_nonce_range()->set_start("nonce1");
+  messages.mutable_nonce_range()->set_end("nonce2");
+
+  SECAGG_ASSERT_OK(accumulator_->ProcessClientMessages(messages));
+
+  SECAGG_ASSERT_OK_AND_ASSIGN(auto serialized_state,
+                              accumulator_->ToSerializedState());
+  ServerAccumulatorState state;
+  ASSERT_TRUE(state.ParseFromString(serialized_state));
+
+  ASSERT_EQ(state.verifier_states_size(), 1);
+  willow::VerifierStateProto verifier_state = state.verifier_states(0);
+
+  // Proto should be non-empty, i.e. the underlying Rust Option should be Some.
+  ASSERT_GE(verifier_state.ByteSizeLong(), 1);
+}
+
 }  // namespace
 }  // namespace secure_aggregation