diff --git a/biometrics/fingerprint/aidl/default/Android.bp b/biometrics/fingerprint/aidl/default/Android.bp
index 24087cf..6b43bff 100644
--- a/biometrics/fingerprint/aidl/default/Android.bp
+++ b/biometrics/fingerprint/aidl/default/Android.bp
@@ -6,9 +6,11 @@
     vintf_fragments: ["fingerprint-default.xml"],
     local_include_dirs: ["include"],
     srcs: [
-        "main.cpp",
+        "CancellationSignal.cpp",
         "Fingerprint.cpp",
         "Session.cpp",
+        "WorkerThread.cpp",
+        "main.cpp",
     ],
     shared_libs: [
         "libbase",
diff --git a/biometrics/fingerprint/aidl/default/CancellationSignal.cpp b/biometrics/fingerprint/aidl/default/CancellationSignal.cpp
new file mode 100644
index 0000000..6598316
--- /dev/null
+++ b/biometrics/fingerprint/aidl/default/CancellationSignal.cpp
@@ -0,0 +1,37 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include "CancellationSignal.h"
+
+#include <android-base/logging.h>
+#include <chrono>
+
+namespace aidl::android::hardware::biometrics::fingerprint {
+
+CancellationSignal::CancellationSignal(std::promise<void>&& cancellationPromise)
+    : mCancellationPromise(std::move(cancellationPromise)) {}
+
+ndk::ScopedAStatus CancellationSignal::cancel() {
+    mCancellationPromise.set_value();
+    return ndk::ScopedAStatus::ok();
+}
+
+bool shouldCancel(const std::future<void>& f) {
+    CHECK(f.valid());
+    return f.wait_for(std::chrono::seconds(0)) == std::future_status::ready;
+}
+
+}  // namespace aidl::android::hardware::biometrics::fingerprint
diff --git a/biometrics/fingerprint/aidl/default/Fingerprint.cpp b/biometrics/fingerprint/aidl/default/Fingerprint.cpp
index 6f89346..67dc34f 100644
--- a/biometrics/fingerprint/aidl/default/Fingerprint.cpp
+++ b/biometrics/fingerprint/aidl/default/Fingerprint.cpp
@@ -15,11 +15,12 @@
  */
 
 #include "Fingerprint.h"
+
 #include "Session.h"
 
 namespace aidl::android::hardware::biometrics::fingerprint {
 namespace {
-
+constexpr size_t MAX_WORKER_QUEUE_SIZE = 5;
 constexpr int SENSOR_ID = 1;
 constexpr common::SensorStrength SENSOR_STRENGTH = common::SensorStrength::STRONG;
 constexpr int MAX_ENROLLMENTS_PER_USER = 5;
@@ -32,7 +33,8 @@
 
 }  // namespace
 
-Fingerprint::Fingerprint() {}
+Fingerprint::Fingerprint()
+    : mEngine(std::make_unique<FakeFingerprintEngine>()), mWorker(MAX_WORKER_QUEUE_SIZE) {}
 
 ndk::ScopedAStatus Fingerprint::getSensorProps(std::vector<SensorProps>* out) {
     std::vector<common::HardwareInfo> hardwareInfos = {
@@ -52,14 +54,21 @@
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Fingerprint::createSession(int32_t /*sensorId*/, int32_t /*userId*/,
+ndk::ScopedAStatus Fingerprint::createSession(int32_t sensorId, int32_t userId,
                                               const std::shared_ptr<ISessionCallback>& cb,
                                               std::shared_ptr<ISession>* out) {
-    *out = SharedRefBase::make<Session>(cb);
+    auto sessionSp = mSession.lock();
+    CHECK(sessionSp == nullptr || sessionSp->isClosed()) << "Open session already exists!";
+
+    auto session = SharedRefBase::make<Session>(sensorId, userId, cb, mEngine.get(), &mWorker);
+    mSession = session;
+    *out = session;
     return ndk::ScopedAStatus::ok();
 }
 
 ndk::ScopedAStatus Fingerprint::reset() {
+    // Crash. The system will start a fresh instance of the HAL.
+    CHECK(false) << "Unable to reset. Crashing.";
     return ndk::ScopedAStatus::ok();
 }
 
diff --git a/biometrics/fingerprint/aidl/default/Session.cpp b/biometrics/fingerprint/aidl/default/Session.cpp
index c928df4..f6a0314 100644
--- a/biometrics/fingerprint/aidl/default/Session.cpp
+++ b/biometrics/fingerprint/aidl/default/Session.cpp
@@ -14,93 +14,216 @@
  * limitations under the License.
  */
 
-#include <aidl/android/hardware/biometrics/common/BnCancellationSignal.h>
+#include "Session.h"
+
 #include <android-base/logging.h>
 
-#include "Session.h"
+#include "CancellationSignal.h"
 
 namespace aidl::android::hardware::biometrics::fingerprint {
 
-class CancellationSignal : public common::BnCancellationSignal {
-  public:
-    ndk::ScopedAStatus cancel() override { return ndk::ScopedAStatus::ok(); }
-};
+Session::Session(int sensorId, int userId, std::shared_ptr<ISessionCallback> cb,
+                 FakeFingerprintEngine* engine, WorkerThread* worker)
+    : mSensorId(sensorId),
+      mUserId(userId),
+      mCb(std::move(cb)),
+      mEngine(engine),
+      mWorker(worker),
+      mScheduledState(SessionState::IDLING),
+      mCurrentState(SessionState::IDLING) {
+    CHECK_GE(mSensorId, 0);
+    CHECK_GE(mUserId, 0);
+    CHECK(mEngine);
+    CHECK(mWorker);
+    CHECK(mCb);
+}
 
-Session::Session(std::shared_ptr<ISessionCallback> cb) : mCb(std::move(cb)) {}
+void Session::scheduleStateOrCrash(SessionState state) {
+    CHECK(mScheduledState == SessionState::IDLING);
+    CHECK(mCurrentState == SessionState::IDLING);
+    mScheduledState = state;
+}
 
-ndk::ScopedAStatus Session::generateChallenge(int32_t /*cookie*/, int32_t /*timeoutSec*/) {
+void Session::enterStateOrCrash(int cookie, SessionState state) {
+    CHECK(mScheduledState == state);
+    mCurrentState = mScheduledState;
+    mScheduledState = SessionState::IDLING;
+    mCb->onStateChanged(cookie, mCurrentState);
+}
+
+void Session::enterIdling(int cookie) {
+    mCurrentState = SessionState::IDLING;
+    mCb->onStateChanged(cookie, mCurrentState);
+}
+
+bool Session::isClosed() {
+    return mCurrentState == SessionState::CLOSED;
+}
+
+ndk::ScopedAStatus Session::generateChallenge(int32_t cookie, int32_t timeoutSec) {
     LOG(INFO) << "generateChallenge";
+    scheduleStateOrCrash(SessionState::GENERATING_CHALLENGE);
+
+    mWorker->schedule(Callable::from([this, cookie, timeoutSec] {
+        enterStateOrCrash(cookie, SessionState::GENERATING_CHALLENGE);
+        mEngine->generateChallengeImpl(mCb.get(), timeoutSec);
+        enterIdling(cookie);
+    }));
+
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::revokeChallenge(int32_t /*cookie*/, int64_t /*challenge*/) {
+ndk::ScopedAStatus Session::revokeChallenge(int32_t cookie, int64_t challenge) {
     LOG(INFO) << "revokeChallenge";
+    scheduleStateOrCrash(SessionState::REVOKING_CHALLENGE);
+
+    mWorker->schedule(Callable::from([this, cookie, challenge] {
+        enterStateOrCrash(cookie, SessionState::REVOKING_CHALLENGE);
+        mEngine->revokeChallengeImpl(mCb.get(), challenge);
+        enterIdling(cookie);
+    }));
+
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::enroll(int32_t /*cookie*/, const keymaster::HardwareAuthToken& /*hat*/,
-                                   std::shared_ptr<common::ICancellationSignal>* /*out*/) {
+ndk::ScopedAStatus Session::enroll(int32_t cookie, const keymaster::HardwareAuthToken& hat,
+                                   std::shared_ptr<common::ICancellationSignal>* out) {
     LOG(INFO) << "enroll";
+    scheduleStateOrCrash(SessionState::ENROLLING);
+
+    std::promise<void> cancellationPromise;
+    auto cancFuture = cancellationPromise.get_future();
+
+    mWorker->schedule(Callable::from([this, cookie, hat, cancFuture = std::move(cancFuture)] {
+        enterStateOrCrash(cookie, SessionState::ENROLLING);
+        if (shouldCancel(cancFuture)) {
+            mCb->onError(Error::CANCELED, 0 /* vendorCode */);
+        } else {
+            mEngine->enrollImpl(mCb.get(), hat);
+        }
+        enterIdling(cookie);
+    }));
+
+    *out = SharedRefBase::make<CancellationSignal>(std::move(cancellationPromise));
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::authenticate(int32_t /*cookie*/, int64_t /*keystoreOperationId*/,
+ndk::ScopedAStatus Session::authenticate(int32_t cookie, int64_t operationId,
                                          std::shared_ptr<common::ICancellationSignal>* out) {
     LOG(INFO) << "authenticate";
-    if (mCb) {
-        mCb->onStateChanged(0, SessionState::AUTHENTICATING);
-    }
-    *out = SharedRefBase::make<CancellationSignal>();
+    scheduleStateOrCrash(SessionState::AUTHENTICATING);
+
+    std::promise<void> cancPromise;
+    auto cancFuture = cancPromise.get_future();
+
+    mWorker->schedule(
+            Callable::from([this, cookie, operationId, cancFuture = std::move(cancFuture)] {
+                enterStateOrCrash(cookie, SessionState::AUTHENTICATING);
+                if (shouldCancel(cancFuture)) {
+                    mCb->onError(Error::CANCELED, 0 /* vendorCode */);
+                } else {
+                    mEngine->authenticateImpl(mCb.get(), operationId);
+                }
+                enterIdling(cookie);
+            }));
+
+    *out = SharedRefBase::make<CancellationSignal>(std::move(cancPromise));
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::detectInteraction(
-        int32_t /*cookie*/, std::shared_ptr<common::ICancellationSignal>* /*out*/) {
+ndk::ScopedAStatus Session::detectInteraction(int32_t cookie,
+                                              std::shared_ptr<common::ICancellationSignal>* out) {
     LOG(INFO) << "detectInteraction";
+    scheduleStateOrCrash(SessionState::DETECTING_INTERACTION);
+
+    std::promise<void> cancellationPromise;
+    auto cancFuture = cancellationPromise.get_future();
+
+    mWorker->schedule(Callable::from([this, cookie, cancFuture = std::move(cancFuture)] {
+        enterStateOrCrash(cookie, SessionState::DETECTING_INTERACTION);
+        if (shouldCancel(cancFuture)) {
+            mCb->onError(Error::CANCELED, 0 /* vendorCode */);
+        } else {
+            mEngine->detectInteractionImpl(mCb.get());
+        }
+        enterIdling(cookie);
+    }));
+
+    *out = SharedRefBase::make<CancellationSignal>(std::move(cancellationPromise));
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::enumerateEnrollments(int32_t /*cookie*/) {
+ndk::ScopedAStatus Session::enumerateEnrollments(int32_t cookie) {
     LOG(INFO) << "enumerateEnrollments";
-    if (mCb) {
-        mCb->onStateChanged(0, SessionState::ENUMERATING_ENROLLMENTS);
-        mCb->onEnrollmentsEnumerated(std::vector<int32_t>());
-    }
+    scheduleStateOrCrash(SessionState::ENUMERATING_ENROLLMENTS);
+
+    mWorker->schedule(Callable::from([this, cookie] {
+        enterStateOrCrash(cookie, SessionState::ENUMERATING_ENROLLMENTS);
+        mEngine->enumerateEnrollmentsImpl(mCb.get());
+        enterIdling(cookie);
+    }));
+
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::removeEnrollments(int32_t /*cookie*/,
-                                              const std::vector<int32_t>& /*enrollmentIds*/) {
+ndk::ScopedAStatus Session::removeEnrollments(int32_t cookie,
+                                              const std::vector<int32_t>& enrollmentIds) {
     LOG(INFO) << "removeEnrollments";
-    if (mCb) {
-        mCb->onStateChanged(0, SessionState::REMOVING_ENROLLMENTS);
-        mCb->onEnrollmentsRemoved(std::vector<int32_t>());
-    }
+    scheduleStateOrCrash(SessionState::REMOVING_ENROLLMENTS);
+
+    mWorker->schedule(Callable::from([this, cookie, enrollmentIds] {
+        enterStateOrCrash(cookie, SessionState::REMOVING_ENROLLMENTS);
+        mEngine->removeEnrollmentsImpl(mCb.get(), enrollmentIds);
+        enterIdling(cookie);
+    }));
+
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::getAuthenticatorId(int32_t /*cookie*/) {
+ndk::ScopedAStatus Session::getAuthenticatorId(int32_t cookie) {
     LOG(INFO) << "getAuthenticatorId";
-    if (mCb) {
-        mCb->onStateChanged(0, SessionState::GETTING_AUTHENTICATOR_ID);
-        mCb->onAuthenticatorIdRetrieved(0 /* authenticatorId */);
-    }
+    scheduleStateOrCrash(SessionState::GETTING_AUTHENTICATOR_ID);
+
+    mWorker->schedule(Callable::from([this, cookie] {
+        enterStateOrCrash(cookie, SessionState::GETTING_AUTHENTICATOR_ID);
+        mEngine->getAuthenticatorIdImpl(mCb.get());
+        enterIdling(cookie);
+    }));
+
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::invalidateAuthenticatorId(int32_t /*cookie*/) {
+ndk::ScopedAStatus Session::invalidateAuthenticatorId(int32_t cookie) {
     LOG(INFO) << "invalidateAuthenticatorId";
+    scheduleStateOrCrash(SessionState::INVALIDATING_AUTHENTICATOR_ID);
+
+    mWorker->schedule(Callable::from([this, cookie] {
+        enterStateOrCrash(cookie, SessionState::INVALIDATING_AUTHENTICATOR_ID);
+        mEngine->invalidateAuthenticatorIdImpl(mCb.get());
+        enterIdling(cookie);
+    }));
+
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::resetLockout(int32_t /*cookie*/,
-                                         const keymaster::HardwareAuthToken& /*hat*/) {
+ndk::ScopedAStatus Session::resetLockout(int32_t cookie, const keymaster::HardwareAuthToken& hat) {
     LOG(INFO) << "resetLockout";
+    scheduleStateOrCrash(SessionState::RESETTING_LOCKOUT);
+
+    mWorker->schedule(Callable::from([this, cookie, hat] {
+        enterStateOrCrash(cookie, SessionState::RESETTING_LOCKOUT);
+        mEngine->resetLockoutImpl(mCb.get(), hat);
+        enterIdling(cookie);
+    }));
+
     return ndk::ScopedAStatus::ok();
 }
 
-ndk::ScopedAStatus Session::close(int32_t /*cookie*/) {
+ndk::ScopedAStatus Session::close(int32_t cookie) {
     LOG(INFO) << "close";
+    CHECK(mCurrentState == SessionState::IDLING) << "Can't close a non-idling session. Crashing.";
+    mCurrentState = SessionState::CLOSED;
+    mCb->onStateChanged(cookie, mCurrentState);
     return ndk::ScopedAStatus::ok();
 }
 
@@ -119,4 +242,5 @@
     LOG(INFO) << "onUiReady";
     return ndk::ScopedAStatus::ok();
 }
+
 }  // namespace aidl::android::hardware::biometrics::fingerprint
diff --git a/biometrics/fingerprint/aidl/default/include/CancellationSignal.h b/biometrics/fingerprint/aidl/default/include/CancellationSignal.h
new file mode 100644
index 0000000..99f2fba
--- /dev/null
+++ b/biometrics/fingerprint/aidl/default/include/CancellationSignal.h
@@ -0,0 +1,42 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <aidl/android/hardware/biometrics/common/BnCancellationSignal.h>
+#include <aidl/android/hardware/biometrics/fingerprint/ISessionCallback.h>
+#include <functional>
+#include <future>
+
+#include "WorkerThread.h"
+
+namespace aidl::android::hardware::biometrics::fingerprint {
+
+class CancellationSignal : public common::BnCancellationSignal {
+  public:
+    explicit CancellationSignal(std::promise<void>&& cancellationPromise);
+
+    ndk::ScopedAStatus cancel() override;
+
+  private:
+    std::promise<void> mCancellationPromise;
+};
+
+// Returns whether the given cancellation future is ready, i.e. whether the operation corresponding
+// to this future should be cancelled.
+bool shouldCancel(const std::future<void>& cancellationFuture);
+
+}  // namespace aidl::android::hardware::biometrics::fingerprint
diff --git a/biometrics/fingerprint/aidl/default/include/FakeFingerprintEngine.h b/biometrics/fingerprint/aidl/default/include/FakeFingerprintEngine.h
new file mode 100644
index 0000000..9343316
--- /dev/null
+++ b/biometrics/fingerprint/aidl/default/include/FakeFingerprintEngine.h
@@ -0,0 +1,76 @@
+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#pragma once
+
+#include <android-base/logging.h>
+
+namespace aidl::android::hardware::biometrics::fingerprint {
+
+class FakeFingerprintEngine {
+  public:
+    void generateChallengeImpl(ISessionCallback* cb, int32_t /*timeoutSec*/) {
+        LOG(INFO) << "generateChallengeImpl";
+        cb->onChallengeGenerated(0 /* challenge */);
+    }
+
+    void revokeChallengeImpl(ISessionCallback* cb, int64_t challenge) {
+        LOG(INFO) << "revokeChallengeImpl";
+        cb->onChallengeRevoked(challenge);
+    }
+
+    void enrollImpl(ISessionCallback* cb, const keymaster::HardwareAuthToken& /*hat*/) {
+        LOG(INFO) << "enrollImpl";
+        cb->onEnrollmentProgress(0 /* enrollmentId */, 0 /* remaining */);
+    }
+
+    void authenticateImpl(ISessionCallback* cb, int64_t /*operationId*/) {
+        LOG(INFO) << "authenticateImpl";
+        cb->onAuthenticationSucceeded(0 /* enrollmentId */, {} /* hat */);
+    }
+
+    void detectInteractionImpl(ISessionCallback* cb) {
+        LOG(INFO) << "detectInteractionImpl";
+        cb->onInteractionDetected();
+    }
+
+    void enumerateEnrollmentsImpl(ISessionCallback* cb) {
+        LOG(INFO) << "enumerateEnrollmentsImpl";
+        cb->onEnrollmentsEnumerated({} /* enrollmentIds */);
+    }
+
+    void removeEnrollmentsImpl(ISessionCallback* cb, const std::vector<int32_t>& enrollmentIds) {
+        LOG(INFO) << "removeEnrollmentsImpl";
+        cb->onEnrollmentsRemoved(enrollmentIds);
+    }
+
+    void getAuthenticatorIdImpl(ISessionCallback* cb) {
+        LOG(INFO) << "getAuthenticatorIdImpl";
+        cb->onAuthenticatorIdRetrieved(0 /* authenticatorId */);
+    }
+
+    void invalidateAuthenticatorIdImpl(ISessionCallback* cb) {
+        LOG(INFO) << "invalidateAuthenticatorIdImpl";
+        cb->onAuthenticatorIdInvalidated(0 /* newAuthenticatorId */);
+    }
+
+    void resetLockoutImpl(ISessionCallback* cb, const keymaster::HardwareAuthToken& /*hat*/) {
+        LOG(INFO) << "resetLockoutImpl";
+        cb->onLockoutCleared();
+    }
+};
+
+}  // namespace aidl::android::hardware::biometrics::fingerprint
\ No newline at end of file
diff --git a/biometrics/fingerprint/aidl/default/include/Fingerprint.h b/biometrics/fingerprint/aidl/default/include/Fingerprint.h
index ce1366c..9b8eef8 100644
--- a/biometrics/fingerprint/aidl/default/include/Fingerprint.h
+++ b/biometrics/fingerprint/aidl/default/include/Fingerprint.h
@@ -18,9 +18,13 @@
 
 #include <aidl/android/hardware/biometrics/fingerprint/BnFingerprint.h>
 
+#include "FakeFingerprintEngine.h"
+#include "Session.h"
+#include "WorkerThread.h"
+
 namespace aidl::android::hardware::biometrics::fingerprint {
 
-class Fingerprint final : public BnFingerprint {
+class Fingerprint : public BnFingerprint {
   public:
     Fingerprint();
 
@@ -31,6 +35,11 @@
                                      std::shared_ptr<ISession>* out) override;
 
     ndk::ScopedAStatus reset() override;
+
+  private:
+    std::unique_ptr<FakeFingerprintEngine> mEngine;
+    WorkerThread mWorker;
+    std::weak_ptr<Session> mSession;
 };
 
 }  // namespace aidl::android::hardware::biometrics::fingerprint
diff --git a/biometrics/fingerprint/aidl/default/include/Session.h b/biometrics/fingerprint/aidl/default/include/Session.h
index 99d806b..adda831 100644
--- a/biometrics/fingerprint/aidl/default/include/Session.h
+++ b/biometrics/fingerprint/aidl/default/include/Session.h
@@ -19,6 +19,9 @@
 #include <aidl/android/hardware/biometrics/fingerprint/BnSession.h>
 #include <aidl/android/hardware/biometrics/fingerprint/ISessionCallback.h>
 
+#include "FakeFingerprintEngine.h"
+#include "WorkerThread.h"
+
 namespace aidl::android::hardware::biometrics::fingerprint {
 
 namespace common = aidl::android::hardware::biometrics::common;
@@ -26,7 +29,8 @@
 
 class Session : public BnSession {
   public:
-    explicit Session(std::shared_ptr<ISessionCallback> cb);
+    Session(int sensorId, int userId, std::shared_ptr<ISessionCallback> cb,
+            FakeFingerprintEngine* engine, WorkerThread* worker);
 
     ndk::ScopedAStatus generateChallenge(int32_t cookie, int32_t timeoutSec) override;
 
@@ -35,7 +39,7 @@
     ndk::ScopedAStatus enroll(int32_t cookie, const keymaster::HardwareAuthToken& hat,
                               std::shared_ptr<common::ICancellationSignal>* out) override;
 
-    ndk::ScopedAStatus authenticate(int32_t cookie, int64_t keystoreOperationId,
+    ndk::ScopedAStatus authenticate(int32_t cookie, int64_t operationId,
                                     std::shared_ptr<common::ICancellationSignal>* out) override;
 
     ndk::ScopedAStatus detectInteraction(
@@ -62,8 +66,29 @@
 
     ndk::ScopedAStatus onUiReady() override;
 
+    bool isClosed();
+
   private:
+    // Crashes the HAL if it's not currently idling because that would be an invalid state machine
+    // transition. Otherwise, sets the scheduled state to the given state.
+    void scheduleStateOrCrash(SessionState state);
+
+    // Crashes the HAL if the provided state doesn't match the previously scheduled state.
+    // Otherwise, transitions into the provided state, clears the scheduled state, and notifies
+    // the client about the transition by calling ISessionCallback#onStateChanged.
+    void enterStateOrCrash(int cookie, SessionState state);
+
+    // Sets the current state to SessionState::IDLING and notifies the client about the transition
+    // by calling ISessionCallback#onStateChanged.
+    void enterIdling(int cookie);
+
+    int32_t mSensorId;
+    int32_t mUserId;
     std::shared_ptr<ISessionCallback> mCb;
+    FakeFingerprintEngine* mEngine;
+    WorkerThread* mWorker;
+    SessionState mScheduledState;
+    SessionState mCurrentState;
 };
 
 }  // namespace aidl::android::hardware::biometrics::fingerprint