feat(appcheck): Add App Check token verification support with replay protection

src/main/java/com/google/firebase/appcheck/DecodedAppCheckToken.java

@@ -0,0 +1,103 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * 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.
+ */
+
+package com.google.firebase.appcheck;
+
+import static com.google.common.base.Preconditions.checkArgument;
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import com.google.common.collect.ImmutableList;
+import com.google.common.collect.ImmutableMap;
+import java.util.Date;
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Represents a verified Firebase App Check token.
+ */
+public class DecodedAppCheckToken {
+
+  private final Map<String, Object> claims;
+
+  /**
+   * Creates an instance of {@link DecodedAppCheckToken} from a map of JWT claims.
+   *
+   * @param claims A map of JWT claims.
+   */
+  public DecodedAppCheckToken(Map<String, Object> claims) {
+    checkNotNull(claims, "Claims map must not be null");
+    checkArgument(claims.containsKey("sub"), "Claims map must contain sub");
+    this.claims = ImmutableMap.copyOf(claims);
+  }
+
+  /**
+   * Returns the issuer identifier for the token.
+   */
+  public String getIssuer() {
+    return (String) claims.get("iss");
+  }
+
+  /**
+   * Returns the subject claim ('sub') of the token.
+   */
+  public String getSubject() {
+    return (String) claims.get("sub");
+  }
+
+  /**
+   * Returns the audience for which this token is intended.
+   */
+  public List<String> getAudience() {
+    Object audience = claims.get("aud");
+    if (audience instanceof String) {
+      return ImmutableList.of((String) audience);
+    } else if (audience instanceof List) {
+      @SuppressWarnings("unchecked")
+      List<String> audienceList = (List<String>) audience;
+      return ImmutableList.copyOf(audienceList);
+    }
+    return ImmutableList.of();
+  }
+
+  /**
+   * Returns the expiration time in seconds since the Unix epoch.
+   */
+  public long getExpirationTime() {
+    Object exp = claims.get("exp");
+    if (exp instanceof Date) {
+      return ((Date) exp).getTime() / 1000L;
+    }
+    return exp instanceof Number ? ((Number) exp).longValue() : 0L;
+  }
+
+  /**
+   * Returns the issued-at time in seconds since the Unix epoch.
+   */
+  public long getIssuedAt() {
+    Object iat = claims.get("iat");
+    if (iat instanceof Date) {
+      return ((Date) iat).getTime() / 1000L;
+    }
+    return iat instanceof Number ? ((Number) iat).longValue() : 0L;
+  }
+
+  /**
+   * Returns the entire map of claims.
+   */
+  public Map<String, Object> getClaims() {
+    return claims;
+  }
+}

src/main/java/com/google/firebase/appcheck/FirebaseAppCheck.java

@@ -0,0 +1,139 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * 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.
+ */
+
+package com.google.firebase.appcheck;
+
+import static com.google.common.base.Preconditions.checkNotNull;
+
+import com.google.api.core.ApiFuture;
+import com.google.common.annotations.VisibleForTesting;
+import com.google.firebase.FirebaseApp;
+import com.google.firebase.ImplFirebaseTrampolines;
+import com.google.firebase.appcheck.internal.AppCheckTokenVerifier;
+import com.google.firebase.internal.CallableOperation;
+import com.google.firebase.internal.FirebaseService;
+
+/**
+ * This class is the entry point for the Firebase App Check service.
+ *
+ * <p>You can get an instance of {@link FirebaseAppCheck} via {@link #getInstance()}
+ * or {@link #getInstance(FirebaseApp)}.
+ */
+public final class FirebaseAppCheck {
+
+  private static final String SERVICE_ID = FirebaseAppCheck.class.getName();
+
+  private final FirebaseApp app;
+  private final AppCheckTokenVerifier tokenVerifier;
+
+  private FirebaseAppCheck(FirebaseApp app) {
+    this(app, new AppCheckTokenVerifier(app));
+  }
+
+  @VisibleForTesting
+  FirebaseAppCheck(FirebaseApp app, AppCheckTokenVerifier tokenVerifier) {
+    this.app = checkNotNull(app, "FirebaseApp must not be null");
+    this.tokenVerifier = checkNotNull(tokenVerifier, "AppCheckTokenVerifier must not be null");
+  }
+
+  /**
+   * Gets the {@link FirebaseAppCheck} instance for the default {@link FirebaseApp}.
+   *
+   * @return The {@link FirebaseAppCheck} instance for the default {@link FirebaseApp}.
+   */
+  public static FirebaseAppCheck getInstance() {
+    return getInstance(FirebaseApp.getInstance());
+  }
+
+  /**
+   * Gets the {@link FirebaseAppCheck} instance for the specified {@link FirebaseApp}.
+   *
+   * @param app The {@link FirebaseApp} instance.
+   * @return The {@link FirebaseAppCheck} instance for the specified {@link FirebaseApp}.
+   */
+  public static synchronized FirebaseAppCheck getInstance(FirebaseApp app) {
+    FirebaseAppCheckService service =
+        ImplFirebaseTrampolines.getService(app, SERVICE_ID, FirebaseAppCheckService.class);
+    if (service == null) {
+      service = ImplFirebaseTrampolines.addService(app, new FirebaseAppCheckService(app));
+    }
+    return service.getInstance();
+  }
+
+  /**
+   * Verifies an App Check token string.
+   *
+   * @param appCheckToken The App Check token string to verify.
+   * @return A {@link VerifyAppCheckTokenResponse} containing the decoded token.
+   * @throws FirebaseAppCheckException If verification fails.
+   */
+  public VerifyAppCheckTokenResponse verifyToken(String appCheckToken)
+      throws FirebaseAppCheckException {
+    return verifyToken(appCheckToken, null);
+  }
+
+  /**
+   * Verifies an App Check token string with options.
+   *
+   * @param appCheckToken The App Check token string to verify.
+   * @param options Verification options specified via {@link VerifyAppCheckTokenOptions}.
+   * @return A {@link VerifyAppCheckTokenResponse} containing the decoded token
+   *     and consumption status.
+   * @throws FirebaseAppCheckException If verification fails.
+   */
+  public VerifyAppCheckTokenResponse verifyToken(
+      String appCheckToken, VerifyAppCheckTokenOptions options) throws FirebaseAppCheckException {
+    return this.tokenVerifier.verifyToken(appCheckToken, options);
+  }
+
+  /**
+   * Asynchronously verifies an App Check token string.
+   *
+   * @param appCheckToken The App Check token string to verify.
+   * @return An {@link ApiFuture} containing the {@link VerifyAppCheckTokenResponse}.
+   */
+  public ApiFuture<VerifyAppCheckTokenResponse> verifyTokenAsync(String appCheckToken) {
+    return verifyTokenAsync(appCheckToken, null);
+  }
+
+  /**
+   * Asynchronously verifies an App Check token string with options.
+   *
+   * @param appCheckToken The App Check token string to verify.
+   * @param options Verification options specified via {@link VerifyAppCheckTokenOptions}.
+   * @return An {@link ApiFuture} containing the {@link VerifyAppCheckTokenResponse}.
+   */
+  public ApiFuture<VerifyAppCheckTokenResponse> verifyTokenAsync(
+      String appCheckToken, VerifyAppCheckTokenOptions options) {
+    return verifyTokenOp(appCheckToken, options).callAsync(this.app);
+  }
+
+  private CallableOperation<VerifyAppCheckTokenResponse, FirebaseAppCheckException> verifyTokenOp(
+      final String appCheckToken, final VerifyAppCheckTokenOptions options) {
+    return new CallableOperation<VerifyAppCheckTokenResponse, FirebaseAppCheckException>() {
+      @Override
+      protected VerifyAppCheckTokenResponse execute() throws FirebaseAppCheckException {
+        return verifyToken(appCheckToken, options);
+      }
+    };
+  }
+
+  private static class FirebaseAppCheckService extends FirebaseService<FirebaseAppCheck> {
+    FirebaseAppCheckService(FirebaseApp app) {
+      super(SERVICE_ID, new FirebaseAppCheck(app));
+    }
+  }
+}

src/main/java/com/google/firebase/appcheck/FirebaseAppCheckException.java

@@ -0,0 +1,55 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * 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.
+ */
+
+package com.google.firebase.appcheck;
+
+import com.google.firebase.ErrorCode;
+import com.google.firebase.FirebaseException;
+import com.google.firebase.IncomingHttpResponse;
+import com.google.firebase.internal.NonNull;
+import com.google.firebase.internal.Nullable;
+
+/**
+ * Generic exception related to Firebase App Check. Check the error code and message for more
+ * details.
+ */
+public class FirebaseAppCheckException extends FirebaseException {
+
+  public FirebaseAppCheckException(
+      @NonNull ErrorCode errorCode,
+      @NonNull String message,
+      @Nullable Throwable cause,
+      @Nullable IncomingHttpResponse response) {
+    super(errorCode, message, cause, response);
+  }
+
+  public FirebaseAppCheckException(
+      @NonNull ErrorCode errorCode,
+      @NonNull String message,
+      @Nullable Throwable cause) {
+    this(errorCode, message, cause, null);
+  }
+
+  public FirebaseAppCheckException(
+      @NonNull ErrorCode errorCode,
+      @NonNull String message) {
+    this(errorCode, message, null, null);
+  }
+
+  public FirebaseAppCheckException(@NonNull FirebaseException base) {
+    this(base.getErrorCode(), base.getMessage(), base.getCause(), base.getHttpResponse());
+  }
+}

src/main/java/com/google/firebase/appcheck/VerifyAppCheckTokenOptions.java

@@ -0,0 +1,78 @@
+/*
+ * Copyright 2026 Google LLC
+ *
+ * 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.
+ */
+
+package com.google.firebase.appcheck;
+
+import java.util.Optional;
+
+/**
+ * Options for verifying a Firebase App Check token.
+ */
+public final class VerifyAppCheckTokenOptions {
+
+  private final Optional<Boolean> consume;
+
+  private VerifyAppCheckTokenOptions(Builder builder) {
+    this.consume = builder.consume;
+  }
+
+  /**
+   * Returns whether to consume the App Check token during verification for replay protection.
+   */
+  public Optional<Boolean> getConsume() {
+    return consume;
+  }
+
+  public static Builder builder() {
+    return new Builder();
+  }
+
+  public static final class Builder {
+
+    private Optional<Boolean> consume = Optional.empty();
+
+    private Builder() {}
+
+    /**
+     * Sets whether to consume the token during verification.
+     *
+     * @param consume Set to true to consume the token.
+     * @return This builder.
+     */
+    public Builder setConsume(boolean consume) {
+      this.consume = Optional.of(consume);
+      return this;
+    }
+
+    /**
+     * Sets whether to consume the token during verification.
+     *
+     * @param consume Optional boolean value.
+     * @return This builder.
+     */
+    public Builder setConsume(Optional<Boolean> consume) {
+      this.consume = consume != null ? consume : Optional.<Boolean>empty();
+      return this;
+    }
+
+    /**
+     * Builds a new {@link VerifyAppCheckTokenOptions} instance.
+     */
+    public VerifyAppCheckTokenOptions build() {
+      return new VerifyAppCheckTokenOptions(this);
+    }
+  }
+}