java_src/examples/mac/MacExample.java

/**
 * Copyright 2021 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.
 */
// [START mac-example]
package mac;

import static java.nio.charset.StandardCharsets.UTF_8;

import com.google.crypto.tink.CleartextKeysetHandle;
import com.google.crypto.tink.JsonKeysetReader;
import com.google.crypto.tink.KeysetHandle;
import com.google.crypto.tink.Mac;
import com.google.crypto.tink.mac.MacConfig;
import com.google.crypto.tink.subtle.Hex;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.GeneralSecurityException;
import java.util.List;

/**
 * A command-line utility for checking file integrity with a Message Authentication Code (MAC).
 *
 * <p>It loads cleartext keys from disk - this is not recommended!
 *
 * <p>It requires the following arguments:
 *
 * <ul>
 *   <li>mode: either 'compute' or 'verify'.
 *   <li>key-file: Read the key material from this file.
 *   <li>input-file: Read the input from this file.
 *   <li>mac-file: name of the file containing a hexadecimal MAC of the input data.
 */
public final class MacExample {
  public static void main(String[] args) throws Exception {
    if (args.length != 4) {
      System.err.printf("Expected 4 parameters, got %d\n", args.length);
      System.err.println("Usage: java MacExample compute/verify key-file input-file mac-file");
      System.exit(1);
    }
    String mode = args[0];
    if (!mode.equals("compute") && !mode.equals("verify")) {
      System.err.println("Incorrect mode. Please select compute or verify.");
      System.exit(1);
    }
    File keyFile = new File(args[1]);
    byte[] msg = Files.readAllBytes(Paths.get(args[2]));
    File macFile = new File(args[3]);

    // Register all MAC key types with the Tink runtime.
    MacConfig.register();

    // Read the keyset into a KeysetHandle.
    KeysetHandle handle = null;
    try {
      handle = CleartextKeysetHandle.read(JsonKeysetReader.withFile(keyFile));
    } catch (GeneralSecurityException | IOException ex) {
      System.err.println("Cannot read keyset, got error: " + ex);
      System.exit(1);
    }

    // Get the primitive.
    Mac macPrimitive = null;
    try {
      macPrimitive = handle.getPrimitive(Mac.class);
    } catch (GeneralSecurityException ex) {
      System.err.println("Cannot create primitive, got error: " + ex);
      System.exit(1);
    }

    if (mode.equals("compute")) {
      byte[] mac = macPrimitive.computeMac(msg);
      try (FileOutputStream stream = new FileOutputStream(macFile)) {
        stream.write(Hex.encode(mac).getBytes(UTF_8));
      }
      System.exit(0);
    }

    List<String> lines = Files.readAllLines(macFile.toPath());
    if (lines.size() != 1) {
      System.err.printf("The MAC file should contain only one line, got %d", lines.size());
      System.exit(1);
    }

    byte[] mac = Hex.decode(lines.get(0).trim());
    try {
      macPrimitive.verifyMac(mac, msg);
    } catch (GeneralSecurityException ex) {
      System.err.println("MAC verification failed.");
      System.exit(1);
    }

    System.exit(0);
  }

  private MacExample() {}
}
// [END mac-example]