content: /*
   * Copyright (C) 2012 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.
   */
  
  package android.content.pm;
  
  import java.io.FilterInputStream;
  import java.io.IOException;
  import java.io.InputStream;
  
  import javax.crypto.Mac;
  
  /**
   * An input stream filter that applies a MAC to the data passing through it. At
   * the end of the data that should be authenticated, the tag can be calculated.
   * After that, the stream should not be used.
   *
   * @hide
   */
  public class MacAuthenticatedInputStream extends FilterInputStream {
      private final Mac mMac;
  
      public MacAuthenticatedInputStream(InputStream in, Mac mac) {
          super(in);
  
          mMac = mac;
      }
  
      public boolean isTagEqual(byte[] tag) {
          final byte[] actualTag = mMac.doFinal();
  
          if (tag == null || actualTag == null || tag.length != actualTag.length) {
              return false;
          }
  
          /*
           * Attempt to prevent timing attacks by doing the same amount of work
           * whether the first byte matches or not. Do not change this to a loop
           * that exits early when a byte does not match.
           */
          int value = 0;
          for (int i = 0; i < tag.length; i++) {
              value |= tag[i] ^ actualTag[i];
          }
  
          return value == 0;
      }
  
      @Override
      public int read() throws IOException {
          final int b = super.read();
          if (b >= 0) {
              mMac.update((byte) b);
          }
          return b;
      }
  
      @Override
      public int read(byte[] buffer, int offset, int count) throws IOException {
          int numRead = super.read(buffer, offset, count);
          if (numRead > 0) {
              mMac.update(buffer, offset, numRead);
          }
          return numRead;
      }
  }
  