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      1 /*
      2  * Copyright (C) 2012 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package android.content.pm;
     18 
     19 import java.io.FilterInputStream;
     20 import java.io.IOException;
     21 import java.io.InputStream;
     22 
     23 import javax.crypto.Mac;
     24 
     25 /**
     26  * An input stream filter that applies a MAC to the data passing through it. At
     27  * the end of the data that should be authenticated, the tag can be calculated.
     28  * After that, the stream should not be used.
     29  *
     30  * @hide
     31  */
     32 public class MacAuthenticatedInputStream extends FilterInputStream {
     33     private final Mac mMac;
     34 
     35     public MacAuthenticatedInputStream(InputStream in, Mac mac) {
     36         super(in);
     37 
     38         mMac = mac;
     39     }
     40 
     41     public boolean isTagEqual(byte[] tag) {
     42         final byte[] actualTag = mMac.doFinal();
     43 
     44         if (tag == null || actualTag == null || tag.length != actualTag.length) {
     45             return false;
     46         }
     47 
     48         /*
     49          * Attempt to prevent timing attacks by doing the same amount of work
     50          * whether the first byte matches or not. Do not change this to a loop
     51          * that exits early when a byte does not match.
     52          */
     53         int value = 0;
     54         for (int i = 0; i < tag.length; i++) {
     55             value |= tag[i] ^ actualTag[i];
     56         }
     57 
     58         return value == 0;
     59     }
     60 
     61     @Override
     62     public int read() throws IOException {
     63         final int b = super.read();
     64         if (b >= 0) {
     65             mMac.update((byte) b);
     66         }
     67         return b;
     68     }
     69 
     70     @Override
     71     public int read(byte[] buffer, int offset, int count) throws IOException {
     72         int numRead = super.read(buffer, offset, count);
     73         if (numRead > 0) {
     74             mMac.update(buffer, offset, numRead);
     75         }
     76         return numRead;
     77     }
     78 }
     79