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
17package android.content.pm;
18
19import java.io.FilterInputStream;
20import java.io.IOException;
21import java.io.InputStream;
22
23import 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 */
32public 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