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/*
* 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;
}
}
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