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/*
* Copyright (C) 2019 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 com.android.server.backup.encryption.keys;
import static com.android.internal.util.Preconditions.checkArgument;
import android.content.Context;
import android.content.SharedPreferences;
import android.util.Slog;
import com.android.internal.annotations.VisibleForTesting;
import java.util.Locale;
/**
* Tracks when a tertiary key rotation is due.
*
* <p>After a certain number of incremental backups, the device schedules a full backup, which will
* generate a new encryption key, effecting a key rotation. We should do this on a regular basis so
* that if a key does become compromised it has limited value to the attacker.
*
* <p>No additional synchronization of this class is provided. Only one instance should be used at
* any time. This should be fine as there should be no parallelism in backups.
*/
public class TertiaryKeyRotationTracker {
private static final int MAX_BACKUPS_UNTIL_TERTIARY_KEY_ROTATION = 31;
private static final String SHARED_PREFERENCES_NAME = "tertiary_key_rotation_tracker";
private static final String TAG = "TertiaryKeyRotationTracker";
private static final boolean DEBUG = false;
/**
* A new instance, using {@code context} to commit data to disk via {@link SharedPreferences}.
*/
public static TertiaryKeyRotationTracker getInstance(Context context) {
return new TertiaryKeyRotationTracker(
context.getSharedPreferences(SHARED_PREFERENCES_NAME, Context.MODE_PRIVATE),
MAX_BACKUPS_UNTIL_TERTIARY_KEY_ROTATION);
}
private final SharedPreferences mSharedPreferences;
private final int mMaxBackupsTillRotation;
/**
* New instance, storing data in {@code sharedPreferences} and initializing backup countdown to
* {@code maxBackupsTillRotation}.
*/
@VisibleForTesting
TertiaryKeyRotationTracker(SharedPreferences sharedPreferences, int maxBackupsTillRotation) {
checkArgument(
maxBackupsTillRotation >= 0,
String.format(
Locale.US,
"maxBackupsTillRotation should be non-negative but was %d",
maxBackupsTillRotation));
mSharedPreferences = sharedPreferences;
mMaxBackupsTillRotation = maxBackupsTillRotation;
}
/**
* Returns {@code true} if the given app is due having its key rotated.
*
* @param packageName The package name of the app.
*/
public boolean isKeyRotationDue(String packageName) {
return getBackupsSinceRotation(packageName) >= mMaxBackupsTillRotation;
}
/**
* Records that an incremental backup has occurred. Each incremental backup brings the app
* closer to the time when its key should be rotated.
*
* @param packageName The package name of the app for which the backup occurred.
*/
public void recordBackup(String packageName) {
int backupsSinceRotation = getBackupsSinceRotation(packageName) + 1;
mSharedPreferences.edit().putInt(packageName, backupsSinceRotation).apply();
if (DEBUG) {
Slog.d(
TAG,
String.format(
Locale.US,
"Incremental backup for %s. %d backups until key rotation.",
packageName,
Math.max(
0,
mMaxBackupsTillRotation
- backupsSinceRotation)));
}
}
/**
* Resets the rotation delay for the given app. Should be invoked after a key rotation.
*
* @param packageName Package name of the app whose key has rotated.
*/
public void resetCountdown(String packageName) {
mSharedPreferences.edit().putInt(packageName, 0).apply();
}
/** Marks all enrolled packages for key rotation. */
public void markAllForRotation() {
SharedPreferences.Editor editor = mSharedPreferences.edit();
for (String packageName : mSharedPreferences.getAll().keySet()) {
editor.putInt(packageName, mMaxBackupsTillRotation);
}
editor.apply();
}
private int getBackupsSinceRotation(String packageName) {
return mSharedPreferences.getInt(packageName, 0);
}
}
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