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A library jar that provides APIs for Applications written for the Google Android Platform.
/*
* 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 android.util;
import android.content.Context;
import android.content.SharedPreferences;
import android.os.Environment;
import android.os.storage.StorageManager;
import android.text.TextUtils;
import com.android.internal.annotations.VisibleForTesting;
import java.io.File;
import java.nio.charset.Charset;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.security.SecureRandom;
/**
* HashedStringCache provides hashing functionality with an underlying LRUCache and expiring salt.
* Salt and expiration time are being stored under the tag passed in by the calling package --
* intended usage is the calling package name.
* @hide
*/
public class HashedStringCache {
private static HashedStringCache sHashedStringCache = null;
private static final Charset UTF_8 = Charset.forName("UTF-8");
private static final int HASH_CACHE_SIZE = 100;
private static final int HASH_LENGTH = 8;
@VisibleForTesting
static final String HASH_SALT = "_hash_salt";
@VisibleForTesting
static final String HASH_SALT_DATE = "_hash_salt_date";
@VisibleForTesting
static final String HASH_SALT_GEN = "_hash_salt_gen";
// For privacy we need to rotate the salt regularly
private static final long DAYS_TO_MILLIS = 1000 * 60 * 60 * 24;
private static final int MAX_SALT_DAYS = 100;
private final LruCache mHashes;
private final SecureRandom mSecureRandom;
private final Object mPreferenceLock = new Object();
private final MessageDigest mDigester;
private byte[] mSalt;
private int mSaltGen;
private SharedPreferences mSharedPreferences;
private static final String TAG = "HashedStringCache";
private static final boolean DEBUG = false;
private HashedStringCache() {
mHashes = new LruCache<>(HASH_CACHE_SIZE);
mSecureRandom = new SecureRandom();
try {
mDigester = MessageDigest.getInstance("MD5");
} catch (NoSuchAlgorithmException impossible) {
// this can't happen - MD5 is always present
throw new RuntimeException(impossible);
}
}
/**
* @return - instance of the HashedStringCache
* @hide
*/
public static HashedStringCache getInstance() {
if (sHashedStringCache == null) {
sHashedStringCache = new HashedStringCache();
}
return sHashedStringCache;
}
/**
* Take the string and context and create a hash of the string. Trigger refresh on salt if salt
* is more than 7 days old
* @param context - callers context to retrieve SharedPreferences
* @param clearText - string that needs to be hashed
* @param tag - class name to use for storing values in shared preferences
* @param saltExpirationDays - number of days we may keep the same salt
* special value -1 will short-circuit and always return null.
* @return - HashResult containing the hashed string and the generation of the hash salt, null
* if clearText string is empty
*
* @hide
*/
public HashResult hashString(Context context, String tag, String clearText,
int saltExpirationDays) {
if (saltExpirationDays == -1 || context == null
|| TextUtils.isEmpty(clearText) || TextUtils.isEmpty(tag)) {
return null;
}
populateSaltValues(context, tag, saltExpirationDays);
String hashText = mHashes.get(clearText);
if (hashText != null) {
return new HashResult(hashText, mSaltGen);
}
mDigester.reset();
mDigester.update(mSalt);
mDigester.update(clearText.getBytes(UTF_8));
byte[] bytes = mDigester.digest();
int len = Math.min(HASH_LENGTH, bytes.length);
hashText = Base64.encodeToString(bytes, 0, len, Base64.NO_PADDING | Base64.NO_WRAP);
mHashes.put(clearText, hashText);
return new HashResult(hashText, mSaltGen);
}
/**
* Populates the mSharedPreferences and checks if there is a salt present and if it's older than
* 7 days
* @param tag - class name to use for storing values in shared preferences
* @param saltExpirationDays - number of days we may keep the same salt
* @param saltDate - the date retrieved from configuration
* @return - true if no salt or salt is older than 7 days
*/
private boolean checkNeedsNewSalt(String tag, int saltExpirationDays, long saltDate) {
if (saltDate == 0 || saltExpirationDays < -1) {
return true;
}
if (saltExpirationDays > MAX_SALT_DAYS) {
saltExpirationDays = MAX_SALT_DAYS;
}
long now = System.currentTimeMillis();
long delta = now - saltDate;
// Check for delta < 0 to make sure we catch if someone puts their phone far in the
// future and then goes back to normal time.
return delta >= saltExpirationDays * DAYS_TO_MILLIS || delta < 0;
}
/**
* Populate the salt and saltGen member variables if they aren't already set / need refreshing.
* @param context - to get sharedPreferences
* @param tag - class name to use for storing values in shared preferences
* @param saltExpirationDays - number of days we may keep the same salt
*/
private void populateSaltValues(Context context, String tag, int saltExpirationDays) {
synchronized (mPreferenceLock) {
// check if we need to refresh the salt
mSharedPreferences = getHashSharedPreferences(context);
long saltDate = mSharedPreferences.getLong(tag + HASH_SALT_DATE, 0);
boolean needsNewSalt = checkNeedsNewSalt(tag, saltExpirationDays, saltDate);
if (needsNewSalt) {
mHashes.evictAll();
}
if (mSalt == null || needsNewSalt) {
String saltString = mSharedPreferences.getString(tag + HASH_SALT, null);
mSaltGen = mSharedPreferences.getInt(tag + HASH_SALT_GEN, 0);
if (saltString == null || needsNewSalt) {
mSaltGen++;
byte[] saltBytes = new byte[16];
mSecureRandom.nextBytes(saltBytes);
saltString = Base64.encodeToString(saltBytes,
Base64.NO_PADDING | Base64.NO_WRAP);
mSharedPreferences.edit()
.putString(tag + HASH_SALT, saltString)
.putInt(tag + HASH_SALT_GEN, mSaltGen)
.putLong(tag + HASH_SALT_DATE, System.currentTimeMillis()).apply();
if (DEBUG) {
Log.d(TAG, "created a new salt: " + saltString);
}
}
mSalt = saltString.getBytes(UTF_8);
}
}
}
/**
* Android:ui doesn't have persistent preferences, so need to fall back on this hack originally
* from ChooserActivity.java
* @param context
* @return
*/
private SharedPreferences getHashSharedPreferences(Context context) {
final File prefsFile = new File(new File(
Environment.getDataUserCePackageDirectory(
StorageManager.UUID_PRIVATE_INTERNAL,
context.getUserId(), context.getPackageName()),
"shared_prefs"),
"hashed_cache.xml");
return context.getSharedPreferences(prefsFile, Context.MODE_PRIVATE);
}
/**
* Helper class to hold hashed string and salt generation.
*/
public class HashResult {
public String hashedString;
public int saltGeneration;
public HashResult(String hString, int saltGen) {
hashedString = hString;
saltGeneration = saltGen;
}
}
}