Susi Kharraz-Post | 14cbfcd | 2019-04-01 11:07:59 -0400 | [diff] [blame] | 1 | /* |
| 2 | * Copyright (C) 2019 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.util; |
| 18 | |
| 19 | import android.content.Context; |
| 20 | import android.content.SharedPreferences; |
| 21 | import android.os.Environment; |
| 22 | import android.os.storage.StorageManager; |
| 23 | import android.text.TextUtils; |
| 24 | |
Susi Kharraz-Post | a7f6145 | 2019-04-04 11:19:20 -0400 | [diff] [blame] | 25 | import com.android.internal.annotations.VisibleForTesting; |
| 26 | |
Susi Kharraz-Post | 14cbfcd | 2019-04-01 11:07:59 -0400 | [diff] [blame] | 27 | import java.io.File; |
| 28 | import java.nio.charset.Charset; |
| 29 | import java.security.MessageDigest; |
| 30 | import java.security.NoSuchAlgorithmException; |
| 31 | import java.security.SecureRandom; |
| 32 | |
| 33 | /** |
| 34 | * HashedStringCache provides hashing functionality with an underlying LRUCache and expiring salt. |
| 35 | * Salt and expiration time are being stored under the tag passed in by the calling package -- |
| 36 | * intended usage is the calling package name. |
Susi Kharraz-Post | 14cbfcd | 2019-04-01 11:07:59 -0400 | [diff] [blame] | 37 | * @hide |
| 38 | */ |
| 39 | public class HashedStringCache { |
| 40 | private static HashedStringCache sHashedStringCache = null; |
| 41 | private static final Charset UTF_8 = Charset.forName("UTF-8"); |
| 42 | private static final int HASH_CACHE_SIZE = 100; |
| 43 | private static final int HASH_LENGTH = 8; |
Susi Kharraz-Post | a7f6145 | 2019-04-04 11:19:20 -0400 | [diff] [blame] | 44 | @VisibleForTesting |
| 45 | static final String HASH_SALT = "_hash_salt"; |
| 46 | @VisibleForTesting |
| 47 | static final String HASH_SALT_DATE = "_hash_salt_date"; |
| 48 | @VisibleForTesting |
| 49 | static final String HASH_SALT_GEN = "_hash_salt_gen"; |
Susi Kharraz-Post | 14cbfcd | 2019-04-01 11:07:59 -0400 | [diff] [blame] | 50 | // For privacy we need to rotate the salt regularly |
| 51 | private static final long DAYS_TO_MILLIS = 1000 * 60 * 60 * 24; |
| 52 | private static final int MAX_SALT_DAYS = 100; |
| 53 | private final LruCache<String, String> mHashes; |
| 54 | private final SecureRandom mSecureRandom; |
| 55 | private final Object mPreferenceLock = new Object(); |
| 56 | private final MessageDigest mDigester; |
| 57 | private byte[] mSalt; |
| 58 | private int mSaltGen; |
| 59 | private SharedPreferences mSharedPreferences; |
| 60 | |
| 61 | private static final String TAG = "HashedStringCache"; |
| 62 | private static final boolean DEBUG = false; |
| 63 | |
| 64 | private HashedStringCache() { |
| 65 | mHashes = new LruCache<>(HASH_CACHE_SIZE); |
| 66 | mSecureRandom = new SecureRandom(); |
| 67 | try { |
| 68 | mDigester = MessageDigest.getInstance("MD5"); |
| 69 | } catch (NoSuchAlgorithmException impossible) { |
| 70 | // this can't happen - MD5 is always present |
| 71 | throw new RuntimeException(impossible); |
| 72 | } |
| 73 | } |
| 74 | |
| 75 | /** |
| 76 | * @return - instance of the HashedStringCache |
| 77 | * @hide |
| 78 | */ |
| 79 | public static HashedStringCache getInstance() { |
| 80 | if (sHashedStringCache == null) { |
| 81 | sHashedStringCache = new HashedStringCache(); |
| 82 | } |
| 83 | return sHashedStringCache; |
| 84 | } |
| 85 | |
| 86 | /** |
| 87 | * Take the string and context and create a hash of the string. Trigger refresh on salt if salt |
| 88 | * is more than 7 days old |
| 89 | * @param context - callers context to retrieve SharedPreferences |
| 90 | * @param clearText - string that needs to be hashed |
| 91 | * @param tag - class name to use for storing values in shared preferences |
| 92 | * @param saltExpirationDays - number of days we may keep the same salt |
| 93 | * special value -1 will short-circuit and always return null. |
| 94 | * @return - HashResult containing the hashed string and the generation of the hash salt, null |
| 95 | * if clearText string is empty |
| 96 | * |
| 97 | * @hide |
| 98 | */ |
| 99 | public HashResult hashString(Context context, String tag, String clearText, |
| 100 | int saltExpirationDays) { |
Susi Kharraz-Post | a7f6145 | 2019-04-04 11:19:20 -0400 | [diff] [blame] | 101 | if (saltExpirationDays == -1 || context == null |
| 102 | || TextUtils.isEmpty(clearText) || TextUtils.isEmpty(tag)) { |
Susi Kharraz-Post | 14cbfcd | 2019-04-01 11:07:59 -0400 | [diff] [blame] | 103 | return null; |
| 104 | } |
| 105 | |
| 106 | populateSaltValues(context, tag, saltExpirationDays); |
| 107 | String hashText = mHashes.get(clearText); |
| 108 | if (hashText != null) { |
| 109 | return new HashResult(hashText, mSaltGen); |
| 110 | } |
| 111 | |
| 112 | mDigester.reset(); |
| 113 | mDigester.update(mSalt); |
| 114 | mDigester.update(clearText.getBytes(UTF_8)); |
| 115 | byte[] bytes = mDigester.digest(); |
| 116 | int len = Math.min(HASH_LENGTH, bytes.length); |
| 117 | hashText = Base64.encodeToString(bytes, 0, len, Base64.NO_PADDING | Base64.NO_WRAP); |
| 118 | mHashes.put(clearText, hashText); |
| 119 | |
| 120 | return new HashResult(hashText, mSaltGen); |
| 121 | } |
| 122 | |
| 123 | /** |
| 124 | * Populates the mSharedPreferences and checks if there is a salt present and if it's older than |
| 125 | * 7 days |
| 126 | * @param tag - class name to use for storing values in shared preferences |
| 127 | * @param saltExpirationDays - number of days we may keep the same salt |
| 128 | * @param saltDate - the date retrieved from configuration |
| 129 | * @return - true if no salt or salt is older than 7 days |
| 130 | */ |
| 131 | private boolean checkNeedsNewSalt(String tag, int saltExpirationDays, long saltDate) { |
| 132 | if (saltDate == 0 || saltExpirationDays < -1) { |
| 133 | return true; |
| 134 | } |
| 135 | if (saltExpirationDays > MAX_SALT_DAYS) { |
| 136 | saltExpirationDays = MAX_SALT_DAYS; |
| 137 | } |
| 138 | long now = System.currentTimeMillis(); |
| 139 | long delta = now - saltDate; |
| 140 | // Check for delta < 0 to make sure we catch if someone puts their phone far in the |
| 141 | // future and then goes back to normal time. |
| 142 | return delta >= saltExpirationDays * DAYS_TO_MILLIS || delta < 0; |
| 143 | } |
| 144 | |
| 145 | /** |
| 146 | * Populate the salt and saltGen member variables if they aren't already set / need refreshing. |
| 147 | * @param context - to get sharedPreferences |
| 148 | * @param tag - class name to use for storing values in shared preferences |
| 149 | * @param saltExpirationDays - number of days we may keep the same salt |
| 150 | */ |
| 151 | private void populateSaltValues(Context context, String tag, int saltExpirationDays) { |
| 152 | synchronized (mPreferenceLock) { |
| 153 | // check if we need to refresh the salt |
| 154 | mSharedPreferences = getHashSharedPreferences(context); |
| 155 | long saltDate = mSharedPreferences.getLong(tag + HASH_SALT_DATE, 0); |
| 156 | boolean needsNewSalt = checkNeedsNewSalt(tag, saltExpirationDays, saltDate); |
| 157 | if (needsNewSalt) { |
| 158 | mHashes.evictAll(); |
| 159 | } |
| 160 | if (mSalt == null || needsNewSalt) { |
| 161 | String saltString = mSharedPreferences.getString(tag + HASH_SALT, null); |
| 162 | mSaltGen = mSharedPreferences.getInt(tag + HASH_SALT_GEN, 0); |
| 163 | if (saltString == null || needsNewSalt) { |
| 164 | mSaltGen++; |
| 165 | byte[] saltBytes = new byte[16]; |
| 166 | mSecureRandom.nextBytes(saltBytes); |
| 167 | saltString = Base64.encodeToString(saltBytes, |
| 168 | Base64.NO_PADDING | Base64.NO_WRAP); |
| 169 | mSharedPreferences.edit() |
| 170 | .putString(tag + HASH_SALT, saltString) |
| 171 | .putInt(tag + HASH_SALT_GEN, mSaltGen) |
| 172 | .putLong(tag + HASH_SALT_DATE, System.currentTimeMillis()).apply(); |
| 173 | if (DEBUG) { |
| 174 | Log.d(TAG, "created a new salt: " + saltString); |
| 175 | } |
| 176 | } |
| 177 | mSalt = saltString.getBytes(UTF_8); |
| 178 | } |
| 179 | } |
| 180 | } |
| 181 | |
| 182 | /** |
| 183 | * Android:ui doesn't have persistent preferences, so need to fall back on this hack originally |
| 184 | * from ChooserActivity.java |
| 185 | * @param context |
| 186 | * @return |
| 187 | */ |
| 188 | private SharedPreferences getHashSharedPreferences(Context context) { |
| 189 | final File prefsFile = new File(new File( |
| 190 | Environment.getDataUserCePackageDirectory( |
| 191 | StorageManager.UUID_PRIVATE_INTERNAL, |
| 192 | context.getUserId(), context.getPackageName()), |
| 193 | "shared_prefs"), |
| 194 | "hashed_cache.xml"); |
| 195 | return context.getSharedPreferences(prefsFile, Context.MODE_PRIVATE); |
| 196 | } |
| 197 | |
| 198 | /** |
| 199 | * Helper class to hold hashed string and salt generation. |
| 200 | */ |
| 201 | public class HashResult { |
| 202 | public String hashedString; |
| 203 | public int saltGeneration; |
| 204 | |
| 205 | public HashResult(String hString, int saltGen) { |
| 206 | hashedString = hString; |
| 207 | saltGeneration = saltGen; |
| 208 | } |
| 209 | } |
| 210 | } |