Android Open Source - conceal Shared Prefs Backed Key Chain

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License

The source code is released under:

BSD License For Conceal software Copyright (c) 2014, Facebook, Inc. All rights reserved. Redistribution and use in source and binary forms, with or without modification, are permitted provided that...

Java Source Code

/*
 *  Copyright (c) 2014, Facebook, Inc./*from   w w w . ja  v a  2 s  .c  o m*/
 *  All rights reserved.
 *
 *  This source code is licensed under the BSD-style license found in the
 *  LICENSE file in the root directory of this source tree. An additional grant
 *  of patent rights can be found in the PATENTS file in the same directory.
 *
 */

package com.facebook.android.crypto.keychain;

import java.security.SecureRandom;
import java.util.Arrays;

import android.content.Context;
import android.content.SharedPreferences;
import android.util.Base64;

import com.facebook.crypto.keychain.KeyChain;
import com.facebook.crypto.cipher.NativeGCMCipher;
import com.facebook.crypto.exception.KeyChainException;
import com.facebook.crypto.mac.NativeMac;

/**
 * An implementation of a keychain that is backed by shared preferences.
 * </p>
 * The implementation tries to cache results as much as possible to avoid
 * having to do expensive lookups to shared preferences. The keys are generated
 * lazily on first use.
 * </p>
 * If your code is sensitive to running shared preference I/O operations on the
 * UI thread, consider calling {@link #getCipherKey()} off the main thread, or
 * providing your own implementation similar to this class using a different
 * backing store.
 */
public class SharedPrefsBackedKeyChain implements KeyChain {

  // Visible for testing.
  /* package */ static final String SHARED_PREF_NAME = "crypto";
  /* package */ static final String CIPHER_KEY_PREF = "cipher_key";
  /* package */ static final String MAC_KEY_PREF = "mac_key";

  private final SharedPreferences mSharedPreferences;
  private final SecureRandom mSecureRandom;

  protected byte[] mCipherKey;
  protected boolean mSetCipherKey;

  protected byte[] mMacKey;
  protected boolean mSetMacKey;

  private static final SecureRandomFix sSecureRandomFix = new SecureRandomFix();

  public SharedPrefsBackedKeyChain(Context context) {
    mSharedPreferences = context.getSharedPreferences(SHARED_PREF_NAME, Context.MODE_PRIVATE);
    mSecureRandom = new SecureRandom();
  }

  @Override
  public synchronized byte[] getCipherKey() throws KeyChainException {
    if (!mSetCipherKey) {
      mCipherKey = maybeGenerateKey(CIPHER_KEY_PREF, NativeGCMCipher.KEY_LENGTH);
    }
    mSetCipherKey = true;
    return mCipherKey;
  }

  @Override
  public byte[] getMacKey() throws KeyChainException {
    if (!mSetMacKey) {
      mMacKey = maybeGenerateKey(MAC_KEY_PREF, NativeMac.KEY_LENGTH);
    }
    mSetMacKey = true;
    return mMacKey;
  }

  @Override
  public byte[] getNewIV() throws KeyChainException {
    sSecureRandomFix.tryApplyFixes();
    byte[] iv = new byte[NativeGCMCipher.IV_LENGTH];
    mSecureRandom.nextBytes(iv);
    return iv;
  }

  @Override
  public synchronized void destroyKeys() {
    mSetCipherKey = false;
    mSetMacKey = false;
    Arrays.fill(mCipherKey, (byte) 0);
    Arrays.fill(mMacKey, (byte) 0);
    mCipherKey = null;
    mMacKey = null;
    SharedPreferences.Editor editor = mSharedPreferences.edit();
    editor.remove(CIPHER_KEY_PREF);
    editor.remove(MAC_KEY_PREF);
    editor.commit();
  }

  /**
   * Generates a key associated with a preference.
   */
  private byte[] maybeGenerateKey(String pref, int length) throws KeyChainException {
    String base64Key = mSharedPreferences.getString(pref, null);
    if (base64Key == null) {
      // Generate key if it doesn't exist.
      return generateAndSaveKey(pref, length);
    } else {
      return decodeFromPrefs(base64Key);
    }
  }

  private byte[] generateAndSaveKey(String pref, int length) throws KeyChainException {
    sSecureRandomFix.tryApplyFixes();
    byte[] key = new byte[length];
    mSecureRandom.nextBytes(key);
    // Store the session key.
    SharedPreferences.Editor editor = mSharedPreferences.edit();
    editor.putString(
        pref,
        encodeForPrefs(key));
    editor.commit();
    return key;
  }

  /**
   * Visible for testing.
   */
  /* package */ byte[] decodeFromPrefs(String keyString) {
    if (keyString == null) {
      return null;
    }
    return Base64.decode(keyString, Base64.DEFAULT);
  }

  /**
   * Visible for testing.
   */
  /* package */ String encodeForPrefs(byte[] key) {
    if (key == null ) {
      return null;
    }
    return Base64.encodeToString(key, Base64.DEFAULT);
  }
}

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