universum.studios.android.preference.crypto

Class CryptoSharedPreferences

java.lang.Object

universum.studios.android.preference.crypto.CryptoSharedPreferences

All Implemented Interfaces:

SharedPreferences

public final class CryptoSharedPreferences
extends Object
implements SharedPreferences

A SharedPreferences implementation that supports encryption for both keys and values that are persisted in shared preferences. New instances of CryptoSharedPreferences may be created via CryptoSharedPreferences.Builder and its CryptoSharedPreferences.Builder.build() method.

Values Encryption

Primary functionality provided by CryptoSharedPreferences is the fact that all values persisted via these preferences implementation are encrypted. Encryption of values stored in shared preferences should improve security standards of an Android application as the encrypted values that are stored within a corresponding preferences Xml file are obscured so any possible attacker that gains access to preferences files containing encrypted values will have hard time reading them. Each instance of CryptoSharedPreferences is required to be created with at least Crypto implementation that will be used for values encryption and decryption. A desired Crypto should be supplied via CryptoSharedPreferences.Builder.valueCrypto(Crypto) otherwise the builder will throw an exception if a new instance of crypto preferences is tried to be created via CryptoSharedPreferences.Builder.build(). See other sections of this description to get more info.

Below snippet shows contents of a shared preferences Xml without and with encryption applied to both keys and values:

 <!-- Preference data without encryption applied. -->
 <?xml version='1.0' encoding='utf-8' standalone='yes'?>
 <map>
     <string name="universum.studios.android.preference.crypto.PREFERENCE.UserName">
          Android User
     </string>

     <long name="universum.studios.android.preference.crypto.PREFERENCE.UserBirthDate">
          1490986235741
     </long>
 </map>

 <!-- Preference data with encryption applied. -->
 <?xml version='1.0' encoding='utf-8' standalone='yes'?>
 <map>
     <string name="dW5pdmVyc3VtLnN0dWRpb3MuYW5kcm9pZC5wcmVmZXJlbmNlLmNyeXB0by5QUkVGRVJFTkNFLlVzZXJOYW1l">
          QW5kcm9pZCBVc2Vy
     </string>

     <string name="dW5pdmVyc3VtLnN0dWRpb3MuYW5kcm9pZC5wcmVmZXJlbmNlLmNyeXB0by5QUkVGRVJFTkNFLlVzZXJCaXJ0aERhdGU=">
          MTQ5MDk4NjIzNTc0MQ==
     </string>
 </map>
 

Keys Encryption

Keys that are used to map with them associated values in shared preferences may be also encrypted in order to improve security standards of an Android application. Unlike values encryption, keys encryption is optional. If keys encryption is required to be enabled a desired Crypto implementation should be supplied via CryptoSharedPreferences.Builder.keyCrypto(Crypto) for CryptoSharedPreferences. The specified crypto will be then used by the crypto preferences for encryption of preference keys whenever a preference value is requested to be persisted or obtained and for decryption whenever a change callback for registered OnSharedPreferenceChangeListener is about to be dispatched (see paragraph below).

In order to receive callback of OnSharedPreferenceChangeListener with instance of crypto preferences along with decrypted key so the listener does not need to deal with either key nor value decryption such listener should be registered via SharedPreferences.registerOnSharedPreferenceChangeListener(OnSharedPreferenceChangeListener) and later unregistered via SharedPreferences.unregisterOnSharedPreferenceChangeListener(OnSharedPreferenceChangeListener). Then each listener's callback will cary instance of the crypto preferences and already decrypted key of which associated value has changed. The changed value may be obtained in a standard fashion via one of get...(...) methods directly via the dispatched preferences instance like shown below:

 CryptoSharedPreferences.registerOnSharedPreferenceChangeListener(new OnSharedPreferenceChangeListener() {

      @Override
      public void onSharedPreferenceChanged(@NonNull SharedPreferences preferences, @NonNull String key) {
          // ... preferences are actually CryptoSharedPreferences and the key is already decrypted
          switch (key) {
              SamplePreferenceKey.USER_NAME:
                  final String userName = preferences.getString(key, "Unknown");
                  // ... userName will contain decrypted value
                  break;
              // ... handle changes for other preference keys
          }
      }
 });
 

Caching

Due to the fact that preference values that are to be returned via one of get...(...) methods need to be always decrypted, obtaining of such values may be potentially a time consuming task. In order to speed up this operation an implementation of SharedPreferencesCache may be supplied via CryptoSharedPreferences.Builder.valueCache(SharedPreferencesCache) for CryptoSharedPreferences. Such cache will be then used by the crypto preferences for storing of already decrypted values which may be retrieved whenever they are requested in a much faster fashion. When cache is used it is simply asked if it contains value for the requested key, if YES, that value is returned, if NO, an encrypted value is obtained from the wrapped preferences, decrypted, stored in the cache and returned. A specific value stored in the cache is invalidated (evicted from the cache) whenever a new edit session is started for key associated with that value, that is whenever edit() is called and followed by put...(...) call with that key. In order to speed up obtaining of preference values that need to be decrypted whenever they are to be returned to the caller where this task may potentially a time consuming operation

Author:

Martin Albedinsky

Nested Class Summary

Nested Classes

Modifier and Type	Class and Description
static class	CryptoSharedPreferences.Builder Builder that may be used to create instances of CryptoSharedPreferences with a desired configuration.

Nested classes/interfaces inherited from interface android.content.SharedPreferences

SharedPreferences.Editor, SharedPreferences.OnSharedPreferenceChangeListener

Method Summary

Modifier and Type	Method and Description
boolean	contains(String key)
SharedPreferences.Editor	edit()
Map<String,?>	getAll() Note that the values contained within the returned map will be either type of String or Set<String>, but already decrypted.
boolean	getBoolean(String key, boolean defValue)
float	getFloat(String key, float defValue)
int	getInt(String key, int defValue)
long	getLong(String key, long defValue)
String	getString(String key, String defValue)
Set<String>	getStringSet(String key, Set<String> defValues)
void	registerOnSharedPreferenceChangeListener(SharedPreferences.OnSharedPreferenceChangeListener listener)
void	unregisterOnSharedPreferenceChangeListener(SharedPreferences.OnSharedPreferenceChangeListener listener)

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Method Detail

registerOnSharedPreferenceChangeListener

public void registerOnSharedPreferenceChangeListener(@NonNull
                                                     SharedPreferences.OnSharedPreferenceChangeListener listener)

Specified by:

registerOnSharedPreferenceChangeListener in interface SharedPreferences

unregisterOnSharedPreferenceChangeListener

public void unregisterOnSharedPreferenceChangeListener(@NonNull
                                                       SharedPreferences.OnSharedPreferenceChangeListener listener)

Specified by:

unregisterOnSharedPreferenceChangeListener in interface SharedPreferences

getAll

public Map<String,?> getAll()

Note that the values contained within the returned map will be either type of String or Set<String>, but already decrypted.

Specified by:

getAll in interface SharedPreferences

contains

public boolean contains(@NonNull
                        String key)

Specified by:

contains in interface SharedPreferences

getString

@Nullable
public String getString(@NonNull
                                   String key,
                                   @Nullable
                                   String defValue)

Specified by:

getString in interface SharedPreferences

getStringSet

@Nullable
public Set<String> getStringSet(@NonNull
                                           String key,
                                           @Nullable
                                           Set<String> defValues)

Specified by:

getStringSet in interface SharedPreferences

getInt

public int getInt(@NonNull
                  String key,
                  int defValue)

Specified by:

getInt in interface SharedPreferences

getLong

public long getLong(@NonNull
                    String key,
                    long defValue)

Specified by:

getLong in interface SharedPreferences

getFloat

public float getFloat(@NonNull
                      String key,
                      float defValue)

Specified by:

getFloat in interface SharedPreferences

getBoolean

public boolean getBoolean(@NonNull
                          String key,
                          boolean defValue)

Specified by:

getBoolean in interface SharedPreferences

edit

public SharedPreferences.Editor edit()

Specified by:

edit in interface SharedPreferences