Concealを使って文字列を暗号化してPreferenceに保存する。 ref: http://qiita.com/Rompei/items/c21c543707510720db2d

getKey.java

public static String getToken(Context context){
        SharedPreferences preferences = context.getSharedPreferences(PreferenceValue.PREF_KEY, Context.MODE_PRIVATE);
        String accessKey = preferences.getString(PreferenceValue.TOKEN_ACCESS_KEY, null);
        String encryptedToken = preferences.getString(PreferenceValue.ENCRYPTED_KEY, null);
        byte[] rawEncryptedToken = null;
        String rawDecryptedToken = null;
        if(encryptedToken!=null&&accessKey!=null) {
            rawEncryptedToken = Base64.decode(encryptedToken, Base64.DEFAULT);

            Crypto crypto = new Crypto(
                    new SharedPrefsBackedKeyChain(context),
                    new SystemNativeCryptoLibrary());
            if (!crypto.isAvailable()) {
                return null;
            }
            try {
                byte[] decryptedToken = crypto.decrypt(rawEncryptedToken, new Entity(accessKey));
                rawDecryptedToken = new String(decryptedToken);
            } catch (UnsupportedEncodingException e) {
                e.printStackTrace();
            } catch (CryptoInitializationException e) {
                e.printStackTrace();
            } catch (KeyChainException e) {
                e.printStackTrace();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return rawDecryptedToken;

    }

storeKey.java

public static void storeToken(Context context, String token){
        SharedPreferences preferences = context.getSharedPreferences(PreferenceValue.PREF_KEY, Context.MODE_PRIVATE);
        SharedPreferences.Editor editor;

        String accessKey = UUID.randomUUID().toString();
        editor = preferences.edit();

        Crypto crypto = new Crypto(
                new SharedPrefsBackedKeyChain(context),
                new SystemNativeCryptoLibrary());
        if(!crypto.isAvailable()){
            return;
        }
        try {
            byte[] encryptedKey = crypto.encrypt(token.getBytes(), new Entity(accessKey));
            editor.putString(PreferenceValue.TOKEN_ACCESS_KEY, accessKey);
            editor.putString(PreferenceValue.ENCRYPTED_KEY, Base64.encodeToString(encryptedKey, Base64.DEFAULT));
        }catch (UnsupportedEncodingException e){
            e.printStackTrace();
        }catch (CryptoInitializationException e){
            e.printStackTrace();
        }catch(KeyChainException e){
            e.printStackTrace();
        }catch(IOException e){
            e.printStackTrace();
        }
        editor.apply();
    }