Medium post for Getting along with credentials in android

code1.kt

val config = TwitterConfig.Builder(context)
    .twitterAuthConfig(TwitterAuthConfig("TODO consumer key comes here", "TODO consumer secret comes here"))
    .build()
Twitter.initialize(config)

code2.kt

val config = TwitterConfig.Builder(context)
    .twitterAuthConfig(TwitterAuthConfig("my-twitter-key", "my-twitter-secret"))
    .build()
Twitter.initialize(config)

code3.kt

val config = TwitterConfig.Builder(context)
    .twitterAuthConfig(TwitterAuthConfig(BuildConfig.TWITTER_COMSUMER_KEY, BuildConfig.TWITTER_COMSUMER_SECRET))
    .build()
Twitter.initialize(config)

code4.kt

import okhttp3.Authenticator
class MyAppAuthenticator(private val preference: SharedPreferences): Authenticator {
	// This callback is called when api request failed in 401.
	override fun authenticate(route: Route?, response: Response?): Request? {
		val currentRefreshToken = preference.getString("REFRESH_TOKEN_KEY", "")
		val (newAccessToken, newRefreshToken) = requestNewTokensToServer(currentRefreshToken).blockingGet()  // TODO error handling
		preference.edit().putString("ACCESS_TOKEN_KEY",  newAccessToken).apply()
		preference.edit().putString("REFRESH_TOKEN_KEY",  newRefreshToken).apply()
		return attachNewTokenOnRequest(response.request(), newAccessToken)
	}
}

code5.kt

companion object {
        private const val ALGORITHM = "AES"
        private const val MODE = "CBC"
        private const val PADDING = "PKCS5Padding"
}

fun savePrivateString(key: String, value: String) {
	prefs.edit().putString(key, encrypt(value)).apply()
}

fun getPrivateString(key: String): String {
	val encrypted = prefs.getString(key, "")
	return decrypt(encrypted)
}

private fun encrypt(plain: String): String {
	val cipher = initializeCipher(Cipher.ENCRYPT_MODE)
    val encrypted = cipher.doFinal(plain.toByteArray()) ?: byteArrayOf()  // TODO catch exception
    return Base64.encodeToString(encrypted, Base64.DEFAULT)
}

private fun decrypt(encrypted: String): String {
    val cipher = initializeCipher(Cipher.DECRYPT_MODE)
    val decrypted = cipher.doFinal(plain.toByteArray()) ?: byteArrayOf()  // TODO catch exception
    return String(decrypted)
}

private fun initializeCipher(mode: Int) = Cipher.getInstance("$ALGORITHM/$MODE/$PADDING").apply {
	val key = BuildConfig.CIPHER_KEY  // !!NOTICE!! Secret key is necessarry for encryption/decryption.
	val iv = BuildConfig.CIPHER_IV  // InitializationVector. 16bytes string.
	val secretKeySpec = SecretKeySpec(key.toByteArray(), ALGORITHM)
    val ivSpec = IvParameterSpec(iv.toByteArray())
    init(mode, secretKeySpec, ivSpec)
}

code6.kt

fun savePrivateString(key: String, value: String) {
	prefs.edit().putString(key, encrypt(key, value)).apply()
}

fun getPrivateString(key: String): String {
	val encrypted = prefs.getString(key, "")
	return decrypt(key, encrypted)
}

private fun encrypt(alias: String, plain: String): String {
	val cipher = initializeCipher(alias, Cipher.ENCRYPT_MODE)
    val encrypted = cipher.doFinal(plain.toByteArray()) ?: byteArrayOf()  // TODO catch exception
    return Base64.encodeToString(encrypted, Base64.DEFAULT)
}

private fun decrypt(alias: String, encrypted: String): String {
    val cipher = initializeCipher(alias, Cipher.DECRYPT_MODE)
    val decrypted = cipher.doFinal(plain.toByteArray()) ?: byteArrayOf()  // TODO catch exception
    return String(decrypted)
}

private fun initializeCipher(alias: String, mode: Int) = Cipher.getInstance("$ALGORITHM/$MODE/$PADDING").apply {
	val iv = BuildConfig.CIPHER_IV  // InitializationVector. 16bytes string.
	val secretKeySpec = buildSecretKey(alias)
    val ivSpec = IvParameterSpec(iv.toByteArray())
    init(mode, secretKeySpec, ivSpec)
}

private fun buildSecretKey(alias: String): SecretKey {
	val keystore = KeyStore.getInstance("AndroidKeyStore").apply { load(null) }
	return if (keystore.containsAlias(alias)) {  // use saved secretKey 
		val entry = (keystore.getEntry(keyAlias, null) as KeyStore.SecretKeyEntry)
		entry.secretKey
	} else {  // generate new secretKey and save it into AndroidKeyStore
		val keyGenerator = KeyGenerator.getInstance(KeyProperties.KEY_ALGORITHM_AES,  "AndroidKeyStore")
		keyGenerator.init(KeyGenParameterSpec.Builder(alias, KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT)
		    .setBlockModes(KeyProperties.BLOCK_MODE_CBC)
		    .setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_PKCS7)
            .build())
        return keyGenerator.generateKey()
}