Posted by Jon Markoff, Staff Developer Advocate, Android Security

Illustration by Virginia Poltrack

Have you ever tried to encrypt data in your app? As a developer, you want to keep data safe, and in the hands of the party intended to use. But if you’re like most Android developers, you don’t have a dedicated security team to help encrypt your app’s data properly. By searching the web to learn how to encrypt data, you might get answers that are several years out of date and provide incorrect examples.

The Jetpack Security (JetSec) crypto library provides abstractions for encrypting Files and SharedPreferences objects. The library promotes the use of the AndroidKeyStore while using safe and well-known cryptographic primitives. Using EncryptedFile and EncryptedSharedPreferences allows you to locally protect files that may contain sensitive data, API keys, OAuth tokens, and other types of secrets.

Why would you want to encrypt data in your app? Doesn’t Android, since 5.0, encrypt the contents of the user’s data partition by default? It certainly does, but there are some use cases where you may want an extra level of protection. If your app uses shared storage, you should encrypt the data. In the app home directory, your app should encrypt data if your app handles sensitive information including but not limited to personally identifiable information (PII), health records, financial details, or enterprise data. When possible, we recommend that you tie this information to biometrics for an extra level of protection.

Jetpack Security is based on Tink, an open-source, cross-platform security project from Google. Tink might be appropriate if you need general encryption, hybrid encryption, or something similar. Jetpack Security data structures are fully compatible with Tink.

Key Generation

Before we jump into encrypting your data, it’s important to understand how your encryption keys will be kept safe. Jetpack Security uses a master key, which encrypts all subkeys that are used for each cryptographic operation. JetSec provides a recommended default master key in the MasterKeys class. This class uses a basic AES256-GCM key which is generated and stored in the AndroidKeyStore. The AndroidKeyStore is a container which stores cryptographic keys in the TEE or StrongBox, making them hard to extract. Subkeys are stored in a configurable SharedPreferences object.

Primarily, we use the AES256_GCM_SPEC specification in Jetpack Security, which is recommended for general use cases. AES256-GCM is symmetric and generally fast on modern devices.


val keyAlias = MasterKeys.getOrCreate(MasterKeys.AES256_GCM_SPEC)

For apps that require more configuration, or handle very sensitive data, it’s recommended to build your KeyGenParameterSpec, choosing options that make sense for your use. Time-bound keys with BiometricPrompt can provide an extra level of protection against rooted or compromised devices.

Important options:

  • userAuthenticationRequired() and userAuthenticationValiditySeconds() can be used to create a time-bound key. Time-bound keys require authorization using BiometricPrompt for both encryption and decryption of symmetric keys.
  • unlockedDeviceRequired() sets a flag that helps ensure key access cannot happen if the device is not unlocked. This flag is available on Android Pie and higher.
  • Use setIsStrongBoxBacked(), to run crypto operations on a stronger separate chip. This has a slight performance impact, but is more secure. It’s available on some devices that run Android Pie or higher.

Note: If your app needs to encrypt data in the background, you should not use time-bound keys or require that the device is unlocked, as you will not be able to accomplish this without a user present.


// Custom Advanced Master Key
val advancedSpec = KeyGenParameterSpec.Builder(
"master_key",
KeyProperties.PURPOSE_ENCRYPT or KeyProperties.PURPOSE_DECRYPT
).apply {
setBlockModes(KeyProperties.BLOCK_MODE_GCM)
setEncryptionPaddings(KeyProperties.ENCRYPTION_PADDING_NONE)
setKeySize(256)
setUserAuthenticationRequired(true)
setUserAuthenticationValidityDurationSeconds(15) // must be larger than 0
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
setUnlockedDeviceRequired(true)
setIsStrongBoxBacked(true)
}
}.build()

val advancedKeyAlias = MasterKeys.getOrCreate(advancedSpec)

Unlocking time-bound keys

You must use BiometricPrompt to authorize the device if your key was created with the following options:

  • userAuthenticationRequired is true
  • userAuthenticationValiditySeconds > 0

After the user authenticates, the keys are unlocked for the amount of time set in the validity seconds field. The AndroidKeystore does not have an API to query key settings, so your app must keep track of these settings. You should build your BiometricPrompt instance in the onCreate() method of the activity where you present the dialog to the user.

BiometricPrompt code to unlock time-bound keys

// Activity.onCreate

val promptInfo = PromptInfo.Builder()
.setTitle("Unlock?")
.setDescription("Would you like to unlock this key?")
.setDeviceCredentialAllowed(true)
.build()

val biometricPrompt = BiometricPrompt(
this, // Activity
ContextCompat.getMainExecutor(this),
authenticationCallback
)

private val authenticationCallback = object : AuthenticationCallback() {
override fun onAuthenticationSucceeded(
result: AuthenticationResult
) {
super.onAuthenticationSucceeded(result)
// Unlocked -- do work here.
}
override fun onAuthenticationError(
errorCode: Int, errString: CharSequence
) {
super.onAuthenticationError(errorCode, errString)
// Handle error.
}
}

To use:
biometricPrompt.authenticate(promptInfo)

Encrypt Files

Jetpack Security includes an EncryptedFile class, which removes the challenges of encrypting file data. Similar to File, EncryptedFile provides a FileInputStream object for reading and a FileOutputStream object for writing. Files are encrypted using Streaming AEAD, which follows the OAE2 definition. The data is divided into chunks and encrypted using AES256-GCM in such a way that it’s not possible to reorder.

val secretFile = File(filesDir, "super_secret")
val encryptedFile = EncryptedFile.Builder(
secretFile,
applicationContext,
advancedKeyAlias,
FileEncryptionScheme.AES256_GCM_HKDF_4KB)
.setKeysetAlias("file_key") // optional
.setKeysetPrefName("secret_shared_prefs") // optional
.build()

encryptedFile.openFileOutput().use { outputStream ->
// Write data to your encrypted file
}

encryptedFile.openFileInput().use { inputStream ->
// Read data from your encrypted file
}

Encrypt SharedPreferences

If your application needs to save Key-value pairs – such as API keys – JetSec provides the EncryptedSharedPreferences class, which uses the same SharedPreferences interface that you’re used to.

Both keys and values are encrypted. Keys are encrypted using AES256-SIV-CMAC, which provides a deterministic cipher text; values are encrypted with AES256-GCM and are bound to the encrypted key. This scheme allows the key data to be encrypted safely, while still allowing lookups.

EncryptedSharedPreferences.create(
"my_secret_prefs",
advancedKeyAlias,
applicationContext,
PrefKeyEncryptionScheme.AES256_SIV,
PrefValueEncryptionScheme.AES256_GCM
).edit {
// Update secret values
}

More Resources

FileLocker is a sample app on the Android Security GitHub samples page. It’s a great example of how to use File encryption using Jetpack Security.

Happy Encrypting!

Gmail protects your incoming mail against spam, phishing attempts, and malware. Our existing machine learning models are highly effective at doing this, and in conjunction with our other protections, they help block more than 99.9% of threats from reaching Gmail inboxes.

One of our key protections is our malware scanner that processes more than 300 billion attachments each week to block harmful content. 63% percent of the malicious documents we block differ from day to day. To stay ahead of this constantly evolving threat, we recently added a new generation of document scanners that rely on deep learning to improve our detection capabilities. We’re sharing the details of this technology and its early success this week at RSA 2020.

Since the new scanner launched at the end of 2019, we have increased our daily detection coverage of Office documents that contain malicious scripts by 10%. Our technology is especially helpful at detecting adversarial, bursty attacks. In these cases, our new scanner has improved our detection rate by 150%. Under the hood, our new scanner uses a distinct TensorFlow deep-learning model trained with TFX (TensorFlow Extended) and a custom document analyzer for each file type. The document analyzers are responsible for parsing the document, identifying common attack patterns, extracting macros, deobfuscating content, and performing feature extraction.

Strengthening our document detection capabilities is one of our key focus areas, as malicious documents represent 58% of the malware targeting Gmail users. We are still actively developing this technology, and right now, we only use it to scan Office documents.

Our new scanner runs in parallel with existing detection capabilities, all of which contribute to the final verdict of our decision engine to block a malicious document. Combining different scanners is one of the cornerstones of our defense-in-depth approach to help protect users and ensure our detection system is resilient to adversarial attacks.
We will continue to actively expand the use of artificial intelligence to protect our users’ inboxes, and to stay ahead of attacks.

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As part of our ongoing efforts — along with help from newly developed technologies — today we’re announcing nearly 600 apps have been removed from the Google Play Store and banned from our ad monetization platforms, Google AdMob and Google Ad Manager, for violating our disruptive ads policy and disallowed interstitial policy.
Mobile ad fraud is an industry-wide challenge that can appear in many different forms with a variety of methods, and it has the potential to harm users, advertisers and publishers. At Google, we have dedicated teams focused on detecting and stopping malicious developers that attempt to defraud the mobile ecosystem. As part of these efforts we take action against those who create seemingly innocuous apps, but which actually violate our ads policies.
We define disruptive ads as ads that are displayed to users in unexpected ways, including impairing or interfering with the usability of device functions. While they can occur in-app, one form of disruptive ads we’ve seen on the rise is something we call out-of-context ads, which is when malicious developers serve ads on a mobile device when the user is not actually active in their app.
This is an invasive maneuver that results in poor user experiences that often disrupt key device functions and this approach can lead to unintentional ad clicks that waste advertiser spend. For example, imagine being unexpectedly served a full-screen ad when you attempt to make a phone call, unlock your phone, or while using your favorite map app’s turn-by-turn navigation.
Malicious developers continue to become more savvy in deploying and masking disruptive ads, but we’ve developed new technologies of our own to protect against this behavior. We recently developed an innovative machine-learning based approach to detect when apps show out-of-context ads, which led to the enforcement we’re announcing today.
As we move forward, we will continue to invest in new technologies to detect and prevent emerging threats that can generate invalid traffic, including disruptive ads, and to find more ways to adapt and evolve our platform and ecosystem policies to ensure that users and advertisers are protected from bad behavior.

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Security keys provide the strongest protection against phishing attacks. That’s why they are an important feature of the Advanced Protection Program that provides Google’s strongest account protections for users that consider themselves at a higher risk of targeted, sophisticated attacks on their personal or work Google Accounts.

Last year, we made the Titan Security Key bundle with USB-A/NFC and Bluetooth/USB/NFC keys available in Canada, France, Japan, the UK, and the US. Starting today, USB-C Titan Security Keys are available in those countries, and the bundle and USB-C Titan Security Keys are now available on the Google Store in Austria, Germany, Italy, Spain, and Switzerland.

Titan Security Keys are now available in 10 countries

Security keys use public-key cryptography to verify your identity and URL of the login page so that an attacker can’t access your account even if they have your username or password. Unlike other two-factor authentication (2FA) methods that try to verify your sign-in, security keys support FIDO standards that provide the strongest protection against automated bots, bulk phishing attacks, and targeted phishing attacks.

We highly recommend users at a higher risk of targeted attacks (e.g., political campaign teams, activists, journalists, IT administrators, executives) to get Titan Security Keys and enroll into the Advanced Protection Program (APP). If you’re working in a federal political campaigns team in the US, you can now request free Titan Security Keys via Defending Digital Campaigns and get help enrolling into the APP. Bulk orders are also available for enterprise organizations in select countries.

You can also use Titan Security Keys for any site where FIDO security keys are supported for 2FA, including your personal or work Google Account, 1Password, Bitbucket, Bitfinex, Coinbase, Dropbox, Facebook, GitHub, Salesforce, Stripe, Twitter, and more.