MASTG-BEST-0066: Implementing Storage Integrity Checks on Android
Implement storage integrity checks in Android apps to detect unauthorized modifications to data stored on the device (for example, in SharedPreferences, files, or databases). These checks raise the cost for attackers who try to tamper with stored data, especially on rooted devices, through backups, or by directly manipulating the app's data directory.
Storage Integrity¶
Compute an HMAC over any data you store on the device before writing it, and verify the HMAC before reading it back. Use a key that is generated and stored in the Android Keystore so that it cannot be extracted from the app package or a backup.
import javax.crypto.Mac
import javax.crypto.spec.SecretKeySpec
fun hmac(data: ByteArray, key: ByteArray): ByteArray {
val mac = Mac.getInstance("HmacSHA256")
mac.init(SecretKeySpec(key, "HmacSHA256"))
return mac.doFinal(data)
}
fun verify(data: ByteArray, tag: ByteArray, key: ByteArray): Boolean {
return hmac(data, key).contentEquals(tag)
}
Alternatively, the Security framework provides java.security.Signature for asymmetric signing and verification of stored data when a public/private key pair is more appropriate than a shared secret.
If you also encrypt the data, follow the Encrypt-then-MAC pattern: encrypt first, then compute the HMAC over the ciphertext.
Warning
Storage integrity checks are bypassable if the attacker can extract the HMAC key (for example, if it is hardcoded in the app or recoverable on a rooted device) or intercept the verification logic at runtime. Treat these as a defense-in-depth control rather than a standalone guarantee. See Shared Preferences for more information on SharedPreferences storage.