No sensitive data stored outside App
No sensitive data stored outside App
To ensure the proper storage of sensitive data, like user credentials, session tokens, and PII, sensitive data must not be stored outside the application container or the system credential storage.
Sensitive data can be exposed through insecure IPC mechanisms or unintentionally leaked to cloud storage, backups, or keyboard cache. Therefore, the risk of losing the mobile device or having it stolen must account for physical access scenarios.
On iOS, developers can leverage the iOS Data Protection API to protect sensitive data. The API relies on
Secure Enclave Processor (SEP) to provide secure cryptographic processing and key management.
Files can be assigned a protection class that offers different levels of protection:
Complete Protection (
NSFileProtectionComplete): A key derived from the user passcode and the device UID protects this class key. The derived key is wiped from memory shortly after the device is locked, making the data inaccessible until the user unlocks the device.
Protected Unless Open (
NSFileProtectionCompleteUnlessOpen): This protection class is similar to Complete Protection, but if the file is opened when unlocked, the app can continue to access the file even if the user locks the device. This protection class is used when, for example, a mail attachment is downloading in the background.
Protected Until First User Authentication (
NSFileProtectionCompleteUntilFirstUserAuthentication): The file can be accessed as soon as the user unlocks the device for the first time after booting. It can be accessed even if the user subsequently locks the device and the class key is not removed from memory.
No Protection (
NSFileProtectionNone): The key for this protection class is protected with the UID only. The class key is stored in "Effaceable Storage", which is a region of flash memory on the iOS device that allows the storage of small amounts of data. This protection class exists for fast remote wiping (immediate deletion of the class key, which makes the data inaccessible).
All class keys except
NSFileProtectionNone are encrypted with a key derived from the device UID and the user's
As a result, decryption can happen only on the device itself and requires the correct passcode.
Since iOS 7, the default data protection class is "Protected Until First User Authentication".
The Keychain can also store small data bits, like encryption keys and session tokens. Access to the keychain is done using a custom API like:
Data stored in the Keychain is protected via a class structure similar to the class structure used for file encryption.
Items added to the Keychain are encoded as a binary plist and encrypted with a 128-bit AES per-item key in
Galois/Counter Mode (GCM). Note that larger blobs of data aren't meant to be saved directly in the Keychain-that's the
purpose of the Data Protection API. You can configure data protection for Keychain items by setting
kSecAttrAccessible key in the call to
configurable accessibility values for kSecAttrAccessible
are the Keychain Data Protection classes:
kSecAttrAccessibleAlways: The data in the Keychain item can always be accessed, regardless of whether the device is locked.
kSecAttrAccessibleAlwaysThisDeviceOnly: The data in the Keychain item can always be accessed, regardless of whether the device is locked. The data won't be included in an iCloud or local backup.
kSecAttrAccessibleAfterFirstUnlock: The data in the Keychain item can't be accessed after a restart until the user has unlocked the device.
kSecAttrAccessibleAfterFirstUnlockThisDeviceOnly: The data in the Keychain item can't be accessed after a restart until the device has been unlocked once by the user. Items with this attribute do not migrate to a new device. Thus, these items will not be present after restoring from a backup of a different device.
kSecAttrAccessibleWhenUnlocked: The data in the Keychain item can be accessed only while the user unlocks the device.
kSecAttrAccessibleWhenUnlockedThisDeviceOnly: The data in the Keychain item can be accessed only while the user unlocks the device. The data won't be included in an iCloud or local backup.
kSecAttrAccessibleWhenPasscodeSetThisDeviceOnly: The data in the Keychain can be accessed only when the device is unlocked. This protection class is only available if a passcode is set on the device. The data won't be included in an iCloud or local backup.
AccessControlFlags define the mechanisms with which users can authenticate the key (
kSecAccessControlDevicePasscode: Access the item via a passcode.
kSecAccessControlBiometryAny: Access the item via one of the fingerprints registered to Touch ID. Adding or removing a fingerprint won't invalidate the item.
kSecAccessControlBiometryCurrentSet: Access the item via one of the fingerprints registered to Touch ID. Adding or removing a fingerprint will invalidate the item.
kSecAccessControlUserPresence: Access the item via either one of the registered fingerprints (using Touch ID) or default to the passcode.
On Android, developers can leverage several capabilities to store data, like Shared Preferences, SQLite, Internal and External Storage but can also leak data through a mechanism like logging, backup, cache ...
Shared Preference is a commonly used API for data storage that can be declared with world-readable permissions. Insecure use of the Shared Preferences API can expose data.
SQLite is another common form to store data and is unencrypted. Developers must prefer encrypted alternatives like SQLCipher that offer improved data privacy.
Internal storage is containerized by default and cannot be accessed by other applications on the device. It is however
possible to set insecure modes
MODE_WORLD_WRITEABLE, which are both deprecated.
External storage is world readable and must not be used to store sensitive data. It is important to note that data
stored out the application container
/data/data/<package-name> will not delete when the application is removed.
Android KeyStore and KeyChain provide secure data storage. KeyStore uses a public key to create an encryption secret to encrypt data. KeyChain is used to store system-wide private keys and will require the user to set a pin or password.