From Apple docs: kSecAttrAccessibleAlways
The data in the keychain item can always be accessed regardless of whether
the device is locked. This is not recommended for application use. Items
with this attribute migrate to a new device when using encrypted backups.
If the data is always accessible regardless of if the device has ever been unlocked, then by definition it must not be encrypted, or at least, it must not be encrypted with any key actually derived from the user's password!Consider, for example, iCloud backups. The "kSecAttrAccessibleAlways" data may be encrypted, but its with a key that Apple knows. So using kSecAttrAccessibleAlways sounds a lot like how Google backups store your WiFi password. As 'itsboncheck' mentions down-thread, a better choice (still a serious compromise) may be... kSecAttrAccessibleAfterFirstUnlock
The data in the keychain item cannot be accessed after a restart until
the device has been unlocked once by the user. After the first unlock,
the data remains accessible until the next restart. This is recommended
for items that need to be accessed by background applications. Items
with this attribute migrate to a new device when using encrypted backups.
That sounds like whats happening is the value is stored encrypted in flash in the protected area which is locked with the key derived from the user's pin/password, which is what you want... But they must cache the plaintext value in some temp storage to be able to provide it while the device is locked. That's just my speculation, I don't see Apple divulging the source code anytime soon...Storing a token in the keychain is preferable, because revoking a token is less onerous than nuking a user's password. Keeping the security set to maximum (kSecAttrAccessibleWhenUnlockedThisDeviceOnly) also seems advisable when we're talking about user password, or even access tokens. Just defer the backup task until the device is unlocked. If you absolutely must proceed with the background task while locked, consider issuing a less-trusted token which is ONLY used for background tasks, and provide the absolute least privileges possible to accomplish the background task when presented with that token. Also consider rotating it. For example, whenever a user connects with their full-access token, indicating their device is unlocked, it's a good time to expire the old background-task token and provide a new one. This also lets you do a "mass delete" of existing tokens in case of any funny business, and you'll have automatic recovery ("background processing will start working again next time you unlock"). |
From what I remember, no, they definitely don't save it in plaintext.
It's late, and I don't remember the exact details, but there are places where Apple talks about how it works.
Here's the gist, I think: When the user unlocks the phone, several encryption keys are generated using the pass code. One's used for Available After First Unlock, and that one's stored in RAM till the device reboots. Another is used for items that are only available when unlocked, and that's thrown away every time the phone is locked. Items that are restricted to the device use a key that is also derived from a private device identifier.
So not plaintext, but the decryption key is hanging around in the device.
You're definitely on target with the suggestion for a background task token though.