[guardian-dev] [briar-devel] Securely saving and restoring state on Android

[Michael Rogers]

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On 06/03/13 13:07, Abel Luck wrote:
> (cc'ing guardian-dev, for those readers that want more context see
> the thread archive [1] )

Hi Guardian people!

> Interesting work Michael! This is definitely something we'd be 
> interested in, both for our own apps and supporting as a general
> utility for the larger developer community. Some questions:
> 
> How is the process key generated?

Using the JVM's SecureRandom; likewise for the random IVs. (In the
long term we're planning to write a Fortuna implementation that mixes
the output of the JVM's SecureRandom with other sources. But for now
we just rely on the JVM.)

> The key is stored in the mutable static field?

I decided to get rid of the mutable static field. Instead, we prevent
the superclass from saving or restoring any state and do the necessary
saving and restoring in the subclass:

* Always pass null to super.onCreate()
* Override onSaveInstanceState() and onRestoreInstanceState()
* Don't call super.onSaveInstanceState() or super.onRestoreInstanceState()

Template for onCreate()/onRestoreInstanceState():

@Override
public void onCreate(Bundle state) {
	super.onCreate(null);
	if(state != null && bundleEncrypter.decrypt(state)) {
		// Restore state to views that need it
		Parcelable p = state.getParcelable(SOME_KEY);
		if(p != null) someView.onRestoreInstanceState(p);
		// Restore activity's state as usual
		someInt = state.getInt(SOME_OTHER_KEY);
	} else {
		// Initialise activity's state as usual
		someInt = 123;
	}
}

Template for onSaveInstanceState():

@Override
public void onSaveInstanceState(Bundle state) {
	// Save state of views that need it
	Parcelable p = someView.onSaveInstanceState();
	state.putParcelable(SOME_KEY, p);
	// Save activity's state as usual
	state.putInt(SOME_OTHER_KEY, someInt);
	// Encrypt the bundle
	bundleEncrypter.encrypt(state);
}

> Is RoboGuice necessary or could a vanilla-android version be
> developed?

RoboGuice isn't necessary - in fact it gets in the way. :-) If you're
not into dependency injection you could just as easily use static
methods to generate the key and encrypt and decrypt bundles.

> Finally, I'd love to see the source :)

Sorry, I forgot to include a link in my previous mail. :-) The Briar
impl is split across two classes: BundleEncrypterImpl and
CryptoComponentImpl (two methods at the end of the file).

http://briar.git.sourceforge.net/git/gitweb.cgi?p=briar/prototype;a=blob_plain;f=briar-android/src/net/sf/briar/android/BundleEncrypterImpl.java;hb=HEAD

http://briar.git.sourceforge.net/git/gitweb.cgi?p=briar/prototype;a=blob_plain;f=briar-core/src/net/sf/briar/crypto/CryptoComponentImpl.java;hb=HEAD

A standalone impl would be easier to read, though - I'll put one
together in the next couple of days.

Cheers,
Michael

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