Thanks Christopher
Fundamentals I still don't get (apologies to the wise):
If I was to employ someone using this QCComposition
+compositionWithData method what do I end up with? Are we building a
plugin that I incorporate into a standard .qtz file in the QC Editor
app, or can Xcode actually build a .qtz file that references encrypted
data so when this .qtz file is opened in the QCEditor most of the
contents remains hidden in an encrypted file. I always thought a .qtz
file was just a .plist describing patches and noodles and a bit of
user data.
The issue is I'm not building my own app, I need a .qtz (or .mov) file
to provide the Theme user something to import onto the Keynote slides.
It is desirable that this .qtz file only reveal user adjustable inputs
and little else when opened in QCEditor. It doesn't want to be loading
from another .qtz file that is a plain text .qtz file obviously,
that's all I'm capable of today.
On 20/06/2011, at 1:14 AM, Christopher Wright wrote:
It's seems important to me that the decrypted composition resides
in memory not written, even temporarily, to HD in plain-text. A
force quit of hosting app like Keynote or QC Editor then search
".qtz" & sort by date will turn up the secret composition in plain
text form otherwise (as pointed out by Christopher on the Kineme
thread).
In memory isn't much better than on-disk to a determined adversary.
And to be honest this doesn't require a particularly determined
adversary at all (more than a casual finder-only attacker, but if
that's all you're trying to thwart then vade's "use an encrypted
NSData" is more than sufficient - in fact, that's what the NSData
API was added for. If you're wanting to prevent even less casual
attackers, you'll need to do some leg work to find out how they do
what they do, and then how to prevent that where possible and how to
make it inconvenient where not possible. There's a cost/benefit
curve between complexity and breakability, and you'll need to find
where you're comfortable on that curve based on your constraints
(how much complexity are you willing to write/purchase from a
developer, how much illicit activity do you intent to prevent, etc.)
Okay so feature request to Apple stands, as good as Apple can make it,
all constraints considered. :-) (Ignorance is bliss)
Casual sharing is the lesser issue, although, if a disincentive to
finder copying is possible, great. Effortless competitor pilfering of
techniques is the main concern. If a competitor can hire a ninja
aquatinted in the black-arts to can-open whatever we can produce in
half a day, then I guess I'm no better off bothering.
Are you talking about writing a QC composition in Xcode that can
load and decrypted the NSData blob from another file? Or are we
talking about a writing a QC plugin that loads the NSData and
decodes it and somehow creates a QCComposition object that can bind
to the plugin patch's ports in QC.
QCComposition has documented API initializers, including
+compositionWithFile: and +compositionWithData:. you can create a
mutable data from a file on disk, decrypt it (using any one of a
variety of algorithms), and then feed it to +compositionWithData:.
Your requirements aren't particularly well-defined:
The encryption key remains secret from a inquisitive competitor!
Where would it be stored or would it auto-generated at runtime and
stay secret somehow? Or would it be stored in the Keychain at
installation time thereby becoming safe.
how inquisitive? Also, you're totally missing out on numerous
attack vectors that happen _after_ decryption in memory, regardless
of how locked down the key is. Stuff in the keychain is user-
visible (via the "show password" check box + enter their keychain
password).
The machine does not isolate man from the great problems of nature but
plunges him more deeply into them.
Antoine de Saint-Exupery
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