[guardian-dev] Sharing encrypted content with other apps

[Mark Murphy]

On Tue, Jan 28, 2014, at 15:54, Michael Rogers wrote:
> I've been pondering whether it's possible for an Android app to share
> encrypted content - for example, an image stored in a SQLCipher
> database - with another app, without first creating an unencrypted
> copy of the content on disk.

Create a ContentProvider that implements openFile(), using
ParcelFileDescriptor and createPipe() to stream your content from heap
space, based upon your SQLCipher query. So long as the other party needs
a forward-only stream (i.e., no rewinding), a content:// Uri pointing at
your provider should work fine. Note that media playback, in particular,
tends to want rewinding, as I think was discussed on this list last
year.

Here's a sample:

https://github.com/commonsguy/cw-omnibus/tree/master/ContentProvider/Pipe

In my case, I am reading the data from a file, to keep the sample
simple, but you could get your data from anything you wanted, including
an encrypted source.

The catch is that you need to be in position to do the decryption, which
means that you have the passphrase. If the trigger for wanting to share
the content is the user doing something in your UI, you can make sure
that you have the passphrase before calling startActivity() to request
the other app to do something with your content://, so you will be able
to decrypt it. This will not work nearly so well if the content can be
requested at arbitrary points in time, as your process may be gone and
you may not be in position to get the passphrase. That's the stumbling
block that I ran into when pondering wrapping IOCipher in my
StreamProvider (an extension of Google's FileProvider for serving
content from more sources), and I haven't gotten back around to try to
deal with this.

-- 
Mark Murphy (a Commons Guy)
http://commonsware.com | http://github.com/commonsguy
http://commonsware.com/blog | http://twitter.com/commonsguy

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