[guardian-dev] zerobin as possible temp store for ChatSecure iOS

Hans of Guardian hans at guardianproject.info
Sun Apr 12 22:59:56 EDT 2015

On Apr 11, 2015, at 6:40 AM, Michael Rogers wrote:

> On 10/04/15 22:17, Hans-Christoph Steiner wrote:
>> I don't know the details of iOS, so I can't really say much there.  So correct
>> me if I get any of that part wrong.  But I think you get the gist of the idea.
>> And to be clear, when I say 'zerobin', I mean any instance of the zerobin
>> software, not only https://zerobin.net.
> If you're encrypting the file yourself rather than using zerobin's JS
> crypto then you could use any pastebin, right?

pastebin.com generally blocks tor.  But yes, your theory generally stands: encrypted base64 text could be posted to any pastebin-like service.  That's actually what Chris and I were originally talking about, but it seemed too difficult to handle all of the variances of the services.

>> We do have to work out how to protect some of the details, and figure out how
>> to integrate with various push services like Apple or Google GCM.  Here's an
>> attempt to flush some of that out, based on your outline:
>> * Phone1 encrypts content with OTR Extra Symmetric Key
> Phone1 could use a fresh key here in order to avoid potentially
> encrypting more than one file with the same key.

As far as I understand it the OTR TLV8 Extra Symmetric Key is generated per session, but I could be wrong.  It has the big advantage of both sides being able to generate it without actually sending it to each other.

>> * Phone1 adds content to background service to upload to zerobin using an
>> upload URL that does not include the key in it (should be possible)
> Must be possible, otherwise zerobin could decrypt the content.

Yup, and hopefully we can post to a zerobin instance using the iOS background service.  That's probably the bigger question mark here.

>> * Phone1 sends a OTR-encrypted message to Phone2 via XMPP that includes the
>> URL to download the content, as well as the key used (we can't rely on OTR's
>> Extra Symmetric Key on Phone2 since the OTR session might be gone by the time
>> Phone2 gets the message to download the content)
>> * Phone2 receives OTR-encypted message and adds URL to background download
>> service, and securely stores the zerobin URL that includes the key
>> * once Phone2 gets the whole file, next time it is in the foreground, it can
>> decrypt and display the file
>> This approach hides the most metadata, especially if one side uses Tor to send
>> or receive.  But it does not work with push services.  Ideally the download
>> URL without the key would be sent via a push service, and that message would
>> be encrypted with a key that both sides have already.  That's the hard part.
> How much work is the push message handler allowed to do? Can you, for
> example, maintain a separate OTR session for push messages?

Hmm, interesting idea.  I think that the timeframe might be too slow for OTR, but maybe there could be an axolotl session via the push framework.  That sounds a lot more complicated to implement though.  TextSecure uses GCM to do all of the message sending, so it should be possible.  I don't know about Apple's push though.


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