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List:       koffice-devel
Subject:    Re: Some details about encrypted documents
From:       Thomas Schaap <t.m.schaap () student ! tudelft ! nl>
Date:       2007-02-19 18:32:32
Message-ID: 45D9ED40.8050503 () student ! tudelft ! nl
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Thomas Zander schreef:
> On Monday 19 February 2007 18:09, Thomas Schaap wrote:
>> Thomas Zander schreef:
>>> I'd like to avoid having just 2 choices.   1) fully encrypted,  2) not
>>> encrypted.
>>> I'd love to eventually have partly encrypted. Just some chapters for
>>> example.
>> What use-cases do you see for partially encrypted documents? Although
>> it's technically possible in the standard, I don't think it's really
>> useful.
> 
> Having an encrypted chapter that contains sensitive data.  Like an appendix 
> chapter that contains a table with the budget.
> When opened the user is prompted for a password, and if he doesn't supply it 
> the chapter is removed from the document (not the file). And the document is 
> marked read only (i.e. the user can only save under a different filename)
> Do note that I don't expect this to work next month, or even this year. I just 
> want to make sure any decisions now will not make it hard to do so later.

In that case I suggest we leave it as is right now. I can't change much 
in the implementation now in anticipation of such features without 
actually implementing them right away. It shouldn't be too hard, though, 
if you're willing to invest the time and effort. In fact, decryption 
already allows for partially encrypted files, only won't allow you to 
open them without the password, currently.

> 
>> A problem you'll have in implementing this is that it means checking
>> from top to bottom whether any part of the document can be read and
>> dealing with that from top to bottom as well. Rather expensive, I'd say.
> 
> Yes, and no.
> Reading a document will read the whole document no matter how you look at it. 
> The trick is to make it possible to let the storage layer find out there is 
> an encrypted section while parsing the document, which happens anyway.
> I would not be surprised if the spec says to mark such a section in the 
> manifest anyway.
> Again; this is not for current concern. :)

Indeed the parts are marked in the manifest file: it's the only place in 
fact where it's visible (apart from unreadable files). But like you 
said: it's of no concern right now. We can fuss over the small details 
in a couple of years when we feel like implementing this ;).

> 
>>> In other words; I'd love to be able to save documents as normal, and have
>>> the 'checkbox' in the koffice-wide Gui somewhere.  The result would be a
>>> normal ODF file with the same mimetype and all.  Just with the contents
>>> encrypted.
>> Effectively, an encrypted ODF already is a normal ODF with the same
>> mimetype. The appearance in the saving-dialogue is made by adding it in
>> KoFileDialog, just like the uncompressed and flat XML versions. In fact,
>> this was the main problem for getting it into the save dialogue: it
>> doesn't have it's own mimetype.
> 
> In that case I don't mind having this extra entry in the file dialog, but I 
> surely would like to see a checkbox in the document-info dialog.

I'll check it out. Might turn out not to be too hard.


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