I'd also be concerned about library support for that form of encryption.
Specifically, if you have a ZIP library in your platform or language
libraries, but it does not support ZIP encryption, it will be far easier
to code a pre-ZIP encryption method than to modify the way your system
library handles ZIPs.
-Rob
Bob Jolliffe <> wrote on 05/12/2010 05:22:08 AM:
>
> Re: [office] Encryption and data leakage
>
> On 12 May 2010 10:06, Malte Timmermann <> wrote:
> > I agree (except for file names, at least OOo doesn't keep file names,
> > but in the end the spec doesn't guide what to do anyway).
> >
> > I would like to avoid way 1+2, but to directly go with way 3.
> >
> > a) It's IMHO the better approach
> > b) Don't introduce interim changes, This breaks compatibility twice,
and
> > I am even not not sure if somebody would implement them at all.
> >
> > Another not yet mentioned approach would be to use/allow standard zip
> > encryption including directory encryption (instead of way 1+2, but not
> > as a replacement for 3).
> > But I don't know if this would allow for different algorithms, nor do
I
> > know if the standard zip encryption is considered to be strong.
> > I guess there are reasons that it hasn't be considered for ODF
> > encryption from the beginning...
>
> I think there may also be IP concerns about using "standard" zip
> encryption. From the application note:
>
> "X. Incorporating PKWARE Proprietary Technology into Your Product
> ----------------------------------------------------------------
>
> PKWARE is committed to the interoperability and advancement of the
> .ZIP format. PKWARE offers a free license for certain technological
> aspects described above under certain restrictions and conditions.
> However, the use or implementation in a product of certain technological
> aspects set forth in the current APPNOTE, including those with regard to
> strong encryption, patching, or extended tape operations requires a
> license from PKWARE. Please contact PKWARE with regard to acquiring
> a license."
>
> Regards
> Bob
>
> >
> > Malte.
> >
> > wrote, On 05/11/10 19:42:
> >> The approach we inherited from ODF 1.1 encrypts each file in the ZIP
> >> independently. Although the contents of the files are not viewable
due to
> >> the encryption, there are bits of information that potential "leak",
such
> >> as:
> >>
> >> 1) The file size
> >> 2) The file date
> >> 3) The file name
> >> 4) The file mime type
> >> 5) The hash of the first 1024 bytes of the file
> >>
> >> For example, even in an encrypted document I could see a file name
called
> >> "big-secret-takeover-june-3.jpg" and know some information that the
person
> >> who wrote the encrypted document might be rather surprised to see in
the
> >> open.
> >>
> >> Although not required by ODF, an implementation, if it is clever, can
> >> avoid some of these leakages. For example, the timestamp of the file
can
> >> be turned into the time of encryption rather than the original time
stamp.
> >> And the file name can be randomized rather than indicate the
original
> >> file name. This might be fine for ODF, since these time stamps and
file
> >> names are not necessary to be preserved. So long as as we preserve
> >> referential integrity of the package, the names of images are not
> >> significant.
> >>
> >> However we still should be concerned here. First, the reason we
split
> >> Part 3 into its own part was the believe that it could be useful for
> >> purposes other than just ODF 1.2. Many of us hoped that it would
other
> >> uses. But I don't think we can assume that all uses can ignore the
> >> original file names and time stamps. These might be significant for
some
> >> uses.
> >>
> >> Second, even within ODF, especially if we allow package extensions,
we
> >> might see items added to packages where the names of files (which may
> >> ultimately end user-defined) cannot safely be renamed to random
names. For
> >> example, there may be referential integrity constraints that a
generic ODF
> >> processor is not aware of. Maybe there is RDF that points to a
contained
> >> image or other package resource. In any case, the approach is very
> >> fragile.
> >>
> >> Finally, even without extensions, and with the use of randomized
names, we
> >> still leak information, based on knowing the size and hash of the
first
> >> 1024 bytes of the file. For example, if I have a copy of "
> >> big-secret-takeover-june-3.jpg" I can easily check to see what
encrypted
> >> documents also contain that same image. I can similarly probe for
any
> >> other resource where I know in advance its size and or contents.
> >>
> >> There are three ways of getting around this problem. (Or at least
two
> >> that come to mind). One is to keep a "shadow directory" for the ZIP,
that
> >> contains the original names, time stamps, and sizes of the files.
Encrypt
> >> this "shadow directory" when the document is encrypted. For example
> >> encrypted file, prepend it with some random bytes (not sure what is
> >> optimal) in order to prevent data leakage of original size and hash
of
> >> first 1024 bytes.
> >>
> >> Another approach is to encode the original full path of the file,
appended
> >> with its timestamp, using the original derived key, base64 encode
that,
> >> and then write that out as the full path for the ZIP entry. That way
you
> >> do not need another file in the ZIP.
> >>
> >> The other way is to move to a whole-package encryption method, rather
than
> >> trying to do this file-by-file.
> >>
> >> -Rob
> >>
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> >>
> >
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