Re: building compatible zip files "by hand" that are encrypted and compressed

Greg,
I do have a follow-up question for you.  Supposing I
do encrypt with AES and compress with (deflate?) on
the client before uploading.  I will then have a
collection of already compressed and already encrypted
files.

Do you think, then, that I could simply ask the user,
then, before building their ZIP file, which format
they wanted it in?

In other words, are the differences and
incompatabilities between the PKZip and WinZip
strictly in how the headers are built in the ZIP file?
 Or is there some other more complicated difference?

Thanks,

ed

ed
--- Greg Roelofs <[email protected]> wrote:
> Ed,
> 
> > Thank you, that is very helpful information.
> 
> You bet.
> 
> > Supposing then I accept the vulnerable but
> universal
> > 96-bit encryption, how formidable do you think my
> two
> > tasks are:  1) Encrypting and compressing the
> files on
> > the "upstream" side, before the client sends them,
> and
> > 2) Building the ZIP file from the (already
> compressed
> > and encrypted) files later?
> 
> In theory, not too bad.  All the source code you
> need for the
> difficult parts already exists; you just need to
> glue it together
> the right way.  Of course, you're pretty much on
> your own for
> that.
> 
> > I can see already that if I compress the file on
> the
> > client before sending it to the server I must also
> > transmit the original file size to the server; and
> if
> > I eventually allow the end user to select from a
> menu
> > of encryption methods I must also transmit which
> > algorithm was used.  I can add those items to my
> > protocol very easily.  I'm wondering what else I
> might
> > need to send to the server so that it can
> construct a
> > valid ZIP file later.
> 
> Just look at the zipfile header layouts in unzip.h
> or unzpriv.h,
> or do "unzip -Zv" on an archive and look at the kind
> of info you
> get.  Most of it isn't too relevant (i.e., you could
> hardcode it),
> but you will need to compute the CRC of the
> uncompressed data.
> 
> > Are there any librarys or classes related to
> info-zip
> > that would make the two tasks easier?
> 
> Not that we make.  The DynaZip DLLs are based on
> Info-ZIP code, but
> I don't know anything beyond that.
> 
> > Any pointers to documentation would be welcome
> too.
> 
> See appnote in
> ftp://ftp.info-zip.org/pub/infozip/docs/.
> 
> Greg
> _______________________________________________
> Info-ZIP mailing list
> [email protected]
> http://lists.wku.edu/mailman/listinfo/info-zip


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