Re: Encrypted File Transfer

From: Chung Leong (chernyshevsky_at_hotmail.com)
Date: 01/13/05 

Date: Wed, 12 Jan 2005 20:02:57 -0500

"Philipp Brune" <philipp.brune@t-online.de> wrote in message
news:cs45u8$s5k$01$1@news.t-online.com...
> Hi all,
>
> the following problem occured to me and I hope somebody can
> help me :
>
> I need to implement an encrypted File download with
> a PHP script on the server side and a c# client application.
>
> I know there is a way to symmetrically encrypt files
> with the php mcrypt routines and already did implement a
> protocol based on xml for querying files from the
> server.
>
> So the problem is, I need to generate and exchange a
> random key for each transfer. This is possible,
> for example, with the Diffie-Hellman Key Exchange
> Procedure. Because I found nothing about this
> procedure in the php function list, I began
> to implement it myself.
>
> The problem in this case is that I need arithmetic
> support for Big Numbers (>=1024 Bits). The bc* functions
> are too slow and do not work in this case. An alternative
> would be the GMP Library, which is not supported by the
> ISP where the script will be running.
>
> Setting up my own Server with GMP support is no alternative either in
> this case.
>
> So perhaps anybody knows how I could achieve an encrypted
> file download with different keys for every "Download Session" in
> php. Is it possible over an ssl / https connection ? Or what am I
> missing here ?
>
> I would be very grateful for any hints, tips, sugesstions, points
> to existing code...

Can't rely on a shared secret?

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