Re: advice needed - secure transfer of client details

[Peter Conrad <conrad () tivano de>]

Hi,

Am Freitag, 29. Oktober 2004 12:18 schrieb Tim James:
> Hi all,
>
> This is a brain teaser. I have an application to
> review which supplies details from the client's
> workstation (derived from files on disk, hostname, IP
> address). It currently implements a Java applet whose
> job is to obtain these details and send them up to the
> server in an ordinary HTTP POST.
>
> This sends alarm bells ringing for me. I have
> developed a simple attack whereby I can replace the
> applet at will with my own code, which can send
> different details for workstation ID, hostname, IP
> address. This falsifies the audit trail from this
> point on and the server is none the wiser.
>
> So, the general problem is this :-
>
> How can a client communicate details that are only
> known to the client, up to a server, in a way that
> cannot be tampered with?

Use encryption. However, I don't think the above is the question you want
to have answered. From what you said before, I suppose the real question is

How can I authenticate the client properly?

Which is much more difficult to answer.

> Why should a server trust the supplied values?

It shouldn't, of course.

> The data for the workstation next to me is known by everyone - why can't 
> I create an applet to reproduce those details, and hence impersonate 
> that workstation ? 

Assuming that, as you say, all data for the workstation next to you is 
known by everyone, you're lost. In other words, if you know all the 
required bits, you can *always* create an applet to impersonate the
workstation next to you.

> I have some ideas but none are totally satisfactory.
>
> 1) Encrypt the data
> This shifts the problem to one of key management.

Encryption is only useful for protecting the communication between client
and server. It does not prevent modification or sniffing of data before 
it is encrypted.

> 2) Checksum the applet

Doesn't work because if you can subvert the applet's code you can also
subvert the integrity protecting code.

> 3) Keep the details on the server in the first place
> and supply some token from the client which cannot be
> impersonated

As I said above, as long as all data from the workstation next to you
is publicly known, anyone can impersonate it. So in order to find a
solution to the problem, you must introduce some kind of secret.

Since you're speaking of "applets" and "HTTP" I suppose your users can
access the machine using a web browser. Which means you cannot store the
secret anywhere on the machine, because any location that can be accessed
by an applet can also be accessed by the browser directly.

Maybe a better option would be to authenticate the people using the
application instead of the workstations used by the people. People can
keep secrets in their heads (as long as they're not unneccessarily
complicated, like more than four digits or so... ;-).

Bye,
        Peter
-- 
Peter Conrad                        Tel: +49 6102 / 80 99 072
[ t]ivano Software GmbH             Fax: +49 6102 / 80 99 071
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