[stunnel-users] key+cert+dh risks

Ludolf Holzheid lholzheid at bihl-wiedemann.de
Sat Feb 12 14:16:00 CET 2011

On Fri, 2011-02-11 23:40:10 +0100, Jean-Yves F. Barbier wrote:
> Hi list,
> I'm using stunnel between two machines only, so is there a risk to reuse the
> server stunnel.pem on the client? (and if it's risky, why?)

I wouldn't want to have our server's private key on a windows netbook
of one of our sales people, but if the two machines are in a trusted
environment and the file permissions are set properly, I don' see any
reason why this could be a problem.

Though I think I understood the concept of PKI, I don't know the first
thing about the mathematics used for asymmetric encryption -- maybe
using the same key on both ends is a strange corner case I'm not aware
of. I don't expect this to be the cases, but I don't know.

As far as I understood, the private key is used at session setup only
(for negotiating the session key, which is newly created for each
session). This is to reduce the amount of data that could be used to
'guess' the private key. If you use the same key pair on both ends,
this doubles this data. However, I don't think this raises the chances
of success for guessing the private key to a real-world likelihood.

If I had to set up an SSL-secured point-to-point tunnel, my first idea
would be to use different key pairs on both sides (thinking of the
point-to-point tunnel as a special case of multi-client tunnels),
but I'm not aware of any real reason not to do so.




Ludolf Holzheid             Tel:    +49 621 339960
Bihl+Wiedemann GmbH         Fax:    +49 621 3392239
Floßwörthstraße 41          e-mail: lholzheid at bihl-wiedemann.de
D-68199 Mannheim, Germany

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