"Pascal Janse van Vuuren" <[EMAIL PROTECTED]> writes:
> I'm not a real crypto expert. But, I'm facing a potential (?)
> problem. I've used OpenSSL to negotiate a secure control channel
> between two nodes of a private network. The generated private keys
> are encrypted with a specific password. Naturally, any secure system
> is only as strong as it's weakest link, but yesterday one of our
> developers raised the following concern. (I've included his email
> below)
>
> > MITM is particularly an issue for a proxy product, particularly with a nat.
> > One could write a proxy that provided this functionality!
>
> > Consider this situation, a standard man in the middle:
>
> > 1 Bob connects to the master.
> > 2 Mary intercepts the connection, and makes her own connection to the master.
>
> > Bob <-----> Mary <-----> Master
>
> > Mary is acting like a transparent proxy, and Bob does not know.
>
> > 3 Master send Bob the public key.
> > 4 Mary grabs it
> > 5 Mary creates her own key pair and send the public one to Bob.
> > 6 Bob Encrypts a new "session key" with Marys public key, that he thinks is
> > Masters key.
> > 7 Mary decrypts the data, re-encrypts it with the Real Qbik master key and
> > sends it.
> > 8 Master is happy, and the session starts with the session key.
>
> > Mary has all the pieces of the puzzle.
>
> > We can easily overcome this by using an extra level of security: Encrypting
> > with a shared secret the initial public key that is transmitted.
>
> Our key pairs are pre-generated, along with the associated, self-signed certifcates.
>They won't be used in any other instance, but for negotiating this connection. After
>the control-channel has been negotiated, we do normal user/node authentication, etc.
It's hard to answer your question because you don't say whether you're
using SSL or just some ad-hoc protocol with OpenSSL for your crypto.
For the sake of the rest of the discussion I'll assume you're using
SSL. If you've invented your own protocol you probably have bigger
problems than this.
> Is this a vulnerability, or something we should be concerned about ?
The usual way SSL prevents man-in-the-middle attacks is by having the
client check the server's certificate against a trusted CA. If you're
using self-signed certificates and the client doesn't have any
independent knowledge of the server's certificate you certainly are
vulnerable to a man-in-the-middle attack.
-Ekr
--
[Eric Rescorla [EMAIL PROTECTED]]
http://www.rtfm.com/
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