Re: Need some education: Man-in-the-Middle Attacks

On 9/1/06, Eygene Ryabinkin <rea-sec@codelabs.ru> wrote:

> > However, SSH stores the host key in the "known_hosts" file so it
> > doesn't need to ask for it on subsequent connection attempts.  The SSH
> > protocol has a mechanism whereby the client sends something (its
> > "challenge") encrypted with the public key for that server.  The
> > server decrypts it and sends an appropriate "response" back to the
> > client.  This lets the client know for certain that the server is the
> > same one as recorded in known_hosts.  The MITM can't decrypt the
> > challenge so it doesn't know how to respond.
> >
> > I've simplified this quite a bit, but I hope this is enough to answer
> > your question without getting too confusing.  ;-)
>
> Please, read the RFC 4253 and do not oversimplify the things: there is
> no challenges in establishing the initial shared secret in SSH transport
> layer.

You are refering to the wrong RFC, we are not talking about the
transport layer here, we are talking about the connection layer, info
on this can be found in RFC 4251(SSH Protocol Architecture - section
4.1).
The initial shared secret is established after you have performed the
host key checking.

Regards,

Nathan