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

On 8/29/06, Christ, Bryan wrote:
> All,
>
> Please pardon my naivete.
>
> I was looking at the diagram on the URL listed below and contemplating
> how host fingerprinting prevents MITM attacks.
>
> http://www.vandyke.com/solutions/ssh_overview/ssh_overview_threats.html
>
> So my question is this... Given the illustration in the URL above, what
> prevents Eve from *first* contacting Alice to obtain a fingerprint which
> then gets passed to Bob on the first connection attempt?

The server passes the client its public key; the client generates a
fingerprint of this public key, and verifies that it matches a known
one from previous connections.

Eve can pass Alice's public key to Bob, but she doesn't possess
Alice's private key, so she has no way to interfere further with the
communications (beyond tampering at a network level - introducing
delay, dropping the connection, etc.)

Only if Eve gets in the way of the very first connection attempt, can
she pass her own public key off as Alice's, without Bob detecting it.
On the first connection, he'd have to either trust what he sees, or
verify the fingerprint offline somehow.  On subsequent connections,
the mismatch would be obvious.