On 30.10.2011 20:03, Ben Kennedy wrote:
> On 30 Oct 2011, at 7:50 am, Alessandro Vesely wrote:
> 
>> We don't need to actually trust the certificate if all what is wanted
>> is to encrypt the channel for privacy.
> 
> I bet Sam is going to say: if you don't trust the cert, how can you be  
> sure that a man in the middle did not provide it and that your  
> conversation isn't actually being observed?

I looked up a known name and connected to that IP.  As far as I trust the
DNS, I can trust I'm connected to the correct server.  Although this is not
unbreakable, I don't see how using TLS could make things worse.

I don't understand why a message must be returned as undeliverable if, for
example, a MITM attack is underway.  A notification is certainly due, but
what's wrong with keeping the message queued until the attack is cleared?

> Therefore, such privacy  is a total illusion.
> 
> On an emotional level I agree with you, however, and certainly an  
> encrypted (read: obfuscated) byte stream would at least deter a casual  
> packet-sniffer.

Untrusted encryption is still stronger than obfuscation.  It can cause
systematic server monitoring to become apparent, because systematic DNS
changes are visible.

An intermediate approach could be to have a "starttls-something" database
anyway, where each host's entry contains the state of the last handshake, any
of "known CA", "auto-trusted" with fingerprint and dates, or "broken", with
suitable rules for state changes

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