Subject: TLS and TN

From: John Hind <[email-masked]>
Date: Fri, 5 Dec 1997 10:13:34 -0500

I just wanted to clarify the two-way authentication point, either two-way
authentication OR one-way server authentication with client refusal to
downgrade to non-secure is needed to prevent a man in the middle attack for
TN3270.

For TN3270 we know that if the back-end 3270 host needs to know the end-users
identity it will present a logon screen to collect a userid and password over
the TN data channel, so the key issue is that the client side know for sure
that it is securely connected to the proper TN3270 server rather than a hacker
site simulating the logon sequence (so the ID and password can be stolen or
perhaps the data by acting as a relay site to the real server - note that this
is exactly the kind of attack that the WEB folks have recently seen so we are
not speculating about obscure situations here). One can see that if the hacker
can convince the client though the TN protocols to not use security or if the
client fails to properly verify the server certificate in the PKI case (or
similar server authentication via 1416)  then  the attack will succeed.

John Hind
[email-masked]@internet