[Security] TLS Certificates Verification

Dirk Meyer dmeyer at tzi.de
Tue Aug 19 13:44:42 CDT 2008


"Eric Rescorla" wrote:
> What Dave is suggesting, I think, would be a garden variety TLS handshake with
> whatever ciphersuites you already support and self-signed certs. Then you'd run
> SASL with some challenge/response protocol and channel bindings (you'd
> almost certainly want mutual auth here) and then on the basis of the C/R
> note that you trusted the peer's self-signed cert.

That does no work, SASL is not man-in-the-middle-proof:

user 1 <--> man-in-the-middle <--> user 2

Now both the keys they do not know. But they are from the attacker. If
they use SASL on that link they only know that somewhere in the line
is the user user, they do not know that they talk direct. In this
scenario you would store the man-in-the-middle keys as verified.

One idea: user 1 sends a challenge to user 2. The min-in-the-middle
can see it. Now user 2 generates an answer with password, his public
key and the challenge. Now it generates the md5 sum and sends it
back. user 1 does the same. A min-in-the-middle can not create a valid
md5 because he is missing the password, if there is a
man-in-the-middle user 2 would use a different public key than user 1. 
This could work.


Dirk

-- 
I just found out that the brain is like a computer.
If that's true, then there really aren't any stupid people.
Just people running Windows. 


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