Re: VNC authentication weakness

[Mike Porter <mike () UDel Edu>]

> To be more specific, there are two things you need in a challenge
> value:  uniqueness and unpredictability.  Lack of uniqueness allows an
> attacker to replay a past response to a future challenge.  Predictability
> allows an attacker to pre-fetch a correct future response from one of the
> parties.
>
> A counter provides perfect uniqueness (up to its maximum range) but easy
> predictability.  A physical random source provides great unpredictability

A counter is acceptable if it and a value from the entropy pool are
run through MD5 or SHA1.  The "seed" or current state of the
entropy pool must of course be kept in a secure fashion and not
revealed.  You must not ever re-issue a challenge, etc.  The
counter must be used properly and not allowed to wrap without some
sort of reseeding operation.  Otherwise, you will violate the
previous condition.

I have hardly covered all the points.  A good paper seems to be:
http://www.counterpane.com/yarrow.html.

Mike