Re: Australian "calculatorcard"

[attila <attila@primenet.com>]

On Mon, 1 Jan 1996, amp wrote:

> DS> I'd think you could have the server safely accept # N, N-60 sec, and
> DS> N+60 seconds; and adjust the server's idea of your card's clock speed
> DS> from that.
> 
> DS> What new risk would that create?
> 
> i would figure the server would give a minute or so for slippage.
> basically the risk is that it would give someone 3 minutes to do a
> brute force attack rather than one. if you have decent security on
> the server side, i.e., disallow the card for 5 minutes or more after 3
> or so failed attempts, brute attacks would be minimized. however, if
> the actual window for a single code is 3 minutes, that increases your
> chance of hitting it as 3 separate numbers would be valid for a given
> card at any given time.
> 

    START <attila>

	Bank wire systems over the SWIFT private wire are time synched
    much closer than a minute although I have never been given more of
    an answer than that.

	given that you have a tolerable high speed link, and are not
    dealing with an overloaded concentrator at the telco -> carrier
    inferface or an overloaded server, I believe you can solve most
    of the windowing problem by:

	1.  client sends number and time to server
	2.  server send what it thinks as time to client
	3.  client can place a delta on servers time for local time
	4.  enter PIN, etc. and you are working with a much narrower
	    window.

	the security risk does not appear to increase from the 
    exchange times and entering the PIN and letting the normal
    progression go forward once v. just monitoring a series of
    successive verifications trying to effect a pattern in the
    hash.

	Secure-ID seems to be a one-time time-based single use
    pad; to me, using a time exchange initiator has the advantage
    of a smaller window, and fewer problems with client machines
    running on strange times which require sloppier time windows.

    END <attila>