[46210] in Cypherpunks
Re: Australian "calculatorcard"
daemon@ATHENA.MIT.EDU (attila)
Mon Jan 1 04:52:55 1996
Date: Mon, 1 Jan 1996 09:44:13 +0000 (GMT)
From: attila <attila@primenet.com>
To: amp <Alan.Pugh@internetMCI.COM>
Cc: David Lesher <wb8foz@nrk.com>, cypherpunks <cypherpunks@toad.com>
In-Reply-To: <01HZH81Y0DKI95P3WV@MAIL-CLUSTER.PCY.MCI.NET>
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>
