Great Circle Associates Firewalls
(October 1994)
 

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Subject: Dial-In security
From: padgett @ tccslr . dnet . mmc . com (A. Padgett Peterson, P.E. Information Security)
Date: Thu, 20 Oct 94 08:39:20 -0400
To: "firewalls @ greatcircle . com"@UVS1.dnet.mmc.com

Have gotten several inquiries so will go into greater depth:
I rote:
| IMNSHO this consists of two parts:
| 
| 1) Caller-ID (Nationwide in '95)
| 2) Software OTP (using FIPS 46-2)

>Are you using caller ID for logging or access control purposes?   The
>first I can see as being useful, the second I'm not so sure about.

First you need to understand that the Caller-ID signal is sent between
the first and second rings (there is a detailed FAQ I wrote in the
TELECOM archives along with my Caller-ID .ASP - got rather deep into
the subject two years ago) so the calling number or blocking indication
is available before the line is answered.

My PROCOMM .ASP records the calling number or other indicator in a log file
then consults a list of "approved" numbers. If there is a match, the line
is answered by the modem, if not the call is either not answered or can
be directed elsewhere (e.g. a answering machine set to pick up on the
fifth ring).

Thus an approved number connects to the system, others never even know that
there is a modem there. This is a very effective defense against "war
dialers" (of course just setting modems to answer on the 5th (ATS0=5) or
6th ring instead of the first is almost as good).

Further, this is not a privacy issue to the caller, nothing is preventing
them from blocking, I am just exercising *my* right not to answer a blocked
call.
					Warmly,
						Padgett

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