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From: jslove@starch.enet.dec.com ("J. Spencer Love; 237-2751; SHR1-3/E29  06-Nov-1990 1006")
Newsgroups: misc.security
Subject: Re: Alarm company leased lines (was: Imposing fines for false alarms)
Message-ID: <9012031739.AA24946@ucbarpa.Berkeley.EDU>
Date: 6 Nov 90 15:06:27 GMT
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Approved: security@rutgers.edu

> When there isn't an alarm the phone company polls your box about twice a
> minute to make sure that he's alive and that the phone line is uncut.

I can immediately see a way to defeat this.  Short the line near the house.  
If the loop is at all long, the resistance of the wires between the short and 
the central office will fairly reliably prevent detection of the short, which 
will look like the line is off-hook.  I *guarantee* that an alarm box squeal 
will not get past a short.

One thing that would get past a nonmalicious short in the event of an alarm 
would be to ground (unbalance) one side of the line (a similar technique is 
used by pay telephones), but once the short is in place the wires between the 
short and the house could be safely cut.  In fact, a 600 ohm resistor could 
thwart both short and open detection.

No special gear or knowledge is needed to use this strategy, which can be 
applied at the side of your house unless you also wire your yard for intruder 
detection (help, officer, there's a killer poodle pissing in my flower bed).  
A separate security company leased line, on the other hand, has at least the 
possibility of being concealed.

In the ISDN era, special hardware will be required to simulate a working line, 
since a continuous digital dialogue is possible over the 16Kbit call-control 
channel even when two other conversations are in progress over the same wires. 
A short and cut wires look the same at the link level.

To defeat Mission Impossible burglars, public key encryption can be used 
between the house and the security company, a la "smart cards".  The security 
company can require a different response to each challenge and the 
transformation can be kept secret.  However, it'll be several years (at least) 
before ISDN is widely available in the U.S. to residential subscribers.

						-- Spencer


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