Military radio signal jams garage doors

I can remember units from about 15 years ago... they had dip switches with 7 (bit) selections to set a code.

...Jim Thompson

| On 4 Dec 2006 21:46:21 -0800, "zeez" wrote: | |>

|>hob wrote: |>

|>> 1) If the military signals are interfering with those digital commands, then |>> the signal must be overwhelming the receivers - they surely are not sending |>> simultaneous multiple sets of "trains" of pulses that fool all those |>> receivers' security, weak as it is. |>

|> I wonder if they sell garage door openers and remotes that use |>challenge-response. It wouldn't be that expensivge to do. | | I have problems with steel doors, and walls that use metal mesh for | the stucco, so my range is crap. | | I'v been considering some kind of IR replacement. Any available | commercially?

Try coax through the wall to an external antenna.

Brings to mind the cell phone extender gimmick... external antenna capacitively-coupled thru the rear vehicle window, then another antenna inside

...Jim Thompson

I don't remember how many switches (IIRC 10) but before the "opener codes now in use, Sears used a trinary code.

All the garage door units I've ever examined had regenerative receivers. Those, of course, overload quite easily, and selectivity is seldom better than a few percent, e.g. a 5-10MHz bandpass @ 300MHz. Can't be more selective than that anyhow--the regenerative detector drifts about that much all by itself.

Best, James Arthur

Contemporary openers use a "rolling code" where the transmitter sends out an changing number every time you press the button. Initially you press a "learn" button on the receiver which allows its to sync up with the transmitter. After that, the receiver will only accept codes that are within some small boundary of where in the sequence it thinks the transmitter

*should* be (it needs some margin in case people accidentally press the button a little early, or if the kids play with it, or...).

This approach prevents capturing & re-transmitting the same signal to get into someone's garage. However, at least in theory if you knew the garage door opener brand you might know the algorithm and hence be able to predict what the next code in the sequence is and generate *that* if you're trying to, e.g., rob the house. Fancier openers then still have DIP switches that must match at the transmitter and the receiver that feed into how the next number in the sequence is created, which prevents that particular attack.

At that point, for the average garage a physical attack becomes much more viable than somehow trying to track multiple transmissions to ascertain the DIP switch settings and break in electronically. It also keeps the garage door opener cheap in that it doesn't have to receive -- still only transmit.

For 'world-wide' arrangements, I think you'd have to consult the ITU.

For other US-Canada, US-Mexico and others: Please do.

RL

ITU has only allocated subauthority, it is WARC that makes the big decisions.

Richard Fynman was a physicist on the Manhattan Project and had a known interest in safes. He was asked to see if he could open a safe - the person with the combination was not there that day. This may not have been the Manhattan Project, but IIRC the safe was to protect classified information. He tried the default combination those safes were shipped with and it worked.

-- bud--

When I was in the Army, years ago in North Africa, I once had to open a safe used for crypto equipment, after the owner had been transferred. The first thing I tried was his birthdate. It worked!

Many hackers get computer access by using "password" as the password.