Schneier on Security

Clive Robinson • January 15, 2013 6:00 PM

@ Y,

Didn’t mythbusters use plexiglass to get past an IR sensor at one point

You can hold up anything that does not alow IR from the object behind it to pass through, provided the surface of the thing you hold up remains at the same temprature as the ambient conditions.

My favourate material for bypassing such alarms is the “100 ohm foam” used to store DIL IC’s. It’s effectivly a conductive carbon loaded foam which is a poor conductor of heat, and is not transmissive of the normal IR frequences, but also as it’s carbon loaded also absorbs microwave radiation as well. As it’s also sufficiently flexible to roll up like an excercise mat it makes quite a protable “stealth shield” for burglars.

However it should always have a significant air gap between it and the burglar to stop conduction issues. You can make it better by getting a roll of the metal foil backed expanded polystyren foam used to provide insulation behind radiator panels and glue it using rubber solution glue foam to fom with the 100 ohm foam. But remember to get the resulting shield the right way around metal foil is usually a very good relflector of microwave energy and IR.

Clive Robinson • January 17, 2013 11:15 AM

@ Wael,

This game has three simple rules

Depending on your (religion/) philosophy there are more or less rules. But fundementaly,

Life’s a game lost when conceived, thus all is in the play

Being an object of surveillance is a subset of life, it’s not a choice you can make, nor as RobertT notes is how it ends.

@ Nick P,

The trick for secure notifications is not doing them on a phone at all. Gotta leave the rest of that idea to your creativity so my obfuscation isn’t blown. 😉

Secure notification requires two things,

1, A secure message obsfuscation (crypto etc).
2, A secure channel to send the secure message.

Of the two message obsfuscation is by far the easiest, even if you are trying to hide it in plain sight (simple code such as “how about a Drink/Coffee/Tea? where D/C/T has another meaning)

There are various reasons why channels are harder to implement and to understand this you need to look at the iideas of Claud Shannon and extend them.

Claud Shannon considered a communication channel to have three basic parts of a transmitter, a medium and a receiver. His original work was based in analysing the medium and it’s effects on communications (Shannon and others went on to develop coding techniques etc from this).

There are two basic channel types,

3, Broadcast or Point to Point.

That are generaly used either singley or in pairs in two basic modes of operation,

4, Simplex or Duplex.

Where the fun starts is when you pipeline a single channel of communication through one or more relays. Relays can work in two basic ways,

5, Forward, Store.

It can be fairly easily seen at this point the basic IP model used for the Internet has a significant advantage over the telephone network in that it can be simplex and packet switched through many relay points and not required to be either circuit switched or duplex as a telephone generaly is.

Nearly all “trade craft” communications is by simplex communications through relays. That is a courier or “cut out” is a forwarding relay and a dead letter drop is a storage relay.

It can be seen that unless latency is an issue a dead letter drop or storage relay is by far the safer option to use as with a little care it can decouple the sending and receiving parties fairly effectivly. Which a go-between / cut-out cannot, as rubber hoses I’m told are still usefull as aids memoir.

One advantage of the internet is you can expand on using one relay to using two or more relays and unlike humans the idiot savants of the internet can provably forget unless deliberatly designed to do otherwise.

Further unlike physical dead letter drops the Internet can have it’s dead letter drops looked at from just about anywhere and fairly anonymously even when under quite intense surveillance.

As I’ve detailed befor anonymous blogs make good cut outs and search engines make good dead letter drops the trick is knowing how to decouple things even when under 100% surveillance [1]. It’s known as the “Prisoners Problem” not to be confused with the “Prisoners Dilemma”.

As an example of what was once possible [4] was to uses a UDP packet where the IP pactet from field was faked. After it had gone through a number of routers it was well neigh impossible to walk back to the original sender.

This was used in a number of attacks one of which was an early DoS amplifier [5], where you would send a ping or other UDP message to a network address (ie binary adress ends in all zeros) with the address of the target in the from field. The result being that each host in the network would send an ack back to the host identified in the from address. With class C networks that could potentialy give 254 acks sent to the victim host for each ping packet sent by the attacker.

@ RobertT,

One of the reasons I still like pagers is the simple fact that their location cannot be easily determined.

Not so long ago I designed and built two gateways one to convert mobile phone SMS to pages, the other to convert pages to mobile phone SMS or send through to the Internet via the AT command set supported by mobile phones to provide “Dialup Modem” equivalence.

You can buy an adrinou board with two serial ports and a couple of Motorola G24 quad band modules for a verry small amount of money and make a dial through gateway the size of a packet of cigerets that with “Super-Market Top-Up” on SIM only service is increadably easy to setup.

To make life even easier some Mobile Broadband USB dongles will do the same job with no soldering involved. Two such dongles and a Rasbery Pi and just a simple script to configure the Linux protocols with an “anonymous pre-pay credit card” to pay for the mobile service providers and you can put a cut-out in a different country well out of jurisdiction. When I’ve a little time I was planning to pop a VoIP relay on such a setup just to see how well it works…

Such a simple device can be built with batteries into a small shoebox size locable weather proof box such as used for external mains power junction boxes. If bolted on the wall behind lift winding equipment or equivalent at the top of a tower block will stay there for years without comment (I put a covert WiFi bridge on a tower block for a client some years ago with a CCTV camera etc and stuck a lable on the outside of it indicating it belonged to a local council Parking Enforcment Company and the last I heard it was still working fine).

[1] For those interested look up the rather interesting subset of covert channels that are Subliminal [2] and Newton channels [3]

[2] Gustavus J. Simmons. The Prisoners Problem and the Subliminal Channel. In Advances in Cryptology – CRYPTO ’83 pages 51–67, New York, 1984. Lecture Notes in Computer Science, ed. D. Chaum.

[3] Ross J. Anderson, Serge Vaudenay, Bart Preneel, and Kaisa Nyberg. The Newton Channel. In Proceedings of the First International Workshop on Information Hiding, pages 151–156 London, UK, 1996. Springer-Verlag.

[4] Many routers sanity check the from address, and even where they don’t quite a few log packet information such that the path can be partialy or fully reverse walked. Some border gateway systems detect and kill such activity not to prevent deliberate attacks but to stop accidental loops building up (which is what the TTL field was designed to do all be it inefficiently).

[5] This attack should nolonger work as most first hop routers these days rather than blindly forward a packet actually sanity check the from address, and drop the packet if the from address is incorrect.