Category Archives: Hardware & signals

Electrical engineering aspects of computer security: tamper resistance, eavesdropping, signal processing, etc.

Debug mode = hacking tool?

Hardware & signals

We have recently been implementing an attack on ZigBee communication. The ZigBee chip we have been using works pretty much like any other — it listens on a selected channel and when there is a packet being transmitted, the data is stored in internal buffer. When the whole packet is received, an interrupt is signalled and micro-controller can read out the whole packet at once.

What we needed was a bit more direct access to the MAC layer. The very first idea was to find another chip as we could not do anything at the level of abstraction described. On the second thought, we carefully read the datasheet and found out that there is an “unbuffered mode” for receiving, as well as transmitting data. There is a sentence that reads “Un-buffered mode should be used for evaluation / debugging purposes only”, but why not to give it a go.

It took a while (the datasheet does not really get the description right, there are basic factual mistakes, and the micro-controller was a bit slower to serve hardware interrupts than expected) but we managed to do what we wanted to do — get interesting data before the whole packet is transmitted.

This was not the first occasion when debug mode or debug information saved us from a defeat when implementing an attack. This made me think a bit.

This sort of approach exactly represents the original meaning of hacking and hackers. It seems that this sort of activity is slowly returning to universities as more and more people are implementing attacks to demonstrate their ideas. It is not so much popular (my impression) to implement complicated systems like role-based access control systems because real life shows that there will be “buffer overflows” allowing all the cleverness to be bypassed. Not many people are interested in doing research into software vulnerabilities either. On the other hand, more attacks on hardware (stealthy, subtle ones) are being devised and implemented.

The second issue is much more general. Is it the case that there will always be a way to get around the official (or intended) application interface? Surely, there are products that restrict access to, or remove, debugging options when the product is prepared for production — smart-cards are a typical example. But disabling debug features introduces very strong limitations. It is very hard or even impossible to check correct functionality of the product (hardware chip, piece of software) — something not really desirable when the product should be used as a component in larger systems. And definitely not desirable for hackers …

23rd Chaos Communication Congress

Banking security, Cryptology, Hardware & signals, Internet censorship, Privacy technology

23C3 logoThe 23rd Chaos Communication Congress will be held later this month in Berlin, Germany on 27–30 December. I will be attending to give a talk on Hot or Not: Revealing Hidden Services by their Clock Skew. Another contributor to this blog, George Danezis, will be talking on An Introduction to Traffic Analysis.

This will be my third time speaking at the CCC (I previously talked on Hidden Data in Internet Published Documents and The Convergence of Anti-Counterfeiting and Computer Security in 2004 then Covert channels in TCP/IP: attack and defence in 2005) and I’ve always had a great time but this year looks to be the best yet. Here are a few highlights from the draft programme, although I am sure there are many great talks I have missed.

It’s looking like a great line-up, so I hope many of you can make it. See you there!

Kish's "totally secure" system is insecure

Hardware & signals

Recently, Kish proposed a “totally secure communication system” that uses only resistors, wires and Johnson noise. His paper—“Totally Secure Classical Communication Utilizing Johnson (-like) Noise and Kirchoff’s Law”—was published on Physics Letters (March 2006).

The above paper had been featured in Science magazine (Vol. 309), reported in News articles (Wired news, Physorg.com) and discussed in several weblogs (Schneier on security, Slashdot). The initial sensation created was that Quantum communication could now be replaced by a much cheaper means. But not quite so …

This paper—to appear in IEE Information Security—shows that the design of Kish’s system is fundamentally flawed. The theoretical model, which underpins Kish’s system, implicitly assumes thermal equilibrium throughout the communication channel. This assumption, however, is invalid in real communication systems.

Kish used a single symbol ‘T’ to denote the channel temperature throughout his analysis. This, however, disregards the fact that any real communication system has to span a distance and endure different conditions. A slight temperature difference between the two communicating ends will lead to security failure—allowing an eavesdropper to uncover the secret bits easily (more details are in the paper).

As a countermeasure, it might be possible to adjust the temperature difference at two ends to be as small as possible—for example, by using external thermal noise generators. However, this gives no security guarantee. Instead of requiring a fast computer, an eavesdropper now merely needs a voltage meter that is more accurate than the equipments used by Alice and Bob.

In addition, the transmission line must maintain the same temperature (and noise bandwidth) as the two ends to ensure “thermal equilibrium”, which is clearly impossible. Kish avoids this problem by assuming zero resistance on the transmission line in his paper. Since the problem with the finite resistance on the transmission line had been reported before, I will not discuss it further here.

To sum up, the mistake in Kish’s paper is that the author wrongly grafted assumptions from one subject into another. In circuit analysis, it is common practice to assume the same room temperate and ignore wire resistance in order to simplify the calculation; the resultant discrepancy is usually well within the tolerable range. However, the design of a secure communication is very different, as a tiny discrepancy could severely compromise the system security. Basing security upon invalid assumptions is a fundamental flaw in the design of Kish’s system.

Hot or Not: Revealing Hidden Services by their Clock Skew

Hardware & signals, Privacy technology

Next month I will be presenting my paper “Hot or Not: Revealing Hidden Services by their Clock Skew” at the 13th ACM Conference on Computer and Communications Security (CCS) held in Alexandria, Virginia.

It is well known that quartz crystals, as used for controlling system clocks of computers, change speed when their temperature is altered. The paper shows how to use this effect to attack anonymity systems. One such attack is to observe timestamps from a PC connected to the Internet and watch how the frequency of the system clock changes.

Absolute clock skew has been previously used to tell whether two apparently different machines are in fact running on the same hardware. My paper adds that because the skew depends on temperature, in principle, a PC can be located by finding out when the day starts and how long it is, or just observing that the pattern is the same as a computer in a known location.

However, the paper is centered around hidden services. This is a feature of Tor which allows servers to be run without giving away the identity of the operator. These can be attacked by repeatedly connecting to the hidden service, causing its CPU load, hence temperature, to increase and so change the clockskew. Then the attacker requests timestamps from all candidate servers and finds the one demonstrating the expected clockskew pattern. I tested this with a private Tor network and it works surprisingly well.

In the graph below, the temperature (orange circles) is modulated by either exercising the hidden service or not. This in turn alters the measured clock skew (blue triangles). The induced load pattern is clear in the clock skew and an attacker could use this to de-anonymise a hidden service. More details can be found in the paper (PDF 1.5M).

Clock skew graph

I happened upon this effect in a lucky accident, while trying to improve upon the results of the paper “Remote physical device fingerprinting“. A previous paper of mine, “Embedding Covert Channels into TCP/IP” showed how to extract high-precision timestamps from the Linux TCP initial sequence number generator. When I tested this hypothesis it did indeed improve the accuracy of clock skew measurement, to the extent that I noticed an unusual peak at about the time cron caused the hard disk on my test machine to spin-up. Eventually I realised the potential for this effect and ran the necessary further experiments to write the paper.

With a single bound it was free!

Banking security, Cryptology, Hardware & signals, Internet censorship, Legal issues, Meta, News coverage, Security economics, Security engineering

My book on Security Engineering is now available online for free download here.

I have two main reasons. First, I want to reach the widest possible audience, especially among poor students. Second, I am a pragmatic libertarian on free culture and free software issues; I believe many publishers (especially of music and software) are too defensive of copyright. I don’t expect to lose money by making this book available for free: more people will read it, and those of you who find it useful will hopefully buy a copy. After all, a proper book is half the size and weight of 300-odd sheets of laser-printed paper in a ring binder.

I’d been discussing this with my publishers for a while. They have been persuaded by the experience of authors like David MacKay, who found that putting his excellent book on coding theory online actually helped its sales. So book publishers are now learning that freedom and profit are not really in conflict; how long will it take the music industry?

New card security problem?

Banking security, Hardware & signals, Legal issues, Security engineering

Yesterday my wife received through the post a pre-approved unsolicited gold mastercard with a credit limit of over a thousand pounds. The issuer was Debenhams and the rationale was that she has a store card anyway – if she doesn’t want to use the credit card she is invited to cut the credit card in half and throw it away. (Although US banks do this all the time and UK banks aren’t supposed to, I’ll leave to the lawyers whether their marketing tactics test the limits of banking regulation.)

My point is this: the average customer has no idea how to ‘cut up’ a card now that it’s got a chip in it. Bisecting the plastic using scissors leaves the chip functional, so someone who fishes it out of the trash might use a yescard to clone it, even if they don’t know the PIN. (Of course the PIN mailer might be in the same bin.)

Here at the Lab we do have access to the means to destroy chips (HNO3, HF) but you really don’t want that stuff at home. Putting 240V through it will stop it working – but as this melts the bonding wires, an able attacker might depackage and rebond the chip.

My own suggestion would be to bisect the whole chip package using a pair of tin snips. If you don’t have those in your toolbox a hacksaw should do. This isn’t foolproof as there exist labs that can retrieve data from chip fragments, but it’s probably good enough to keep out the hackers.

It does seem a bit off, though, that card issuers now put people to the trouble of devising a means of the secure disposal of electronic waste, when consumers mostly have neither the knowledge nor the tools to do so properly

Persec 2006 and Naccache on tapping mobile phones

Hardware & signals, Privacy technology

Over the past couple of months I attended about half a dozen events around the world (Brussels, Pisa (x3), Tokyo, Cambridge, York, Milan), often as invited speaker, but failed to mention them here. While I won’t promise that I will ever catch up with the reporting, let me at least start.

I was, with Ari Juels of RSA Labs, program chair of IEEE PerSec 2006, the security workshop of the larger PerCom conference, held in March 2006 in Pisa, Italy. I previously mentioned the rfid virus paper by Rieback et al when it got the (second) best paper award: that was the paper I found most enjoyable of the ones in the main track.

Ari and I invited David Naccache as the keynote speaker of our workshop. This was, if I may say so myself, an excellent move: for me, his talk was by far the most interesting part of the whole workshop and conference. Now a professor at the École Normale Supérieure in Paris, David was until recently a security expert at leading smartcard manufacturer Gemplus. Among other things, his talents allow him to help law enforcement agencies tap the bad guys’s cellphones, read the numbers in their phone books and find out where they have been.

His talk was very informative and entertaining, full of fascinating war stories such as the tricks used to steal covertly an expired session key from the phone of a suspect to decrypt a recorded phone call that had been intercepted earlier as cyphertext. The target was asleep in a hotel room, with his phone under recharge on his bed table, and the author and his agents were in the next room, doing their electronic warfare from across the wall. What do you do in a case like this? pretend to be the base station, reissue the old challenge so that the SIM generates the same session key, and then listen to the electromagnetic radiation from the pads of the SIM while the key is being transmitted to the handset via the SIM’s electric contacts. Brilliant. And just one in a rapid-fire sequence of other equally interesting real life stories.

David, like many of the other speakers at the workshop, has kindly allowed me to put up his paper and presentation slides on the workshop’s web site. It won’t be as good as his outstanding live talk, but you may still find it quite interesting.

On the same page you will also find two more papers by members of the Cambridge security group: one on multi-channel protocols by Ford-Long Wong and yours truly, and one attacking key distribution schemes in sensor networks by Tyler Moore.

Video eavesdropping demo at CeBIT 2006

Hardware & signals

If you happen to be at CeBIT 2006 in Hanover this week, don’t miss a little demonstration of compromising video emanations that I developed (Halle 6, Stand A42, booth of GBS). It shows how easily now cheap FPGA DSP evaluation boards can be turned into impressive home-brew eavesdropping devices.

COVISP demonstration setup at CeBIT 2006

The system shown consists of a log-periodic antenna (not on the photo), a Dynamic Sciences R1250 wideband receiver, and an Altera FPGA DSP Development Kit, Stratix II Edition. The FPGA board is the implementation platform for my COVISP-1 (compromising video emanations processor) circuit. It receives the 30 MHz intermediate-frequency output signal from the UHF tuner, samples it with 12-bit resolution at 120 MHz, applies a number of signal-processing steps (AM demodulation, gain control, clipping, blanking), and outputs the result – along with sync-pulses – onto the connected VGA monitor. It implements all the controls necessary to adjust it precisely and comfortably to the video mode of the eavesdropping target, including a video clock synthesizer with a frequency-resolution of about 1 part-per-billion, necessary for accurate synchronization of the image.
The eavesdropping target to which the demo setup is tuned in on the above picture is a PC with a flat-panel display:
Eavesdropping target of COVISP demonstration at CeBIT 2006

It belongs to a nearby Russian stand, is about 25 meters away from our antenna. Its PowerPoint presentation is clearly readable on our eavesdropping system, which managed to isolate this signal from the many hundred PCs located in the same room.