The RISKS Digest
Volume 17 Issue 60

Friday, 5th January 1996

Forum on Risks to the Public in Computers and Related Systems

ACM Committee on Computers and Public Policy, Peter G. Neumann, moderator

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o Spy Viruses
o German Telecom Software forgets holiday
Debora Weber-Wulff
o Re: Timing cryptanalysis of RSA, DH, DSS
Paul C. Kocher with an interspersed note from Jonathan Epstein
o Risks of installed bases (was: Timing Cryptanalysis)
Barry Jaspan
o IP mistakes and stealing network traffic
John F. Whitehead
o Re: X-31 crash follow up
Pete Mellor
o Computer Security Gathering
Hank Wolfe
o "Physical Attack On Hard Disk Drives" [Anonymized contributor]
o Re: Floating-Point Numbers
Wade Bowmer
o Giving CMU the finger...
Peter da Silva
o Re: Nearest-match alphabetic metrics
Kerry Buckley
o Gasoline Pump Receipt Risks
Charles P. Schultz
o ABRIDGED info on RISKS (comp.risks)

Spy Viruses (Edupage, 31 December 1995)

Educom <>
Mon, 1 Jan 1996 19:48:38 -0500 (EST)

Syndicated columnist Gina Smith predicts a proliferation of computer "spy" viruses similar to Microsoft Windows 95's registration wizard that can zip around your CPU and determine whether you've legally registered all the software you've got loaded on there: "It's already possible to do this sort of scanning without alerting the user, so it doesn't take much of a futurist to imagine the same sort of stealth technology being used on unknowing bulletin board and Internet users. In fact, I think a trend away from juvenile-prank computer viruses to information-seeking `spy' viruses isn't merely likely, it's inevitable." (Popular Science Dec 95 p12)

German Telecom Software forgets holiday

Debora Weber-Wulff <>
5 Jan 1996 13:57:50 GMT

The German Telecom, a private telecommunication system with a monopoly in Germany for 2 more years, has upped considerably the prices for telephone usage, some say to fill the coffers before the competition wars. Local calls now cost 12pf for 90 seconds [and you can't say anything interesting in German under 90 seconds...], calls to Mom and Dad and Aunt Sally are more expensive except when called between 9pm and 5am, etc. The only time one can really afford to call is on weekends and holidays.

The tariffs went into effect on 1 Jan 1996, a holiday [used to sleep off all that alcohol from the night before]. Lucikly, some suspicious people used stopwatches at phone boots to clock the units — and they were being used up at the rate of 1 unit per 90 seconds, just like a normal Monday. Enraged customers flooded the Telecom hotlines; one man tried to have the police take down a report of thievery (they refused); politicians and news commentators tsk-tsk-ed. The next day the Telecom somewhat red-faced admitted that they had "forgotten" to program the first of January as a holiday. They assured that all persons damaged would be compensated - how they are going to do that when we don't have itemized bills in Germany will probably be the basis for one of my next RISKS reports from Germany.

Debora Weber-Wulff, Technische Fachhochschule Berlin, Luxemburger Str. 10, 13353 Berlin, Germany
[Also reported by Thomas Fuckner <>. PGN]

Re: Timing cryptanalysis of RSA, DH, DSS (Tomazic, RISKS-17.59)

Paul C. Kocher <>
Wed, 3 Jan 1996 01:09:49 -0800

> 1) Security of practical cryptosystems do not rest solely on security of
> crypt algorithm. [...]

Using this logic, the fact that there are often be other weaknesses should make people panic even more. While there are usually additional ways to attack a cryptosystem, a weakness in the cryptography will often completely destroy the system's security.

The primary reason the attack isn't a cause for panic is that (1) it can be fixed and (2) there aren't many sophisticated cryptosystems online yet that use high-value certificates.

> 2.) It is not so difficult to rewrite algorithms to be resistant to timing
> attacks, i.e., to have execution time independent of secret key. For
> example, the algorithm to compute R = y^x mod n given in the Kocher paper
> can be simply rewritten as: [program deleted; see RISKS-17.59. PGN]
> to be resistant to timing attacks.

This does not work. As I explain in the abstract, this will not eliminate the RISK and actually tends to make the problem worse.

The problem is that doing the multiply regardless of whether the exponent bit is set does not eliminate the timing variations. It just pushes the variation forward until the next iteration of the exponentiation loop. The amount of time required for iteration i now depends on exponent bit from iteration i-1. This will often help the attacker since zero bits can now be identified from positive correlations instead of the lack of a correlation.

There has been a disturbing amount of technically inaccurate talk about the timing attack. A quick rule of thumb when trying to figure out how many samples are required is to assume that the number of measurements is proportional to:

(timing_signal_size / measurement_error)^2

where timing_signal_size is the size of the signal being detected (in the example above, this would be the standard deviation of the modular multiplication function) and measurement_error is the standard deviation of the error being detected (including both measurement errors and errors induced by the not-yet-attacked portions of the modexp function). Good local network connections seem to have a S.D. of about 1ms, while well-connected Internet sites typically have a S.D. of a few milliseconds.

As explained in the abstract ( there are four good solutions to the timing attack RISK:

  1. Don't reuse keys (impractical with certificates)
  2. Prevent attackers from making timing measurements (i.e. have a standard deviation of at least several seconds)
  3. Make modular exponentiation take exactly constant time.
  4. Blind before the modexp and unblind afterward.
Note that (3) is extremely difficult to do on most computers since factors like system load, system response time, etc. can leak timing information. Even on cryptographic tokens, details like power consumption can still leak timing information. Option (4) is thus the simplest solution.
Paul Kocher, Crypto consultant 415-354-8004, FAX: 415-321-1483
[Jonathan Epstein <> also commented on Tomazik's program, warning about using an optimizing compiler that outsmarts you (in this case, realizing that the "B" computation need not be performed). ``This is one of the reasons why it is hard to build a portable, secure solution; a solution that works securely in *your* environment may be unsecure in other environments.'' PGN]

Risks of installed bases (was: Timing Cryptanalysis)

Barry Jaspan <>
Tue, 2 Jan 96 14:34:52 EST

I am dismayed by the large number of competent people say that the timing cryptanalysis attack is not really a problem because "it is easy to rework the implementation to avoid the attack."

The fact that implementations CAN be fixed is not the point. There are many systems ALREADY DEPLOYED that have this problem. Those systems are already at risk. The fact that it takes an engineer a few hours to develop a technical solution is irrelevant. History shows that it takes a long time to update systems in the field.

There are good reasons not to get too worried about this kind of attack. My point is that "it is easy to fix" is NOT among them.

Barry Jaspan,

IP mistakes and stealing network traffic (Bereza, RISKS-17.59)

"John F. Whitehead" <>
Wed, 3 Jan 1996 10:51:33 -0500

As I was about to send my message, Bill Bereza <> wrote about "Dynamic IP mistakes" to RISKS 17.59:

>I think the biggest risk from this is that I was receiving dozens of possibly
>private messages intended for someone else. Mistakes in configuring name
>servers or assigning IP addresses could cause anyone's mail to be

A similar situation happened to my service provider, not to an individual address, but an entire C-class network.

Last night, an Internet service provider in New York added a block of addresses for themselves to use. They chose addresses already in use by my North Carolina-based ISP, announced them to their provider, who in turn announced them to MAE-East, the major east coast routing information center. The new addresses were established, even though they were already assigned to us. Until they fixed the problem, all traffic for our network got sent to New York. The problem in double-assigning our addresses was compounded by the fact that neither of the N.Y. ISPs had contact people available for 24-hour support.

The risks could be enormous. Not only is there a potential for accidents, but also for sabotage. What if instead of our block of addresses, the ISP had chosen that of the FBI, or the White House? What if someone rerouted a block of addresses to which people were sending credit card order information, for 10 minutes? We have a way to go before we can trust the Internet to our most important messages.

John F. Whitehead OnLine Technical Director 919-821-8605

Re: X-31 crash follow up (Gomez and Fuller, RISKS-17.46)

Pete Mellor <>
Thu, 28 Dec 95 18:23:24 GMT

I am a bit behind with my e-mail, so have only just seen the forwarded mail between Martin Gomez <> and Andy Fuller <> in RISKS-17.46. I would like to take issue with
Martin's view.

Martin disagrees with the statement in the original posting that:- "It is apparent to me that this crash is the result of a bug in the flight control software and sensor hardware of the X-31 aircraft. The computer was unable to compensate for a loss of the airspeed indicator."

and writes:-

> That is a non-sequitur. A "bug" is not the same as a missing feature.
> Microsoft Word 6.0 can't do my taxes for me, but that's not a bug.
> To me, a bug is when the computer does something other than what
> the designer intended. If the designer intended the computer to detect
> and compensate for the loss of a sensor, then you're right, it was a bug.
> My understanding, from talking to engineers on the X-31 project at Dryden,
> is that the flight computer was NOT meant to detect such failures.
> It therefore acted correctly, in the sense that it did what it was
> designed to do. Obviously, it would have been nice if it could detect
> that the airspeed sensor had failed, but it's inability to do that is
> not a "bug" as you suggest.

and later:-

> I assume your article ... is a warning to software designers to be
> careful what they assume, etc. You must realize, though, that
> "it was a software problem" is the perennial cry of the hardware
> engineer who asked for the wrong software, and got what he asked for.

and still later:-

> If a pilot crashes the remaining X-31 because he tried to land it
> vertically, or to fly coast-to-coast, that's not a software problem,
> so neither is failure to detect a sensor error.

Andy offers an (unnecessary, IMHO) apology in response:-

> [Guilty as charged. I agree with you, the X-31 crashed because of a
> SYSTEM DESIGN error, not a software bug. It would have taken more than
> a software change to prevent the loss of control.

Having extracted what I think are the key statements, where do I start?

Martin's attitude sounds horribly like the perennial cry of the airframe manufacturer after an accident: "All systems were behaving as specified!", with the (implicit or explicit) conclusion: "Therefore it must have been pilot error!", except that in this case Martin concludes: "Therefore it was a system requirements specification defect." and furthermore (*TOTALLY* incorrectly, IMHO): "Therefore the programmer is absolved of all blame."

Both Martin and Andy seem to believe that there is a world of difference between a "system design error" and a "software bug".

In fact, a "software bug" is simply a "system design fault" which happens to be located in software rather than in hardware. (I prefer the term "fault" rather than "error" to denote "something wrong with the way the system has been designed".)

In many cases, the argument as to whether it was a hardware or software design fault that caused an accident is about as futile as arguing about the number of angels that can dance on the head of a pin (except perhaps during an internal post-mortem to decide whose head is going to roll!). The system design includes both hardware and software, and the required functions of the system are shared between them. Different designs may provide the same overall function by partitioning the sub-functions differently between hardware and software. A given design fault might be corrected *either* by a software modification *or* by a hardware modification.

For example, the Warsaw A320 crash would probably not have occurred if the "on ground" condition had been recognised by the "compressed" signal being received from *either* of the main landing gear struts instead of from *both* of them. It might also not have occurred if the "Weight on Wheels" (WOW) switches had been more sensitive. The logic whereby the "on ground" condition is recognised could have been altered by a software modification. In fact, a hardware modification to the struts to enable the WOW switches to send a signal at a pressure of 2.5 tonne instead of 6.5 tonne has now been retrofitted to all of Lufthansa's A320 fleet.
This will also reduce the probability of a similar accident, and has been available to airlines operating the A320 since before that particular crash, although at the airlines' own expense. (Airbus, of course, state that the retrofit is in no way connected with the conclusions of the crash investigation, and that the modification was originally devised for reasons of comfort rather than safety.)

I am very strongly of the following opinions:-

  1. If the function of a system has been so specified that it behaves
    in an obviously undesirable way in some circumstances, then THAT CONSTITUTES A FAULT IN ITS DESIGN. The fact that the fault originated from the requirements specification phase merely indicates when and at what level a mistake was made, not that there is no fault.

    When I was active in supporting operating systems, a proportion of the incident reports we received were closed with a classification which meant "The system was behaving as specified but the customer still has justifiable grounds for complaint." It was realised that customers would not be fobbed off with the excuse "But that's what the spec. says it should do!", and such reports were treated in exactly the same way as those which were diagnosed as being due to "software bugs" as more narrowly defined, i.e., "The system departed from specified behaviour because the programmer made a mistake while coding." In many cases, design modifications were made at a later release to remove such faults.

  2. The omission of an obviously desirable feature IS A DESIGN FAULT.
    It is just one example of the type of fault that originates in the requirements specification phase.

    In the X-31 case, it is preposterous to argue that the inability of the system to respond in a safe way to a sensor failure of a type which occurs quite frequently is anything other than a FAULT.

  3. What constitutes "required behaviour" is not merely "what is in the
    specification", but "WHAT A REASONABLE USER WOULD EXPECT".

    Systems operate in the real world, not in some abstract model of it. If they are defined according to an inaccurate model of reality, they must be modified to take account of a more accurate model.

    A reasonable pilot would expect a flight control system to react in a safe way to an air-speed sensor failure. Only an unreasonable idiot would expect it to convert a horizontally landing aircraft into a vertically landing aircraft.


    This is one of the main characteristics (IMHO) which distinguishes a "software engineer" from a "monkey see - monkey do" programmer, and is one of the things I try to drum into the heads of my students on the software engineering modules. One of the first things they are taught is that it is essential to find out what the *real* requirements are (as opposed to what the customer initially *thinks* they are), and to use common sense and engineering judgement to elicit a COMPLETE, consistent and unambiguous specification.

    If it did not occur to the people who wrote that code to ask the hardware team what action the system should take in the event of a sensor failure, then they should be drummed into the manager's office, where, in front of a parade of their colleagues, their software engineer's badges are ceremonially torn from their designer T-shirts and their flow-chart templates broken across the manager's knee, before they are marched off to spend the rest of their careers maintaining COBOL programs in the Commercial Division.


    Q: "How many software engineers does it take to change a light-bulb? A: "It can't be done; it's a hardware problem!"

    Sorry! That attitude won't do where real-time safety-critical embedded software is concerned!

I assume that Martin, being a pilot, would not have perpetrated a similar cock-up if he had been handed a duff spec. from which to code?

Cheers! (and a Happy New Year everyone!)

Pete Mellor, Centre for Software Reliability, City University, Northampton Sq. London EC1V 0HB, UK. Tel: +44 (171) 477-8422, Fax: +44 (171) 477-8585

Computer Security Gathering

Hank Wolfe <>
Fri, 22 Dec 95 12:19:50 1300

[Courtesy of John Rushby <>.]

We are holding a computer security conference, called The Gathering, in Queenstown, New Zealand on the 13-15 February 1996 at the Millennium Resort Hotel. Your readers might be interested in visiting our WEB site which is located at:

Hank (Dr. H.B. Wolfe - Chairman: The Gathering)

"Physical Attack On Hard Disk Drives"

<[Anonymized contributor]>
Thu, 21 Dec 1995 18:01:22 -0800

In the September/October 1995 issue of _IDEMA_Insight_ (a magazine for people in the hard disk drive business), there's an article "Adhesive Dispensing Components: A Hidden Source of Silicone Contamination" by Larry Sutton which I find deeply disturbing. The article can be summarized as decrying the fact that you can't buy certified silicone-free dispensing syringes.

This is important because even tiny levels of silicone contamination are fatal to hard disk drives. They screw up the interface between the disk and the head, causing errors and crashes. This article is a complaint that the use of silicone oils as mold release agents in the plastic injection molding of adhesive-dispensing syringes is a cause of reduced manufacturing yield and product lifetime for hard disk drives. Even when the vendor asserts that silicone mold release agents were not used for the syringes, there can be cross-contamination from other presses in the same facility in which silicone oils are used. Just that tiny amount of contamination, on the inside surface of a syringe used to dispense, for example, an adhesive used to attach a disk head to a positioning mechanism, can result in premature failure of the disk drive.

Add to this the fact that the air inside a hard disk drive is at equilibrium with the atmosphere. Every hard disk drive has a vent or "breather hole" through which air can pass. The vent is filtered, so no particles can pass through, but the pores in the filter are much larger than even the largest molecules.

When the drive starts up, it gets warm, which causes the air inside the disk drive to expand. Excess air goes out the breather hole. When the disk is shut down, it cools off, and the air contracts drawing in more air through the breather hole. Even on days when the disk drive is not used, there is passage of air through the breather hole caused by the daily fluctuation in barometric pressure.

So, a water balloon filled with low molecular weight silicones, such as a silicone mold release agent, thrown against an air-conditioning system intake, seems to me to be a potential hard disk drive killer. Even better might be a pure silicone oligomer. E.g., octamethylcyclotetrasiloxane is widely used as a precursor for making silicones such as adhesives and sealants so it's fairly cheap, about $20 a pound for industrial grade. It's both volatile and reactive, undergoing a ring-opening polymerization.

What would it be like to be attacked? If a slow releasing system like an oil were used, the target might not even notice that instead of serving for 5 or 10 years, the hard drives were crashing after 2 or 3 years. If a pure
volatile silicone oligomer were used, who knows? All the drives might crash on the same day.

Re: Floating-Point Numbers

Wade Bowmer <>
Wed, 03 Jan 1996 18:03:46 +1000

Peter Denning mentioned the problems of binary representation of decimal numbers. Bleah! But since it *is* an IEEE standard, the very group who created it would (should?) have known the Risks of that! (Or am I assuming too much?) Of course, I'm sure the real reason that that IEEE standard is so popular is because Intel's co-processors use it. Hmm...

The first computer I owned, a TI-99/4a, used a base-100 encoding scheme for floating point - so it had a *guaranteed accuracy* for 13-digits. True, it was not an efficient use of memory, like the IEEE format, but the IEEE format cannot guarantee accuracy.

Microsoft had their own floating point format for a while. What was it? Was it binary, too? Or did it have a BCD-type storage? Perhaps they should have stuck with it.


Re: Nearest-match alphabetic metrics (Baker, RISKS-17.59)

"Andrew C. Plotkin" <erkyrath+@CMU.EDU>
Fri, 5 Jan 1996 16:28:44 -0500 (EST)

Subject: Giving CMU the finger...

In fact, this *is* a phonetic match, as I understand the technology.
In the world of phonetics...
J and Y are regarded as the same
W and GH are pretty close
B is B
and vowels are ignored. So the CMU white pages system really was doing its best. You would have been much more appreciative if you'd been trying to find H. Yaghoobi, and tried "finger jahuby".

Of course, unless you've had a lesson in the CMU finger system, there's no way to know that it uses such generous phonetic matching, when no exact user-id matches. There is a formulation which does what you want: "finger j.webb"; but there is no way for outsiders to learn this fact.

I guess the moral is that it's risky to extend standard protocols like finger, even if the extension adds functionality.

[Also noted by Leslie M. Barstow III <>, who added that `ghoti' does match `fish' on their system, to use a standard example. (MODERATOR'S NOTE added for nonAnglophones: that is "gh" as in "enough", "o" as in the second "o" in "colonel" (perhaps), and "ti" as in Andrew's "formulation" rather than in "phonetic".) PGN]

Re: Nearest-match alphabetic metrics (Baker, RISKS-17.59)

Peter da Silva <>
Wed, 3 Jan 1996 09:53:36 -0600

[Retitled from Peter da Silva's "Giving CMU the finger"]

> finger: No names match "jwebb" , 1 nearest match is:
> Hooman Yaghoobi (hooman)

It's possible that the string "jwebb" matched some other part of the user's password file entry. Possibly, the encrypted password... (I ran into this problem with a quick shell-script finger clone on Xenix).

[Interesting potential source of inference channels! PGN]

Re: Nearest-match alphabetic metrics (Baker, RISKS-17.59)

Kerry Buckley <>
Wed, 03 Jan 96 16:54:40 0000

This reminds me of a bizarre piece of behaviour exhibited by the Microsoft Word grammar checker (this was in version 5 — I don't know if it's still there):

I ran the grammar checker over a project proposal containing the phrase "Regular QA reviews will take place" (or something similar). The grammar checker highlighted "QA reviews", suggested that this might be a foreign phrase, and asked "Is the expression 'rigor mortis'?".

No particular RISK here, other than that associated with blindly accepting default decisions from grammar or spelling checkers — I wonder what the customer would have made of a proposal stating that "regular rigor mortis will take place"?


Gasoline Pump Receipt Risks

3 Jan 96 06:56:36 -0600

Over the last few months, I have pulled up to self-serve gasoline pumps that accept credit card payment, and noticed that a previous customer has left behind the receipt that gets printed at the end of the transaction. Some pumps make you explicitly hit a button to get a receipt, but others do it automatically.

So what's the risk? The risk lies in the information that you leave behind if you drive away without taking the receipt, or if you simply toss it in the trash nearby. The receipts from different gas companies have different information, but the worst risk I have seen so far is on the Amoco receipt where the account number and CUSTOMER NAME appear on the receipt. I also found a Chevron receipt that has someone's account number on it, and the gas station's name and station number. Since this particular gas station is "Juan's Chevron" I suppose a spoofer could call up Chevron posing as Juan and give their station number to legitimize their spoof, so here there is also a risk to the station owner. Mobil receipts have less information than the Amoco or Chevron ones. But as long as someone gets a legitimate account number, this is probably enough information to perpetrate some damaging fraud (for example, setting up a bogus gas station, then turning in credit card receipts and getting paid for them by the gas provider - this actually happened down here a number of years ago).

Here's a list of the information provided on the receipts (besides the amount of gas and price per gallon) from the three providers I mentioned above. I don't know if the gas companies use different models of pumps in different parts of the country (I'm in South Florida), so your receipts may differ. Perhaps we could collect information from other gas providers, and urge them to be more sensitive to their customers' privacy.

AMOCO: Station name and address, date, time, CUSTOMER NAME, CARD ACCOUNT NUMBER, reference number

CHEVRON: Date, station name and address, station number, CARD ACCOUNT NUMBER, invoice number, authorization number

MOBIL: CARD ACCOUNT NUMBER, invoice number, date, station name and city

Charles P. Schultz

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