The RISKS Digest
Volume 16 Issue 64

Saturday, 10th December 1994

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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Re: Interesting product claim (more Pentium stuff)
Paul N. Hilfinger
Re: Formal Methods ... Intel FDIV bug and verifying FPUs
Miriam Leeser
Re: Formal verification of the AAMP5
Re: Multicast backbone blunder
Derek Atkins
Re: Digital Cash
A. Padgett Peterson
Re: Cellular One roaming in NYC
Alan Clegg
Re: "Good Times" virus
Steve Summit
Susanne Forslev
Re: Fun with your phone company
Andy K
Re: Digital cash on the web
Hal Pomeranz
German Telecom: technical risks/crime
Klaus Brunnstein
Info on RISKS (comp.risks)

Re: Interesting product claim (more Pentium stuff)

"Paul N. Hilfinger" <hilfingr@CS.Berkeley.EDU>
Fri, 09 Dec 1994 16:31:47 -0800

Well, William Kahan informs me that

(1) Inmos used formal methods for the FP on the T800, but that they
    got the specification wrong.

(2) Intel actually had in its possession a test that Kahan had
    designed that uncovers the flaw after about 2 minutes of testing.
    The test (as I understand it) is sort of "gray box" in that it
    makes certain assumptions about the class of algorithm used.

These points notwithstanding, however, Kahan still agrees that there
is no substitute for formal proof in these cases.

Paul Hilfinger

Formal Methods ... Intel FDIV bug and verifying FPUs

Miriam Leeser <mel@ultrastar.EE.CORNELL.EDU>
Thu, 8 Dec 94 13:33:22 EST

The Intel FDIV bug occurs both in single and in double precision
divide.  It is NOT a double precision error.  The best source of
information on this is the web site:

The Pentium uses a subtractive division algorithm based on a radix-4
Booth SRT algorithm.  Similar algorithms are used for square root and
division in this and several other processors.  (We are in the process
of working on a survey article of division and square root
implementations in recent microprocessors.)

The closest verification work I know of is a proof of
a radix 2 subtractive square root chip I did with some students here
at Cornell.  The paper describing this proof
appeared in the Conference on Theorem Provers in Circuit Design in
September 1994.  We are working on a proof of a radix 2 subtractive
division implementation.  The two proofs are very similar.

The square root paper is available by anonymous ftp from

Verification of radix 4 is somewhat more difficult, largely because
the implementation depends on a lookup table to guess the next digit.
I have heard rumors that the error is in the translation of a table
used for digit selection, but no substantiation that this is the case.
It is highly likely that the error is related to using the output of
the carry-save adder. Since we have not done a proof of a radix 4
algorithm, we do not consider the lookup table in our proof.

Our proof shows that the basic algorithm does in fact perform a square
root, and then relates this to the register-transfer level
implementation of the hardware. We do not prove the correctness of the
implementation of each of the blocks of hardware (adders, shifters, etc.)
As several people have pointed out, errors occur at several different
levels.  Formal verification is very useful at high levels, but will
not preclude the need for lower level testing or simulation.

Miriam Leeser  Cornell University

Re: Formal verification of the AAMP5 (Rushby, RISKS-16.62)

srivas <>
Fri, 9 Dec 94 13:07:00 -0800

   [John Rushby was incorrect when he said that the AAMP5 did not have
   floating point.  The following message from the authors of the cited
   report gives a more accurate summary of the project.  PGN]

Recently, John Rushby sent a message to this forum giving a reference to a
paper about an application of mechanized verification to a commercial
microprocessor.  I am giving below a summary of the work reported in the
paper.  For those interested in finding out more details about the project,
the paper is available over the web through

or by ftp from

(Note that name of the ftp site has an "ftp" at its head.)

        Formal Verification of AAMP5 Microprocessor:
   A Case Study in the Industrial Use of Formal Methods

Steve Miller                            Manadayam Srivas
Collins Commercial Avionics             Computer Science Laboratory
Rockwell International                  SRI International
Cedar Rapids, Iowa 52498                Menlo Park, CA 94025

The AAMP5 verification was a project conducted to explore how formal
techniques for specification and verification could be introduced into an
industrial process. Sponsored by the Systems Validation Branch of NASA
Langley and Collins Commercial Avionics, a division of Rockwell
International, it was conducted by Collins and the Computer Science Research
Lab at SRI International. The project consisted of specifying in the PVS
language developed by SRI a portion of a Rockwell proprietary
microprocessor, the AAMP5, at both the instruction set and register-transfer
levels and using the PVS theorem prover to show the microcode correctly
implemented the specified behavior for a representative subset of
instructions. The formal verification was performed in parallel with the
development of the AAMP5 and did not replace any production verification

The central result of this project was to demonstrate the feasibility of
formally specifying a commercial microprocessor and the use of mechanical
proofs of correctness to verify microcode. This is particularly significant
since the AAMP5 was not designed for formal verification, but to provide a
more than three-fold performance improvement while remaining object code
compatible with the earlier AAMP2. As a consequence, the AAMP5 is one of the
most complex microprocessors to which formal methods have been applied.  The
AAMP architecture is specifically designed for use with block-structured,
high level languages like Ada in real-time embedded applications.  It is
based on a stack architecture and provides hardware support for many
features normally provided by the compiler run-time environment, such as
procedure state saving, parameter passage, return linkage, and reentrancy.
AAMP5 supports floating point calculations implemented via microcode.

Besides demonstrating the verification of a subset of AAMP5 microcode, an
equally important accomplishment of the project was the development of an
effective methodology that can be used by practicing engineers to apply
formal verification technology to a complex microprocessor design.  This
includes techniques for decomposing the complex microprocessor verification
problem into a set of verification conditions that the engineers can
formulate, and proof strategies to automate the proof of the verification

This methodology was used to formally verify a core set of eleven AAMP5
instructions representative of several instruction classes.  The core set
did not include floating point instructions.  Although the number of
instructions verified is small, the methodology and the formal machinery
developed are adequate to cover most of the remaining AAMP5 microcode.  The
success of this project has lead to a sequel in which the same methodology
is being reused to verify another member of the AAMP family of processors.

Another key result was the discovery of both actual and seeded errors. Two
actual microcode errors were discovered during development of the formal
specification, illustrating the value of simply creating a precise
specification. Both errors were specific to the AAMP5 and corrected prior to
first fabrication. Two additional errors seeded by Collins in the microcode
were systematically uncovered by SRI while doing correctness proofs. One of
these was an actual error that had been discovered by Collins during testing
of an early prototype but left in the microcode provided to SRI. The other
error was designed to be unlikely to be detected by walkthroughs, testing,
or simulation.

Re: Multicast backbone blunder (Clegg, 16.63)

Fri, 9 Dec 94 16:05:31 EST

> The risk turns out to be that the newest version of the NV video
> transmission software allows you to transmit video across the mbone
> without a camera... any window or cursor-followed region will be happily
> transmitted to the world... It is not obvious in ANY way that you are
> actually transmitting any information...

This is not true.  Having played with the latest version of nv last night,
trying to get Multicast to work with Linux 1.1.72, I can attest to the fact
that nv does provide you with a clue that it is transmitting.

First, the button on the bottom of the nv window will change between "click
to start sending" and "click to stop sending", depending on the current
state.  Second, when you are transmitting, you should see your transmission
in the nv session window, which displays all the video feeds currently in

Also, it is possible to configure everything such that nv will be started
with the -recvOnly flag, in which case it wont even let you send video (our
version of sd does this automatically).

I don't think this is necessarily a program error or a program bug, although
it is clearly an interface issue and possibly a clueless user not knowing
what all these neat buttons do!


Re: Digital Cash (Wells, RISKS-16.63)

A. Padgett Peterson <>
Fri, 9 Dec 94 15:27:15 -0500

Sounds a lot like some things I was discussing with some friends. Consider
the following (going to use PGP for an example). A store sends out a
catalogue via E-Mail at the start of the catalogue is the store's PGP public
key (package obtained from ViaCrypt (plug) so legal for commercial use).

The subscriber has a program that on choosing a selection from a menu is
able to use the header to encrypt the selections, credit card number, and
shipping instructions with the store's public key and sign it with the
shopper's private key (Addison Fisher was discussing this sort of thing
quite a few years ago, just now technology has (almost) caught up).

The completed order is then sent to the store. Note that since the shopper
just signed the message, it does not need the shopper's public key to
extract the order, just to authenticate it if repudiated later, but the
store's private key is needed to read it.

Concerned that the header key is someone else's and the whole thing is a
scam ? Call this 800 number and press 9 to get a voice mail certificate
number (eight hex digits and built into PGP now). And the "Home Shopping
MIME" will flash it every thirty seconds on the bottom of the screen. Spice
will know the most popular by heart anyway.

Of course this leads to the question of revoking a key that is compromised -
obviously with the age-old tradition of a classified ad: "I will no longer
be responsible for any debts incurred by..." - not so much a revocation as a

Re: Cellular One roaming in NYC

Alan Clegg <>
Thu, 8 Dec 1994 13:44:18 -0500 (EST)

Recently, wrote about Cellular One turning off roaming in

A friend of mine, after reading the article had this response:

  Funny thing is the fact that the same people that are scanning the ESN
  are also getting that Credit Card number when you make the call.  :)

Hummm... guess that does not burn the cellular company...  Caller Beware.

- -abc [tuning out of NYC]

"Good Times" virus (RISKS 16.62)

Steve Summit <>
Fri, 9 Dec 1994 14:56:10 -0800

I'm also not sure where the real virus is.  I have now received more copies
of the "it's a hoax" message from well-meaning friends (and my own spouse)
than the original "warning."

We should also be careful about shouting too loudly that "There's no such
thing as an e-mail virus!"  As RISKS readers know, there are several
perfectly workable ways of wreaking mayhem via e-mail, and "active" e-mail
is such a seductively attractive concept that sooner or later some
misguideded person will actually implement it (with inadequate safeguards),
and then the fun will really begin.  (Nat Borenstein has written a few
papers about the idea of active e-mail, although his papers of course DO
mention the glaring risks, and propose methods of implementing active e-mail
reasonably safely, if anyone dared to.)

Steve Summit

AOL "Virus"

Susanne Forslev <>
Fri, 9 Dec 1994 16:57:30 -0600 (CST)

After reading in RISKS about the supposed AOL virus, I logged on to my
account to see if I had any mail.  I didn't and I was pleased to find a
button on my opening screen for e-mail information.  I pushed it and found a
very coherent explanation from the management of what can and cannot be done
with e-mail and what to do if someone sends you a questionable file
attachment.  Now I just hope people bother to push the button and read the
message.  With luck, the AOL "Inoculation" has worked.

Sue Forslev

Re: Fun with your phone company

Fri, 09 Dec 1994 18:27:47 EST

A RISK uncovered here is the weakness of the underlying Data Model utilized
by the phone company in question.  Phone companies, in general, seem to have
insisted on a business rule that requires that the telephone number itself
also be the customers account number.

If one were to make an entity-relationship model of this enterprise there
would be several entity-types that utilize the same identifier.
expected to be uniquely identifiable by the telephone number.
Overwhelmingly this is the case, but there are enough exceptions - multiple
lines for one customer, people moving, etc. that it weakens the data model
(or increases its complexity unnecessarily) that the business processes
begin to suffer.  What ought to be merely a change to a relationship between
two entities, or perhaps a value change to an attribute, results in a change
to the existence of the entity itself.

I imagine, in the eyes of the Data Model, that when Russell Stewart moved -
one CUSTOMER entity was deleted (or marked inactive) and another instance
was created.  No wonder they couldn't allocate the money properly!  Each
customer service rep probably has developed their own individual set of
business rules to apply in the case of finding an appropriate instance of a
CUSTOMER entity when the original one no longer exists.

Re: Digital cash on the web [Wells, RISKS-16.63]

Hal Pomeranz <>
Fri, 9 Dec 1994 23:52:52 -0500

In RISKS-16.63, Justin Wells confuses two different methods of paying
for goods and services over the Internet.

First, he talks about submitting credit card information via the FORMS
interface supported by most recent Web browsers.  Assuming both the
client and server support some secure encryption mechanism, it is safe
to transmit your credit card information over the Internet.  The
NetMarket Company, for which I work, is the first company to do secure
encrypted commercial transactions via the Web, though several other
companies are also providing this functionality now.

Note that this is distinct from "digital cash".  The scheme described
above is simply a mechanism for transmitting your credit card
information and is in fact not very different from simply calling a
mail-order house on the phone and using your credit card.

Okamoto and Ohta ["Universal Electronic Cash", Crypto '91] propose six
properties for an "ideal" digital cash system.  Of these properties,
credit card transactions are neither private (anonymous or
untraceable), transferrable (a merchant can't pay somebody else with
your "credit" without a bank doing arbitrage), nor divisible (you
can't "make change" with credit).

Note that there are several competing digital cash standards under
development at this very moment.  Not all of these standards implement
all of the features that Okamoto and Ohta outline in their paper.  I
fervently hope that we will end up with an "ideal" digital cash
standard, but I am not hopeful.

Hal Pomeranz, Engineering Project Leader, The NetMarket Company

German Telecom: technical risks/crime

Klaus Brunnstein <>
Sat, 10 Dec 1994 13:33:11 +0100

German media's awareness about Telecom related crimes was raised
significantly when International Herald Tribune reported on its front page
earlier this week that "several thousands of German Telecom employees" are
suspected to have participated in criminal activities which damaged German
Telecom in the order of 500 million DM. According to this report, telephone
lines were switched to service providers in areas such as Netherlands
Antillas where services such as astrologic reports (horoscopes) and taped
sex conversations are regularly offered at high prices (up to 12 DM/minute);
such services are usually announced in German boulevard newspapers on
specific pages.

Income from such telephone calls is usually divided between German Telecom
which bills it's resp. international tariff, and the related PTT (e.g.
NL-Antillas PTT) which subtracts its tariff from the amount sent and
distributes the rest to the service provider. This trade between the PTTs is
calculated by counting the *total volume of connect time* between related
Telecoms. This implies that Telecom pays more to another PTT than it can
charge to individual customers if some pirates succeed to generate
communication between PTTs even when a real connection was NOT established,
or with other criminal tricks. In cases reported by Herald Tribune, Telecom
employees and service providers worked together to generate a significant
volume of communication. Such procedures are a modern version of earning
real money with "virtual communication" :-)

As German media (with few exemptions) are not well informed about details of
Telecom procedures and systems, much noise was generated, where some
"experts" (e.g. a misinformed member of the Chaos Computer Club :-) said
that hackers may have hacked Telecom computers (which is nonsense, both in
the sense of telephone hacking=phreaking and computer hacking). While
Telecom admitted that investigations were underway (one day later, 2 Telecom
employees and 1 service provider were jailed, being accused with having
damaged German Telecom in the order of 2 million DM), spokesmen immediately
rejected that damage be in the order of 500 million DM. More cases are
evidently underway (both in jailing and reporting :-).

Since some time, public is growingly concerned about Telecom bills as
steadily growing numbers of Telecom customers complain about unexpectedly
high telephone bills. With estimated 600,000 customers (of 35 mio private
customers) complaining this year, and a roughly estimated mean damage of
1,000 DM (as many customers report too high figures over months, with single
bills adding to over 200,000 DM!), the *overall damage for private
customers* may sum up to about 600 million DM! Despite of some recent
damages to enterprise switching systems, discussion concerning potential
economic damage has not reached the media so far.

Unfortunately, German telecom customers so far cannot control their bills
amount and so argue whether they really connected to such service providers.
Different from other technically advanced countries, German customers
receive monthly bills with *sums of telephone units and the total price
which they have to pay*. This is a relic from ancient technologies when
units were counted in electromechanical counters whose actual figures were
photographed for documentation purposes; the photos of a new and the last
month were compared to calculate the difference as the basis of the new
bill. Since some time, digital switching systems (esp. Siemens' EWSD and
Alcatel' S12) are installed in most regional switching offices
(Vermittlungsstellen), where a log-record is stored for each call containing
all essential billing data. While German Telecom only recently offered to
list details of each telephone call if customers apply for this service and
pay a monthly price in the order of 10 DM), a federal parliament's
commission (Petitionsausschuss) recently suggested to the ministry of
Telecommunications that detailed bills should be given and that such service
should be free of a fee (as e.g. in US and Canada).

Presently, a growing number of customers are seeking legal help against such
Telecom bills. In few cases, courts (assisted by technical expertises about
potential faults and points-of-attacks) have sentenced the bills as
irregular.  As in many cases of digital technologies, complexity of Telecom
networks has grown so rapidly that new risks have appeared, e.g. in software
and management of complex switching systems. In several cases, software bugs
were not detected in Telecom's very detailed test process; in one case,
billing records were store doubly, which was only detected "in the field".
Management of such systems has never been analysed for any reasonable
"quality" (even an ISO 9000-based analysis which is not very adequate would
lead to improvements).

In cases of growingly complex systems with growing bugs and management
faults, more customer protection is needed. As customers are rarely able to
relate overly high bills to technical problems of any kind, it should belong
to the professional duties of related experts and their organisations
(international as IFIP; national as ACM, BCS, GI/Germany, IEEE) to provide
expertise for the public in cases such as Telecom criminality (from which
side whatsoever). This may also help to produce better insight of public
media about technologies.

Klaus Brunnstein (Dec.10,1994)

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