The RISKS Digest
Volume 12 Issue 57

Monday, 28th October 1991

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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DSA/DSS — Digital Signatures
Ron Rivest
Porn-Sabotage in Italian newspaper
Enrico Musio
Re: MCI Friends & Family
Allan Meers
Do floor vibrations damage disks?
Magnus Redin
Re: Software migration at Johnson Space Center
Doug Burke
A New Twist on "Speed Controlled by Radar"
Andrew C. Green
Call for Papers ESORICS-92
Yves Deswarte
Info on RISKS (comp.risks)

DSA/DSS — Digital Signatures

Ron Rivest <>
Sat, 26 Oct 91 23:12:33 EDT
Director, Computer Systems Laboratory
ATTN: Proposed FIPS for DSS
Technology Building, Room B-154
National Institute of Standards and Technology
Gaithersburg, MD  20899

Dear Director,

I'm writing to comment on the public-key digital signature algorithm, which you
call ``DSA'', that you have proposed to become a standard.  This letter is in
response to your request for comments to this proposal.  Although I have other
comments about this algorithm (to be made in another letter), and I believe
that an RSA-based standard would serve the country much better, I will restrict
my comments here to just a discussion of your proposed key size of 512 bits for
the DSA.  I believe that a national standard based on such a fixed small key
size would serve our country very poorly---you are unnecessarily risking
catastrophic failure of the integrity of our financial, industrial, and
governmental information-processing systems.

To begin with, I question the rationale for having a fixed key-size at all as
part of your proposal.  Clearly, one needs a minimum key-size to prevent users
from choosing keys-sizes that are too short to be secure.  And one might want a
maximum key-size so one can design efficient yet fully-compatible
implementations.  Yet there is no obvious reason why the minimum required
key-size and the maximum allowed key-size should be the same.

Indeed, your ``one size fits all'' proposal is a very poor match to the
engineering and security needs of most public-key applications.  Typically,
there are many users, each of whom has a public key used by others to verify
his digital signatures.  Such applications are invariably based on the use of
``certificates,'' so verifiers can assure themselves that they are verifying
signatures with the appropriate public key.  A certificate is a message
associating a user's name with his public key; this message is itself signed by
a ``certifying authority'' whose public key is widely known.  A typical
signature verification then normally consists of two verifications: one to
verify the certifying authority's signature on the user's certificate, and then
another to actually verify the user's signature.

(As a side note, I observe that this typical structure contradicts your claim
that signing is done more often than verification. In my experience,
verification is typically done at least twice as often as signing.)

A certificate-based application thus incorporates two basic kinds of
signatures: ordinary user signatures and signatures by a ``certifying
authority.''  The latter kind form the backbone of the integrity of the entire
application, since the ability to forge certificates would give an attacker
essentially unlimited power to corrupt the system.  I consider it essential in
secure public-key system design to have certifying authorities use the maximum
allowable key-size.  This size is typically much larger than what an ordinary
user might select for his own public-key size.  As an example, in an RSA-based
scheme, certifying authorities might use keys of 1024 bits, whereas ordinary
users might choose key sizes of 500--800 bits.

In a typical application, the trade-off between security and performance
mandates the use of different key-sizes in different parts of the system.
Certifying authorities, or users with very valuable data, must use very long
keys to achieve the highest possible security level.  Other users, with reduced
security requirements and/or more stringent performance requirements, will use
shorter keys.  Trying to make ``one size fit all'' results either in
unacceptably low security for all users (because all certificates will be
suspect) or unacceptably poor performance for some users.

In a public-key system based on number theory, there is no valid technical
reason for requiring a fixed key size.  The underlying number-theoretic
algorithms can support arbitrary key sizes.  Users and certifying authorities
should be able to choose key sizes according to their requirements.

I now turn to a discussion of the particular key size you have chosen: 512
bits.  (By key size, here, I refer to the size of the prime modulus p.)  I
argue here that if you are going to insist on having a fixed key size, then 512
bits is far too short.

I note that you provide no rationale for the choice of your key size.  While it
is my belief that you have been co-opted by the NSA (who fears the use of
widely distributed public keys as a basis for encryption algorithms), I will
restrict my discussion to technical matters rather than political speculation.

In order to estimate the key size necessary, one needs to understand the
computational resources available to an imagined potential attacker, and the
computational difficulty of the underlying cryptanalytic problem.  Let me
address each of these issues in turn.

How much computational power can an imagined attacker bring to bear to
``break'' the system?  This depends on the time period we are talking about
(since technology is rapidly evolving) and the financial resources of the
attacker (to purchase the necessary computing power).

It is necessary to know the expected lifetime of the proposed standard in order
to know what level of security to aim for.  A scheme that is considered
``secure'' today may not be secure in the year 2000, and a scheme considered
secure in the year 2000 may not be secure in the year 2010.  Computer
technology is evolving at an incredible pace, and is likely to continue to do
so for the next few decades.  The security of cryptographic schemes thus tends
to ``erode'' steadily over time, and the design of cryptographic systems must
envision and plan for such erosion.

I would suggest that a digital signature standard should be designed with a
minimum expected lifetime of at least 25 years.  That is, one should design so
that a system adopted in the year 1992 should still be secure in the year 2017.
It should not be possible for an attacker in 2017 to forge a signature, using
the computers available then.

Where does ``25 years'' come from?  To consider the only available precedent of
the lifetime of a NIST cryptographic standard, I note that the DES was adopted
in 1976 and seems likely to still be in widespread use by 1996, twenty years
later.  After a cryptographic signature standard has been terminated, one needs
to have an additional period of time where the validity of signatures can still
be assured.  For example, it is not uncommon to require that signed documents
be retained and be considered legally binding for seven years.  A signature
produced in the year 2010 should still be verifiable in the year 2017, with an
understood assurance that it wasn't just recently forged.  I consider a 25-year
expected lifetime a minimum reasonable requirement for a digital signature

What kind of computational power will be available to an attacker in the year
2017?  It is widely asserted that computational power (per dollar spent) is
increasingly at approximately 40% per year.  Some of my colleagues assert that
45% is a better estimate, but I'll stick to the more conservative estimate.
This means that we have an approximate doubling of computer power (per dollar)
every two years, and an approximate increase of a factor of 4500 after
twenty-five years.  Let's round this off to 5000 for our back-of-the-envelope
calculations (corresponding to 25.3 years at 40% growth/year).  In the year
2017, I expect computer power will be about 5000 times cheaper than it is now.

How big an attack should one prepare for?  Let me suggest that a national
digital signature standard should, at a minimum, be able to withstand an attack
costing an attacker $25 million.  This amount of money is easily available to
large corporations, drug dealers, and third-world countries.  There is no
reason that our national security, in terms of the integrity of our electronic
business, financial, and governmental information-processing systems, should be
vulnerable to an attack costing only $25 million.  Indeed, it is easy to make
an argument for a much higher threshold; it is not hard to imagine scenarios in
which the benefit of a successful attack exceeds $25 million.  However, I'll
continue our back-of-the-envelope calculation with the $25 million figure.

How much computing power can one buy for $25 million?  Today, a workstation
with 100 MIPS (million instructions per second) can be probably be purchased in
quantity for about $5,000.  An attacker wouldn't need all of the peripherals
(screen, floppy disk, mouse, nice cabinent, etc.), and could economize by
sharing power supplies, fans, etc.  He is basically interested in having many
processors, each with a reasonable amount of memory.  Let me estimate that such
a ``stripped-down'' 100-MIPS processor would have an amortized cost today of

A convenient unit of computation is a ``MIPS-year''---the amount of computation
performed by a one-MIPS processor running for a year.  A MIPS-year thus
corresponds to about 32 trillion basic operations.  If we assume that a
100-MIPS processor lasts for about 10 years, we obtain an amortized cost
estimate for today of $100 per MIPS-year of computation.  (Here we are buying
``computation by the yard''; our yard is one MIPS-year, and it should cost
about $100 in quantity.  The details of buying computational power in 2017 I
leave to your imagination; a simple cost-effective way might be to spend
considerably more than \$25 million to purchase hardware, and then to resell
the hardware after the computation is done.)

We therefore can estimate that an attacker with $25 million to spend today
could purchase about 250,000 MIPS-years of computation.  In the year 2017, he
will be able to purchase about 5000 times as much, or 1.25 billion MIPS-years.
I believe that a digital signature standard adopted today should, at a minimum,
be able to withstand an attack of 1.25 billion MIPS-years.  (This sounds like a
lot of computation, but you can see from my arguments above that this is in
fact a rather conservative estimate of the security requirement for such a

How large a key-size is needed to withstand an attack of 1.25 billion
MIPS-years? This depends, of course, on the cryptanalytic problem to be solved.
In the case of your proposed DSA, the basic cryptanalytic problem is the
``discrete logarithm problem'': computing x, given g, p and g^x mod p.  Using
the best-known algorithms for this problem, the number of operations required
is approximately

    L(p) = e^{ sqrt{ ln p  ln ln p}} .

The state of the art in algorithms for the discrete logarithm problem is still
evolving, but the above formula certainly seems like a very conservative
estimate of what will be possible in 2017, since it represents what is possible
today.  For example, with a 512-bit prime p (as in your proposal), we see that

    L(2^{512}) = 6.7 x 10^19 operations
           = 2.1 million MIPS-years

of computation are required to ``break'' a 512-bit problem.  Thus, we see that
your proposed DSA is over 500 times weaker than a conservative analysis
suggests is required.

Another way of stating the above result is that the DSA, as proposed, has a
maximum expected secure lifetime of approximately six years.  (Since 250,000 x
1.4^6.33 is greater than 2.1 million.)

Setting L(p) equal to 1.2 billion MIPS-years, and solving for p, we find that
the DSA should be using keys of at least 640 bits, minimum.

This is, as noted, a conservative estimate.  It doesn't plan for improvements
in the state of the art of algorithms for solving the discrete logarithm
problem, which can have a dramatic effect on the key size required.  It has no
margin built-in for faster-than-expected improvements in hardware,
longer-than-expected use of the DSA, or richer-than-expected adversaries.  The
ability to harness for free ``unused'' processor cycles over large networks of
workstations could also dramatically increase the computational ability of an
adversary, thereby altering the equation further in his favor.  For these
reasons, and as a matter of sound conservative design, I feel that a
substantial ``margin of safety'' should be built into the standard.  Most
importantly, certifying authorities should have generous key-sizes allowed.  I
would strongly recommend allowing key sizes of at least 1024 bits for
certifying authorities, and at least 800 bits for users, in any digital
signature standard. I feel that anything less is short-sighted and risky,
possibly verging on the irresponsible.  In cryptographic systems, responsible
design is conservative design; generous allowance must be made for unforeseen

For all the above reasons, I feel very strongly that your DSA proposal, with
its proposed 512-bit keys, is not sufficiently secure to be acceptable as a
national signature standard.


Ronald L. Rivest, Professor of Computer Science, MIT, Cambridge, Mass. 02139

Porn-Sabotage in Italian newspaper

Enrico Musio <>
Mon, 28 Oct 91 13:12:47 MET
Two national newspapers (Corriere Della Sera and La Repubblica) reported on
25,26,27 October on a series of incidents occured to a third Italian
newspaper,La Notte, circulated in Milan metropolitan area.

On Thursday 24 October someone (probably an insider) altered an advertisement
for a coffee brand,exploiting the lack of acces control of the computer
system used by the editorial staff to prepare the journal.

Each occurrence of the word 'coffee', including the headline, was changed to
the four-letter (in Italian too.. :-) bad word commonly used to denote the
female sexual organ.

The fact was discovered too late to block distribution of the first printing
of the morning edition (35.000 copies).

The day after,the prankster stroke back,twice.  He (or she) turned a definition
in a crossword puzzle into an obscene phrase, and in the horoscope suggested
to Capricorn-born :"explain as soon as possible a misunderstanding with a
colleague:just put your hands on her ***" (politely: 'her buttocks'). The
horoscope modify was caught in time by an emergency revision task-force,but the
crossword wasn't.

The journalists have been denouncing the RISKy situation since last winter, and
are ready to withdraw their signatures from articles if lasts the present
situation in which everyone with minimal skills can modify everything,even the
camera-ready files.

An internal inquiry was open and a denouncement versus unknown presented to law

Enrico Musio, Politecnico di Milano , Italy

re: MCI Friends & Family

Allan Meers - Sun Education/Professional Services <>
Fri, 25 Oct 91 11:07:07 PDT
Calling the 800-FRIENDS number lets you quickly build a list of all the
listed "friends and family" of anyone currently on the service, and with
the zip code, you can build an "ever-widening" circle of connections.

If I were a skip-tracer, tracking down a pastdue bill payer, OR trying to find
an estranged spouse, or anyone who wishes not to be found, this could be an
absolute boon.  Even if you don't subscribe to the MCI service, people who call
you do, and I only need to know of one of your family or friends phone numbers
- and if they use MCI, and have your number listed, it becomes all but "public"

I would expect that any phone company like MCI, or the 800-FRIENDS number to
access their database, would utilize CALLER-ID to track who is calling them.  I
am a fan of CALLER-ID, and believe it to be a valuable tool against this kind
of possible abuse.  If only they would use it for security instead of just
phone marketing.

There are some other interesting risks when scanning the databases of your
friends and family for occurences of your phone number:

        I actually felt insulted that my older brother had all of our
        9 other brothers and sisters listed, BUT NOT ME !   I wonder
        why he didn't feel that I should be listed ?

        A couple of the phone numbers listed were wrong, either because
        of a transposed digit, or because the phone number has changed.
        This means that you think you are getting a discount when you
        call your brother, but in fact, since the number is wrong, you
        are not.  You might have been better off with another company that
        doesn't require a pre-planned list of numbers.

        Not only do you have to have them on your list, but they also
        have to be an MCI customer for you to get the discount.  I think
        you also have to be on their list for that discount.  This means
        that you don't save if you are making calls to any companies or
        people unless they are pre-planned, and inserted on your F&F list.
        There may be a delay between the time you ask that they be listed,
        and the listing becomes effective.

        I was listed on 6 different F&F databases.  So far, MCI has called
        me 3 different times for 3 of those 6 F&F's.  They tell you
        whether or not your listed F&F has accepted (they are on the
        "MEMBERS" list).  If they are on the "NOMINEES" list, there is
        a notation for "Not called/accepted yet", or "Did Not Accept",
        which means you told the MCI salesman no.

        1 of the entries for me is listed "Did Not Accept" (it must be
        they way they list "he said no thanks and hung up on me").
        Even with that entry, I am still receiving other calls.  I expect
        a call for EVERY person who lists me, because apparently they
        don't cross-reference F&F lists for your number to see if you
        have been contacted already.  Either that, or they are hoping
        to wear me down.

        There is a look up service were you can check your account for
        the inclusion of a specific number, rather than just relisting
        all of them.  Every number I tried was marked "not in your
        calling circle", even tho they are listed on the big list.  MCI
        must have serious problems with their database lookup scheme.

        2 numbers listed on the members list were reported as
        "number not in the MCI plan" when I tried to look that number
        and zipcode up for their list.  This is another occurence
        which leads you to believe you are saving when you call them,
        but in reality, you are not.  Both of those people were listed
        on other lists as "Did Not Accept", and I know that neither
        has chosen MCI as their plan.  The people calling them
        are quite fooled however into thinking they are discount calls.

        MCI has a very aggressive phone marketing strategy, and very much
        a part of their tactics, is to call you once a family member
        has been signed-up and say "Don't you want them to save money?".
        If you don't sign up, you make your mom pay a lot more for calls.
        Of course, the hazards, pitfalls and misconceptions arn't well
        explained on the phone or in the literature.

        The phone company can be helpful in translating a phone number
        into a geographical area, while the post office will help
        translate geo-info into zip-code info.

This service isn't for me - not with all the other flat-rate discount plans
from other companies that don't require pre-planned, constantly updated,
limited use, publicly available lists of your personal contacts.

Do floor vibrations damage disks?

Magnus Redin <>
Mon, 28 Oct 1991 01:03:02 GMT
Has anyone had experience with locating a computer hall on the same floor (slab
of concrete) as a machine shop?  Do the vibrations damage the disks?

Magnus Redin, Lysator Computer Club Magnus redin, Rydsv{gen 240C26,
582 51 LINK|PING, SWEDEN         Phone: Sweden (0)13 260046 (Answering machine)

Re: Software migration at Johnson Space Center (Bouchard, RISKS-12.48)

"Doug Burke" <>
Sun, 27 Oct 91 19:05:36 PST
>2) Unisys A-Series (Burroughs) and 1100-Series (Univac/Sperry) ... go from
>desktop processing to major mainframe class processing power with NO required
>changes to the software...

As I said, I am not a salesman.  However, it's necessary that the record be set
straight.  Being conservative, the VAX line spans more than 50 specific
machines from Desktop, to Mainframe and SMP Mainframe Clusters, including
server, Fault Tolerant, and high availability systems all using exactly the
same operating system VAX/VMS.  The range extends from roughly under .8 MIPS,
to over 5000 MIPS, in increments of 2 to 3 MIPS.  All code written on VAX/VMS
is binary compatible across all of these lines with "NO required changes to the
software".  I have been lead to believe that this gives Digital Equipment the
"largest RANGE" given the above criteria.

I must, unfortunately beg ignorance of the Unisys line.  Making claims of
"largest RANGE" though, can be highly subjective, and must be extensively
qualified when dealing in the information processing environment.  This is a
risk that everyone working in the this field deals with on a daily basis.

Doug Burke, Senior Software Specialist, Digital Equipment (Malaysia),

A New Twist on "Speed Controlled by Radar"

Mon, 28 Oct 1991 09:54:29 CST
The current discussions regarding bridge warning signs that don't work and
AT&T's problems with warning signals being ignored bring to mind a recent
experience of mine. I'll leave the evaluation of RISKS to the readership.

There was until recently a rather bad intersection on Route 41 north of
Chicago, where an expressway was interrupted by a traffic light at Clavey Rd.
(This has finally been replaced by an overpass.) As this stop was very
unexpected for northbound traffic speeding out of the city, the intersection
was one of the deadliest in the state. Numerous crashes occurred when 3-D
drivers (Drugged, Drowsy or Drunk) failed to notice the stoplight and plowed
into stopped vehicles. Adding strobe lights to the red traffic lights gave
too-little advance warning, and bump strips (like those at toll plazas) kept
nearby residents awake at all hours.

As an additional remedy, radar transmitters were mounted behind signs on
overpasses near the intersection. The idea was that they would trigger radar
detectors in oncoming traffic and slow it down. I can attest from first-hand
experience that they worked awfully well; my detector would go completely
bonkers about two miles before the intersection, giving me plenty of warning to
slow down.

So far, so good, but after a short time, the transmitters apparently failed (or
were shut down; unfortunately I don't have the details).  Nevertheless, the
experiment raises some questions in my mind: How would anyone in an official
capacity such as the State Police discover that the transmitters had suddenly
failed and the hazard had escalated? (I rather doubt that they have their own
radar detectors in the squad cars!) More to the point, how would they be able
to enforce the speed limits or get coherent radar gun readings in an area
flooded with bogus signals? I'm not taking a position on the 55 mph speed limit
here (and I've had no difficulties with the local constabulary :-), but we know
how difficult it is to fight a ticket imposed by radar gun. I wonder if the
transmitted radar "flood" interfered with speed radar guns, and if so, how they
knew where the limits of the range were. The basic idea of slowing a portion of
the oncoming traffic with radar seems viable, but the attendant RISKS of
error-detection (false indicated speeds and system failure) seem a bit unclear.

Andrew C. Green, Datalogics, Inc., 441 W. Huron, Chicago, Il. 60610
(312) 266-4431  UUCP: ..!uunet!dlogics!acg   Internet:

Call for Papers ESORICS-92

Yves Deswarte <>
Mon, 28 Oct 91 17:28:16 +0100
ESORICS-92                                               CALL FOR PAPERS

        European Symposium on Research in Computer Security

              Toulouse, France, November 23-25, 1992
                        Sponsored by AFCET

AIMS AND TOPICS: The aim of this symposium is to further the progress
of research in computer security by bringing together researchers in
this area, by promoting the exchange of ideas with system developers
and by encouraging links with researchers in areas related to computer
science, information theory and artificial intelligence.

Papers are solicited in the following areas:

Theoretical Foundations of Security
- security models, contribution of models for knowledge representation
- contribution of formal logic and information theory
- formal development techniques

Secure Computer Systems
- operating system security, network security
- security management
- virus and worms
- contribution of artificial intelligence
- contribution of new architectures and new technologies

Applications Requesting Security
- data bases, knowledge bases, transaction systems
- process control, real time
- distributed applications

- applications
- validation of protocols
- authentication: protocols, key management, processes

Security Verification and Evaluation
- formal methods
- measure and evaluation of risks
- measure and evaluation of security
- criteria

Software Development Environments for Security

Operation of Secure Systems
- management
- intrusion detection

This list is not exhaustive. Research papers, position papers and panel
proposals will be welcomed.

SUBMISSIONS: Five copies of papers or panel proposals should be submitted
to the program chair by April 3, 1992 at the following address:

                      Jean-Jacques Quisquater
                        AFCET - ESORICS-92
                      156, boulevard Pereire
                       75017 Paris - France

The texts must be submitted in French or in English. Papers should be limited
to 6000 words, full page figures being counted as 300 words.  Each paper must
in clude a short abstract and a list of keywords indicating subject
classification. Papers will be refereed and the final choice will be made by
the Program Committee. Notification of acceptance will be sent by June 15,
1992, and camera-ready copy will be due on September 1, 1992.

Panel proposals should include title, proposed chair, tentative panelists, a 2
or 3 paragraphs description of the subject, format of the presentation, and
rationale for the panel.

For further information and/or copy of the advance program when available, send
E-mail to:
or write to:
              AFCET, 156 bd Pereire, 75017 Paris, France.

Submission deadline: April 3, 1992
Acceptance notification: June 15, 1992
Camera-ready copy due: September 1, 1992

GENERAL CHAIR: Gerard Eizenberg (ONERA-CERT, France)

CHAIR: Jean-Jacques Quisquater (UCL, Belgium)
Bruno d'Ausbourg (ONERA-CERT, France)
Joachim Biskup (Universitat Hildesheim, Germany)
Peter Bottomley (RSRE, United Kingdom)
David Chaum (CWI, Netherlands)
Yvo Desmedt (University of Wisconsin-Milwaukee, USA)
Yves Deswarte (LAAS-CNRS & INRIA, France)
Gerard Eizenberg (ONERA-CERT, France)
Amos Fiat (University of Tel-Aviv, Israel)
Dieter Gollmann (University of London, United Kingdom)
Franz-Peter Heider (GEI, Germany)
Jeremy Jacob (Oxford University, United Kingdom)
Helmut Kurth (IABG, Germany)
Peter Landrock (Aarhus University, Denmark)
Jean-Claude Laprie (LAAS-CNRS, France)
Teresa Lunt (SRI, USA)
John McDermid (University of York, United Kingdom)
John McLean (NRL, USA)
Catherine Meadows (NRL, USA)
Jonathan Millen (MITRE, USA)
Emilio Montolivo (Fondazione Ugo Bordoni, Italy)
Alfredo de Santis (Universita di Salerno, Italy)
Einar Snekkenes (NDRE, Norway)
Marie-Jeanne Toussaint (Universite de Liege, Belgium)
Kioumars Yazdanian (ONERA-CERT, France)

CHAIR: Yves Deswarte (LAAS-CNRS & INRIA)
Laurent Cabirol (SCSSI)
Jean-Francois Cornet (CORESIA)
Michel Dupuy (ENST)
Marie-Therese Ippolito (LAAS-CNRS)
Paul Richy (CNET)
Pierre Rolin (ENSTA)
Kioumars Yazdanian (ONERA-CERT)

===== Yves Deswarte - LAAS-CNRS & INRIA - 31077 Toulouse (France) =====
==== - Tel:+33/61336288 - Fax:+33/61336411 ====

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