The Risks Digest

The RISKS Digest

Forum on Risks to the Public in Computers and Related Systems

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

Volume 18 Issue 54

Monday 21 October 1996


o A new attack on DES
Adi Shamir
o "Key Recovery" Replaces "Key Escrow" in Encryption Plan
o Apology/Explanation for BBN-Planet outage
John Hight
o Snail causes Liechtenstein's cable TV system to fail
Henning Holtschneider
o Re: Rats take down Stanford ...
William Hugh Murray
o Re: Computers miss $1.2M in ATM withdrawals
William Hugh Murray
o Re: Health Info Database Misused
William Hugh Murray
o People Security versus Computer Security
Li Gong
o Info on RISKS (comp.risks)

A new attack on DES

Shamir Adi <>
Fri, 18 Oct 1996 16:58:50 +0200
You have recently referred in RISKS [18.50, 18.52] to the ingenious new
attack against public key cryptosystems developed at Bellcore. All the
published information on the subject (including Bellcore's press release)
stress that the attack is not applicable to secret key cryptosystems.  Well,
Eli Biham and I have just released a research announcement in which we show
that an extension of the attack can, under the same realistic fault model,
break almost any secret-key algorithm, including DES, multiple DES, IDEA,
etc. The attack on DES was actually implemented on a PC, and it found the
key by analysing fewer than 200 ciphertexts generated from unknown

Adi Shamir

= = = = = =

Research announcement: A new cryptanalytic attack on DES

Eli Biham                                 Adi Shamir
Computer Science Dept.                    Applied Math Dept.
The Technion                              The Weizmann Institute
Israel                                    Israel

                 18 October 1996

In September 96, Boneh Demillo and Lipton from Bellcore announced an
ingenious new type of cryptanalytic attack which received widespread
attention (see, e.g., John Markoff's 9/26/96 article in the New York Times).
Their full paper had not been published so far, but Bellcore's press release
and the authors' FAQ (available at specifically state that
the attack is applicable only to public key cryptosystems such as RSA, and
not to secret key algorithms such as the Data Encryption Standard (DES).
According to Boneh, "The algorithm that we apply to the device's faulty
computations works against the algebraic structure used in public key
cryptography, and another algorithm will have to be devised to work against
the nonalgebraic operations that are used in secret key techniques." In
particular, the original Bellcore attack is based on specific algebraic
properties of modular arithmetic, and cannot handle the complex bit
manipulations which underly most secret key algorithms.

In this research announcement, we describe a related attack (which we call
Differential Fault Analysis, or DFA), and show that it is applicable to
almost any secret key cryptosystem proposed so far in the open literature.
In particular, we have actually implemented DFA in the case of DES, and
demonstrated that under the same hardware fault model used by the Bellcore
researchers, we can extract the full DES key from a sealed tamperproof DES
encryptor by analysing fewer than 200 ciphertexts generated from unknown
cleartexts.  The power of Differential Fault Analysis is demonstrated by the
fact that even if DES is replaced by triple DES (whose 168 bits of key were
assumed to make it practically invulnerable), essentially the same attack
can break it with essentially the same number of given ciphertexts.

We would like to greatfully acknowledge the pioneering contribution of Boneh
Demillo and Lipton, whose ideas were the starting point of our new attack.

In the rest of this research announcement, we provide a short technical
summary of our practical implementation of Differential Fault Analysis of
DES. Similar attacks against a large number of other secret key cryptosystems
will be described in the full version of our paper.


The attack follows the Bellcore fundamental assumption that by exposing a
sealed tamperproof device such as a smart card to certain physical effects
(e.g., ionizing or microwave radiation), one can induce with reasonable
probability a fault at a random bit location in one of the registers at some
random intermediate stage in the cryptographic computation. Both the bit
location and the round number are unknown to the attacker.

We further assume that the attacker is in physical possession of the
tamperproof device, so that he can repeat the experiment with the same
cleartext and key but without applying the external physical effects. As a
result, he obtains two ciphertexts derived from the same (unknown) cleartext
and key, where one of the ciphertexts is correct and the other is the result
of a computation corrupted by a single bit error during the computation. For
the sake of simplicity, we assume that one bit of the right half of the data
in one of the 16 rounds of DES is flipped from 0 to 1 or vice versa, and
that both the bit position and the round number are uniformly distributed.

In the first step of the attack we identify the round in which the fault
occurred.  This identification is very simple and effective: If the fault
occurred in the right half of round 16, then only one bit in the right half
of the ciphertext (before the final permutation) differs between the two
ciphertexts. The left half of the ciphertext can differ only in output bits
of the S box (or two S boxes) to which this single bit enters, and the
difference must be related to non-zero entries in the difference
distribution tables of these S boxes.  In such a case, we can guess the six
key bit of each such S box in the last round, and discard any value which
disagree with the expected differences of these S boxes (e.g., differential
cryptanalysis). On average, about four possible 6-bit values of the key
remain for each active S box.

If the faults occur in round 15, we can gain information on the key bits
entering more than two S boxes in the last round: the difference of the
right half of the ciphertext equals the output difference of the F function
of round 15.  We guess the single bit fault in round 15, and verify whether
it can cause the expected output difference, and also verify whether the
difference of the right half of the ciphertext can cause the expected
difference in the output of the F function in the last round (e.g., the
difference of the left half of the ciphertext XOR the fault).  If
successful, we can discard possible key values in the last round, according
to the expected differences.  We can also analyse the faults in the 14'th
round in a similar way.  We use counting methods in order to find the key.
In this case, we count for each S box separately, and increase the counter
by one for any pair which suggest the six-bit key value by at least one of
its possible faults in either the 14'th, 15'th, or 16'th round.

We have implemented this attack on a personal computer.  Our analysis
program found the whole last subkey given less than 200 ciphertexts,
with random single-faults in all the rounds.

This attack finds the last subkey.  Once this subkey is known, we can
proceed in two ways: We can use the fact that this subkey contains 48 out of
the 56 key bits in order to guess the missing 8 bits in all the possible
2^8=256 ways. Alternatively, we can use our knowledge of the last subkey to
peel up the last round (and remove faults that we already identified), and
analyse the preceding rounds with the same data using the same attack. This
latter approach makes it possible to attack triple DES (with 168 bit keys),
or DES with independent subkeys (with 768 bit keys).

This attack still works even with more general assumptions on the fault
locations, such as faults inside the function F, or even faults in the key
scheduling algorithm.  We also expect that faults in round 13 (or even prior
to round 13) might be useful for the analysis, thus reducing the number of
required ciphertext for the full analysis.


Differential Fault Analysis can break many additional secret key
cryptosystems, including IDEA, RC5 and Feal.  Some ciphers, such as Khufu,
Khafre and Blowfish compute their S boxes from the key material.  In such
ciphers, it may be even possible to extract the S boxes themselves, and the
keys, using the techniques of Differential Fault Analysis.  Differential
Fault Analysis can also be applied against stream ciphers, but the
implementation might differ by some technical details from the
implementation described above.

"Key Recovery" Replaces "Key Escrow" in Encryption Plan (Edupage)

Edupage Editors <>
Fri, 18 Oct 1996 09:21:58 -0400 (EDT)
The latest government proposal for encryption software controls touts a new
approach called "key recovery."  This provision would allow law enforcement
officials to rebuild, or "recover" the mathematical key to encoded messages
with the help of third-party code-breakers.  The new policy reflects
suggestions made in a National Research Council report released earlier this
year.  Under the Clinton plan, encryption keys would be expanded from 40
bits to 56 bits in products to be exported, provided the company agrees to
the key recovery process.  In addition, authority to issue licenses for
overseas sales of such products would move from the State Department, where
they're handled as "munitions," over to the Commerce Department.  The
Business Software Alliance, however, is still not completely happy with the
compromise.  "We expect to go back to Congress," says a BSA spokeswoman.
"Although the announcement was clearly a step in the right direction, it's
not at all what the industry was looking for in its entirety."  (*Investor's
Business Daily* 17 Oct 1996 A4; Edupage 17 October 1996)

Apology/Explanation for BBN-Planet outage

John Hight <>
Fri, 18 Oct 1996 10:15:41 -0700
Here's the official apology and explanation of the BBN-Planet outage last
week.  I'm a little put off by Paul Gudonis's offer for credit for two days
of service in his last paragraph.  As everyone knows, the cost of not being
able to do business is vastly greater than the cost of keeping up our single
T1 line here at SRI.

John Hight  SRI International

>Date: Wed, 16 Oct 1996 16:26:29 -0500
>From: Paul Gudonis <>
>Subject: Palo Alto Power Outage
>This letter is to our customer administrative and technical contacts
>who were affected by the outage last week at BBN Planet's Palo Alto
>POP.  I know how detrimental even a short outage of Internet service
>is to your business; I have spoken to many of you and understand how
>drastic an impact the lengthy outage on Friday had.  I want to
>communicate what we know about the cause of the outage and what steps
>we took and are taking.
>BBN Planet's Palo Alto POP is collocated at Stanford University's data
>center.  This facility, in addition to handling the University's data
>systems, hosts systems for the Stanford University Hospital.  Stanford
>has redundant power systems, drawing on both Stanford's co-generation
>facility and PG&E power. Stanford has had continuous power at this
>facility for over 10 years, including during earthquakes and during
>the brownout that the western region experienced in August.
>The equipment that switches between these power sources failed last
>Thursday at 7 pm PDT.  Battery backup allowed us to continue powering
>equipment through 9 pm PDT.  Stanford's power was restored at 12:30 am
>PDT Friday. BBN restored service to the majority of customers affected
>by 1:20 PDT.  Stanford's power failed again at 6 am PDT.  Due to the
>previous outage, the backup batteries were not fully recharged at this
>point and BBN Planet's power was lost.
>Two backup generators were ordered and in place by 2 pm PDT.  One
>generator was required for the cooling systems, the second for the POP
>equipment.  When the generators started running, one failed.  To avoid
>further failures, two additional generators were ordered and one was
>in place by 3:30 pm PDT.  As the second generator was being started
>up, main power was restored to Stanford at 4:30.  Over the next hour,
>the temperature in the POP was stabilized, the POP equipment was
>restarted and service restored to customers.
<>From the time the first outage occurred, BBN Planet field service staff
>remained on site.  I was personally at the Palo Alto facility to ensure
>that our local staff had any resources -- internal or external --
>required to restore service to you as quickly as possible.  At both
>headquarters and our Western Region offices, additional staff were
>recruited to personally call each customer and inform them of our
>plans.  Our 800 632 7638 line was updated as new information was
>I am proud of dedication the BBN Planet staff showed in addressing
>customer concerns and striving to restore service as fast as possible.
>However, none of us are satisfied with our performance.  Although the
>redundant power at Stanford had performed with 100% reliability for
>the last 10 years, we are taking additional steps to ensure that power
>outages do not impact our customers. The generators will be kept on
>site to avoid further outages until they are replaced with a permanent
>generator.  They are being manned 7 x 24 to ensure we can start them
>up before the batteries are exhausted.
>While BBN Planet's staff did its best to contact customers, we were
>not able to call as often or as early as we, or you, would like.  We
>will have an operational Emergency Broadcast System in place in
>December that will allow us to contact all affected customers within a
>few minutes.  We'll update you as this service comes on line.
>We have also made substantial investments in our network
>infrastructure to deliver the service quality that you expect.  In the
>last six months, we deployed a high-capacity national backbone with
>redundant facilities, expanded our server operations, and implemented
>new systems for customer support.  We are continuing to install more
>capacity, more redundancy, and new systems to meet your evolving
>needs.  We'll keep you informed of these improvements.
>We are automatically issuing credits for two days of service to all
>customers who lost service due to the Palo Alto power failure.  We
>have made significant investments and continue to strive to be the
>most reliable Internet Service Provider.  After this incident, we have
>a ways to go to regain your confidence, but will make every effort to
>do so.
>Paul R. Gudonis
>BBN Planet

Snail causes Liechtenstein's cable TV system to fail

Henning Holtschneider <>
Fri, 18 Oct 1996 22:30:16 +0200 (MET DST)
As far as I can see from the archives, this is the slowest animal ever to
make it into the RISKS gallery of "rodent"-caused outages ;-)

According to a local newspaper, Soccer fans in Liechtenstein were unable
to watch the last couple of minutes of a soccer match between the French
team of Auxerre and Switzerland's Zuerich Grasshoppers when a snail
crawled into a socket. The resulting short-circuit caused the entire cable
TV network of Liechtenstein to fail.

Henning Holtschneider * Bauernkamp 41 * 44339 Dortmund * Germany

  [Don't forget the GraceHopper bug, although it
  had the potential for higher speed.  PGN]

Re: Rats take down Stanford ... (RISKS-18.53)

William Hugh Murray <>
Fri, 18 Oct 96 11:03 EST
PGN's request for redundancy brings to mind the story of the infrastructure
computer center in Trumbull, Connecticut.  It is an old story but bears

Seems that a squirrel got into a transformer and brought down the external
power supply.  The UPS kicked in, engine generators came on line, and the
center operated in this mode for about an hour and a half.  At the end of
that time the external power was restored.  The external power, the UPS, and
the engine generators went inot a deadly embrace.  The whole thing came down
and would not come back up.

I take two lessons from this.  First, redundancy adds some complexity and a
lot of redundancy adds a lot of complexity.  At some point the redundancy
begins to introduce failure modes and failure events that would not have
exited in its absence.  There is an upper bound to such redundancy.

Second, test redundant systems through to resumption of normal operations.
In this case, the operators had tested to ensure that the redundant systems
would come online in the event of a failure of the primary system.  They had
not tested to see what would happen when the primary system was restored to
normal operation.

Who would have even thought about it?  I confess that I would not have.

William Hugh Murray, New Canaan, Connecticut

Re: Computers miss $1.2M in ATM withdrawals (Fenner, RISKS-18.53)

William Hugh Murray <>
Fri, 18 Oct 96 11:03 EST
Some older readers of RISKS may recall the Franklin National Bank.  An
officer, speculating in foreign currency, lost fifty million dollars, a
small percentage of the bank's capital.  While the bank was able to keep
this peculation secret for ninety days, it inevitably leaked and the leak
resulted in an article in the Wall Street Journal.  In the next ninety days
the bank lost a billion and a half dollars in deposits and ultimately

The lesson that most bankers take from the story is that publicity about
losses is often worse than the losses themselves.  In the electronic era,
bank runs do not involve panic stricken crowds in the streets.

William Hugh Murray, New Canaan, Connecticut

Re: Health Info Database Misused (Fickeisen, RISKS-18.53)

William Hugh Murray <>
Fri, 18 Oct 96 11:03 EST
> a public health worker took a laptop and disks with confidential lists

This story would be very troubling if true.  Just as troubling is the
failure to provide a complete citation.  In the absence of such citations,
this story is merely hearsay, another modern myth.

I am reminded of the rumors, for example, of viruses, that are more
destructive than the thing that they report.  Another example is the report
that Lexis/Nexis would return social security number for name.

In the electronic era, when gossip and rumors spread with unprecedented
speed, it essential to know the primary source and to err on the side of not
repeating that which you do not know first-hand to be both true and

William Hugh Murray, New Canaan, Connecticut

People Security versus Computer Security

Li Gong <li.gong@Eng.Sun.COM>
Fri, 18 Oct 1996 21:52:29 -0700
Once again we are reminded that computer security should be considered
within a wider context that includes people security.  The *Manchester
Guardian Weekly* (vol.155, no.14, for week ending Oct.6, 1996) in its
Washington Post section (p.16) reported that a spy named Robert Chaegon Kim
in the Washington D.C. area had been able to gather sensitive information
and pass it on to a South Korean embassy official.  The report said that

    "... Kim was an ideal source ... His computer work in what is
    known as a SCIF (Secure Compartmented Information Facility) behind
    two secure doors at the Office of Naval Intelligence afforded him
    extraordinary access to a wide range of classified studies and
    analysis by many intelligence agencies."

The report went on to say that "Kim routinely removed the classified labels"
before sending out the files.  One might reasonably argue that the computer
system was a significant help for Kim to do his "job".  Bob Morris often
says that "there will always be a Walker", and he is proven correct so far!

Li Gong, JavaSoft,

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