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THE CLIPPER CHIP: A TECHNICAL SUMMARY
Dorothy Denning
Revised, April 21, 1993
INTRODUCTION
On April 16, the President announced a new initiative that will bring
together the Federal Government and industry in a voluntary program
to provide secure communications while meeting the legitimate needs of
law enforcement. At the heart of the plan is a new tamper-proof encryption
chip called the "Clipper Chip" together with a split-key approach to
escrowing keys. Two escrow agencies are used, and the key parts from
both are needed to reconstruct a key.
CHIP CONTENTS
The Clipper Chip contains a classified single-key 64-bit block
encryption algorithm called "Skipjack." The algorithm uses 80 bit keys
(compared with 56 for the DES) and has 32 rounds of scrambling
(compared with 16 for the DES). It supports all 4 DES modes of
operation. The algorithm takes 32 clock ticks, and in Electronic
Codebook (ECB) mode runs at 12 Mbits per second.
Each chip includes the following components:
the Skipjack encryption algorithm
F, an 80-bit family key that is common to all chips
N, a 30-bit serial number (this length is subject to change)
U, an 80-bit secret key that unlocks all messages encrypted with the chip
The chips are programmed by Mykotronx, Inc., which calls them the
"MYK-78." The silicon is supplied by VLSI Technology Inc. They are
implemented in 1 micron technology and will initially sell for about
$30 each in quantities of 10,000 or more. The price should drop as the
technology is shrunk to .8 micron.
ENCRYPTING WITH THE CHIP
To see how the chip is used, imagine that it is embedded in the AT&T
telephone security device (as it will be). Suppose I call someone and
we both have such a device. After pushing a button to start a secure
conversation, my security device will negotiate an 80-bit session key K
with the device at the other end. This key negotiation takes place
without the Clipper Chip. In general, any method of key exchange can
be used such as the Diffie-Hellman public-key distribution method.
Once the session key K is established, the Clipper Chip is used to
encrypt the conversation or message stream M (digitized voice). The
telephone security device feeds K and M into the chip to produce two
values:
E[M; K], the encrypted message stream, and
E[E[K; U] + N; F], a law enforcement field ,
which are transmitted over the telephone line. The law enforcement
field thus contains the session key K encrypted under the unit key U
concatenated with the serial number N, all encrypted under the family
key F. The law enforcement field is decrypted by law enforcement after
an authorized wiretap has been installed.
The ciphertext E[M; K] is decrypted by the receiver's device using the
session key:
D[E[M; K]; K] = M .
CHIP PROGRAMMING AND ESCROW
All Clipper Chips are programmed inside a SCIF (Secure Compartmented
Information Facility), which is essentially a vault. The SCIF contains
a laptop computer and equipment to program the chips. About 300 chips
are programmed during a single session. The SCIF is located at
Mykotronx.
At the beginning of a session, a trusted agent from each of the two key
escrow agencies enters the vault. Agent 1 enters a secret, random
80-bit value S1 into the laptop and agent 2 enters a secret, random
80-bit value S2. These random values serve as seeds to generate unit
keys for a sequence of serial numbers. Thus, the unit keys are a
function of 160 secret, random bits, where each agent knows only 80.
To generate the unit key for a serial number N, the 30-bit value N is
first padded with a fixed 34-bit block to produce a 64-bit block N1.
S1 and S2 are then used as keys to triple-encrypt N1, producing a
64-bit block R1:
R1 = E[D[E[N1; S1]; S2]; S1] .
Similarly, N is padded with two other 34-bit blocks to produce N2 and
N3, and two additional 64-bit blocks R2 and R3 are computed:
R2 = E[D[E[N2; S1]; S2]; S1]
R3 = E[D[E[N3; S1]; S2]; S1] .
R1, R2, and R3 are then concatenated together, giving 192 bits. The
first 80 bits are assigned to U1 and the second 80 bits to U2. The
rest are discarded. The unit key U is the XOR of U1 and U2. U1 and U2
are the key parts that are separately escrowed with the two escrow
agencies.
As a sequence of values for U1, U2, and U are generated, they are
written onto three separate floppy disks. The first disk contains a
file for each serial number that contains the corresponding key part
U1. The second disk is similar but contains the U2 values. The third
disk contains the unit keys U. Agent 1 takes the first disk and agent
2 takes the second disk. Thus each agent walks away knowing
an 80-bit seed and the 80-bit key parts. However, the agent does not
know the other 80 bits used to generate the keys or the other 80-bit
key parts.
The third disk is used to program the chips. After the chips are
programmed, all information is discarded from the vault and the agents
leave. The laptop may be destroyed for additional assurance that no
information is left behind.
The protocol may be changed slightly so that four people are in the
room instead of two. The first two would provide the seeds S1 and S2,
and the second two (the escrow agents) would take the disks back to
the escrow agencies.
The escrow agencies have as yet to be determined, but they will not
be the NSA, CIA, FBI, or any other law enforcement agency. One or
both may be independent from the government.
LAW ENFORCEMENT USE
When law enforcement has been authorized to tap an encrypted line, they
will first take the warrant to the service provider in order to get
access to the communications line. Let us assume that the tap is in
place and that they have determined that the line is encrypted with the
Clipper Chip. The law enforcement field is first decrypted with the
family key F, giving E[K; U] + N. Documentation certifying that a tap
has been authorized for the party associated with serial number N is
then sent (e.g., via secure FAX) to each of the key escrow agents, who
return (e.g., also via secure FAX) U1 and U2. U1 and U2 are XORed
together to produce the unit key U, and E[K; U] is decrypted to get the
session key K. Finally the message stream is decrypted. All this will
be accomplished through a special black box decoder.
CAPSTONE: THE NEXT GENERATION
A successor to the Clipper Chip, called "Capstone" by the government
and "MYK-80" by Mykotronx, has already been developed. It will include
the Skipjack algorithm, the Digital Signature Standard (DSS), the
Secure Hash Algorithm (SHA), a method of key exchange, a fast
exponentiator, and a randomizer. A prototoype will be available for
testing on April 22, and the chips are expected to be ready for
delivery in June or July.
ACKNOWLEDGMENT AND DISTRIBUTION NOTICE. This article is based on
information provided by NSA, NIST, FBI, and Mykotronx. Permission to
distribute this document is granted.
April 16, 1993 Washington, DC
COMPUTER PROFESSIONALS CALL FOR PUBLIC
DEBATE ON NEW GOVERNMENT ENCRYPTION INITIATIVE
Computer Professionals for Social Responsibility (CPSR) today called
for the public disclosure of technical data underlying the government's
newly-announced "Public Encryption Management" initiative. The new
cryptography scheme was announced today by the White House and the National
Institute for Standards and Technology (NIST), which will implement the
technical specifications of the plan. A NIST spokesman acknowledged that the
National Security Agency (NSA), the super-secret military intelligence agency,
had actually developed the encryption technology around which the new
initiative is built.
According to NIST, the technical specifications and the Presidential
directive establishing the plan are classified. To open the initiative to
public review and debate, CPSR today filed a series of Freedom of Information
Act (FOIA) requests with key agencies, including NSA, NIST, the National
Security Council and the FBI for information relating to the encryption plan.
The CPSR requests are in keeping with the spirit of the Computer Security Act,
which Congress passed in 1987 in order to open the development of non-military
computer security standards to public scrutiny and to limit NSA's role in the
creation of such standards.
CPSR previously has questioned the role of NSA in developing the
so-called "digital signature standard" (DSS), a communications authentication
technology that NIST proposed for government-wide use in 1991. After CPSR
sued NIST in a FOIA lawsuit last year, the civilian agency disclosed for the
first time that NSA had, in fact, developed that security standard. NSA is
due to file papers in federal court next week justifying the classification of
records concerning its creation of the DSS.
David Sobel, CPSR Legal Counsel, called the administration's apparent
commitment to the privacy of electronic communications, as reflected in
today's official statement, "a step in the right direction." But he
questioned the propriety of NSA's role in the process and the apparent secrecy
that has thus far shielded the development process from public scrutiny. "At
a time when we are moving towards the development of a new information
infrastructure, it is vital that standards designed to protect personal
privacy be established openly and with full public participation. It is not
appropriate for NSA — an agency with a long tradition of secrecy and
opposition to effective civilian cryptography — to play a leading role in the
development process."
CPSR is a national public-interest alliance of computer industry
professionals dedicated to examining the impact of technology on society.
CPSR has 21 chapters in the U.S. and maintains offices in Palo Alto,
California, Cambridge, Massachusetts and Washington, DC. For additional
information on CPSR, call (415) 322-3778 or e-mail <cpsr@csli.stanford.edu>.
If all of the privacy concerns about the Clipper chip magically disappeared, the chip will still encounter widespread economic resistance. Why? Because almost everything can be done cheaper in software and the secrecy surrounding the algorithm effectively prohibits software implementations. Why would a computer designer add a high-speed encryption chip to the machine? Even if the chips cost about $25 in large quantities, they could still add about $100 to the final cost after everyone takes their markups. The computer designer must ask whether people are willing to spend extra to buy a box when the clone manufacturer in the garage down the street isn't going to be putting one in. Adding encryption in software is a different proposition. There is a one-time cost of engineering and a small extra cost for increased support. Once the code is written, the manufacturing costs do not increase. Also, software can retrofit machines for no extra cost and add widespread compatibility after the update is finished. This is why Novell, Apple and Microsoft choose to add encryption software in their latest rev of the system software. It is not even clear that the standard has much of a chance in the phone system. DSP chips and digital designs are becoming more and more part of cellular standards. Why pay extra for another chip if it can be done in the DSP? Weight and power consumption are important considerations for these applications. I'm sure that the algorithm designers and NSA committee considered the RISKS of exposing the algorithm. Scrutiny weakens the code because it makes it easier for people to attack the system. It is obvious that the committee tried to consider some of the economic RISKS involved in promulgating a "Big Brother" standard. That is why they arranged for the chips to be presented as a fait accompli as part of AT&T's latest phones. But they face an uphill battle against the forces of economics. --Peter Wayner
I mailed the following letter to the President today:
To: 0005895485@MCIMAIL.COM (White House)
Subject: Second thoughts about your encryption proposal
17 April 1993
Dear Mr. President --
Since writing my initial reaction I have given considerable second thought to
your encryption proposal, announced yesterday. I must withdraw my initial
partial support for your plan, pending the release of further details.
My initial assumption was that you were mandating the replacement of every
telephone handset in the USA with one which would digitize the person's voice
and encrypt it. I assumed that this replacement would start with cellular
handsets and proceed through wireless and wired — in order of severity of
vulnerability. Given that the government would mandate such a change and that
that change would interfere with the FBI's current ability to tap voice
telephone calls on the public networks, it made sense to propose an encryption
method which would allow the FBI to continue in court-ordered wiretaps --
specifically via key escrow.
While it would be beneficial from the point of view of improving the
privacy and security of citizens from illegal eavesdropping, I now believe
that this proposal is far too costly to undertake at this time. The
federal government is facing a huge debt and deficit and the private sector
is far from thriving. The proposal to pay for some of this equipment with
funds from civil forfeiture adds insult to injury, since abuses of civil
forfeiture have led me to conclude that law enforcement's right to such
funds should be severely restricted if not removed.
If this proposal is only for limited use of such encryption, then it does
little to advance the cause of citizen's privacy and it is in direct
competition with existing products which already service the small market
of citizens who are aware of their vulnerability and who are willing to pay
for assurance of their privacy. It is especially disturbing that the press
release suggests that this proposal is not merely a call for action but an
already designed implementation which some agency of the administration is
attempting to impose upon the American people. The talent exists in the
private sector to address these security concerns.
Meanwhile, there is a danger that the key escrow provision is intended to
imply that all cryptosystems used by citizens in the lawful course of their
daily personal and business lives must include key registration. This
would be an unacceptable erosion of our current rights, especially of the
fundamental right of privacy which you supported so strongly during your
campaign. Legislation to this effect would be unenforceable. It would be
easily and frequently broken — leading to the danger that some law
enforcement officer with a private grudge would have an easy method of
filing a criminal complaint against the innocent victim of his grudge. A
requirement for key registration would also come directly into conflict
with certain uses of cryptography in advanced computer system design. In
those cases, both key registration and use of some government-designed chip
are unacceptable.
Meanwhile, there is the additional danger that this proposal would serve as
a vehicle for advancing the FBI's wiretap proposal which was rejected by
Congress last year and which I oppose on several grounds.
I look forward to full technical details of your proposal and to a public
debate on its merits.
Sincerely,
Carl M. Ellison
Senior Technical Consultant - Advanced Development Group
Stratus Computer Inc.
55 Fairbanks Boulevard
Marlborough MA 01752-1298
TEL: (508) 460-2783
FAX: (508) 624-7488
E-mail: cme@sw.stratus.com
cme@vos.stratus.com
--
- <<Disclaimer: All opinions expressed are my own, of course.<>
- Carl Ellison cme@sw.stratus.com
- Stratus Computer Inc. M3-2-BKW TEL: (508)460-2783
- 55 Fairbanks Boulevard ; Marlborough MA 01752-1298 FAX: (508)624-7488
Sometime last week, electronic sign boards along Interstate 95 in Connecticut were hacked to say "You all suck." These boards are normally used to announce construction, fog, and whatnot. Apparently it was several hours before State Police and the Highway Department were able to clear the messages. Well, it happened again a day or two ago, with a different message attacking the Governer. A teenager has been caught; he said that there was no password on the Highway Department's computer system. I originally heard this on a short news blurb on WHJY, a rock station in Providence RI, so I'm sure the accuracy is all you would expect it to be. Confirming reports welcomed! I'm just waiting for the day that the speed limit signs go electronic. They'll probably only allow two digits, though...
In RISKS 14:50, dww@math.fu-berlin.de (Debora Weber-Wulff) writes: DWW> ...The computer reacted by setting the outbound signal DWW> correctly to "halt". The overseer believed that this DWW> was a defect in the system, and overrode the signal... DWW> without telephoning anyone to investigate the supposed error. The last line, of course, explain the proximate cause and points a flashing red arrow toward the root cause. I read RISKS 14:50 just a couple of days after a lessons-learned article on "lockout/ tagout" in Occupational Safety OBSERVER, a newsletter from the Department of Energy's Office of Safety and Quality Assurance. (Lockout/tagout is a set of rules and procedures meant to keep electricians and others who work with stored energy from releasing it through themselves.) The article pointed out, strenuously, that the cardinal rule of lockout/tagout is, "*never* remove a tag affixed by someone else." When you have virtual "tags" affixed by a computer, and operators who perhaps were not exposed to the subset of technical endeavors where the lockout/tagout discipline is used, you have a risk added to another risk! The irony, of course, is that the computer system took the right action but was subverted by its user. I doubt that there will ever be a system so sophisticated that some dumb bunny can't cause a disaster. The lesson: designers of man-in-the-loop systems have to understand operator psychology, and owners of such systems have to provide appropriate training that accounts for such risks and then maintain a culture that places importance on avoiding them. --Joe
In this morning's (4/15) Minneapolis Star-Tribune was a small blurb on a new problem discovered on the recent Discovery shuttle mission. Apparently, the crew that goes out to pickup the Solid Rocket Boosters (SRBs) after they come back down to the ocean via parachute, noted a pair of pliers still "attached" to one of the SRBs. They did not say if they were "jammed" in to some spot or if they were in fact holding on to something. If the latter, one would have to assume that they are probably "Vise-Grips" or a generic version of them. I can see the commercial advertisements now.... "The pliers that grip so tight, they will keep holding on even through the G-Forces of a space shuttle launch". Apparently NASA is scrambling to figure out what went wrong. One would think so given the sensitivity to another possible failure of the SRBs. Marc Schwartz Director of Clinical Services Summit Medical Systems, Inc. Minneapolis, MN.
>From: viking@iastate.edu (Dan Sorenson) >Subject: Re: Columbia and Discovery shuttle problems (RISKS-14.47) > > The "fix" is to bypass the sensor, fooling the computer into >thinking the valve is properly closed. Love it - a high-tech implementation of the venerable "black tape" remedy well known to Car Talk listeners. You know: Q: "My Check Engine / alternator / oil pressure light just came on." A: "Get some black tape..." Steve Bacher (Batchman) Draper Laboratory Internet: seb@draper.com Cambridge, MA, USA
I work for a large company with badge entry systems for all buildings and all internal labs. This is normally great, because I can get into my building any time I want, day or night, and I can get into the labs that I need to use, but it keeps me out of places I don't belong. It also does the same for others. About once a year, however, I forget my badge. This is not a problem for the normal person who comes in after 8:00 in the morning when the building is open and there is a receptionist, but I carpool from quite a distance away, before traffic gets heavy, and I get in about 6:45. This morning was one of those rare occasions when I forgot my badge. No problem, each building has a phone at the front door that rings through directly to security. I picked it up and told them that I needed to get into the building. They asked me a few questions, like what is my name, my badge number, my phone extension and two or three other such pieces of information, then they opened the door remotely and I was let into the building. As I was walking to my office, I realized that all of the information I was required to give them is contained on-line in our computer network and anyone with access to a workstation could get this information about anyone in the company in just a few seconds. I'm not recommending that the database be changed, because it is a great aid to employee intercommunication. However, I expect that I could easily get into some building where I don't belong by gathering this information about someone who works in that building. I don't even need to know a name to get this information, the database allows searches by building. Not too long ago, when I needed to be let in a building, someone from security would come by, check a photo ID, such as a drivers license, then let me in with a key. Even though my company has become much more security conscious during the 5 years I've worked here, I believe it is a security risk to allow building entry based solely on information given over the phone that is contained in a computer database readily accessible to anyone who can access the network. I'm going to recommend a change to this procedure.
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