Denning's Technical Summary of Clipper Chip
Here is a revised version of my summary which corrects some errors and provides some additional information and explanation.
THE CLIPPER CHIP: A TECHNICAL SUMMARY
Revised, April 21, 1993
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.
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:
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.
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