Clipper Chip

August 1995

AT&T description of the Clipper Chip technology

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(919) 279-6841 (fax)
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CLIPPER is an NSA developed, hardware oriented, cryptographic device that implements a symmetric encryption/decryption algorithm and a law enforcement satisfying key escrow system. While the key escrow management system design is not completely designed, the cryptographic algorithm (SKIPJACK) is completely specified (and classified SECRET).

The crytographic algorithm (called CA in this PAPER) has the following characteristics:

1. Symmetric, 80-bit key encryption/decryption algorithm;
2. Similar in function to DES (i.e., basically a 64-bit code book transformation that can be used in the same four modes of operation as specified for DES in FIPS 81);
3. 32 rounds of processing per single encrypt/decrypt operation;
4. Design started by NSA in 1985; evaluation completed in 1990.

The CLIPPER CHIP is just one implementation of the CA. The CLIPPER CHIP designed for the AT&T commercial secure voice products has the following characteristics:

1. Functions specified by NSA; logic designed by MYKOTRONX; chip fabricated by VLSI, INC.: manufactured chip programmed (made unique) by MYKOTRONX to security equipment manufacturers willing to follow proper security procedures for handling and storage of the programmed chip; equipment sold to customers;

2. Resistant to reverse engineering against a very sophisticated, well funded adversary;

3. 15-20 MB/S encryption/decryption constant throughout once cryptographic synchronization is established with distant CLIPPER Chip;

4. The chip programming equipment writes (one time) the following information into a special memory (called VROM or VIA-Link) on the chip:
   1. (unique) serial number
   2. (unique) unit key
   3. family key
   4. specialized control software

5. Upon generation (or entry) of a session key in the chip, the chip performs the following actions:

   1. Encrypts the 80-bit session key under the unit key producing an 80-bit intermediate rsult;

   2. Concatenates the 80-bit result with the 25-bit serial number and a 23-bit authentication pattern (total of 128 bits);

   3. Enciphers this 128 bits with family key to produce a 128-bit cipher block chain called the Law Enforcement Field (LEF);

   4. Transmits the LEF at least once to the intended receiving CLIPPER chip;

   5. The two communicating CLIPPER chips use this field together with a random IV to establish Crytographic Synchronization.

6. Once synchronized, the CLIPPER chips use the session key to encrypt/decrypt data in both directions;

7. The chips can be programmed to not enter secure mode if the LEF field has been tampered with (e.g., modified, superencrypted, replaced);

8. CLIPPER chips will be available from a second source in the future;

9. CLIPPER chips will be modified and upgraded in the future;

10. CLIPPER chips presently cost $16.00 (unprogrammed) and $26.00 (programmed).

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