Cryptography Reference
In-Depth Information
The Clipper Chip
The Clipper Chip was designed to be resistant to reverse engineering even
against sophisticated and well-funded attackers. It was developed by the de-
fence contractor, Mykotronx of Torrence, California as a secure voice device for
AT&T. Mykotronx called the chips MYK-78.
The chip was to be used to protect private voice communications while al-
lowing government agents to employ what was called key escrow to access the
encrypted communications. The reason was to provide access for law enforce-
ment to be readily able to obtain information in criminal cases from voice com-
munications. The proposal was that the Clipper Chip would hold a master key
“in escrow” for release to government agents at a later time as a “secret door”
for them to access the encrypted conversations without having to use methods of
decryption to get them. This secret door was called LEAF or Law Enforcement
Access Field . To accomplish this, the master key would be in the protected,
secure custody of the government. Then the (classified) encryption algorithm
on the Clipper Chip, called Skipjack , would use the session key to encipher the
sounds as they left the transmitting source, and decrypted as they arrived at
the received target. There was also to be a unique chip identifier, a unique
chip key. LEAF would be generated by Skipjack using the session key and the
unique chip key to produce an enciphered session key and the unique chip key.
Then the master key would encrypt it all. Below is a stepwise version of how
this all works. First we look at Skipjack to see how it is an improvement over
DES, and examine what the Clipper Chip contains.
Chip Contents : Each Clipper Chip contains the following.
The classified SKC, Skipjack, employing 80-bit keys (DES has only 64), and
32 rounds (DES has only 16), supporting DES modes of operation (see
Section 3.3).
An 80-bit family (master key), F , common to all Clipper Chips.
A unique serial number, N .
A unique 80-bit secret key, U , for decrypting all messages enciphered with
the chip.
The following is a simplified version of the employment of the Skipjack SKC
used in conjunction with the Clipper Chip for secure voice communications
between Alice and Bob.
Encrypting with the Clipper Chip : Alice and Bob will use the Clipper
Chip for an encrypted telephone conversation as follows. We will assume that
the conversation is digitized voice.
1. First there is a negotiation of a session key between Alice and Bob that
takes place outside the chip, which is embedded in the AT&T security
device for each phone. Alice activates her security device to call Bob, and
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