Effective CISSP Questions

Voice and data messages in telecommunication are encrypted for security concerns. Which of the following cryptographic algorithms was designed to protect confidentiality while preserved a back door for law enforcement?
A. Rijndael
B. Clipper
C. Skipjack
D. Twofish

Kindly be reminded that the suggested answer is for your reference only. It doesn’t matter whether you have the right or wrong answer. What really matters is your reasoning process and justifications.

My suggested answer is C. Skipjack.

The Clipper chip was a chipset that used the data encryption algorithm called Skipjack to transmit information.

MYK-78 Clipper chip markings
MYK-78 Clipper chip markings (Image Credit: Wikipedia)


The Clipper chip was a chipset that was developed and promoted by the United States National Security Agency as an encryption device that secured “voice and data messages” with a built-in backdoor that was intended to “allow Federal, State, and local law enforcement officials the ability to decode intercepted voice and data transmissions.” It was intended to be adopted by telecommunications companies for voice transmission. Introduced in 1993, it was entirely defunct by 1996.


The Clipper chip used a data encryption algorithm called Skipjack to transmit information and the Diffie–Hellman key exchange-algorithm to distribute the cryptokeys between the peers. Skipjack was invented by the National Security Agency of the U.S. Government; this algorithm was initially classified SECRET, which prevented it from being subjected to peer review from the encryption research community. The government did state that it used an 80-bit key, that the algorithm was symmetric, and that it was similar to the DES algorithm. The Skipjack algorithm was declassified and published by the NSA on June 24, 1998. The initial cost of the chips was said to be $16 (unprogrammed) or $26 (programmed), with its logic designed by Mykotronx, and fabricated by VLSI Technology, Inc.

Key Escrow

At the heart of the concept was key escrow. In the factory, any new telephone or other device with a Clipper chip would be given a cryptographic key, that would then be provided to the government in escrow. If government agencies “established their authority” to listen to a communication, then the key would be given to those government agencies, who could then decrypt all data transmitted by that particular telephone. The newly formed Electronic Frontier Foundation preferred the term “key surrender” to emphasize what they alleged was really occurring.

Source: Wikipedia



My new book, The Effective CISSP: Security and Risk Management, helps CISSP aspirants build a solid conceptual security model. It is not only a tutorial for information security but also a study guide for the CISSP exam and an informative reference for security professionals.

基於安全考慮,電信通訊的語音和數據訊息都會進行加密。 以下哪種加密演算法旨在保護機密性,同時又保留了執法的後門?
A. Rijndael
B. Clipper
C. Skipjack
D. Twofish

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