definition of Wikipedia
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. (March 2009)|
|First published||1998 (declassifed)|
|Key sizes||80 bits|
|Block sizes||64 bits|
|Structure||unbalanced Feistel network|
|Best public cryptanalysis|
|31 rounds are susceptible to impossible differential cryptanalysis.|
In cryptography, Skipjack is a block cipher—an algorithm for encryption—developed by the U.S. National Security Agency (NSA). Initially classified, it was originally intended for use in the controversial Clipper chip. Subsequently, the algorithm was declassified and now provides a unique insight into the cipher designs of a government intelligence agency.
Skipjack was proposed as the encryption algorithm in a US government-sponsored scheme of key escrow, and the cipher was provided for use in the Clipper chip, implemented in tamperproof hardware. Skipjack is used only for encryption; the key escrow is achieved through the use of a separate mechanism known as the Law Enforcement Access Field (LEAF).
The design was initially secret, and was regarded with considerable suspicion by many in the public cryptography community for that reason. It was declassified on 24 June 1998.
To ensure public confidence in the algorithm, several academic researchers from outside the government were called in to evaluate the algorithm (Brickell et al., 1993). The researchers found no problems with either the algorithm itself or the evaluation process. Moreover, their report gave some insight into the (classified) history and development of Skipjack:
Eli Biham and Adi Shamir discovered an attack against 16 of the 32 rounds within one day of declassification, and (with Alex Biryukov) extended this to 31 of the 32 rounds (but with an attack only slightly faster than exhaustive search) within months using impossible differential cryptanalysis.
A truncated differential attack was also published against 28 rounds of Skipjack cipher. This was later complemented by a slide attack on all 32 rounds. Biham, Shamir and Biryukov's attack continues to be the best cryptanalysis of Skipjack known to the public.
An algorithm named Skipjack forms part of the back-story to Dan Brown's 1998 novel Digital Fortress. In Brown's novel, Skipjack is proposed as the new public-key encryption standard, along with a back door secretly inserted by the NSA ("a few lines of cunning programming") which would have allowed them to decrypt Skipjack using a secret password and thereby "read the world's email". When details of the cipher are publicly released, programmer Greg Hale discovers and announces details of the backdoor. This is arguably similar to the Dual EC DRBG NSA controversy.
Skipjack is also mentioned in the novels Hostile Intent and Shock Warning, both by author Michael Walsh.
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