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Skipjack (cipher)
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Skipjack (cipher)

In cryptography, Skipjack is a block cipher — an algorithm for encryption — developed by the US 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 as a unique insight into the cipher designs of a government intelligence agency.

Table of contents
1 History of Skipjack
2 Description
3 Cryptanalysis
4 See also
5 References
6 External Links

History of Skipjack

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. The design was originally 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:

[Skipjack] is representative of a family of encryption algorithms developed in 1980 as part of the NSA suite of "Type I" algorithms... SKIPJACK was designed using building blocks and techniques that date back more than forty years. Many of the techniques are related to work that was evaluated by some of the world's most accomplished and famous experts in combinatorics and abstract algebra. SKIPJACK's more immediate heritage dates to around 1980, and its initial design to 1987...The specific structures included in SKIPJACK have a long evaluation history, and the cryptographic properties of those structures had many prior years of intense study before the formal process began in 1987. — SKIPJACK Review, Interim Report, 1993.

Description

Skipjack uses an
80-bit key to encrypt 64-bit data blocks. It is an unbalanced Feistel network with 32 rounds.

Cryptanalysis

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 within months using impossible differential cryptanalysis.

As of 2004, no better attack has been discovered.

See also

References

External Links


Block ciphers
Algorithms: 3-Way | AES | Blowfish | Camellia | CAST-128 | CAST-256 | CMEA | DEAL | DES | DES-X | FEAL | G-DES | GOST | IDEA | Iraqi | KASUMI | KHAZAD | Khufu and Khafre; | LOKI89/91 | LOKI97 | Lucifer | MacGuffin | Madryga | MAGENTA | MARS | MISTY1 | MMB | NewDES | RC2 | RC5 | RC6 | Red Pike; | S-1 | SAFER | Serpent | SHARK | Skipjack | Square | TEA | Triple DES; | Twofish | XTEA
Design: Feistel network; | Key schedule; | Product cipher; | S-box | SPN   Attacks: Brute force; | Linear / Differential cryptanalysis | Mod n; | XSL   Standardisation: AES process; | CRYPTREC | NESSIE   Misc: Avalanche effect | Block size; | IV | Key size; | Modes of operation; | Piling-up lemma; | Weak key;