- Skipjack (cipher)
Infobox block cipher
name = Skipjack
designers = NSA
publish date = 1998 (declassifed)
key size = 80 bits
block size = 64 bits
structure = unbalanced Feistel network
rounds = 32
cryptanalysis = 31 rounds are susceptible to
impossible differential cryptanalysis.
cryptography, Skipjack is a block cipher— an algorithmfor 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 cipherdesigns of a government intelligence agency.
History of Skipjack
Skipjack was proposed as the encryption algorithm in a US government-sponsored scheme of
key escrow, and the cipherwas 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:
: " [Skipjack] is representative of a family of encryption algorithms developed in
1980as 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 combinatoricsand 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.
Eli Bihamand Adi Shamirdiscovered 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.
Truncated differentials and later a complementation
slide attackwas published against all 32 rounds of Skipjack cipher. It was found, however, that the attacks are flawed.fact|date=August 2008Biham, Shamir and Biryukov's attack continues to be the best cryptanalysis of Skipjack known to the public.
In pop culture
An algorithm named Skipjack forms part of the
back-storyto Dan Brown's 1998 novel " Digital Fortress". In Brown's novel, Skipjack is proposed as the new " public-key encryptionstandard", 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". However, when Skipjack is released for public peer review, a programmer discovers and announces the existence of the back door, effectively ending the chances of the standard being adopted.
Additionally, in the
Half-Life 2modification Dystopia, the "encryption" program used in cyberspace apparently uses both Skipjack and Blowfish algorithms.
* Biham, E., Biryukov, A., Shamir, A. (1999). Cryptanalysis of Skipjack reduced to 31 rounds using impossible differentials. EUROCRYPT 1999, pp12–23.
* E.F.Brickell, D.E.Denning, S.T.Kent, D.P.Mahler, W.Tuchman, "SKIPJACK Review ", Interim Report, July 28, (1993), 8 pages. Available at: http://www.cs.georgetown.edu/~denning/crypto/clipper/SKIPJACK.txt
* L.R.Knudsen, M.J.B. Robshaw, D. Wagner, "Truncated differentials and Skipjack", CRYPTO 1999.
* L.Granboulan, "Flaws in differential cryptanalysis of Skipjack", FSE 2001.
* R.Chung-Wei Phan, "Cryptanalysis of full Skipjack block cipher", Electronics Letters, Volume 38, Issue 2, p. 69--71, 2002.
* [http://www.cs.technion.ac.il/~biham/Reports/SkipJack/note1.html Initial observations on Skipjack (Biham et al.)]
* [http://csrc.nist.gov/groups/ST/toolkit/documents/skipjack/skipjack.pdf Specification of Skipjack] (PDF)
* [http://www.users.zetnet.co.uk/hopwood/crypto/scan/cs.html#SKIPJACK SCAN's entry for the cipher]
* [http://www.schneier.com/crypto-gram-9807.html#skip Bruce Schneier's comments on declassification]
* [http://www.itl.nist.gov/fipspubs/fip185.htm fip185 Escrowed Encryption Standard EES]
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