Skipjack (cipher)

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Block cipher

title: "Skipjack (cipher)" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["type-2-encryption-algorithms", "national-security-agency-cryptography"] description: "Block cipher" topic_path: "technology/algorithms" source: "https://en.wikipedia.org/wiki/Skipjack_(cipher)" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0

::summary Block cipher ::

::data[format=table title="Infobox block cipher"]

FieldValue
nameSkipjack
designersNSA
publish date1998 (declassified)
key size80 bits
block size64 bits
structureunbalanced Feistel network
rounds32
ECRYPT II recommendations note that, as of 2012, ciphers with a key size of 80 bits provide only "Very short-term protection against agencies".[http://www.ecrypt.eu.org/documents/D.SPA.20.pdf Yearly Report on Algorithms and Keysizes] (2012), D.SPA.20 Rev. 1.0, ICT-2007-216676 ECRYPT II, 09/2012. {{webarchiveurl
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|name = Skipjack |designers = NSA |publish date = 1998 (declassified) |key size = 80 bits |block size = 64 bits |structure = unbalanced Feistel network |rounds = 32 |cryptanalysis = ECRYPT II recommendations note that, as of 2012, ciphers with a key size of 80 bits provide only "Very short-term protection against agencies". NIST recommends not to use Skipjack after 2010. Impossible differential cryptanalysis breaks 31 rounds (but only slightly faster than exhaustive search).

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.

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. 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 algorithm was initially secret, and was regarded with considerable suspicion by many for that reason. It was declassified on 24 June 1998, shortly after its basic design principle had been discovered independently by the public cryptography community.

To ensure public confidence in the algorithm, several academic researchers from outside the government were called in to evaluate the algorithm. 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:

::quote [Skipjack] is representative of a family of encryption algorithms developed in 1980 as part of the NSA suite of "[[Type 1 product ::

In March 2016, NIST published a draft of its cryptographic standard which no longer certifies Skipjack for US government applications.

Description

Skipjack uses an 80-bit key to encrypt or decrypt 64-bit data blocks. It is an unbalanced Feistel network with 32 rounds. It was designed to be used in secured phones.

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 (but with an attack only slightly faster than exhaustive search) within months using impossible differential cryptanalysis.{{cite book |first1=Eli |last1=Biham |author-link1=Eli Biham |first2=Adi |last2=Shamir |author-link2=Adi Shamir |first3=Alex |last3=Biryukov |title=Advances in Cryptology — EUROCRYPT '99 |chapter=Cryptanalysis of Skipjack Reduced to 31 Rounds Using Impossible Differentials |series=Lecture Notes in Computer Science |author-link3=Alex Biryukov |pages = 12–23 |chapter-url = https://www.iacr.org/cryptodb/archive/1999/EUROCRYPT/15920012.pdf |year = 1999 |volume=1592 |url-status = dead |archive-url = https://web.archive.org/web/20120627202429/http://www.iacr.org/cryptodb/archive/1999/EUROCRYPT/15920012.pdf |archive-date = 2012-06-27 |doi=10.1007/3-540-48910-X_2 |isbn=978-3-540-65889-4 }}

A truncated differential attack was also published against 28 rounds of Skipjack cipher.

A claimed attack against the full cipher was published in 2002,{{cite journal |first = Raphaël Chung-Wei |last = Phan |title = Cryptanalysis of full Skipjack block cipher |journal = Electronics Letters |volume = 38 |issue = 2 |pages = 69–71 |url = http://csis.bits-pilani.ac.in/faculty/murali/netsec-10/seminar/refs/apoorv2.pdf |year = 2002 |doi = 10.1049/el:20020051 |bibcode = 2002ElL....38...69P |url-status = dead |archive-url = https://web.archive.org/web/20130921055228/http://csis.bits-pilani.ac.in/faculty/murali/netsec-10/seminar/refs/apoorv2.pdf |archive-date = 2013-09-21

In pop culture

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. In real life there is evidence to suggest that the NSA has added back doors to at least one algorithm; the Dual_EC_DRBG random number algorithm may contain a backdoor accessible only to the NSA.

Additionally, in the Half-Life 2 modification Dystopia, the "encryption" program used in cyberspace apparently uses both Skipjack and Blowfish algorithms.{{cite web | title = Dystopia Wiki | url = http://www.dystopia-game.com/wiki/index.php?title=Cyberspace_Interaction#Defensive_Programs

References

References

  1. (2010). "Advances in Cryptology – CRYPTO 2010". Springer.
  2. [http://www.ecrypt.eu.org/documents/D.SPA.20.pdf Yearly Report on Algorithms and Keysizes] (2012), D.SPA.20 Rev. 1.0, ICT-2007-216676 ECRYPT II, 09/2012. {{webarchive. link. (July 21, 2013)
  3. (January 2011). "Transitions: Recommendation for Transitioning the Use of Cryptographic Algorithms and Key Lengths". NIST.
  4. Schneier, Bruce. (July 15, 1998). "Declassifying Skipjack".
  5. Savard, John J. G.. (1999). "Skipjack".
  6. (July 28, 1993). "SKIPJACK Review Interim Report The SKIPJACK Algorithm".
  7. (June 25, 1998). "Initial Observations on the SkipJack Encryption Algorithm".
  8. Barker, Elaine. (March 2016). "NIST Special Publication 800-175B Guideline for Using Cryptographic Standards in the Federal Government: Cryptographic Mechanisms". [[National Institute of Standards and Technology.
  9. Schneier, Bruce. (April 15, 2016). "New NIST Encryption Guidelines".
  10. (May 29, 1998). "SKIPJACK and KEA Algorithm Specifications".
  11. (1999). "Advances in Cryptology — CRYPTO' 99".
  12. (2009). "Advanced Differential-Style Cryptanalysis of the NSA's Skipjack Block Cipher". Figshare.

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type-2-encryption-algorithmsnational-security-agency-cryptography