Initial observations on skipjack: Cryptanalysis of skipjack-3XOR

Eli Biham, Alex Biryukov, Orr Dunkelman, Eran Richardson, Adi Shamir

Research output: Chapter in Book/Report/Conference proceedingConference contributionpeer-review


Skipjack is the secret key encryption algorithm developed by the NSA for the Clipper chip and Fortezza PC card. It uses an 80-bit key, 128 table lookup operations, and 320 XOR operations to map a 64-bit plaintext into a 64-bit ciphertext in 32 rounds. This paper describes an efficient attack on a variant, which we call Skipjack- 3XOR (Skipjack minus 3 XORs). The only difference between Skipjack and Skipjack-3XOR is the removal of 3 out of the 320 XOR operations. The attack uses the ciphertexts derived from about 500 plaintexts and its total running time is equivalent to about one million Skipjack encryptions, which can be carried out in seconds on a personal computer. We also present a new cryptographic tool, which we call the Yoyo game, and efficient attacks on Skipjack reduced to 16 rounds. We conclude that Skipjack does not have a conservative design with a large margin of safety.

Original languageEnglish
Title of host publicationSelected Areas in Cryptography - 5th Annual International Workshop, SAC 1998, Proceedings
EditorsStafford Tavares, Henk Meijer
PublisherSpringer Verlag
Number of pages14
ISBN (Print)3540658947, 9783540658948
StatePublished - 1999
Externally publishedYes
Event5th Annual Workshops on Selected Areas in Cryptography, SAC 1998 - Kingston, Canada
Duration: 17 Aug 199818 Aug 1998

Publication series

NameLecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics)
ISSN (Print)0302-9743
ISSN (Electronic)1611-3349


Conference5th Annual Workshops on Selected Areas in Cryptography, SAC 1998

Bibliographical note

Publisher Copyright:
© Springer-Verlag Berlin Heidelberg 1999.


  • Clipper chip
  • Cryptanalysis
  • Fortezza PC card
  • Skipjack
  • Yoyo game

ASJC Scopus subject areas

  • Theoretical Computer Science
  • General Computer Science


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