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,

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

A claimed attack against the full cipher was published in 2002, but a later paper with attack designer as a co-author clarified in 2009 that no attack on the full 32 round cipher was then known.

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.

References

Further reading

  • SCAN's entry for the cipher
  • fip185 Escrowed Encryption Standard EES