Skipjack (cipher)

Skipjack belongs to the symmetric DES block ciphers. The key has a fixed length of 80 bits. Skipjack was developed by the NSA in the Clipper initiative and the Clipper chip ( on voice encryption ) and Capstone chip implemented ( for data encryption ). The research, the result is Skipjack, began around the beginning of the 80s. 1987, a first design was created in 1993, the algorithm was done.

The specifications of the algorithm have long been kept secret and only released on 24 June 1998.

The Skipjack algorithm used to encrypt 32 rounds. Here are two different round functions "Rule A" and "Rule B" use. Each round is defined as a linear feedback shift register with an additional key-dependent non-linear G function. In addition, place a counter use, which is counted in each round. The entire algorithm is composed of 8 rounds "Rule A", then 8 rounds "Rule B", followed by again 8 rounds "Rule A" and 8 rounds "Rule B". The key-dependent, non-linear G- function is the heart of the algorithm. G provides 16 inputs on output from 16 bits. Here, a vierrundiges Feistel network F as a solid table designated substitution ( S-box ) is used, which contains a permutation of the numbers from 0 to 255.

A few hours after the publication published several scientists - including Eli Biham and Adi Shamir - a preliminary analysis. Already in their first post "Initial Observations on the Skipjack Encryption Algorithm " on 25 June 1998 they demonstrated an attack on a Skipjack variant with reduced number of rounds to 16, which is faster than the full scan of the key space. In the days following the attacks have been further improved. Even after this short analysis, there were several criticisms such as low key length of 80 bits, successful differential and linear attacks on Skipjack reduced number of rounds and statistical weaknesses against Related -key cryptanalysis.