Skipjack
| Skipjack | |
|---|---|
| developer | NSA |
| Released | 1998 |
| Key length | 80 bits |
| Block size | 64 bit |
| structure | Feistel cipher (unbalanced) |
| Round | 32 |
Like DES, Skipjack belongs to the symmetrical block ciphers . The key has a fixed length of 80 bits. SkipJack was developed by the NSA in the Clipper Initiative and implemented in the Clipper chip (for voice encryption) and Capstone chip (for data encryption). The research, the result of which is SkipJack, began around the early 1980s. A first design was created in 1987, and the algorithm was ready in 1993.
The specifications of the algorithm were kept secret for a long time and only released on June 24, 1998.
The skipjack algorithm uses 32 rounds for encryption. Two different round functions "Rule A" and "Rule B" are used. Each lap is defined as a linear feedback shift register with an additional key-dependent non-linear G function. In addition, a counter is used, which is counted up in each round. The entire algorithm consists of 8 rounds according to "Rule A", then 8 rounds according to "Rule B", followed by another 8 rounds according to "Rule A" and 8 rounds according to "Rule B". The key-dependent, non-linear G function forms the heart of the algorithm. G maps 16 input to 16 output bits. A four-round Feistel network with a fixed substitution (S-box) called F-table is used, which contains a permutation of the numbers from 0 to 255.
A few hours after publication, several scientists - including Eli Biham and Adi Shamir - published an initial analysis . In their first contribution, "Initial Observations on the SkipJack Encryption Algorithm" from June 25, 1998, they demonstrated an attack on a SkipJack variant with the number of rounds reduced to 16, which is faster than a complete search of the key area. In the days that followed, the attacks were further improved. Even after this brief analysis, several points of criticism emerged, such as B. small key length of 80 bits, successful differential and linear attacks against SkipJack with reduced number of rounds and statistical weaknesses against related key cryptanalysis.