Block Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 109

1. Set the new w 1 value to be the G -1 permutation of the old w 2 value.
2. Set the new w 2 value to be the old w 3 value.
3. Set the new w 3 to be the old w 4 .
4. Set the new w 4 to be the old w 1 , XORed with the old w 2 value as well as the counter.
5. Decrement the counter.
Skipjack Rule B -1
Rule B -1 works similarly to Rule B.
1. Set the new w 1 value to be the G -1 permutation of the old w 2 value.
2. Set the new w 2 value to be the counter XORed with the old w 3 value, again XORed with the G -1 per-
mutation of the old w 2 .
3. Set the new w 3 to be the old w 4 .
4. Set the new w 4 to be the old w 1 .
5. Decrement the counter.
4.11.3 Permutations
The above encryption and decryption relied on functions called the G and G -1 permutations. See Figures 4-19
and 4-20 for their structure.
Figure 4-19 Skipjack G k transform.