Block Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 108

2. Set the new w 2 value to be the G permutation of the old w 1 .
3. Set the new w 3 to be the old w 2 .
4. Set the new w 4 to be the old w 3 .
5. Increment the counter.
Figure 4-17 shows this process.
Figure 4-17 Skipjack Rule A.
Skipjack Rule B
Rule B works similarly to Rule A.
1. Set the new w 1 value to be the old w 4 value.
2. Set the new w 2 value to be the G permutation of the old w 1 .
3. Set the new w 3 to be the old w 1 XORed with the counter and XORed with the old w 2 value.
4. Set the new w 4 to be the old w 3 .
5. Increment the counter.
Figure 4-18 shows Rule B.
Figure 4-18 Skipjack Rule B.
4.11.2 Skipjack Decryption Algorithm
Decryption of a Skipjack ciphertext is fairly straightforward: Every operation from above is reversible, includ-
ing the G permutation. The rules are replaced with two new rules: A -1 and B -1 . The decryption is performed by
running Rule B -1 eight times, followed by Rule A -1 eight times, Rule B -1 eight times again, and finally Rule
A -1 eight times.
Please note that the counter needs to run backwards. The keys are also submitted in reverse order k = 31, 30,
... , 0. That is, we start knowing the values for k = 32 (this corresponds to the ciphertext) and calculate the values
for k = 0 (this corresponds to the plaintext).
Skipjack Rule A -1
Rule A -1 follows a similar structure to Rule A of the encryption.