Block Ciphers - Modern Cryptanalysis: Techniques for Advanced Code Breaking - page 96

1. Initialize the P -values (left-to-right) with the hexadecimal digits of π — that is, the digits to the right
of the “hexadecimal point.” They start off as 24 3f 6a 88 85 a3 08 d3 .... After the P -values are
filled, fill the S-boxes, in order with the digits of π. The pre-computed values can be found easily on the
Internet.
2. Starting with P 1 , calculate P 1 = P 1 ⊕ K 1 , P 2 = P 2 ⊕ K 2 , and so forth. Here, the K -values represent the
originally inputted key values, up to 448 bits. There may be up to 14 K -values, to correspond to the P -
values. It is necessary to XOR a K -value with each P -value, so repeat the values as necessary by starting
over with K 1 , then K 2 , and so on, again. For example, with a 128-bit key (representing K 1 , ... , K 4 values),
after calculating P 4 = P 4 ⊕ K 4 , then perform P 5 = P 5 ⊕ K 1 , and so on.
3. Encrypt a 64-bit block consisting of all zeros (00 00 00 00 00 00 00 00) using the Blowfish
algorithm, with the P -values from the previous step.
4. Replace P 1 and P 2 with the output from the previous step's Blowfish run.
5. Take the output from Step 3 and encrypt it (with the new, modified P -values).
6. Replace P 3 and P 4 with the output from the previous step.
7. Repeat this process (encrypting the previous Blowfish output and replacing the next set of P -values),
filling all P -values and then the S-boxes (i.e., the 32-bit outputs of the S-box entry) in order. The order of
the S-boxes is defined to be S 1,0 , S 1,1 , ... , S 1,255 , S 2,0 , S 2,1 , ... , S 4255 .
This will require a total of 521 iterations in order to compute all values.
4.8.2 Blowfish Algorithm
The algorithm is based on the Feistel structure, with a total of 16 rounds. The key to the algorithm, as mentioned
above, is that the algorithm is kept very simple: For each round, only three XORs, two additions, and four S-box
lookups are required.
Blowfish Encryption Algorithm. The basic cryptographic algorithm operates on a 64-bit input and produces a
64-bit output. The following shows the encryption portion. To obtain the decryption code, simply replace P i in
the following with P 19- i (with P 17 and P 18 at the end being replaced with P 2 and P 1 , respectively):
1. Split the plaintext into two halves: the left half ( L 0 ) and the right half ( R 0 ).
2. For each of 16 rounds ( i = 1, 2, ... , 16):
(a) Set L i = L i -1 ⊕ P i .
(b) Set R i = f ( L i ) ⊕ R i .
(c) Swap L i and R i .
3. Swap L 16 and R 16 (undoing the previous swap).
4. Set R 17 = R 16 ⊕ P 17 .
5. Set L 17 = R 17 ⊕ P 18 .
The output is the block obtained by recombining L 17 and R 17 . This main round procedure is shown in Figure
4-9 .