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# What are the operations of Blowfish Algorithm?

Blowfish is the symmetric block cipher algorithm and it encrypts the block information of 64-bits at a time. It follows the Feistel network and the working procedure of this algorithm is divided into two parts.

**Subkey Generation**− This process transform the key upto 448 bits long to subkeys adding 4168 bits.**Data-Encryption**− In the data encryption process, it will iterate 16 times of network. Each round includes the key-dependent permutation, and the keyand data- dependent substitution. The operations in the algorithms are XORs or additions on 32-bit words. The only additional operations are four indexed array information lookups per round.

Let us discuss these two parts are as follows −

**Sub-key Generation**− Blowfish cryptographic algorithm uses huge number of sub keys. These keys are creating earlier to any of the data encryption or the decryption.The p-array includes 18, 32-bit sub keys −

P1,P2,………….,P18.

There are four 32-bit S-Boxes includes 256 entries each −

S1,0, S1,1,………. S1,255

S2,0, S2,1,……….. S2,255

S3,0, S3,1,……….. S3,255

S4,0, S4,1,………... S4,255

**Steps to Generate Sub Keys**Initialize first, the P-array and therefore the four S-boxes, in order, with a fixed string and also this string also includes the hexadecimal digits of π.

P1=0x243f6a88, P2=0x85a308d3, P3=0x13198a2e, P4=0x3707344, etc.

XOR P1 with the first 32 bits of the key, XOR P2 with second which is 32-bits of the key, etc. for all bits of the key (conceivably up to P14). Repeatedly cycles the procedure through the key bits until the complete P-array has been XORed with key bits. (For each short key, there is partly one equivalent longer key. For example, if A is a 64-bit key, then AA, AAA, etc., are same keys.)

It can encrypt the all-zero string with the Blowfish algorithm, utilizing the subkeys defined in step 1 and step 2.

It can restore P1 and P2 with the 64 bit output of step (3).

It can encrypt the output of step (3) using the algorithm with the changed subkeys.

It can restore P3 and P4 with the output of step (5).

It can be used to continue the procedure, restoring all entries of P array, and then all four S-boxes in order, with the output of the continuously changing algorithm.

In total 521 iterations are needed to make all required subkeys. Application can save the subkeys instead of execute this derivation process, multiple times.

**Data Encryption**− Blowfish is a Feistel network including 16 rounds.The input is a 64-bit data element, x.

Divide x into two 32-bit halves : x

_{L}, x_{R}.Then, for i = 1 to 16;

x

_{L}= x_{L}XOR P_{i}x

_{R}= F(x_{L}) XOR x_{R}Swap x

_{L}and x_{R}After the 16

^{th}round, Swap x_{L}and x_{R}again to undo the last swap.Then, ciphertext = concatenation of x

_{L}and x_{R}, x_{R}= x_{R}XOR P_{17}and x_{L}= x_{L}XOR P_{18}.Finally, recombine x

_{L}and x_{R}to get the ciphertext. Decryption is the equivalent as encryption, other than P1, P2,……P18 are utilized in the reverse order.

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