Blowfish encryption is made as follows:
1 - XOR the round key with half of the plaintext.
2 - Split the 32-bits word in 4 bytes and insert each one in the S-Boxes.
3 - Sum the result of byte #1 with #2 and XOR in the result of byte #3
4 - Sum the above result with result of byte #4.
5 - XOR the result of the above operations in the other half of plaintext.
6 - Repeat the steps above 15 times more.
7 - XOR the two halves of the ciphertext with 2 whitening keys.
I have been thinking if something similar could be done with larger halves of ciphertext other than 32-bits.
For a 256-bits block size (128-bits each half) It could be:
1 - XOR the 128-bits round key with half of the plaintext.
2 - Split the 128-bits resulting array in 16 bytes and insert each one the S-Boxes.
2 - Concatenate the first 128-bits chunk and sum with next 128-bits chunk #2.
3 - XOR the result above with the chunk #3
4 - Sum the above result with chunk #4.
5 - XOR the result of the above operations in the other half of plaintext.
6 - Repeat the steps above 15 times more.
7 - XOR the two halves of the ciphertext with 2 whitening keys of 128-bits size each one.
Resuming the scheme above: results of the insertions of bytes in the S-Boxes was concatenated and modular and XOR operations was increased.
Obviously the sum and XOR operations for a 256-bits block must be done in mod 2^128 instead of mod 2^32 as regular Blowfish.
My question is:
Can Blowfish be securely extended to larger key/block sizes if only increasing the level modular and XOR operations as I proposed in the scheme above?
