Schneier and Blaze based MacGuffin on DES , their main change was spliting the DES 64 bits data block into two unequal halves in the Feistel network , 48 bits of the 64-bit data block are fed through the round function , whose output is XORed with the other 16 bits of the data block . Schneier and Blaze based MacGuffin on DES , their main change being that the data block is not split into equal halves in the Feistel network . Instead , 48 bits of the 64-bit data block are fed through the round function , whose output is XORed with the other 16 bits of the data block .