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Your XOR equations are wrong. You wrote: 1110 XOR 0000 = 0001 But it's not how the XOR works. 1 xor 0 = 1, and in your example it somehow turns into 0 wich is an error. Reference the xor table: https://en.wikipedia.org/wiki/Exclusive_or So, the correct answer for S1(x1) XOR S1(x2) is: 1110 XOR 0000 = 1110 and for x1 xor x2: 0000000 ...


The original is completely broken and would be regardless of the insecurity of DES. The ECB encryption of a single block message (with a secure cipher) would be a secure MAC, but XORing the message blocks means that an attacker can modify any block of the message simply by making the same change to another block so that they cancel out. The modified ...

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