Consider the following encryption scheme to provide confidentially A and B want

Question

Consider the following encryption scheme to provide confidentially A and B want to communicate with each other. A and B sphere a secret two bit number T i.e T is known only to A and B. A gets a 56 bit DES key K by repeating T 28 times. For example, if T was 01, the K would be 010101 independently B also gets the 56 bit DES key K in the same way, Now if A wants to send M to B, A will calculate C = E_K (M) and send C to B. B will recover the plaintext by doing M = D_x (C). Is this a good scheme? Give a YES/NO answer. If you said NO explain what you think is the single biggest meanness of the scheme.

Explanation / Answer

The answer is NO. It is not a good scheme.

Both the possible keys are semi weak keys of DES. The semi-weak keys are keys that, when you use them to generate the 16 subkeys, generate only two different subkeys instead of 16 different ones.

DES also has six pairs of "semi-weak keys", for which encryption with one of the keys in the pair is equivalent to decryption with the other (and vice versa). Basically, these are the keys for which the bit patterns of KL and KR repeat with a period of 2, i.e.

KL,KR{000000…00,010101…01,101010…10,111111…11}

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