QUESTION 1 In last class we discussed an encryption scheme that achieves computa

Question

QUESTION 1 In last class we discussed an encryption scheme that achieves computational indistinguishability with key space smaller than the plaintext space. Assume k E(0,1)" and m (0, 1J2. Which of the following describes the encryption algorithm? Enc(k,m) m XOR (k | | k), where "||" denotes string concatenation Enc(k,m) m XOR G(k), where G) is a pseudorandom generator that outputs strings of size double the seed size. Enck,m) = (m1 XOR k) l l (m2 XOR k), where m-m1 1 1 m2, i.e. mi is of the first part of the message m, i.e. Im1l-n, and m2 the second part of the message. Enc(k,m) k XOR G(m), where G) is a pseudorandom generator that outputs strings of size double the seed size.

Explanation / Answer

Actuall in any encryption what we generally do is:

Divide the message M into m1,m2,m3,m4...........mn, where each mi size is equal to the size of key.

in other word every generated small block should be the same size of Key, if key contains n bits so each message block will contain n bits, in this way we divide, Then each block is used to encrypt with Key.

So OPTION 1 is false, Because i already told you, message are divided, not keys are concatinated.

OPTION 2 is false, because if from key random numbers are generated so at decryption time, you will not know how priviously random number were generated during encryption.

OPTION 3 is correct, as i told you earlier, message are divide, here m is divided to m1 and m2, and their lengths are equal, so this is valid answer.

OPTION 4 is also false.

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