What can you learn about m0 and m1

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Reference no: EM132164243

When using block-cipher mode of opeartion, in particular, CTR and CBC modes, it is important that the Initialization Vectors (IV) is chosen uniformly and randomly upon every invocation of the encryption algorithm, so that, the probability that the same IV is chosen in two invocation of the encryption algorithm is very small. So what really happens when the IVs do collide?

Let E : {0, 1} k × {0, 1} l ? {0, 1} l be the block cipher used in the CTR and CBC mode. Consider encrypting two messages M0 = M0[1]M0[2] · · · M0[q] and M1 = M1[1]M1[2] · · · M1[q], each consisting of exactly q blocks of length l.

1. Encrypt M0 and M1 using CTR mode to obtain ciphertexts C0 = C0[0]C0[1] · · · C0[q] and C1 = C1[0]C1[1] · · · C1[q]. If it happens that C0[0] = C1[0], what can you learn about M0, M1 from C0, C1?

2. Instead of CTR mode, the CBC mode is used to encrypt the messages. Again, if it happens that C0[0] = C1[0], what can you learn about M0, M1 from C0, C1?

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Reference no: EM132164243

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