Suppose that Alice and Bob use CBC mode encryption.
a. What security problems arise if they always use a fixed initialization vector (IV), as opposed to choosing IVs at random? Explain.
b. Suppose that Alice and Bob choose IVs in sequence, that is, they first use 0 as an IV, then they use 1 as their IV, then 2, and so on. Does this create any security problems as compared to choosing the IVs at random?
Recall that a MAC is given by the CBC residue, that is, the last ciphertext block when the data is encrypted in CBC mode. Given data X, key K, and an IV, define F(X) to be the MAC of X.
a. Is F one-way, that is, given F(X) is it possible to determine ΧΊ
b. Is F collision resistant, that is, given F(X) is it possible to find a value Y such that F(Y) = F(X)7
Obtain the files Alice.bmp and Alice.jpg from the textbook website.
a. Use the TEA cipher to encrypt Alice.bmp in ECB mode, leaving the first 10 blocks unencrypted. View the encrypted image. What do you see? Explain the result.
b. Use the TEA cipher to encrypt Alice.jpg in ECB mode, leaving the first 10 blocks unencrypted. View the encrypted image. What do you see? Explain the result.
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