t is possible to use a hash function to construct a block cipher with a structure similar to DES.

Because a hash function is one way and a block cipher must be reversible $($to decrypt$),$ how is it

possible?

Question $4$

In the discussion of subkey generation in CMAC, it states that the block cipher is applied to the

block that consists entirely of $0$ bits. The first subkey is derived from the resulting string by a left

shift of one bit and, conditionally, by XORing a constant that depends on the block size. The

second subkey is derived in the same manner from the first subkey.

$$ What constants are needed for block sizes of $64$ and $128$ bits?

$$ Explain how the left shift and XOR accomplishes the desired result.