4.1 A. In Section 4.1, under the subsection on the motivation for the Feistel cipher structure, it was stated that, for a block of n bits, the number of different reversible mappings for the ideal block cipher is 2 n !.  Justify

 

7.1 You want to build a hardware device to do block encryption in the cipher block chaining (CBC) mode using an algorithm stronger than DES. 3DES is a good candidate. Figure 7.17 shows two possibilities, both of which follow from the definition of CBC. Which of the two would you choose:

1.    For security?

2.    For performance?

7.4 With the ECB mode, if there is an error in a block of the transmitted ciphertext, only the corresponding plaintext block is affected. However, in the CBC mode, this error propagates. For example, an error in the transmitted C 1  (Figure 7.4) obviously corrupts P 1   and P 2 . 

1.    Are any blocks beyond P 2   affected?

2.    Suppose that there is a bit error in the source version of P 1 .  Through how many ciphertext blocks is this error propagated? What is the effect at the receiver?

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