Suppose Alice is using a block cipher to send the message
THE ORDER IS KARL, ANDY, FRED AND IAN. IAN AND ANDY HAVE LEFT.
to Bob. Assume that
the block cipher is used in ECB mode,
the English is divided into plaintext blocks of 2 letters(ignore spaces and punctuation)
the ciphertext blocks are denoted C1,C2,...,C23
(a) Write down the 23 plaintext blocks.
(b) Will any of the ciphertext blocks be repeated? If so, whichones?
(c) Suppose an attacker manipulates the ciphertext so that insteadof receiving C1,C2,...,C23.
Bob receives
C1, C2, C3, C4, C5, C10, C11, C6, C7, C8, C9, C12, C13, C14,C15, C16, C17, C6, C7, C20, C21, C22, C23
What does Bob think the message Alice sent is? (add spaces andpunctuation)
(d) If Bob receives
C10, C8, C6, C17, C6, C7, C20, C21, C22, C23
what does he think the message is? (add spaces and punctuation)
(e) Find another sequence of ciphertext blocks that decrypts toa plausible plaintext sentence of at least five words (add spacesand punctuation to your sentence).
