Suppose Alice is using a block cipher to send the message THE ORDER IS KARL, ANDY,...

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, which ones?
(c) Suppose an attacker manipulates the ciphertext so that instead of 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 and punctuation)

(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 to a plausible plaintext sentence of at least five words (add spaces and punctuation to your sentence).

Solutions

Expert Solution

Colby Messinger answered 1 year ago

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