Question

Assume that Alice sends a message of 640 bits encrypted using DES to Bob and that the 65th bit of the ciphertext is flipped during transmission. How many blocks of the plaintext at the Bob’s side are affected if the message is encrypted under ECB, CBC, 8-bit CFB, 8-bit OFB, and CTR modes, respectively?

Answer 1

Solution

ECB

ECB: 8 bits. Only bits in that block will be affected.

CBC

CBC: 575 bits, all corresponding cipher blocks are affected

CFB

CFB – 16 bits, the corresponding block and the next are affected.

OFB

OFB: 8 bits, Only affects the corresponding bit

CTR

CTR: None

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