Question

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 C1 (Figure 20.6) obviously corrupts
P1 and P21.
a. Are any blocks beyond P2 affected?
b. Suppose that there is a bit error in the source version of P1. Through how many
ciphertext blocks is this error propagated? What is the effect at the receiver?

Answer 1

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Answer 2

Answer:

a) No. The output block P3 depends only on the input blocks $\mathrm{C} 2$  $\mathrm{C} 3$.

b) An error in $\mathrm{P} 1$ effects C1. But since $\mathrm{C} 1$ the input to the calculation  $\mathrm{C} 2, \mathrm{C} 2$is also affected. This effect carries through indefinitely so that all ciphertext blocks are affected. However, at the receiving end, the decryption algorithm restores the correct plaintext for blocks except the one in error. Therefore, the error only affects the corresponding decrypted plaintext block.

Explanation:

• The initial code is given by codec.png
• if C1 is wrong, it only involves P1 and P2
• All of the ciphers will totally different with the original cipher text as given below in image

• The image is attached as codechange.png
• That means all plain text block will depend on P1
• the receiver will get the P1 with a bit error and the rest plain text will not be changed at all the reason is plain text does not involve the cipher text
• It just like transmits an bit different plain text in this case