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 6.4) obviously corrupts P₁ and P₂.

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?

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IV K K IV Encrypt C C Decrypt P K Encrypt C (a) Encryption Decrypt P (b) Decryption Figure 6.4 Cipher Block Chaining (CFB) Mode CN-1 K CN-1 PN Encrypt CN CN Decrypt